Wire Ampacity Calculator

Determine the allowable ampacity of a copper or aluminum conductor from NEC Table 310.16, then apply ambient temperature correction (Table 310.15(B)(1)) and adjustment for more than three current-carrying conductors (Table 310.15(C)(1)). Results include the NEC 110.14(C) termination temperature limit. NEC 2020.

Conductor and conditions

Base ampacities from NEC Table 310.16 at the conductor's insulation column. Derating per Tables 310.15(B)(1) and 310.15(C)(1) starts from that column; the result is capped at the termination column per NEC 110.14(C). Ampacity values are identical across the 2017/2020/2023 editions.

How to use this calculator

  1. Select conductor material, size, and insulation rating (THHN/THWN-2 is 90°C; THWN/THW is 75°C; TW is 60°C).
  2. Set the termination rating of the lowest-rated terminal in the circuit per NEC 110.14(C) (75°C for most modern equipment).
  3. Enter the ambient temperature and the number of current-carrying conductors in the raceway or cable.
  4. Read the final usable ampacity. The breakdown shows base ampacity, each derating factor, and which limit governs.

NEC reference

This calculator uses NEC Table 310.16 (base ampacities), NEC Table 310.15(B)(1) (ambient temperature correction), NEC Table 310.15(C)(1) (adjustment for more than three current-carrying conductors), and NEC 110.14(C) (termination temperature limitation), NEC 2020. Table 310.16 was designated Table 310.15(B)(16) in NEC 2017; ampacity values are unchanged across the 2017, 2020, and 2023 editions. NEC 240.4(D) small-conductor overcurrent limits are shown as a note where applicable.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

How conductor ampacity is calculated

Allowable ampacity starts as a table lookup, not a formula. NEC Table 310.16 gives the base ampacity for a conductor by material and insulation column (60/75/90°C), assuming 30°C (86°F) ambient and not more than three current-carrying conductors in the raceway. Two derating steps and one cap modify that base: 

Final usable ampacity = min( base × ambient × count , termination limit )

The two derating factors multiply; they are not alternatives.

Worked example

6 AWG THHN copper (90°C insulation), landing on 75°C-rated terminations, in a raceway at 104°F (40°C) ambient with six current-carrying conductors: 

Base (90°C col, Table 310.16)        75 A
Ambient (36-40°C, ×)               0.91
Conductors 4-6 (×)                 0.80
Derated  75 × 0.91 × 0.80   =     54.6 A
Termination cap (75°C, 110.14(C))    65 A
Final  min(54.6, 65)           =     54.6 A

Derating governs here: 54.6 A is below the 65 A termination limit, so the conductor is good for 54.6 A. Drop the bundle to three conductors and the count factor disappears — the 65 A termination limit then governs instead.

Common mistakes

Frequently asked questions

Can I use the 90°C column for ampacity?

You can use the 90°C column as the starting point for derating, per NEC 310.15, when the conductor insulation is rated 90°C (THHN, THWN-2, XHHW-2). You generally can’t use the 90°C value as the final ampacity: NEC 110.14(C) limits the conductor to the termination temperature rating, which is 60°C or 75°C on most equipment. The 90°C column buys derating headroom, not a higher final ampacity, unless every termination in the circuit is rated 90°C — which is rare.

What’s the difference between the 60, 75, and 90°C columns?

The column is the conductor insulation’s maximum operating temperature. Higher-rated insulation carries more current in the same size because it tolerates more heat. Which column you may use is set by two things: the conductor’s own insulation rating and the termination rating per NEC 110.14(C). TW is 60°C; THW/THWN is 75°C; THHN/THWN-2 is 90°C. The 90°C column is used mainly as the derating starting point, not the final ampacity.

Does the neutral count as a current-carrying conductor?

It depends on the load. Per NEC 310.15(E), a neutral carrying only the unbalanced current from the other conductors — the typical case on a multiwire branch circuit or a balanced 3-phase wye — is not counted for conductor-count adjustment. A neutral on a 3-wire circuit from a single-phase, 3-wire system does count. A neutral carrying harmonic current from significant nonlinear loads (electronic ballasts, VFDs, switching supplies) counts. The equipment grounding conductor never counts.

How many amps can 12 AWG copper carry?

12 AWG copper is 20 A at 60°C, 25 A at 75°C, and 30 A at 90°C in NEC Table 310.16, before derating. NEC 240.4(D) separately caps overcurrent protection for 12 AWG copper at 20 A regardless of insulation, which is why 12 AWG copper is almost always on a 20 A breaker. The 30 A (90°C) figure is only a derating starting point, not a usable circuit rating.

How do I correct ampacity for high ambient temperature?

Multiply the base ampacity by the correction factor from NEC Table 310.15(B)(1) for your ambient range, read from the same insulation column you’re using as the derating start. Table 310.16 assumes 30°C (86°F); above that, factors drop below 1.0. A 90°C conductor at 40°C ambient gets ×0.91; at 50°C, ×0.82. Apply the ambient correction and the more-than-three-conductors adjustment together — they multiply — then cap at the termination rating.

What size wire do I need for a 100 A feeder?

At the 75°C termination column of NEC Table 310.16, 3 AWG copper (100 A) or 1 AWG aluminum (100 A) meets 100 A before derating. Full feeder sizing also has to account for continuous load (125% per NEC 215.3), ambient and conductor-count derating, and voltage drop over the run, so the conductor often lands larger than the bare ampacity match. Use the Wire Size Calculator to size for load, distance, and voltage drop together.

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