Wire Size Calculator
Size a branch circuit or feeder in one pass: ampacity with derating, terminal ratings, continuous loads and voltage drop — the larger answer wins.
Result
Correction ×1.00 (ambient) · adjustment ×1.00 (conductors), from the 90 °C column, capped at the 75 °C terminal rating.
Based on NFPA 70 (NEC) 2023 tables. An estimating aid, not an engineering design: confirm sizes against the code edition your authority having jurisdiction enforces, local amendments and manufacturer data.
How the wire size is chosen
- Required current = load × 1.25 if continuous.
- For each size, take the Table 310.16 ampacity for the insulation rating, apply the ambient and conductor-count factors, then cap it at the terminal column. The first size that carries the required current — and that 240.4(D) lets you protect at that current — is the ampacity answer.
- Separately, find the smallest size whose voltage drop over the run stays under your limit.
- Use the larger of the two.
Frequently asked questions
What size wire do I need for 40 amps?
8 AWG copper at 75 °C terminals (50 A) for a 40 A non-continuous load on a short run — 10 AWG copper is only 35 A at 75 °C, and NEC 240.4(D) limits its breaker to 30 A. If the 40 A is continuous it must be sized for 50 A, which 8 AWG copper still meets. Long runs may need a bigger wire for voltage drop; enter the length to check.
Why can’t I use the 90 °C column?
You start derating from the 90 °C column when the insulation is rated 90 °C (THHN, XHHW-2), but the final ampacity can’t exceed the rating of the terminals it lands on — usually 75 °C, and 60 °C for many devices rated 100 A or less (NEC 110.14(C)). The calculator applies both.
What is the 125 % rule?
A continuous load — maximum current expected for three hours or more — must be served by conductors rated at least 125 % of that load (NEC 210.19(A)(1) and 215.2(A)(1)). A 32 A EV charger therefore needs a conductor good for 40 A.
How do more wires in a conduit change the size?
With more than three current-carrying conductors in a raceway, NEC Table 310.15(C)(1) reduces each conductor’s ampacity: 80 % for 4–6, 70 % for 7–9, 50 % for 10–20 and so on. Equipment grounds don’t count; a neutral counts only when it carries unbalanced or harmonic current.
Copper or aluminum?
Aluminum carries less current per size — roughly two AWG sizes larger than copper for the same ampacity — and has higher voltage drop, but it is lighter and much cheaper for feeders. It is common from about 1 AWG up; terminals must be rated AL/CU.
Sizing one run is easy. Counting a whole job isn't.
Aginera reads your electrical drawings and counts devices, fixtures and panels, measures conduit and feeder runs, and builds the takeoff with a sheet reference for every number.
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