Service & Feeder Sizing Calculator

How service and feeder conductor sizing works

NEC 230 governs service conductors (from the utility to the service equipment) and NEC 215 governs feeders (from service equipment to downstream panelboards). In both cases the minimum conductor size is determined by the calculated load — you pick the smallest conductor from Table 310.16 whose ampacity equals or exceeds that load at the applicable temperature column.

Table 310.16 lists allowable ampacities for conductors in conduit, cable, or earth — the three columns correspond to the conductor's temperature rating: 60 °C, 75 °C, and 90 °C. The column you use is governed by the termination rating, not the conductor's own rating. NEC 110.14(C)(1) limits most 100 A-and-below equipment to the 60 °C column; 110.14(C)(1)(b) allows the 75 °C column for equipment rated 100 A and above (which covers most service and feeder terminations). The 90 °C column is only available when both the conductor and every termination are rated 90 °C — uncommon in service work.

Overcurrent protection for feeders (NEC 215.3) and services (NEC 230.90) must not exceed the conductor's ampacity. The overcurrent device must be a standard size listed in NEC 240.6 — if the calculated load doesn't land exactly on a standard size, round up to the next standard size. This calculator finds the smallest standard OCPD that is at least as large as the calculated load.

Note: dwelling-unit services may use the optional 83% demand factor in NEC 310.12, which allows a smaller conductor than Table 310.16 would otherwise require. This calculator applies Table 310.16 directly without that allowance. No derating for conduit fill, ambient temperature, or continuous loads is applied here — those adjustments are required in practice per NEC 310.15.