Electrical Sizing Guides on AmpSizer

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.

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.

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.

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.

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.

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 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.

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 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.