Wire Size Chart: Conductor Ampacity Under NEC 310.16

The ampacity table is the most-used table in the code book, and it is also the one people most often use wrong. Not because the numbers are hard to read, but because the number you read off the table is almost never the number you are allowed to use.

There are three things that happen to a table value before it becomes your actual ampacity: ambient temperature correction, conductor bundling adjustment, and the termination temperature limit. Skip any of them and you have a conductor that is legal on paper and undersized in reality.

The table

These are the ampacities from NEC 310.16 for conductors rated 0 to 2000 volts, not more than three current-carrying conductors in a raceway, at an ambient of 30 degrees C. Values have been stable across recent editions, but verify against the edition your jurisdiction has adopted.

Copper

Size	60°C (TW, UF)	75°C (THW, THWN, XHHW)	90°C (THHN, THWN-2, XHHW-2)
14 AWG	15	20	25
12 AWG	20	25	30
10 AWG	30	35	40
8 AWG	40	50	55
6 AWG	55	65	75
4 AWG	70	85	95
3 AWG	85	100	115
2 AWG	95	115	130
1 AWG	110	130	145
1/0	125	150	170
2/0	145	175	195
3/0	165	200	225
4/0	195	230	260

Aluminum and copper-clad aluminum

Size	60°C	75°C	90°C
12 AWG	15	20	25
10 AWG	25	30	35
8 AWG	30	40	45
6 AWG	40	50	55
4 AWG	55	65	75
2 AWG	75	90	100
1/0	100	120	135
2/0	115	135	150
3/0	130	155	175
4/0	150	180	205

Small conductor rule: overcurrent protection for 14, 12 and 10 AWG copper is separately limited to 15, 20 and 30 amperes respectively, regardless of what the ampacity table allows, with specific exceptions. This is why a 12 AWG conductor rated 30 amps at 90 degrees C still lands on a 20 amp breaker.

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Step one: the termination rule

This is the rule that overrides everything else, and it is the reason the 90 degree C column exists mostly as a starting point rather than an answer.

Table value, then correction, then adjustment, then the termination limit. In that order.

Conductor ampacity is limited by the lowest temperature rating of any connected termination, conductor or device. In practice:

Equipment rated 100 amperes or less, and conductors 14 AWG through 1 AWG, are generally limited to the 60 degree C column unless the equipment is listed and identified for higher.
Equipment rated over 100 amperes, and conductors larger than 1 AWG, are generally limited to the 75 degree C column.
Most modern breakers and panelboards are listed for 75 degrees C, which is why the 75 column is where most real work lands.

You are permitted to begin from the 90 degree C column when applying correction and adjustment factors, because those factors are about the conductor's ability to shed heat along its length. But the final answer, after all the math, still cannot exceed the termination-limited value.

Read the equipment, not the internet. The termination rating is on the equipment label or in the listing. A 90 degree C conductor into a 75 degree C lug is a 75 degree C connection. The weakest link sets the limit.

Step two: ambient temperature correction

The table assumes 30 degrees C, which is 86 degrees F. Run conductors through an attic in July and that assumption is fiction. Correction factors from Table 310.15(B)(1) reduce the ampacity accordingly.

Ambient	60°C conductor	75°C conductor	90°C conductor
21 to 25°C (70 to 77°F)	1.08	1.05	1.04
26 to 30°C (79 to 86°F)	1.00	1.00	1.00
31 to 35°C (88 to 95°F)	0.91	0.94	0.96
36 to 40°C (97 to 104°F)	0.82	0.88	0.91
41 to 45°C (106 to 113°F)	0.71	0.82	0.87
46 to 50°C (115 to 122°F)	0.58	0.75	0.82
51 to 55°C (124 to 131°F)	0.41	0.67	0.76

Notice how much better the 90 degree C conductor holds up as ambient climbs. That is the real reason to start from the 90 column: not to get a bigger final number, but to lose less to correction.

Step three: bundling adjustment

Once more than three current-carrying conductors share a raceway or cable, they cannot shed heat as well and ampacity is reduced again. From Table 310.15(C)(1):

Current-carrying conductors	Percent of table value
4 to 6	80%
7 to 9	70%
10 to 20	50%
21 to 30	45%
31 to 40	40%
41 and above	35%

What counts as current-carrying is its own small subject. A neutral that carries only the unbalanced current of a multiwire branch circuit generally is not counted. A neutral on a circuit supplying substantial nonlinear load generally is. Equipment grounding conductors are never counted.

Raceways not exceeding 24 inches in length are exempt from the adjustment factors entirely.

Putting it together

Worked example — six conductors in a hot attic

Six current-carrying 10 AWG THHN copper conductors in EMT through an attic space at 45 degrees C. Terminations rated 75 degrees C. What is the usable ampacity?

Start: 10 AWG copper at 90°C = 40 amps

Ambient correction at 41 to 45°C for a 90°C conductor = 0.87
40 × 0.87 = 34.8 amps

Bundling adjustment for 6 current-carrying conductors = 0.80
34.8 × 0.80 = 27.8 amps

Termination limit: 10 AWG at 75°C = 35 amps. Our corrected value of 27.8 is below that, so the corrections govern.

Usable ampacity: 27.8 amps. The small conductor rule caps 10 AWG overcurrent protection at 30 amps anyway, and 27.8 is below 30, so this conductor supports a load up to 27.8 amps and would typically land on a 30 amp device only if the calculated load stays under the corrected ampacity and the next-size-up rules are satisfied.

A conductor that reads 40 amps in the table delivers 27.8 in this installation. That gap is the entire reason the corrections exist, and it is what separates an ampacity lookup from an ampacity calculation.

What the table does not tell you

Ampacity is a thermal question: can this conductor carry this current without cooking its insulation. It says nothing about whether the voltage arriving at the far end is usable.

On long runs, voltage drop frequently drives a larger conductor than ampacity alone would require. The two calculations are independent, they are both real, and the larger of the two answers is the one you install. A conductor that passes ampacity and fails voltage drop is a callback waiting to happen.

Fill is a third, separate question. Once you know the conductor size, check that they physically fit using the conduit fill tables.

Quick reference: common circuits

These are the answers people are usually looking for. They assume copper, 75 degree C terminations, three or fewer current-carrying conductors, 30 degree C ambient, and a short enough run that voltage drop is not controlling. Change any of those and the answer changes.

Circuit	Typical copper conductor	Watch for
15 A lighting	14 AWG	Small conductor rule caps at 15 A
20 A general purpose	12 AWG	Small conductor rule caps at 20 A
30 A dryer or water heater	10 AWG	Small conductor rule caps at 30 A
40 A range	8 AWG	Demand factors apply to the load calc
50 A range or EVSE	8 AWG at 75°C	Continuous load rule for EVSE
60 A subpanel	6 AWG	Separate grounds and neutrals downstream
100 A feeder	3 AWG	Termination rating, then voltage drop

Treat that table as a sanity check on an answer you calculated, never as a substitute for calculating it.

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Frequently asked questions

Which temperature column of 310.16 do I use?

You may use the higher column for correction and adjustment calculations, but the final ampacity is limited by the temperature rating of the terminations at both ends. For most equipment rated 100 amps and below, that means the 60 degree C column; above that it is commonly 75 degrees C. Check the equipment listing rather than assuming.

Can I use the 90 degree C column?

Usually only as a starting point. Most conductors sold today are rated 90 degrees C, and you are allowed to begin from that column when applying ambient correction and bundling adjustment, but the final answer still cannot exceed what the terminations are rated for.

What size wire do I need for a 50 amp circuit?

It depends on the conductor material, the insulation, the termination rating, the ambient temperature, how many conductors share the raceway, and the length of the run. A common answer for a 50 amp copper circuit at 75 degrees C terminations is 8 AWG, but treat that as a starting point and run the actual conditions.

Do I have to derate for conduit fill?

Not for fill itself, but you do have to apply the adjustment factor once more than three current-carrying conductors share a raceway or cable. Neutrals that carry only unbalanced current generally are not counted; neutrals on certain nonlinear loads are.

Does voltage drop change the required wire size?

It can. Ampacity and voltage drop are independent checks, and on long runs voltage drop frequently drives a larger conductor than ampacity alone would require.

Keep reading

Calculators

// DisclaimerThe National Electrical Code® (NEC®) and NFPA 70® are registered trademarks and copyrighted publications of the National Fire Protection Association (NFPA). This article is an independent educational resource and is not affiliated with, endorsed by, or sponsored by the NFPA. All code references in this article are paraphrased for educational purposes only and do not reproduce the official text of any NEC edition. This content is not a substitute for the official NFPA 70 publication. Electricians, contractors, and students are encouraged to purchase the official National Electrical Code from the NFPA at nfpa.org for complete and authoritative code text. Always verify requirements with the code edition adopted by your jurisdiction and your Authority Having Jurisdiction (AHJ).