Wire Size Calculator - Softotech

Wire Size Calculator

Pick the right wire gauge. A 40 amp load at 240 V over 100 ft needs 8 AWG copper wire with 2.59% (6.22 V) voltage drop.

How the wire size is chosen

Two checks must pass. First, the wire must carry the current safely: its ampacity (from the NEC 75 C column) must be at least the load, or 125% of a continuous load. Second, the voltage drop over the run must stay within a limit, usually 3% for a branch circuit and 5% total.

Voltage drop (single phase) = 2 x length x current x resistance per 1,000 ft / 1,000. For 40 A over 100 ft, 8 AWG copper drops 2.59% (6.22 V) of 240 V and carries 50 A.

Important safety notes

  • This tool is for estimating. Electrical work must follow the current NEC and local code, and often needs a licensed electrician and permit.
  • Small conductors have breaker limits (14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A for copper).
  • Temperature, conduit fill and bundling can require derating.
  • Aluminum needs compatible lugs and anti-oxidant compound.

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    Frequently asked questions

    What size wire do I need for 40 amps?

    Typically 8 AWG copper for short runs and 6 AWG copper if the run is long. Check voltage drop and code.

    What is acceptable voltage drop?

    3% on a branch circuit and 5% overall is the common recommendation.

    Copper or aluminum wire?

    Aluminum is cheaper for large feeders but needs larger gauge and the right connectors.

    Does this replace the NEC?

    No. Always verify with the current code and a licensed electrician.

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