🌡️How many amps can a wire carry?

✓ Tested & verified Updated: Aug 10, 2026 How we calculate this

Wire ampacity calculator

Conductor

Metal

Temperature column

Ambient

°F

Current-carrying conductors together

25 amps usable

12 AWG reads 25 A in the table, then × 1.00 for heat × 1.00 for bundling

No derating at these conditions

Metal	60 °C	75 °C	90 °C
Copper	20 A	25 A	30 A
Aluminium	15 A	20 A	25 A

Largest device this supports: 20 A
The same ambient in Celsius: 30 °C

240.4(D) caps this size at 20 A whatever the table says.

Count only current-carrying conductors — the ground never counts.

A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.

Try also

How many amps can a wire carry?

The AmpSizer ampacity result for 12 AWG copper at a 75 °C terminal is 25 amps — and the circuit is still limited to a 20 amp breaker, because 240.4(D) caps small conductors regardless of what the table allows.

Both corrections come off that 25. Put six current-carrying conductors in one raceway and it becomes 20 A. Raise the ambient to 104 °F as well and it becomes 17.6 A, which is below the breaker the circuit was designed around.

This is why the 90 °C column rarely helps. THHN is a 90 °C conductor, but 110.14(C) makes you use the temperature rating of the terminals it lands on, and almost every breaker is listed at 75 °C. The 90 °C figure is the number you derate from, not the one you use.

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.

Two circuits in one conduit. That is four current-carrying conductors, so everything in the pipe drops to 80%. It is the cheapest way to lose a circuit rating and the easiest to avoid at rough-in.

A run through insulation or an attic. Attic temperatures above a roof deck are routinely 120 °F in summer, and at 122 °F a 75 °C conductor is down to 75% of its table value.

A panel feed sharing a raceway with everything else. Feeders and sub-feeds in a common conduit stack up fast, and at ten conductors the factor is 50% — half the table.

None of these are unusual, and none of them show up on a wall chart.

Frequently asked questions

How many amps can 12 gauge wire carry?

20 A at a 60 °C terminal, 25 A at 75 °C and 30 A at 90 °C — and the circuit is a 20 A circuit in all three cases, because 240.4(D) holds 12 AWG copper to a 20 amp device. The extra table capacity is not spare current you can use; it is headroom that derating eats into before it reaches the ceiling.

When do I have to derate for conduit fill?

Above three current-carrying conductors in the same raceway or cable, and then the factor is 80% for four to six, 70% for seven to nine, and 50% for ten to twenty. Two circuits sharing a pipe is four conductors, so it starts sooner than people expect. Grounds never count, and the neutral of a balanced multiwire branch circuit does not count either.

Does conduit in the sun change the ampacity?

Yes, and by more than most people allow for. A raceway in direct sunlight on a rooftop runs well above air temperature, and once the ambient goes over 86 °F the correction factors start biting: at 104 °F a 75 °C conductor keeps 88% of its rating, at 122 °F only 75%. Underground and in a cool crawlspace it works the other way and you get a little back.

Do the two corrections multiply or do I use the worse one?

They multiply. A hot attic with six conductors in the pipe is the ambient factor times the bundling factor, not whichever is smaller — 0.88 × 0.80 is 0.70, so 25 A becomes 17.6. Applying only the worse of the two is the most common derating mistake.

Why is aluminium two sizes bigger for the same amps?

Because it carries about 61% of copper’s current for the same cross-section. 4/0 aluminium and 2/0 copper are both 180 to 175 A at 75 °C — one step apart in ampacity, two steps apart in size. On feeders the extra diameter usually still costs less than the copper it replaces.

How we calculate this

Table ampacity is NEC Table 310.16 — not more than three current-carrying conductors in a raceway, cable or directly buried, at an ambient of 30 °C (86 °F) — for the metal and temperature column you choose. The ambient correction factor is Table 310.15(B)(1), keyed on the conductor’s temperature rating rather than the terminal’s. The bundling adjustment is Table 310.15(C)(1), counted on current-carrying conductors only, with equipment grounding conductors excluded and the neutral of a balanced multiwire branch circuit excluded per 310.15(E). The two factors multiply. The largest device shown is the standard rating from 240.6(A) that does not exceed the corrected ampacity, held down by the small-conductor ceiling of 240.4(D) for 14, 12 and 10 AWG.

Worked example

With Conductor 12 AWG, Metal Copper, Temperature column 75 °C — THWN, most terminals, Ambient 86, Current-carrying conductors together 3, this page works out 25 amps usable. 12 AWG reads 25 A in the table, then × 1.00 for heat × 1.00 for bundling

Largest device this supports20 A
The same ambient in Celsius30 °C

Sources:

What ampacity is actually measuring

It is a temperature limit, not a fuse. The table current is the current at which the insulation reaches its rated temperature under standard conditions. Nothing dramatic happens one amp above it; the insulation simply ages faster, and it does not recover.

Standard conditions are three conductors at 86 °F. Every real installation departs from that in at least one direction, which is what the correction factors are for.

The lowest-rated part of the path governs. Conductor, terminal, splice, device: the circuit is rated at whichever of them is listed lowest, and it is almost always a terminal.