What size wire do I need for 50 amps?

For a 50 A circuit, 8 AWG copper at the 75 degree C column carries 50 A and is the usual answer, or 6 AWG aluminium. If the terminations are only rated 60 degrees C, 8 AWG copper is listed at 40 A and you need 6 AWG copper instead. Long runs may need a size up for voltage drop.

Why does the same wire have three different ampacities?

Because the limit is temperature, not current. A conductor with 90 degree C insulation can run hotter before the insulation degrades, so it carries more current. You can only use the higher column if every termination in the circuit is also rated for that temperature, which NEC 110.14(C) usually prevents.

Does Table 310.16 apply to wire in free air?

No. Table 310.16 is for conductors in a raceway, cable or directly buried. Single conductors in free air use Table 310.17, which gives substantially higher values because heat escapes more easily.

Gauge Charts

Ampacity Chart

Ampacity is the current a conductor can carry continuously without exceeding its temperature rating. NEC Table 310.16 is the starting point for conductors in a raceway or cable, and it assumes not more than three current-carrying conductors and an ambient temperature of 30 degrees C. Every real installation then needs three further checks before the table value can be used.

Wire size calculator

Load / breaker (A)

Insulation rating

One-way run (ft)

System volts

Minimum wire size8AWG

Voltage drop3.3%

Drop7.8V

Drop over 3%, size up 8 AWG copper is rated 50 A in NEC Table 310.16 at this insulation rating. The 3% figure is the NEC’s informational recommendation for branch circuits, not a requirement.

NEC Table 310.16 ampacity, copper

AWG	60 °C	75 °C	90 °C	Ω/1000 ft
18	–	–	14	7.95
16	–	–	18	4.99
14	15	20	25	3.14
12	20	25	30	1.98
10	30	35	40	1.24
8	40	50	55	0.778
6	55	65	75	0.491
4	70	85	95	0.308
3	85	100	115	0.245
2	95	115	130	0.194
1	110	130	145	0.154
1/0	125	150	170	0.122
2/0	145	175	195	0.0967
3/0	165	200	225	0.0766
4/0	195	230	260	0.0608

This is a starting point, not a final answer. Table 310.16 assumes not more than three current-carrying conductors in a raceway at 30 °C ambient. The installed ampacity must still be corrected for ambient temperature (310.15(B)(1)), derated for conductor count (310.15(C)(1)), and limited by the lowest-rated termination in the circuit (110.14(C)), which is 60 °C on most equipment at 100 A or less. Verify against the code edition your jurisdiction has adopted, and have anything you are unsure about checked by a licensed electrician.

NEC Table 310.16 ampacity

Values are for insulated conductors rated 0 to 2000 V, not more than three current-carrying conductors in a raceway, cable or earth, at an ambient of 30 degrees C.

Allowable ampacity, copper and aluminium, by insulation temperature rating
AWG	Cu 60 °C	Cu 75 °C	Cu 90 °C	Al 60 °C	Al 75 °C	Al 90 °C
18	–	–	14	–	–	–
16	–	–	18	–	–	–
14	15	20	25	–	–	–
12	20	25	30	15	20	25
10	30	35	40	25	30	35
8	40	50	55	35	40	45
6	55	65	75	40	50	55
4	70	85	95	55	65	75
3	85	100	115	65	75	85
2	95	115	130	75	90	100
1	110	130	145	85	100	115
1/0	125	150	170	100	120	135
2/0	145	175	195	115	135	150
3/0	165	200	225	130	155	175
4/0	195	230	260	150	180	205

The three corrections the table does not include

A number read straight off Table 310.16 is almost never the installed ampacity. These are the adjustments the code requires, and skipping them is the most common way a correct-looking chart lookup produces a non-compliant circuit.

Correct for ambient temperature
NEC 310.15(B)(1). Above 30 degrees C the conductor carries less. An attic at 50 degrees C derates a 90 degree C conductor to 82 percent of the table value.
Adjust for conductor count
NEC 310.15(C)(1). Four to six current-carrying conductors in one raceway derate to 80 percent, seven to nine to 70 percent, ten to twenty to 50 percent. Neutrals that carry only unbalanced current do not count; neutrals on a three-phase four-wire circuit supplying non-linear loads do.
Limit to the lowest-rated termination
NEC 110.14(C). Equipment rated for 60 degrees C forces you to use the 60 degree C column even if the wire itself is 90 degree C THHN. Most breakers and devices at 100 A or less are 60 or 75 degrees C rated. This single rule catches more mistakes than the other two combined.

NEC 240.4(D) adds a fourth constraint that is not a derating at all but a hard cap: overcurrent protection for 14 AWG copper may not exceed 15 A, for 12 AWG 20 A, and for 10 AWG 30 A, whatever the ampacity table says. This is why a 12 AWG conductor listed at 25 A in the 75 degree column is still protected by a 20 A breaker on an ordinary branch circuit.

Common questions

What size wire do I need for 50 amps?: For a 50 A circuit, 8 AWG copper at the 75 degree C column carries 50 A and is the usual answer, or 6 AWG aluminium. If the terminations are only rated 60 degrees C, 8 AWG copper is listed at 40 A and you need 6 AWG copper instead. Long runs may need a size up for voltage drop.
Why does the same wire have three different ampacities?: Because the limit is temperature, not current. A conductor with 90 degree C insulation can run hotter before the insulation degrades, so it carries more current. You can only use the higher column if every termination in the circuit is also rated for that temperature, which NEC 110.14(C) usually prevents.
Does Table 310.16 apply to wire in free air?: No. Table 310.16 is for conductors in a raceway, cable or directly buried. Single conductors in free air use Table 310.17, which gives substantially higher values because heat escapes more easily.

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