NEC 2026

Wire Size Calculator

Find the smallest copper or aluminum conductor for a load, with the termination, ambient and conductor-count rules applied. The result updates as you type.

Updated

Default is 60 °C for circuits of 100 A or less and for 14-1 AWG conductors (110.14(C)(1)); 75 °C otherwise. Choose 75 °C only if all terminations are listed/marked for 75 °C (110.14(C)(1)).

Enter the load in amps to see the smallest conductor.

Voltage drop is not included; long runs may need a larger conductor.

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For reference only. Always verify with the NEC edition adopted by your AHJ.

How the size is chosen

The calculator walks up from the smallest conductor and keeps the first size that passes two separate checks (210.19(A)(1) and (2), 215.4(A)(1) and (2)). First, a continuous load multiplied by 125% must fit the Table 310.16 ampacity of the limiting temperature column, with no correction or adjustment factors. Second, the full load must fit the ampacity after the corrections below.

For each size the ampacity is the Table 310.16 value for your insulation rating, corrected for ambient temperature (310.15(B)(1)) and for more than 3 current-carrying conductors (310.15(C)(1)). The result can never exceed the termination column from 110.14(C), which is why the termination temperature matters.

Small conductors also carry a 240.4(D) overcurrent limit, shown as Max OCPD. Voltage drop is not included; long runs may need a larger conductor.

Browse the ampacity chart, or see worked answers for 50 amp and 200 amp service circuits.

Wire size questions

Which termination temperature should I use?
Under 110.14(C)(1), circuits rated 100 A or less, and circuits using 14 through 1 AWG conductors, are limited to the 60 °C column unless the equipment is listed and marked for 75 °C. Use 75 °C only if all terminations are listed/marked for 75 °C. The calculator defaults to 60 °C for those circuits and to 75 °C only for circuits over 100 A that need conductors larger than 1 AWG.
Why does a #12 copper result say Max OCPD 20A?
240.4(D) caps overcurrent protection for small conductors (30 A for 10 AWG copper, 20 A for 12 AWG copper, 15 A for 14 AWG copper, 25 A for 10 AWG aluminum, 15 A for 12 AWG aluminum) no matter how many amps the table column allows. The calculator will not select one of these sizes if the load is above its limit.
How is a continuous load handled?
A continuous load runs 3 hours or more. 210.19(A)(1) and 215.4(A)(1) require the conductor to carry 125% of it, so a 40 A continuous load is sized as 50 A. That 125% figure is checked on its own, without ambient or conductor-count factors; the full load is then checked separately against the derated ampacity (210.19(A)(2), 215.4(A)(2)), and the size must pass both.
What do ambient temperature and conductor count do?
Table 310.16 assumes 30 °C and no more than 3 current-carrying conductors. Above that, 310.15(B)(1) applies a temperature correction and 310.15(C)(1) an adjustment factor, which lower the allowable ampacity. The calculator applies both to the full load, not to the 125% figure, before it picks a size.
Does the result include voltage drop?
No. Voltage drop is not included; long runs may need a larger conductor. The NEC treats voltage drop as a recommendation in the informational notes of 210.19 and 215.4(A)(2), so check it separately.
Is this the size for a dwelling service?
No. Service and feeder conductors for a one-family dwelling can use the 83% rule of 310.12, which gives smaller sizes. See the 100, 150 and 200 amp service pages for that case.

You can read the NEC 2026 for free at NFPA free access.