How the size is chosen
A wire has to pass two separate checks. It must carry the load current without overheating, which is its ampacity from Table 310.16. And it must deliver enough voltage at the far end, which depends on its resistance and the length of the run. The calculator finds the smallest size that passes each check and gives you the larger of the two.
Formula
- Ampacity
- Table 310.16 value ≥ load current
- Voltage drop
- VD ÷ V × 100 ≤ target %
- Result
- the larger of the two smallest sizes
Voltage drop uses the same method as the voltage drop calculator: Chapter 9 Table 9 for AC, Table 8 for DC.
Worked example
A 50 A load at 240 V single-phase, 150 ft from the panel, copper in PVC conduit, 3% drop target.
- Ampacity: 6 AWG copper is rated 55 A in the 60 °C column, enough for 50 A.
- Voltage drop: at 150 ft, 6 AWG drops 3.06%, over the target. 4 AWG drops 4.65 V, or 1.94%.
Answer: 4 AWG copper. The run is long enough that voltage drop, not ampacity, sets the size.
60 °C or 75 °C?
Table 310.16 lists three ampacities for each wire, at 60, 75 and 90 °C. The one you may use is limited by the lowest-rated termination on the circuit. Section 110.14(C) says equipment for circuits of 100 A or less, or for 14 through 1 AWG conductors, is treated as 60 °C unless it is listed and marked for 75 °C. Above that it is 75 °C. The calculator applies this by default. If every breaker and lug on your circuit is marked 75 °C, you can choose the 75 °C column instead.
Small conductors have one more limit. Section 240.4(D) caps the breaker for 14, 12 and 10 AWG copper at 15, 20 and 30 A, whatever the table says.
References
- NFPA 70, National Electrical Code, 2023 edition: Table 310.16 and Section 110.14(C).
- NFPA 70, 2023: Section 240.4(D), small conductors; Section 310.10(G), conductors in parallel.
- NFPA 70, 2023: Chapter 9, Tables 8 and 9.