Wire Size Calculator

Enter the load current and get the minimum conductor size from the NEC 310.16 ampacity table — copper or aluminum, with an optional voltage-drop check for the actual run length. The wire size amp chart for every listed gauge is right below the tool. No sign-up, instant results.

Enter the load current, then press Calculate.

Wire ampacity chart — NEC Table 310.16 (A), not more than 3 conductors, 30 °C ambient

Size (AWG/kcmil)Area (mm²)Cu 60 °CCu 75 °CCu 90 °CAl 60 °CAl 75 °CAl 90 °C
14 AWG2.08202025———
12 AWG3.31252530202025
10 AWG5.26303540253035
8 AWG8.37405055304045
6 AWG13.3556575405055
4 AWG21.2708595556575
3 AWG26.785100115657585
2 AWG33.6951151307590100
1 AWG42.411013014585100115
1/0 AWG53.5125150170100120135
2/0 AWG67.4145175195115135150
3/0 AWG85165200225130155175
4/0 AWG107185230260150180205
250 kcmil127195255290170205230
300 kcmil152215285320190230260
350 kcmil177240310350210250280
400 kcmil203260335380225270305
500 kcmil253310380430260310350

Ampacities from NEC Table 310.16 for insulated conductors rated 60 °C (TW, UF), 75 °C (THW, THWN, RHW, XHHW) and 90 °C (THHN, THWN-2, XHHW-2) in raceway, cable or earth. NEC 240.4(D) additionally limits overcurrent protection to 15 A on 14 AWG copper, 20 A on 12 AWG and 30 A on 10 AWG. Derate for ambient temperatures above 30 °C (310.15(B)) and for more than three current-carrying conductors in a raceway (310.15(C)(1)). Verify against the NEC edition adopted in your jurisdiction.

How the Calculator Works

Wire sizing runs two checks in sequence, and the conductor must pass both:

1. Ampacity (NEC 310.16). The table lists allowable ampacity for copper and aluminum conductors at 60, 75 and 90 °C insulation ratings — reproduced in full below. This calculator suggests from the 60 °C column first because it is the most conservative: it is valid at any termination, while the 75 and 90 °C columns are only usable when every termination and device in the circuit carries the matching rating. For a continuous load (on for 3 hours or more), NEC 210.20(A) requires sizing the conductor to 125 % of the load — enable the switch above and the tool applies it.

2. Voltage drop. Enter the one-way run length and the tool computes the drop of the suggested size (Vd = 2·L·I·ρ/A single-phase, √3·L·I·ρ/A three-phase, resistivity at 20 °C). If it exceeds the 3 % branch-circuit recommendation, the result steps up to the smallest size that passes both checks.

Two further deratings sit outside this calculator and must be applied manually when they apply: ambient temperature above 30 °C (NEC 310.15(B) correction factors) and more than three current-carrying conductors in one raceway or cable (NEC 310.15(C)(1) adjustment factors, e.g. 80 % for 4-6 conductors, 70 % for 7-9). At high ambient the usual technique is to start from the 90 °C column, apply the correction and adjustment factors, then verify the final figure against the termination temperature limit.

When Engineers Use This Calculator

Branch circuits and appliance feeds. "What size wire for 40 amps?" is the canonical use: the 60 °C column of Table 310.16 puts 40 A at 8 AWG copper (or 6 AWG aluminum), and the continuous-load switch applies the 125 % factor when the load runs for hours.

Panel and feeder schedules. A distribution feeder sized on ampacity alone can still sag over a long run — the voltage-drop check shows immediately whether one size up is warranted, before the cable order is placed.

Aluminum feeders. Aluminum conductors save material cost on large services but need one to two sizes larger; the chart below puts the copper and aluminum columns side by side for the same AWG.

Checking a design assumption. Equipment datasheets often state minimum supply conductor sizes for nameplate current; running the same numbers here confirms whether the project specification, a long run or a high ambient room forces a larger conductor than the datasheet minimum.

Frequently Asked Questions

What size wire do I need for 40 amps?

8 AWG copper is the standard answer — the 60 °C column of NEC Table 310.16 rates it 40 A, and it stays valid at any termination. For a continuous 40 A load (50 A after the 125 % factor) move to 6 AWG copper, or use 8 AWG THHN at its 75 °C rating of 50 A only when every termination is 75 °C rated. In aluminum, 6 AWG (50 A at 75 °C) is the usual choice.

Which temperature column of the ampacity table should I use?

The lowest rating of any connected termination or device governs. Residential equipment is commonly 60 °C (or 60/75 °C mixed), most modern commercial gear is 75 °C, and the 90 °C column is only a starting point for derating calculations — you may not use 90 °C ampacity at a 75 °C lug. That is why this calculator recommends from the 60 °C column and displays the 75 and 90 °C values beside it.

Does a long run change the wire size?

Yes — through voltage drop, not ampacity. Enter the one-way length above and the calculator checks the suggested size against the 3 % recommendation; a 40 A circuit that fits 8 AWG on ampacity may need 6 AWG or larger once the run exceeds a hundred feet. The result always shows which constraint drove the final size.

How many current-carrying conductors can share a conduit before derating?

Up to three at full Table 310.16 ampacity. From four to six the ampacity drops to 80 %, seven to nine to 70 %, and beyond that further (NEC 310.15(C)(1)) — apply these to the 90 °C column for THHN/THWN-2, then verify against the termination limit. The conduit fill calculator on this site handles the physical side of the same run.

Can I use a 90 °C conductor on a 60 °C breaker lug?

You can install it, but the circuit ampacity is still limited by the 60 °C column at that termination. Higher-temperature insulation is mainly a derating budget: it gives you a higher starting ampacity before applying ambient-temperature and bundling corrections, not extra capacity at the lug.

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