Wire Ampacity Chart — NEC Table 310.16

What ampacity is, and what it is not

Ampacity is the current a conductor can carry continuously without its insulation exceeding the temperature it is rated for. It is a safety limit, not a performance target: a conductor running at its ampacity is not "working hard", it is at the edge of how hot its insulation is allowed to get. Every number on this page describes a thermal limit, which is why the answer changes when the ambient temperature changes or when conductors are bundled together and cannot shed heat.

What ampacity does not tell you is whether the load at the far end will get enough voltage. A 10 AWG copper conductor carrying 30 A over 250 ft is perfectly legal on ampacity and will leave a 120 V load seeing about 106 V — a different failure with a different fix. Ampacity decides whether a conductor is legal; voltage drop decides whether it works. You size for the larger of the two.

Which of the three columns is legal on your job

Table 310.16 answers with three columns because conductor insulation comes in three temperature ratings. TW and UF are 60°C conductors. THWN, THHW, RHW and XHHW are 75°C. THHN, THWN-2, RHW-2, XHHW-2 and USE-2 are 90°C. A 90°C conductor is not automatically a licence to use the 90°C column, because the conductor is only one part of the circuit.

NEC 110.14(C) is the rule that governs the column. The ampacity you may use is limited by the lowest temperature rating of any termination, conductor or device in the circuit. For circuits of 100 A or less the ampacity is based on the 60°C column unless the equipment is listed and identified for use at 75°C — and most breakers, receptacles, switches and wire connectors are not listed for 90°C. That is why the practical answer for a residential circuit is the 60°C or 75°C column, and why "8 gauge is good for 55 amps" (the 90°C cell) is a statement about the wire, not about your circuit.

NEC Table 310.16 — full wire ampacity chart

Allowable ampacities of insulated conductors rated up to 2000 V, not more than three current-carrying conductors in a raceway, cable or earth (directly buried), based on an ambient temperature of 30°C (86°F). Values in amperes. 14 AWG aluminium is not listed.

Three things are worth reading off that chart before you use it. First, doubling the cross-sectional area roughly doubles the ampacity: 12 AWG copper (20 A at 60°C) to 6 AWG (55 A) is a factor of 2.75 across five sizes. Second, aluminium sits about two sizes below copper — 2 AWG aluminium at 75°C is 90 A, between 4 AWG copper (85 A) and 3 AWG copper (100 A). Third, the 90°C column is much wider than the 60°C column at the same size — 12 AWG copper is 20 A at 60°C and 30 A at 90°C, a 50% difference. Almost every "the internet says this wire handles this many amps" argument is two people reading two different columns of this one table.

240.4(D) — the small-conductor caps that override the table

For 14, 12 and 10 AWG copper, and 12 and 10 AWG aluminium, the overcurrent device may not exceed the values below — regardless of what the 75°C or 90°C column says, and with no next-size-up allowance. The cap applies after any ambient-temperature and conductor-count correction factors have been applied, which is what makes a hot attic hard on small conductors.

This is why a 20 A breaker on 12 AWG copper is the standard residential 20 A circuit, and why 14 AWG copper cannot be put on a 20 A breaker no matter how short the run — the cap is about the overcurrent device, not the load. Note that these are ceilings, not floors: they say how large a device may protect the conductor, not that the conductor can carry that much current in every environment.

The other half of that pairing — which device rating to round up to, and how the continuous-load rule and the 240.4(B) next-size-up change the answer — is worked through on the breaker size chart, including the per-appliance rows for water heaters, dryers, ranges and air conditioners.

How many amps can 12-gauge wire handle?

12 AWG copper is 20 A at 60°C, 25 A at 75°C and 30 A at 90°C in Table 310.16. 12 AWG aluminium is 15 A, 20 A and 25 A in the same three columns. Those are the raw table values — the number you may actually build to is set by two more rules, and together they are why 12 AWG copper is the standard 20 A circuit wire.

Read the two rules in order. First 110.14(C) fixes the column: on a circuit of 100 A or less the ampacity is taken from the 60°C column unless every termination, conductor and device is listed for 75°C, so a typical 12 AWG copper circuit is a 20 A conductor, not a 30 A one. Then 240.4(D) caps the overcurrent device at 20 A regardless of the column, and there is no next-size-up for 12 AWG copper. The 90°C figure of 30 A is the insulation rating of THHN and nothing else — a starting point for derating, never the rating of the circuit.

The gap between those columns is where most of the arguments start. "12 gauge is good for 30 amps" is true of the insulation and false of the circuit. "12 gauge is good for 20 amps" is the working answer for copper at 60°C terminations. Both statements are read off this one table.

Derating can pull even the 20 A answer down. A 12 AWG copper circuit in a 40°C attic, at 60°C terminations, is 20 × 0.82 = 16.4 A after the ambient correction — below the 20 A device protecting it, so the breaker no longer matches the conductor's corrected ampacity and the run needs 10 AWG (30 × 0.82 = 24.6 A). Start from the 75°C column instead (25 A) and the same conductor lands at 25 × 0.88 = 22 A, which holds, because the terminations are what the column depends on.

So the short answer: 20 A for 12 AWG copper, 15 A for 12 AWG aluminium, on an ordinary circuit of 100 A or less. If you need a 30 A circuit, that is 10 AWG copper (30 A at 60°C, itself capped at 30 A). The device rating you may actually use is worked through on the breaker size chart, and the run-length half of the question — where voltage drop forces a larger conductor than ampacity does — is on the voltage drop calculator.

Ambient temperature changes the answer — Table 310.15(B)(1)(1)

The base table is built on a 30°C ambient. For anything hotter or colder, multiply the table value by the correction factor for the ambient temperature and the conductor's insulation rating. Factors below 1 reduce the ampacity; a conductor in a hot attic can lose a quarter of its rating.

A worked case makes the size of this effect obvious. A 20 A circuit in a 40°C attic at 60°C terminations: 12 AWG copper is 20 A in the base table, and 20 × 0.82 = 16.4 A after correction — below the 20 A device protecting it. Start from the 75°C column instead (25 A) and the same conductor lands at 25 × 0.88 = 22 A, which holds. Same wire, same attic, two different answers, decided entirely by what the terminations are rated for. That is the whole reason 110.14(C) exists.

Bundled conductors — Table 310.15(C)(1)

The base table already accounts for three current-carrying conductors. Put more than three in one raceway or cable and they cannot shed heat as freely, so the allowable ampacity is reduced by the factor below. Count only conductors that carry current — a grounding conductor does not count, and neither does a neutral that is not a current-carrying conductor for the application. (How many conductors physically fit in the raceway at all is a separate check — the conduit fill chart answers that from Chapter 9.)

Ambient correction and bundle adjustment multiply together. A feeder for 100 A at 75°C terminations normally needs 3 AWG copper (100 A). Put eight current-carrying conductors in the same conduit — four 240 V circuits, say — and the factor is 0.70: 100 × 0.70 = 70 A, too small. 1 AWG gives 130 × 0.70 = 91 A, still short; 1/0 AWG gives 150 × 0.70 = 105 A, which holds. The bundle, not the load, drove the conductor from 3 AWG to 1/0 AWG.

Worked example: is 8 AWG good for 50 amps?

This is the most common single question about this table, and the honest answer is "it depends on the column". 8 AWG copper is 40 A at 60°C, 50 A at 75°C and 55 A at 90°C.

• At 60°C terminations — the default for circuits of 100 A or less — 40 A is short of 50 A, so 8 AWG fails and 6 AWG (55 A) is the answer.
• At 75°C terminations — that is, every breaker, connector and device in the circuit listed for 75°C — 8 AWG is exactly 50 A. Legal, with zero headroom, so most electricians go up a size.
• The 90°C column (55 A) is the insulation rating of THHN. It is a starting point for derating maths, not a circuit rating — 110.14(C) will not let you hand 50 A to a 60°C terminal on the strength of it.

The same logic applied to aluminium: 6 AWG aluminium is 40 A at 60°C and 50 A at 75°C, and 4 AWG aluminium is 55 A and 65 A — so the equivalent 50 A answer in aluminium is 6 AWG at 75°C or 4 AWG at 60°C.

Worked example: a 100 A feeder to a garage subpanel

Reading straight across the 75°C column, 3 AWG copper is 100 A and 1 AWG aluminium is 100 A. Those are the sizes the code permits under base conditions. Two extra checks decide whether they survive: distance (voltage drop) and bundling.

On distance, the ampacity answer is not the whole story — a 100 A load 200 ft out needs 1 AWG copper to hold a 3% drop, not 3 AWG. The distance tables and the 310.12 "83% rule" trap are worked through in the subpanel wire size guide, and the 200 A version — where the 83% rule does apply to a service — is in the 200 amp service wire size guide. The voltage drop calculator runs the arithmetic for your own run length. What this page settles is the floor: nothing smaller than 3 AWG copper or 1 AWG aluminium may be protected by a 100 A device, because 240.4(B) does not reach far enough to let 4 AWG copper (85 A) sit on a 100 A breaker.

Common mistakes with this table

• Reading the 90°C column as the wire's ampacity. It is the insulation's rating. The circuit is limited by its weakest termination (110.14(C)).
• Forgetting 240.4(D). 14, 12 and 10 AWG have hard overcurrent caps with no next-size-up allowance, and the caps apply after derating.
• Ignoring ambient temperature. A 40°C attic or mechanical room turns a comfortable table value into a failing one; the 0.82 factor for 60°C conductors is not a rounding error.
• Treating four conductors in a conduit as three. The base table is "not more than three current-carrying conductors". The fourth conductor puts you in the 80% row of 310.15(C)(1).
• Using the free-air column. Table 310.17 rates single conductors in free air much higher than 310.16 rates conductors in a raceway, cable or earth. They are not interchangeable, and mixing them up oversizes the wire by one or two sizes.
• Confusing ampacity with voltage drop. Ampacity is what makes the installation legal; voltage drop is what makes the equipment work. Long runs often need a conductor two or three sizes above the ampacity minimum.
• Assuming aluminium is not allowed. It is listed in the same table, sits roughly two sizes below copper, is not listed at all at 14 AWG, and must be terminated on devices listed for aluminium.
• Designing at exactly the limit. An ampacity calculation is a model of a worst case; a loose terminal, a warmer day or an added load has nowhere to go. Take the next size.

Wire ampacity FAQ

Related pages

• Voltage drop & wire size calculator — the distance half of conductor sizing, built on NEC Chapter 9 Table 8
• What size wire for a 100 amp subpanel? — ampacity, distance and the ground wire, worked end to end
• Breaker size chart — NEC 240.6(A) and 240.4(D) — the device rating that goes with the conductor, the 125% rule and the per-appliance circuits
• Reference tables — every code table on the site, with the source and the verification date on each page
• All calculators — 11 free tools, no signup

Table 310.16 values, the 310.15(B)(1)(1) ambient correction factors and the 310.15(C)(1) conductor adjustment factors on this page were verified 2026-09-25 against two independent full-table reproductions — the HELUKABEL "Allowable Ampacity Tables NFPA 70: NEC 2023" document and lugsdirect.com's Table 310.16 reproduction — which agree cell for cell from 14 AWG to 2000 kcmil. The 60°C and 75°C copper and aluminium columns also match the set already published in this site's voltage drop calculator (verified 2026-09-14). Adopted code editions and local amendments vary, and none of this replaces a licensed electrician or your AHJ for work that has to pass inspection.