How to use the wire size calculator
The method
First, the allowed drop in volts: circuit volts × limit%. Then the conductor area needed: CM = 2 × K × I × L ÷ allowed drop. The calculator picks the smallest standard AWG size with at least that many circular mils, then runs an ampacity sanity check using common 75°C ratings — if the wire cannot carry the load current, it moves up in size until it can.
How to use it
Enter the real load current (not just the breaker size), the circuit voltage, and the one-way distance from source to load. Leave the limit at 3% unless you have a reason to change it.
What it does not do
Breaker selection, temperature and bundling derating, conduit fill, and motor starting surge are separate checks. Treat the result as a solid starting point, then verify against your code book.
Example
A 20 A load, 120 V, 50 ft away, copper, 3% limit: allowed drop is 3.6 V, so CM needed = 2 × 12.9 × 20 × 50 ÷ 3.6 = 7,167 CM. 12 AWG (6,530 CM) is just short, so the answer is 10 AWG (10,380 CM) — and its 35 A ampacity comfortably covers 20 A.
Frequently asked questions
How does the calculator choose a size?
It works out the circular-mil area needed to keep the drop at or under your chosen limit (CM = 2 × K × I × L ÷ allowed drop in volts), rounds up to the next standard AWG size, then checks that size's 75°C ampacity against your load — and keeps going up until both tests pass.
Why is 3% the default limit?
3% for a branch circuit (and 5% total including the feeder) is the widely used design guideline in the NEC's informational notes and most training material.
Why did it recommend a bigger wire than the breaker size suggests?
Ampacity is only half the story. On long runs, voltage drop often forces a larger conductor than the current alone would need.
Does this cover three-phase or DC?
This sizing tool uses the single-phase AC formula. The same method with √3 instead of 2 applies to balanced three-phase; DC uses the same round-trip formula as shown here.
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