Wire Size Calculator — Ampacity and Voltage Drop Together

Two questions, one answer

Sizing a conductor requires answering two separate questions, and the larger of the two answers wins.

• Will it overheat? The conductor must be able to carry the load current continuously without exceeding its insulation temperature rating. That is the ampacity question, and it is answered by the tables in Article 310.
• Will the load still work? The voltage that arrives at the far end must be usable. That is the voltage drop question, and it depends on distance.

This calculator does both and reports which one drove the final size. In short residential runs ampacity almost always governs. In long runs, and in low-voltage circuits, voltage drop almost always governs — often by a surprisingly large margin.

How continuous load changes the number

A continuous load is one expected to run for three hours or more. For those, NEC 210.19(A)(1) requires the conductor to be sized at 125% of the load current, and NEC 210.20(A) applies the same factor to the overcurrent device. A 16 A continuous load is therefore designed as 20 A.

This is why an EV charger drawing 32 A is installed on a 40 A circuit with 8 AWG copper, not on a 32 A circuit. It also explains why a water heater or a commercial coffee machine frequently needs larger cable than a first estimate suggests.

Ampacity versus overcurrent limits

Note the third column: for the three smallest sizes the breaker is limited below the ampacity. A 14 AWG conductor in 90°C insulation has an ampacity of 25 A, but it may still not be protected above 15 A. Inspectors check this constantly.

Three worked examples

1. A 50 A range 60 ft from the panel

A 240 V single-phase circuit carrying 40 A continuous, 60 ft one way. Design current is 40 × 1.25 = 50 A, so 8 AWG copper (50 A at 75°C) satisfies ampacity. The voltage drop on 8 AWG over 60 ft is about 1.9 V, or 0.8% — comfortably inside 3%. Answer: 8 AWG with a 50 A breaker.

2. A 30 A circuit feeding a subpanel 200 ft away

The load is 24 A and the distance is 200 ft. Ampacity alone asks for 10 AWG. But 10 AWG over 200 ft drops about 10.4 V on a 240 V circuit, or 4.3% — outside the 3% target. The calculator moves the answer to 6 AWG, which brings the drop to about 1.6%. The cable cost roughly doubles, and that is the correct outcome.

3. A 480 V three-phase motor feeder

60 A on 480 V three-phase, 300 ft. Three-phase uses the square root of 3 factor, so the drop is lower than a single-phase circuit of the same length and current. 4 AWG gives about 2.6% drop with an ampacity of 85 A at 75°C, so a 70 A breaker works. Here ampacity governs, because the higher voltage keeps the percentage drop small.

What this calculator does not include

• Ambient temperature correction. Ampacities in the table assume 30°C ambient. A raceway in a 50°C mechanical room needs the correction factors in Table 310.15(B)(2)(a).
• Conductor bundling. More than three current-carrying conductors in one raceway requires the adjustment factors in Table 310.15(C)(1) — 80% for four to six conductors.
• Termination temperature limits. A 90°C conductor connected to a 75°C rated breaker may only be used at its 75°C ampacity. The lowest-rated component in the circuit sets the ceiling.
• Motor branch circuits. Motors have their own sizing rules in Article 430, including 125% of the table full-load current and separate short-circuit and overload protection.

Treat the result as the starting point for a design, not as a substitute for the code book.

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