NEC Wire Size Calculator — Ampacity, Breaker and Volt Drop

Three rules that change the answer

Sizing a conductor to the NEC is not a table lookup, though it is often treated as one. Three separate rules can each push the answer up a size, and a calculator that applies only the ampacity table will hand you something the code does not allow.

1. The terminals, not the cable — 110.14(C)

THHN is rated 90 °C, and its 90 °C ampacity is the number people reach for. But a conductor is only as hot as its weakest connection, and breaker lugs are typically listed for 75 °C, or 60 °C on older and smaller equipment. Land 90 °C wire on a 75 °C lug and you have a 75 °C circuit.

The 90 °C column may still be used as the starting point for ambient and bundling corrections — that is the value of buying 90 °C insulation — but the final ampacity may not exceed the terminal-rated column. Eight AWG copper is 55 A at 90 °C, 50 A at 75 °C and 40 A at 60 °C. Which of those you may use depends on the equipment, not the wire.

2. The small conductor rule — 240.4(D)

Fourteen AWG copper has an ampacity of 20 A in the 75 °C column. You still cannot protect it at more than 15 A. Twelve AWG is capped at 20 A and ten AWG at 30 A, whatever the table says. For aluminium it is 15 A at twelve and 25 A at ten.

This is the rule that separates a working calculator from a plausible one. A 20 A load on 14 AWG passes an ampacity check and fails an inspection. Small conductors are the ones that get nicked, over-torqued and fatigued, and the code declines to let ampacity alone protect them.

3. Continuous loads — 210.19(A)(1)

A load expected to run for three hours or more is a continuous load, and both the conductor and the overcurrent device are sized at 125 % of it. Lighting, HVAC and EV charging are the usual cases. A 40 A continuous load is a 50 A design current, and the difference is frequently a whole size.

Then the two corrections

Ampacities are tabulated at 30 °C ambient with no more than three current-carrying conductors. Real installations are hotter and more crowded, and both corrections multiply.

• Ambient — Table 310.15(B)(1). Each insulation column derates differently, because a 90 °C conductor has more headroom above ambient than a 60 °C one. At 40 °C the factors are 0.82, 0.88 and 0.91 respectively.
• Bundling — Table 310.15(C)(1). Four to six conductors take 80 %, seven to nine 70 %, ten to twenty 50 %. Count current-carrying conductors: a neutral carrying only imbalance does not count, but a neutral on a circuit full of harmonics does.

A 100 A load on THHN copper needs 3 AWG in ideal conditions. Put six conductors in a raceway at 40 °C and the same load needs 1 AWG — two sizes up, from the conditions alone.

Volt drop is a recommendation, not a rule

The NEC does not mandate a volt drop limit. Informational Notes to 210.19(A) and 215.2(A) suggest 3 % on a branch circuit and 5 % overall, and that is what this calculator checks against.

On long runs it is usually volt drop, not ampacity, that sets the size — a 20 A circuit at 120 V over 250 ft needs 4 AWG for volt drop where 12 AWG would carry the current. The calculator says which of the two governed.

Resistances here are computed from the conductor area and the temperature coefficient of the metal at 75 °C, not read from a table — which is why 12 AWG copper comes out at 1.93 Ω per 1000 ft, agreeing with Chapter 9 Table 8 to three figures.

Ampacity table

Table 310.16 conditions: raceway, cable or direct burial, 0–2000 V, 30 °C ambient, not more than three current-carrying conductors.

What this does not do

• Motor circuits. Article 430 sizes motor conductors from the FLC tables at 125 %, not from the nameplate, and protects them quite differently. Do not use this for a motor feeder.
• Parallel conductors. Where no single conductor carries the load, 310.10(G) allows paralleling from 1/0 upward with rules about equal length and termination. The calculator tells you when you have reached that point.
• Conduit fill, grounding conductors and tap rules. All separate calculations.
• Your local amendments. The NEC is adopted state by state, often with changes and often an edition or two behind. Check what your jurisdiction has actually adopted.

Working in metric too

If you are reading an American datasheet against an IEC design, the AWG to mm² converter handles the conversion and warns you when the nearest metric size is smaller than the AWG one — which happens on thirteen of the forty-four sizes. For sizing to IS 732 or IEC 60364 instead, use the cable sizing calculator, which applies that method properly rather than translating this one.