AmpSizer

12 Volt Wire Size Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The table below is the shape of the problem: the same 10 amps, the same 3% target, at three system voltages.

Run	12 V	24 V	48 V
10 ft	10 AWG	14 AWG	14 AWG
20 ft	8 AWG	12 AWG	14 AWG
30 ft	6 AWG	10 AWG	14 AWG
50 ft	3 AWG	8 AWG	12 AWG
100 ft	1/0 AWG	4 AWG	8 AWG

At 10 amps and a 3% target, copper. The right-hand column is the argument for higher-voltage battery banks in one glance: a 100 ft run at 48 V wants 8 AWG, and the same run at 12 V wants 1/0.

About

Mon, 10 Aug 2026 00:00:00 +0000

AmpSizer answers one kind of question: how big does this have to be? The wire, the breaker, the ground, the conduit, the generator, the transformer. Each gets its own calculator, because the arithmetic is shared and everything after it — which table applies, which ceiling bites first — is not.

Why it exists

There is no shortage of electrical calculators. There is a shortage of ones that finish the job. Wire manufacturers publish good tools and stop at a single number, because the tool exists to sell cable. General calculator sites do voltage drop and nothing else. And the biggest cluster of searches in this subject — people looking for a wire gauge chart — is answered almost entirely by articles with a table pasted into them and no way to check your own numbers against it.

Ampacity Derating Explained

Mon, 10 Aug 2026 00:00:00 +0000

The two tables

Ambient temperature correction — NEC Table 310.15(B)(1), based on 86 °F (30 °C).
Ambient	60 °C wire	75 °C wire	90 °C wire
50–77 F (10–25 C)	× 1.08	× 1.05	× 1.04
78–86 F (26–30 C)	× 1	× 1	× 1
87–95 F (31–35 C)	× 0.91	× 0.94	× 0.96
96–104 F (36–40 C)	× 0.82	× 0.88	× 0.91
105–113 F (41–45 C)	× 0.71	× 0.82	× 0.87
114–122 F (46–50 C)	× 0.58	× 0.75	× 0.82
123–131 F (51–55 C)	× 0.41	× 0.67	× 0.76
132–140 F (56–60 C)	not permitted	× 0.58	× 0.71

Adjustment for more than three current-carrying conductors bundled together — NEC Table 310.15(C)(1).
Current-carrying conductors	Multiply ampacity by
1 to 3	× 1
4 to 6	× 0.8
7 to 9	× 0.7
10 to 20	× 0.5
21 to 30	× 0.45
31 to 40	× 0.4
41 and more	× 0.35

Worked examples

A garage circuit in a hot attic. 12 AWG THHN, 20 A load, attic at 113 °F. Starting from the 90 °C column at 30 A: the ambient factor at 113 °F for a 90 °C conductor is 0.87, giving 26.1 A. Then 110.14(C) caps it at the 75 °C value of 25 A. Result: 25 A, and the 20 A circuit is fine. Had we started from 75 °C, 25 × 0.82 would be 20.5 A — still fine, but with almost nothing left.

Amps to Watts Calculator

Mon, 10 Aug 2026 00:00:00 +0000

What each circuit in a house can deliver, as watts, at the two voltages that matter.

Circuit	At 120 V	At 240 V	Continuous ceiling at 240 V
15 A	1,800 W	3,600 W	2,880 W
20 A	2,400 W	4,800 W	3,840 W
30 A	3,600 W	7,200 W	5,760 W
40 A	4,800 W	9,600 W	7,680 W
50 A	6,000 W	12,000 W	9,600 W
100 A	12,000 W	24,000 W	19,200 W

The last column is the honest design figure for anything running for hours. The middle column is where the protection operates.

AWG to mm² Conversion

Mon, 10 Aug 2026 00:00:00 +0000

The conversion table

Every size, both scales, with the diameter and the insulated area that conduit fill is worked out from.

Conductor size, area, resistance and allowable ampacity. Ampacity is NEC Table 310.16 — not more than three current-carrying conductors in a raceway, cable or earth, at 86 °F (30 °C) ambient. Correct it for heat and for bundling before you use it.
Size	kcmil	mm²	Diameter, in	THHN area, in²
14 AWG	4.11	2.08	0.073	0.0097
12 AWG	6.53	3.31	0.092	0.0133
10 AWG	10.38	5.261	0.116	0.0211
8 AWG	16.51	8.367	0.146	0.0366
6 AWG	26.24	13.3	0.184	0.0507
4 AWG	41.74	21.15	0.232	0.0824
3 AWG	52.62	26.67	0.26	0.0973
2 AWG	66.36	33.62	0.292	0.1158
1 AWG	83.69	42.41	0.332	0.1562
1/0 AWG	105.6	53.49	0.373	0.1855
2/0 AWG	133.1	67.43	0.419	0.2223
3/0 AWG	167.8	85.01	0.47	0.2679
4/0 AWG	211.6	107.2	0.528	0.3237
250 kcmil	250	127	0.575	0.397
300 kcmil	300	152	0.63	0.4608
350 kcmil	350	177	0.681	0.5242
400 kcmil	400	203	0.728	0.5863
500 kcmil	500	253	0.813	0.7073
600 kcmil	600	304	0.893	0.8676
700 kcmil	700	355	0.964	0.9887
750 kcmil	750	380	0.998	1.0496

Nearest stocked metric size

The two systems interleave rather than align. This is where each common gauge falls between the metric sizes that are actually sold, and which one to buy for a like-for-like replacement.

Backup Power Load Plan

Mon, 10 Aug 2026 00:00:00 +0000

Every load you intend to back up, with its running watts and starting surge, totalled into the generator size that covers both.

Box Fill Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The allowances are short enough to memorise, and worth memorising because they turn a box question into mental arithmetic on site.

Conductor	Allowance each	Device yoke	Clamps, all together	Grounds, all together
14 AWG	2.00 cu in	4.00 cu in	2.00 cu in	2.00 cu in
12 AWG	2.25 cu in	4.50 cu in	2.25 cu in	2.25 cu in
10 AWG	2.50 cu in	5.00 cu in	2.50 cu in	2.50 cu in
8 AWG	3.00 cu in	6.00 cu in	3.00 cu in	3.00 cu in
6 AWG	5.00 cu in	10.00 cu in	5.00 cu in	5.00 cu in

The right-hand three columns are all keyed to the largest conductor in the box, so one 10 AWG conductor in a box otherwise full of 12s raises the cost of the device, the clamps and the grounds as well as its own allowance.

Breaker and Wire Size Chart

Mon, 10 Aug 2026 00:00:00 +0000

The chart

Breaker rating against the smallest conductor that carries it at 75 °C, with no derating and a short run. Computed from NEC Table 310.16, 240.4(D) and Table 250.122.
Breaker	Copper	Aluminium	Copper ground	What it usually feeds
15 A	14 AWG	12 AWG	14 AWG	Lighting and general receptacles
20 A	12 AWG	12 AWG	12 AWG	Kitchen, bathroom and garage receptacles
30 A	10 AWG	10 AWG	10 AWG	Dryer, small water heater, RV outlet
40 A	8 AWG	8 AWG	10 AWG	Range, 40 A EV charger
50 A	8 AWG	6 AWG	10 AWG	Range, welder, 50 A EV charger, hot tub
60 A	6 AWG	4 AWG	10 AWG	Sub panel, large hot tub, tankless heater
70 A	4 AWG	3 AWG	8 AWG	Sub panel, larger tankless heater
100 A	3 AWG	1 AWG	8 AWG	Sub panel, or a small service
125 A	1 AWG	2/0 AWG	6 AWG	Sub panel
150 A	1/0 AWG	3/0 AWG	6 AWG	Service, or a large sub panel
200 A	3/0 AWG	250 kcmil	6 AWG	Typical whole-house service

The three rules that move a row

240.4(D), the small-conductor rule. 14 AWG copper is held to 15 A, 12 AWG to 20 A and 10 AWG to 30 A — whatever Table 310.16 says they can carry. It is why 12 AWG reads 25 A in the table and is still a 20 amp circuit, and it is the difference between the ampacity chart and this one.

Breaker Size Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The chart below pairs every common device rating with the conductor it is allowed to protect, computed from the same tables as the calculator. The 240.4(D) ceiling is already applied, which is why 20 A shows 12 AWG rather than the 25 A that Table 310.16 would suggest.

Breaker rating against the smallest conductor that carries it at 75 °C, with no derating and a short run. Computed from NEC Table 310.16, 240.4(D) and Table 250.122.
Breaker	Copper	Aluminium	Copper ground	What it usually feeds
15 A	14 AWG	12 AWG	14 AWG	Lighting and general receptacles
20 A	12 AWG	12 AWG	12 AWG	Kitchen, bathroom and garage receptacles
30 A	10 AWG	10 AWG	10 AWG	Dryer, small water heater, RV outlet
40 A	8 AWG	8 AWG	10 AWG	Range, 40 A EV charger
50 A	8 AWG	6 AWG	10 AWG	Range, welder, 50 A EV charger, hot tub
60 A	6 AWG	4 AWG	10 AWG	Sub panel, large hot tub, tankless heater
70 A	4 AWG	3 AWG	8 AWG	Sub panel, larger tankless heater
100 A	3 AWG	1 AWG	8 AWG	Sub panel, or a small service
125 A	1 AWG	2/0 AWG	6 AWG	Sub panel
150 A	1/0 AWG	3/0 AWG	6 AWG	Service, or a large sub panel
200 A	3/0 AWG	250 kcmil	6 AWG	Typical whole-house service

The standard ratings, and why they matter

240.6(A) is a list, and a device that is not on it effectively does not exist for sizing purposes:

Cable Tray Fill Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The table below is how many single conductors of each large size fit side by side in each standard tray width — the single-layer rule turned into a lookup.

Conductor	Diameter	6 in	12 in	18 in	24 in	36 in
4/0 AWG	0.528 in	11	22	34	45	68
250 kcmil	0.575 in	10	20	31	41	62
350 kcmil	0.681 in	8	17	26	35	52
500 kcmil	0.813 in	7	14	22	29	44
750 kcmil	0.998 in	6	12	18	24	36

Diameters are from Chapter 9, Table 8. In practice you will run out of ampacity or out of budget long before you run out of tray width at these numbers — which is the useful thing to know, because it means the width decision is usually about future circuits rather than about this one.

Circuit Load Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The four numbers that matter for the circuits in a house, in one table.

Circuit	Trips at	Continuous ceiling	Typical real load
15 A at 120 V	1,800 W	1,440 W	Lighting and receptacles, a few hundred watts
20 A at 120 V	2,400 W	1,920 W	Kitchen counter, one large appliance at a time
30 A at 240 V	7,200 W	5,760 W	Dryer, small water heater
40 A at 240 V	9,600 W	7,680 W	Range, mid-size EV charger
50 A at 240 V	12,000 W	9,600 W	Large range, hot tub, 50 A charger

The middle column is the honest one to design against. The left column is where the protection operates, which is not the same thing as where the circuit is comfortable.

Conduit Fill Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The chart below is the whole of EMT at the 40% limit, computed from the code tables rather than retyped from one. Every column is a trade size, every row a conductor, and every cell is how many of that conductor the pipe legally takes.

EMT (thin-wall steel) — how many THHN or THWN-2 conductors of one size fit at the 40 % fill limit. Computed from NEC Chapter 9, Table 4 and Table 5.
Conductor	1/2"	3/4"	1"	1-1/4"	1-1/2"	2"	2-1/2"	3"	3-1/2"	4"
14 AWG	12	21	35	61	83	138	241	364	476	608
12 AWG	9	16	25	44	61	100	176	266	347	443
10 AWG	5	10	16	28	38	63	111	167	218	279
8 AWG	3	5	9	16	22	36	64	96	126	161
6 AWG	2	4	6	11	16	26	46	69	91	116
4 AWG	1	2	4	7	9	16	28	42	56	71
3 AWG	1	2	3	6	8	13	24	36	47	60
2 AWG	1	1	2	5	7	11	20	30	39	50
1 AWG	—	1	2	3	5	8	15	22	29	37
1/0 AWG	—	1	1	3	4	7	12	19	24	31
2/0 AWG	—	—	1	2	3	6	10	15	20	26
3/0 AWG	—	—	1	2	3	5	8	13	17	22
4/0 AWG	—	—	1	1	2	4	7	10	14	18
250 kcmil	—	—	—	1	2	3	5	8	11	14
300 kcmil	—	—	—	1	1	2	5	7	10	12
350 kcmil	—	—	—	1	1	2	4	6	8	11
400 kcmil	—	—	—	1	1	2	3	6	7	10
500 kcmil	—	—	—	—	1	1	3	5	6	8
600 kcmil	—	—	—	—	—	1	2	4	5	6
700 kcmil	—	—	—	—	—	1	2	3	4	5
750 kcmil	—	—	—	—	—	1	2	3	4	5

PVC Schedule 40

Underground and outdoors, PVC is the usual choice, and its internal area is slightly smaller than EMT of the same trade size — so a pull that just fits in EMT can be one conductor over the line in PVC.

Contact

Mon, 10 Aug 2026 00:00:00 +0000

Questions, suggestions or corrections? Email us:

info@ampsizer.com

We usually reply within a few working days.

If you are writing about a number, the fastest way to get it fixed is to say which table you checked it against — every calculator names its tables under How we calculate.

What we cannot do: tell you whether a specific installation is safe or compliant. We cannot see your panel, your terminals, your conditions of use or your local amendments, and a wrong answer to that question is not an inconvenience. That is what a licensed electrician and your inspector are for.

Copper vs Aluminum Wire

Mon, 10 Aug 2026 00:00:00 +0000

Size for size, and amp for amp

Conductor size, area, resistance and allowable ampacity. Ampacity is NEC Table 310.16 — not more than three current-carrying conductors in a raceway, cable or earth, at 86 °F (30 °C) ambient. Correct it for heat and for bundling before you use it.
Size	kcmil	mm²	Cu 60 °C	Cu 75 °C	Cu 90 °C	Al 75 °C	Cu Ω/kft	Al Ω/kft
14 AWG	4.11	2.08	15 A	20 A	25 A	—	3.14	—
12 AWG	6.53	3.31	20 A	25 A	30 A	20 A	1.98	3.18
10 AWG	10.38	5.261	30 A	35 A	40 A	30 A	1.24	2
8 AWG	16.51	8.367	40 A	50 A	55 A	40 A	0.778	1.26
6 AWG	26.24	13.3	55 A	65 A	75 A	50 A	0.491	0.808
4 AWG	41.74	21.15	70 A	85 A	95 A	65 A	0.308	0.508
3 AWG	52.62	26.67	85 A	100 A	115 A	75 A	0.245	0.403
2 AWG	66.36	33.62	95 A	115 A	130 A	90 A	0.194	0.319
1 AWG	83.69	42.41	110 A	130 A	145 A	100 A	0.154	0.253
1/0 AWG	105.6	53.49	125 A	150 A	170 A	120 A	0.122	0.201
2/0 AWG	133.1	67.43	145 A	175 A	195 A	135 A	0.0967	0.159
3/0 AWG	167.8	85.01	165 A	200 A	225 A	155 A	0.0766	0.126
4/0 AWG	211.6	107.2	195 A	230 A	260 A	180 A	0.0608	0.1
250 kcmil	250	127	215 A	255 A	290 A	205 A	0.0515	0.0847
300 kcmil	300	152	240 A	285 A	320 A	230 A	0.0429	0.0707
350 kcmil	350	177	260 A	310 A	350 A	250 A	0.0367	0.0605
400 kcmil	400	203	280 A	335 A	380 A	270 A	0.0321	0.0529
500 kcmil	500	253	320 A	380 A	430 A	310 A	0.0258	0.0424
600 kcmil	600	304	350 A	420 A	475 A	340 A	0.0214	0.0353
700 kcmil	700	355	385 A	460 A	520 A	375 A	0.0184	0.0303
750 kcmil	750	380	400 A	475 A	535 A	385 A	0.0171	0.0282

Read that table twice. Once across a row, to see what the same size gives you in each metal — 4 AWG is 85 A in copper and 65 A in aluminium. Then down the columns, to find the aluminium size that matches a copper one: 85 A in aluminium is 2 AWG, two steps up from 4.

Dwelling Load Worksheet

Mon, 10 Aug 2026 00:00:00 +0000

The standard-method load calculation for a house, line by line, totalled into the service size — the sheet a permit application asks for.

Electrical Test Schedule

Mon, 10 Aug 2026 00:00:00 +0000

Set the interval once and get the next twelve dates for GFCI tests, panel checks and thermal scans, with an email before each one.

Electrician Quote Comparison

Mon, 10 Aug 2026 00:00:00 +0000

Materials, labour, permit and making good for each quote in one table — the total that decides, not the number on the front page.

EV Charger Circuit Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The chargers people actually buy, with the circuit each one legally needs.

Charger output	Circuit at 125%	Breaker	Copper	Aluminium	Ground	Approx. miles per hour
16 A	20 A	20 A	12 AWG	12 AWG	12 AWG	~13
24 A	30 A	30 A	10 AWG	10 AWG	10 AWG	~19
32 A	40 A	40 A	8 AWG	8 AWG	10 AWG	~25
40 A	50 A	50 A	8 AWG	6 AWG	10 AWG	~31
48 A	60 A	60 A	6 AWG	4 AWG	10 AWG	~37
64 A	80 A	80 A	4 AWG	2 AWG	8 AWG	~49
80 A	100 A	100 A	3 AWG	1 AWG	8 AWG	~61

Copper at 75 °C on a short run. Range added assumes 240 V and about 3.5 miles per kWh, which is a mid-size sedan in mild weather.

Extension Cord Size Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The table below is the practical version: how much voltage each common cord gauge loses at a real load, at the three lengths cords are actually sold in.

Load	25 ft	50 ft	100 ft	Buy
5 A — lamp, laptop, fan	16 ga: 1.0%	16 ga: 2.1%	16 ga: 4.2%	16 ga short, 14 ga long
10 A — circular saw, vacuum	14 ga: 1.3%	14 ga: 2.6%	14 ga: 5.2%	14 ga short, 12 ga long
12 A — mitre saw, pressure washer	14 ga: 1.6%	12 ga: 2.0%	10 ga: 2.5%	12 ga to 50 ft, 10 ga beyond
15 A — table saw, compressor	12 ga: 1.2%	12 ga: 2.5%	10 ga: 3.1%	12 ga to 50 ft, 10 ga beyond
20 A — welder, large compressor	12 ga: 1.7%	12 ga: 3.3%	10 ga: 4.1%	10 ga, and keep it short

Percentages are at 120 V with copper conductors. The “buy” column is the size that keeps the loss near or under 3% at that length, which is the same target used for fixed wiring.

Free and Pro

Mon, 10 Aug 2026 00:00:00 +0000

Everything that calculates here is free and always will be — no account, no sign-up, no limits on how often you use it. That is not a trial. Search engines send people here to get an answer, and an answer behind a wall is not an answer.

Pro is for what happens around the answer. It remembers your setups on every device you sign in on, turns any result into a page you can send someone, lets you put a tool on your own site, emails you when a date you care about is coming, and takes the ads off every site we run — with one account and one price.

Generator Size Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The loads people actually back up, with the two numbers each one needs.

Load	Running watts	Starting surge
Refrigerator or freezer	150–800 W	1,200–2,200 W
Furnace blower, 1/2 HP	600–900 W	1,800–2,400 W
Well pump, 1/2 HP	800–1,000 W	2,000–3,000 W
Sump pump	800–1,050 W	1,500–2,900 W
Window air conditioner, 10k BTU	900–1,200 W	2,000–3,000 W
Microwave	1,000–1,500 W	none
Lighting, whole floor in LED	100–300 W	none
Television and router	100–200 W	none
Coffee maker or kettle	1,000–1,500 W	none
Electric water heater	4,500 W	none
Electric range, one element	2,000–3,000 W	none
Electric dryer	5,400 W	none

The top half of that table is what a portable generator is for. The bottom half is why an outage still feels like an outage.

Ground Wire Size Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The whole table is below. It is short, it has not changed materially in decades, and it is worth knowing by heart for the sizes you use every week: 15 A takes 14, 20 A takes 12, everything from 25 to 60 A takes 10, and 100 A takes 8.

Minimum equipment grounding conductor by the rating of the device ahead of the circuit — NEC Table 250.122. Upsize it in proportion when the circuit conductors are upsized for voltage drop, per 250.122(B).
Device rating	Copper	Aluminium	Copper in mm²
Up to 15 A	14 AWG	12 AWG	2.08 mm²
Up to 20 A	12 AWG	10 AWG	3.31 mm²
Up to 60 A	10 AWG	8 AWG	5.261 mm²
Up to 100 A	8 AWG	6 AWG	8.367 mm²
Up to 200 A	6 AWG	4 AWG	13.3 mm²
Up to 300 A	4 AWG	2 AWG	21.15 mm²
Up to 400 A	3 AWG	1 AWG	26.67 mm²
Up to 500 A	2 AWG	1/0 AWG	33.62 mm²
Up to 600 A	1 AWG	2/0 AWG	42.41 mm²
Up to 800 A	1/0 AWG	3/0 AWG	53.49 mm²
Up to 1000 A	2/0 AWG	4/0 AWG	67.43 mm²
Up to 1200 A	3/0 AWG	250 kcmil	85.01 mm²

Why it is sized on amps and not on the wire

The grounding conductor has one job: carry enough fault current, for long enough, to make the overcurrent device operate. What decides that is the device — how much current it needs to see and for how long — not how thick the circuit conductors happen to be.

Hot Tub Wire Size Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The spa sizes on the market, with the circuit each one needs at a normal distance.

Nameplate	Breaker	Copper	Ground	Conduit, 4 wires	Drop at 100 ft
20 A — plug-in 120 V spa	20 A	12 AWG	12 AWG	1/2 in	6.6% at 120 V
30 A	30 A	10 AWG	10 AWG	3/4 in	3.1%
40 A	40 A	8 AWG	10 AWG	3/4 in	2.6%
50 A	50 A	8 AWG	10 AWG	3/4 in	3.2%
60 A	60 A	6 AWG	10 AWG	1 in	2.5%

Copper at 75 °C, 240 V except the first row. The 120 V plug-in spa is the one with a real voltage-drop problem, and it is also the one nobody calculates.

How Voltage Drop Is Calculated

Mon, 10 Aug 2026 00:00:00 +0000

Working it through, line by line

Take a detached garage: a 20 amp circuit, 100 feet of 12 AWG copper, 120 volts.

Step 1 — find the resistance. Chapter 9, Table 8 gives 12 AWG stranded copper as 1.98 ohms per 1,000 ft.

Step 2 — account for both conductors. The current goes out and comes back, so it passes through 200 feet of copper. That is the factor of two.

How We Calculate

Mon, 10 Aug 2026 00:00:00 +0000

Every calculator on this site runs the same three steps in the same order an electrician does them: work out the current, look up what the conductor can carry, then check whether the run is too long for it. Whichever of the last two asks for more copper is the answer.

The formulas

Voltage drop: k × amps × R × one-way feet ÷ 1,000, where k is 2 for single-phase and DC and 1.732 for balanced three-phase, and R is ohms per 1,000 ft.
Percent drop: volts lost ÷ nominal system voltage × 100.
Longest run at a target: target% ÷ 100 × volts × 1,000 ÷ (k × amps × R).
Continuous load: amps × 1.25, per 210.19(A)(1) and 215.2(A)(1). Lighting, EV charging and process heat are continuous; a general receptacle circuit is not.
Usable ampacity: table ampacity × ambient factor × bundling factor.
Conduit fill: total conductor area ÷ internal raceway area. The ceiling is 53% for one conductor, 31% for two, 40% for more than two, and 60% in a nipple no longer than 24 in.
Box fill: a volume allowance for every conductor, twice the largest conductor for each device yoke, one allowance for all internal clamps together, one for all grounding conductors together.
Motors: conductors at 125% of the table full-load current (430.22), inverse-time breaker up to 250% (430.52), overload relay at 115–125% of the nameplate current (430.32).
Transformers: kVA × 1,000 ÷ (k × volts) on each side, then 125% of that current for the protective device.

The tables we read, and where they are

What	NEC table	What we use it for
Allowable ampacity	Table 310.16	60, 75 and 90 °C columns, copper and aluminium
Ambient correction	Table 310.15(B)(1)	anything hotter or colder than 86 °F (30 °C)
Bundling adjustment	Table 310.15(C)(1)	more than three current-carrying conductors together
Small-conductor ceiling	240.4(D)	14 AWG at 15 A, 12 at 20, 10 at 30
Standard device ratings	240.6(A)	rounding a load up to a breaker that exists
Equipment grounding conductor	Table 250.122	the ground wire, and 250.122(B) when the circuit is upsized
Conductor area and resistance	Chapter 9, Table 8	kcmil, mm², diameter, ohms per 1,000 ft
Insulated conductor area	Chapter 9, Table 5	THHN/THWN-2 and XHHW-2 cross-sections
Raceway internal area	Chapter 9, Table 4	EMT, PVC 40 and 80, RMC, IMC, ENT, FMC
Fill percentages	Chapter 9, Table 1	53 / 31 / 40 %, and the nipple note
Box volume allowances	314.16(B) and Table 314.16(A)	cubic inches needed, and the boxes that are sold
Flexible cord ampacity	Table 400.5(A)(1)	extension cords, which 310.16 does not cover
Motor full-load current	Tables 430.248 and 430.250	single-phase and three-phase motors

The full conductor table

This is the file every calculator on the site reads. The reference pages draw the same rows, so a chart here and an answer in a widget cannot disagree.

kVA to Amps Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The transformer and generator sizes that actually exist, in amps, at the voltages they are usually connected to.

Rating	240 V, 1-phase	208 V, 3-phase	480 V, 3-phase
5 kVA	20.8 A	13.9 A	6.0 A
10 kVA	41.7 A	27.8 A	12.0 A
15 kVA	62.5 A	41.6 A	18.0 A
25 kVA	104.2 A	69.4 A	30.1 A
45 kVA	187.5 A	124.9 A	54.1 A
75 kVA	312.5 A	208.2 A	90.2 A
112.5 kVA	468.8 A	312.3 A	135.3 A

The pattern worth internalising: at 480 V three-phase, amps are roughly 1.2 × kVA. At 208 V three-phase, amps are roughly 2.8 × kVA. Those two shortcuts cover most of commercial work.

kW to Amps Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The same kilowatts at the four voltages that cover almost all US work, at unity power factor.

Load	120 V, 1-ph	240 V, 1-ph	208 V, 3-ph	480 V, 3-ph
1 kW	8.3 A	4.2 A	2.8 A	1.2 A
3 kW	25.0 A	12.5 A	8.3 A	3.6 A
5 kW	41.7 A	20.8 A	13.9 A	6.0 A
7.5 kW	62.5 A	31.3 A	20.8 A	9.0 A
10 kW	83.3 A	41.7 A	27.8 A	12.0 A
15 kW	125.0 A	62.5 A	41.6 A	18.0 A
20 kW	166.7 A	83.3 A	55.5 A	24.1 A

Multiply everything in the last two columns by 1.25 for a power factor of 0.8, which is the honest number for motor loads.

Mini Split Wire Size Calculator

Mon, 10 Aug 2026 00:00:00 +0000

Typical nameplate figures by system size, with the circuit each one wants. Always read your own nameplate — these vary by manufacturer and by efficiency rating far more than they vary by capacity.

System	Typical MCA	Typical MOCP	Copper	Ground	Conduit
9,000 BTU, 240 V	10–12 A	15–20 A	14 AWG	14 AWG	1/2 in
12,000 BTU, 240 V	13–16 A	20 A	14 AWG	14 AWG	1/2 in
18,000 BTU, 240 V	17–20 A	25–30 A	12 AWG	12 AWG	1/2 in
24,000 BTU, 240 V	21–25 A	30–35 A	10 AWG	10 AWG	1/2 in
36,000 BTU, 240 V	28–32 A	40–50 A	8 AWG	10 AWG	3/4 in
Multi-zone, 48,000 BTU	35–45 A	50–60 A	8 AWG	10 AWG	3/4 in

Note how far apart MCA and MOCP drift as the system gets larger. At 36,000 BTU the conductor is sized for about 30 amps and the device may be 50 — which looks wrong on paper and is exactly what 440.22 intends, because the breaker’s job there is to survive the compressor start rather than to protect against overload.

Motor Amps Calculator

Mon, 10 Aug 2026 00:00:00 +0000

Both code tables, with the conductor each motor needs at 125% of its table current, computed rather than retyped.

Single-phase motors, Table 430.248

Single-phase motors — NEC Table 430.248. These are the currents the code makes you size conductors and devices from, whatever the nameplate says (430.6(A)(1)).
Motor	115 V	208 V	230 V	Copper at 125 %, 230 V
1/6 HP	4.4 A	2.4 A	2.2 A	14 AWG
1/4 HP	5.8 A	3.2 A	2.9 A	14 AWG
1/3 HP	7.2 A	4 A	3.6 A	14 AWG
1/2 HP	9.8 A	5.4 A	4.9 A	14 AWG
3/4 HP	13.8 A	7.6 A	6.9 A	14 AWG
1 HP	16 A	8.8 A	8 A	14 AWG
1-1/2 HP	20 A	11 A	10 A	14 AWG
2 HP	24 A	13.2 A	12 A	14 AWG
3 HP	34 A	18.7 A	17 A	10 AWG
5 HP	56 A	30.8 A	28 A	8 AWG
7-1/2 HP	80 A	44 A	40 A	8 AWG
10 HP	100 A	55 A	50 A	6 AWG

Three-phase motors, Table 430.250

Three-phase squirrel-cage induction motors — NEC Table 430.250. These are the currents the code makes you size conductors and devices from, whatever the nameplate says (430.6(A)(1)).
Motor	208 V	230 V	460 V	Copper at 125 %, 230 V
1/2 HP	2.4 A	2.2 A	1.1 A	14 AWG
3/4 HP	3.5 A	3.2 A	1.6 A	14 AWG
1 HP	4.6 A	4.2 A	2.1 A	14 AWG
1-1/2 HP	6.6 A	6 A	3 A	14 AWG
2 HP	7.5 A	6.8 A	3.4 A	14 AWG
3 HP	10.6 A	9.6 A	4.8 A	14 AWG
5 HP	16.7 A	15.2 A	7.6 A	12 AWG
7-1/2 HP	24.2 A	22 A	11 A	10 AWG
10 HP	30.8 A	28 A	14 A	8 AWG
15 HP	46.2 A	42 A	21 A	6 AWG
20 HP	59.4 A	54 A	27 A	4 AWG
25 HP	74.8 A	68 A	34 A	4 AWG
30 HP	88 A	80 A	40 A	3 AWG
40 HP	114 A	104 A	52 A	1 AWG
50 HP	143 A	130 A	65 A	2/0 AWG
60 HP	169 A	154 A	77 A	3/0 AWG
75 HP	211 A	192 A	96 A	250 kcmil
100 HP	273 A	248 A	124 A	350 kcmil

Why three phase halves the wire

Compare the two tables at 5 HP: 28 amps single phase at 230 V, 15.2 amps three phase at the same voltage. Same work, 46% of the current, two conductor sizes smaller.

NEC Conduit Fill Chart

Mon, 10 Aug 2026 00:00:00 +0000

EMT — thin-wall steel

The most common raceway in commercial interiors, and the reference everyone quotes.

EMT (thin-wall steel) — how many THHN or THWN-2 conductors of one size fit at the 40 % fill limit. Computed from NEC Chapter 9, Table 4 and Table 5.
Conductor	1/2"	3/4"	1"	1-1/4"	1-1/2"	2"	2-1/2"	3"	3-1/2"	4"
14 AWG	12	21	35	61	83	138	241	364	476	608
12 AWG	9	16	25	44	61	100	176	266	347	443
10 AWG	5	10	16	28	38	63	111	167	218	279
8 AWG	3	5	9	16	22	36	64	96	126	161
6 AWG	2	4	6	11	16	26	46	69	91	116
4 AWG	1	2	4	7	9	16	28	42	56	71
3 AWG	1	2	3	6	8	13	24	36	47	60
2 AWG	1	1	2	5	7	11	20	30	39	50
1 AWG	—	1	2	3	5	8	15	22	29	37
1/0 AWG	—	1	1	3	4	7	12	19	24	31
2/0 AWG	—	—	1	2	3	6	10	15	20	26
3/0 AWG	—	—	1	2	3	5	8	13	17	22
4/0 AWG	—	—	1	1	2	4	7	10	14	18
250 kcmil	—	—	—	1	2	3	5	8	11	14
300 kcmil	—	—	—	1	1	2	5	7	10	12
350 kcmil	—	—	—	1	1	2	4	6	8	11
400 kcmil	—	—	—	1	1	2	3	6	7	10
500 kcmil	—	—	—	—	1	1	3	5	6	8
600 kcmil	—	—	—	—	—	1	2	4	5	6
700 kcmil	—	—	—	—	—	1	2	3	4	5
750 kcmil	—	—	—	—	—	1	2	3	4	5

PVC Schedule 40

Underground, outdoors, and in slabs. Slightly less room than EMT of the same trade size.

Ohm's Law Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The twelve formulas, arranged by what you are solving for. This is the wheel, written out.

To find	From V and I	From V and R	From I and R	From P and V	From P and I	From P and R
Volts	—	—	V = I × R	V = P ÷ I	V = P ÷ I	V = √(P × R)
Amps	—	I = V ÷ R	—	I = P ÷ V	—	I = √(P ÷ R)
Ohms	R = V ÷ I	—	—	R = V² ÷ P	R = P ÷ I²	—
Watts	P = V × I	P = V² ÷ R	P = I² × R	—	—	—

Everything in that table is V = IR and P = VI rearranged. There is nothing else to learn.

Ohm's Law Explained

Mon, 10 Aug 2026 00:00:00 +0000

The two statements

V = I × R. Push harder and more current flows. Add resistance and less does. Georg Ohm published it in 1827 after measuring wires of different lengths, which is the same experiment as measuring voltage drop.

P = V × I. Power is the rate of energy transfer: how much voltage, moved through how much current.

Everything else is substitution. Put the first into the second and you get P = I² × R. Put it in the other direction and you get P = V² ÷ R.

Panel Circuit Schedule

Mon, 10 Aug 2026 00:00:00 +0000

Every circuit in the panel on one sheet — load, breaker and conductor per row, totalled against the service, saved and shareable.

Privacy

Mon, 10 Aug 2026 00:00:00 +0000

Who is responsible for your data

Operated by: maicrosites.com
Established in: Slovakia, European Union
Email: info@ampsizer.com
Site: ampsizer.com

There is no data protection officer. The site is run by one person and meets none of the conditions in Article 37 GDPR that would require one, so the address above reaches the person who actually decides what happens to your data — not a queue.

Short version: the calculators do not send anything anywhere. Every number you type is worked out in your own browser and forgotten when you close the tab.

Refunds and withdrawal

Mon, 10 Aug 2026 00:00:00 +0000

Last updated 2026-08-10.

The short version

You have 14 days from buying Pro to change your mind and get your money back. Press Withdraw from contract on your account page. We confirm by email straight away and the refund goes back to the payment method you used, within 14 days of that email.

You do not need a reason, and we do not ask you to give one up.

Your right of withdrawal

If you are a consumer in the EU, you have a statutory right to withdraw from this contract within 14 days without giving any reason. The period ends 14 days after the day the contract was concluded — that is, 14 days after you bought Pro.

Romex Wire Size Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The short version of this whole page is a five-row table, and it is worth memorising because it covers almost every circuit in a house.

Cable	Copper	Breaker	What it normally feeds
14/2	14 AWG	15 A	Lighting, bedroom and living room receptacles
12/2	12 AWG	20 A	Kitchen and bathroom receptacles, garage, laundry
12/3	12 AWG	20 A	Three-way switching, shared-neutral pairs
10/3	10 AWG	30 A	Electric dryer, small water heater
8/3	8 AWG	40 A	Electric range, cooktop
6/3	6 AWG	55 A	Large range, some sub feeds

The breaker column is the 60 °C ampacity capped by 240.4(D), which for 14, 12 and 10 gauge is the same number either way. From 8 AWG up the cap disappears and the 60 °C figure governs on its own — which is why 8/3 is a 40 A cable and not a 50 A one, even though 8 AWG THHN in conduit carries 50.

Sub Panel Feeder Size Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The panel sizes people actually install, with the conductors that feed them at short range and the distance at which voltage drop takes over.

Panel	Copper	Aluminium	Ground	Conduit for 4 wires	Drop takes over at
60 A	6 AWG	4 AWG	10 AWG	3/4 in	122 ft
100 A	3 AWG	1 AWG	8 AWG	1 in	147 ft
125 A	1 AWG	2/0 AWG	6 AWG	1-1/4 in	148 ft
150 A	1/0 AWG	3/0 AWG	6 AWG	1-1/4 in	156 ft
200 A	3/0 AWG	250 kcmil	6 AWG	2 in	187 ft

Copper conductors at 75 °C, 240 V, 3% target. Conduit is EMT for two hots, a neutral and a ground of the sizes shown.

Terms of use

Mon, 10 Aug 2026 00:00:00 +0000

Last updated 2026-08-10.

These terms cover ampsizer.com (“the site”). Using the site means you accept them. If you buy Pro, the Pro section below is part of your contract.

Who you are dealing with

Operated by: maicrosites.com
Established in: Slovakia, European Union
Email: info@ampsizer.com
Site: ampsizer.com

1. What the site is

AmpSizer is a set of calculators and reference pages. Everything that calculates runs in your browser and is free to use, with no account.

Three Phase Power Calculator

Mon, 10 Aug 2026 00:00:00 +0000

What a given three-phase current is worth in power, at the two systems that cover most work. Unity power factor; multiply by your own for real loads.

Current	208 V, 3-phase	480 V, 3-phase	600 V, 3-phase
10 A	3.6 kVA	8.3 kVA	10.4 kVA
20 A	7.2 kVA	16.6 kVA	20.8 kVA
30 A	10.8 kVA	24.9 kVA	31.2 kVA
50 A	18.0 kVA	41.6 kVA	51.9 kVA
100 A	36.0 kVA	83.1 kVA	103.9 kVA
200 A	72.1 kVA	166.3 kVA	207.8 kVA
400 A	144.1 kVA	332.6 kVA	415.7 kVA

The two shortcuts worth remembering: at 208 V, kVA ≈ amps ÷ 2.8. At 480 V, kVA ≈ amps ÷ 1.2.

Transformer Sizing Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The transformer sizes that exist, at the voltage pair that covers most US commercial work.

Rating	480 V primary, 3-ph	208 V secondary, 3-ph	Primary device	Secondary device
15 kVA	18.0 A	41.6 A	25 A	60 A
25 kVA	30.1 A	69.4 A	40 A	90 A
30 kVA	36.1 A	83.3 A	45 A	110 A
45 kVA	54.1 A	124.9 A	70 A	175 A
75 kVA	90.2 A	208.2 A	125 A	300 A
112.5 kVA	135.3 A	312.3 A	175 A	400 A
150 kVA	180.4 A	416.4 A	225 A	600 A

Devices are 125% of the calculated current rounded up to a standard rating. Local conditions, inrush behaviour and coordination with upstream devices can all move those figures, which is why the number on a drawing is sometimes different from the number here.

Watts to Amps Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The conversions people look up most, in one place. Everything here is at unity power factor, which is correct for resistive loads.

Watts	120 V	208 V	240 V	480 V, 3-phase
500 W	4.2 A	2.4 A	2.1 A	0.6 A
1,000 W	8.3 A	4.8 A	4.2 A	1.2 A
1,500 W	12.5 A	7.2 A	6.3 A	1.8 A
2,000 W	16.7 A	9.6 A	8.3 A	2.4 A
3,000 W	25.0 A	14.4 A	12.5 A	3.6 A
5,000 W	41.7 A	24.0 A	20.8 A	6.0 A
10,000 W	83.3 A	48.1 A	41.7 A	12.0 A

The three-phase column is the reason industrial equipment is 480 V: the same power moves at a seventh of the current, which means a seventh of the copper and a fortieth of the voltage-drop problem.

What Size Wire for a 100 Amp Service?

Mon, 10 Aug 2026 00:00:00 +0000

The two answers side by side

The run	Rule that applies	Copper	Aluminium	Ground
Service entrance, one-family dwelling	310.12, 83%	4 AWG	2 AWG	see 250.66
Main feeder carrying the whole dwelling	310.12, 83%	4 AWG	2 AWG	8 AWG
Feeder to a separate structure, whole load	310.12, 83%	4 AWG	2 AWG	8 AWG
Sub panel inside the house	Ordinary feeder	3 AWG	1 AWG	8 AWG
Sub panel in a detached garage, partial load	Ordinary feeder	3 AWG	1 AWG	8 AWG
100 A branch circuit to one appliance	Ordinary branch circuit	3 AWG	1 AWG	8 AWG

The 83% allowance exists because a dwelling’s demand is heavily diversified — nothing in a house runs everything at once, and the load calculation in Article 220 already accounts for that. The code recognises it a second time for the conductors that carry the lot.

What Size Wire for a 50 Amp Circuit?

Mon, 10 Aug 2026 00:00:00 +0000

The answer by distance

At 240 volts, 50 amps, copper, 3% target:

Run	Conductor	Drop	Why
25 ft	8 AWG	0.8%	Ampacity decides
50 ft	8 AWG	1.6%	Ampacity decides
75 ft	8 AWG	2.4%	Ampacity decides
93 ft	8 AWG	3.0%	The crossover
100 ft	6 AWG	2.0%	Voltage drop decides
150 ft	6 AWG	3.1%	Just over — 4 AWG for margin
200 ft	4 AWG	2.6%	Voltage drop decides

Halve every distance for a 120 volt circuit. The crossover at 120 V is about 46 feet, which is why 50 amp circuits at 120 volts are rare and unpleasant.

Wire Ampacity Calculator

Mon, 10 Aug 2026 00:00:00 +0000

Both correction tables are below, so you can see the shape of them rather than one factor at a time. The pattern worth remembering is that ambient corrections are gentle until about 100 °F and then steep, while the bundling adjustment falls off a cliff at four conductors and again at ten.

Ambient temperature correction — NEC Table 310.15(B)(1), based on 86 °F (30 °C).
Ambient	60 °C wire	75 °C wire	90 °C wire
50–77 F (10–25 C)	× 1.08	× 1.05	× 1.04
78–86 F (26–30 C)	× 1	× 1	× 1
87–95 F (31–35 C)	× 0.91	× 0.94	× 0.96
96–104 F (36–40 C)	× 0.82	× 0.88	× 0.91
105–113 F (41–45 C)	× 0.71	× 0.82	× 0.87
114–122 F (46–50 C)	× 0.58	× 0.75	× 0.82
123–131 F (51–55 C)	× 0.41	× 0.67	× 0.76
132–140 F (56–60 C)	not permitted	× 0.58	× 0.71

Adjustment for more than three current-carrying conductors bundled together — NEC Table 310.15(C)(1).
Current-carrying conductors	Multiply ampacity by
1 to 3	× 1
4 to 6	× 0.8
7 to 9	× 0.7
10 to 20	× 0.5
21 to 30	× 0.45
31 to 40	× 0.4
41 and more	× 0.35

The full ampacity table

Conductor size, area, resistance and allowable ampacity. Ampacity is NEC Table 310.16 — not more than three current-carrying conductors in a raceway, cable or earth, at 86 °F (30 °C) ambient. Correct it for heat and for bundling before you use it.
Size	kcmil	mm²	Cu 60 °C	Cu 75 °C	Cu 90 °C	Al 75 °C	Cu Ω/kft	Al Ω/kft
14 AWG	4.11	2.08	15 A	20 A	25 A	—	3.14	—
12 AWG	6.53	3.31	20 A	25 A	30 A	20 A	1.98	3.18
10 AWG	10.38	5.261	30 A	35 A	40 A	30 A	1.24	2
8 AWG	16.51	8.367	40 A	50 A	55 A	40 A	0.778	1.26
6 AWG	26.24	13.3	55 A	65 A	75 A	50 A	0.491	0.808
4 AWG	41.74	21.15	70 A	85 A	95 A	65 A	0.308	0.508
3 AWG	52.62	26.67	85 A	100 A	115 A	75 A	0.245	0.403
2 AWG	66.36	33.62	95 A	115 A	130 A	90 A	0.194	0.319
1 AWG	83.69	42.41	110 A	130 A	145 A	100 A	0.154	0.253
1/0 AWG	105.6	53.49	125 A	150 A	170 A	120 A	0.122	0.201
2/0 AWG	133.1	67.43	145 A	175 A	195 A	135 A	0.0967	0.159
3/0 AWG	167.8	85.01	165 A	200 A	225 A	155 A	0.0766	0.126
4/0 AWG	211.6	107.2	195 A	230 A	260 A	180 A	0.0608	0.1
250 kcmil	250	127	215 A	255 A	290 A	205 A	0.0515	0.0847
300 kcmil	300	152	240 A	285 A	320 A	230 A	0.0429	0.0707
350 kcmil	350	177	260 A	310 A	350 A	250 A	0.0367	0.0605
400 kcmil	400	203	280 A	335 A	380 A	270 A	0.0321	0.0529
500 kcmil	500	253	320 A	380 A	430 A	310 A	0.0258	0.0424
600 kcmil	600	304	350 A	420 A	475 A	340 A	0.0214	0.0353
700 kcmil	700	355	385 A	460 A	520 A	375 A	0.0184	0.0303
750 kcmil	750	380	400 A	475 A	535 A	385 A	0.0171	0.0282

Where the derating actually bites

Three situations account for most real derating on residential and light commercial work.

Wire and Conduit Takeoff

Mon, 10 Aug 2026 00:00:00 +0000

Every run with its length, conductor count and price per foot, totalled into a material cost — the sheet that becomes the quote.

Wire Gauge Chart

Mon, 10 Aug 2026 00:00:00 +0000

The complete chart

Size, area, ampacity in all three temperature columns for both metals, and resistance. This is the table the rest of this site is built on.

Conductor size, area, resistance and allowable ampacity. Ampacity is NEC Table 310.16 — not more than three current-carrying conductors in a raceway, cable or earth, at 86 °F (30 °C) ambient. Correct it for heat and for bundling before you use it.
Size	kcmil	mm²	Cu 60 °C	Cu 75 °C	Cu 90 °C	Al 75 °C	Cu Ω/kft	Al Ω/kft
14 AWG	4.11	2.08	15 A	20 A	25 A	—	3.14	—
12 AWG	6.53	3.31	20 A	25 A	30 A	20 A	1.98	3.18
10 AWG	10.38	5.261	30 A	35 A	40 A	30 A	1.24	2
8 AWG	16.51	8.367	40 A	50 A	55 A	40 A	0.778	1.26
6 AWG	26.24	13.3	55 A	65 A	75 A	50 A	0.491	0.808
4 AWG	41.74	21.15	70 A	85 A	95 A	65 A	0.308	0.508
3 AWG	52.62	26.67	85 A	100 A	115 A	75 A	0.245	0.403
2 AWG	66.36	33.62	95 A	115 A	130 A	90 A	0.194	0.319
1 AWG	83.69	42.41	110 A	130 A	145 A	100 A	0.154	0.253
1/0 AWG	105.6	53.49	125 A	150 A	170 A	120 A	0.122	0.201
2/0 AWG	133.1	67.43	145 A	175 A	195 A	135 A	0.0967	0.159
3/0 AWG	167.8	85.01	165 A	200 A	225 A	155 A	0.0766	0.126
4/0 AWG	211.6	107.2	195 A	230 A	260 A	180 A	0.0608	0.1
250 kcmil	250	127	215 A	255 A	290 A	205 A	0.0515	0.0847
300 kcmil	300	152	240 A	285 A	320 A	230 A	0.0429	0.0707
350 kcmil	350	177	260 A	310 A	350 A	250 A	0.0367	0.0605
400 kcmil	400	203	280 A	335 A	380 A	270 A	0.0321	0.0529
500 kcmil	500	253	320 A	380 A	430 A	310 A	0.0258	0.0424
600 kcmil	600	304	350 A	420 A	475 A	340 A	0.0214	0.0353
700 kcmil	700	355	385 A	460 A	520 A	375 A	0.0184	0.0303
750 kcmil	750	380	400 A	475 A	535 A	385 A	0.0171	0.0282

Size, area and the metric equivalent

The same conductors by area and diameter, with the space each one occupies as insulated THHN — which is what conduit fill is calculated from. The mm² column is the bridge for anyone working from a metric drawing or buying cable by cross-section; one kcmil is 0.5067 mm².

Wire Size Calculator

Mon, 10 Aug 2026 00:00:00 +0000

The chart below is ampacity only — no distance in it, because a chart cannot know your distance. Use it to sanity-check the calculator, and use the calculator when the run is long.

Breaker rating against the smallest conductor that carries it at 75 °C, with no derating and a short run. Computed from NEC Table 310.16, 240.4(D) and Table 250.122.
Breaker	Copper	Aluminium	Copper ground	What it usually feeds
15 A	14 AWG	12 AWG	14 AWG	Lighting and general receptacles
20 A	12 AWG	12 AWG	12 AWG	Kitchen, bathroom and garage receptacles
30 A	10 AWG	10 AWG	10 AWG	Dryer, small water heater, RV outlet
40 A	8 AWG	8 AWG	10 AWG	Range, 40 A EV charger
50 A	8 AWG	6 AWG	10 AWG	Range, welder, 50 A EV charger, hot tub
60 A	6 AWG	4 AWG	10 AWG	Sub panel, large hot tub, tankless heater
70 A	4 AWG	3 AWG	8 AWG	Sub panel, larger tankless heater
100 A	3 AWG	1 AWG	8 AWG	Sub panel, or a small service
125 A	1 AWG	2/0 AWG	6 AWG	Sub panel
150 A	1/0 AWG	3/0 AWG	6 AWG	Service, or a large sub panel
200 A	3/0 AWG	250 kcmil	6 AWG	Typical whole-house service

Why two rules and not one

Ampacity is about heat. A conductor carrying current warms up, and the table is the current at which the insulation reaches its rated temperature under standard conditions. It has nothing to do with how far the wire goes.