Wire Size Calculator

What is the Wire Size Calculator?

A conductor has to satisfy several limits at once, and the correct size is whichever limit demands the most copper. This calculator applies all of them: the Table 310.16 ampacity, the ambient temperature correction, the adjustment for bundled conductors, the termination temperature ceiling in NEC 110.14(C), the small-conductor overcurrent limits in 240.4(D), and your voltage drop target. It returns one size, and it tells you which limit decided it.

How to Use the Wire Size Calculator

1. 1Enter the load current in amperes, and tick continuous if the load runs for three hours or more
2. 2Pick copper or aluminum, and the insulation rating of the conductor you intend to use
3. 3Set the termination rating. Equipment terminals are commonly 60 °C at 100 A or less and 75 °C above, whatever the wire is rated
4. 4Enter the ambient temperature if it is not the 30 °C the tables assume. An attic or a boiler room is not 30 °C
5. 5Enter how many current-carrying conductors share the raceway, so the bundling adjustment applies
6. 6Add the system, voltage and one-way length to include voltage drop, or leave all three out to size on ampacity alone

Key features

Why Sizing on Ampacity Alone Goes Wrong

The ampacity table is the part everybody knows, and on its own it is not enough. Three separate rules can force a larger conductor than the table suggests, and each has its own failure mode. Skipping the termination rule in 110.14(C) returns a conductor one size too small, because 90 °C wire terminating on 75 °C lugs does not get to use its own column. Skipping 240.4(D) puts a 20 A circuit on 14 AWG, which is the classic residential error. Skipping derating hides the fact that a full conduit in a hot space loses a third of its rating. This calculator applies all of them and reports which one bound the result.

Common use cases

• Size a branch circuit correctly and see whether ampacity or drop set the answer
• Size a feeder to a subpanel, including the run length
• Check whether an existing conductor is adequate at the ambient it actually runs in
• Work out what a full conduit costs you in conductor size
• Compare copper against aluminum for the same load and run
• Confirm a 20 A receptacle circuit needs 12 AWG and understand exactly why

The three ceilings, and why all of them apply

• Start with the ampacity from Table 310.16 for the conductor’s own temperature rating. Multiply by the ambient correction from Table 310.15(B)(1), then by the bundling adjustment from 310.15(C)(1) if more than three current-carrying conductors share the raceway. That gives the derated ampacity.
• Then apply NEC 110.14(C). Equipment terminations have their own temperature rating, usually 60 °C for circuits at 100 A or less and 75 °C above, and the conductor may not be used above the ampacity of that column no matter how good its insulation is. A 90 °C conductor derates on the 90 °C column and is then capped at the 75 °C figure. Using the 90 °C column end to end is the single most common way a wire size comes out one size too small.
• Finally apply NEC 240.4(D). For 14, 12 and 10 AWG the overcurrent protection is capped below the table ampacity: 15, 20 and 30 A for copper, and 15 and 25 A for 12 and 10 AWG aluminum. This is why 14 AWG copper, which Table 310.16 rates at 20 A, cannot serve a 20 A circuit. Unlike the 3% voltage drop figure, this one is enforceable code text rather than an Informational Note.
• The usable ampacity is the smallest of those three results, and the correct conductor is the first size whose usable ampacity meets the load.

Continuous loads and the 125% factor

• A continuous load is one expected to run at its maximum for three hours or more. NEC 210.19(A) and 215.2(A) require the conductor to be sized at 125% of the continuous portion, which is why a 40 A continuous load is sized as 50 A.
• The reason is thermal. A conductor at its rated ampacity is at its rated temperature only once it has reached equilibrium, and three hours is roughly where that happens. A load that cycles never gets there. Lighting circuits, EV chargers, electric heating and most commercial loads are continuous; a domestic receptacle circuit generally is not.

Why ampacity and voltage drop disagree

• Ampacity is about heat, and heat depends on current, not distance. Voltage drop is about resistance over length, so it grows with the run while ampacity does not move at all. On a short run ampacity almost always decides. Past roughly 100 feet at typical branch-circuit currents, drop starts to take over, and on a long run it can demand two or three sizes more than the thermal limit.
• This is why sizing on ampacity alone is safe but not sufficient, and why a conductor that passes inspection can still leave a motor running badly at the far end. Give the calculator the run length and it will report which limit bound the result, so you know whether the fix is a bigger conductor or a shorter route.

NEC Table 310.16 ampacity, copper and aluminum

Allowable ampacity at 30 °C ambient with not more than three current-carrying conductors in a raceway. These are the starting figures, before any derating and before the 110.14(C) and 240.4(D) ceilings.

Aluminum is not tabulated at 14 AWG. Remember that 240.4(D) separately caps 14, 12 and 10 AWG below these figures, so the 20 A shown for 14 AWG copper is not usable as 20 A of overcurrent protection.

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