Motor Circuit Calculator

This calculator looks up a motor's full-load amperage (FLA) from Table 430.248 (single-phase) or 430.250 (three-phase) in the National Electrical Code, NFPA 70 (NEC), then derives every other result from that one figure: Max OCPD size at 250% of FLA for a standard inverse-time breaker or 175% for a dual-element fuse (NEC 430.52), Min conductor ampacity at 125% of FLA (NEC 430.22), and Overload relay trip at 115% (NEC 430.32). Motor horsepower and Voltage both feed the FLA lookup, and Motor horsepower now moves Motor FLA (amps) further than an equivalent percentage change to Voltage does -- at the default inputs, a 10% increase in Motor horsepower (10 to 11 HP) moves Motor FLA by 50%, roughly four to five times the ~9-11% swing from an equivalent 10% change to Voltage, because horsepower selects between sparse NEC table entries (each in-between value rounding up to the next standard size) while Voltage scaling is a smooth, continuous ratio. Raising Voltage still lowers Motor FLA (amps) for the same horsepower, since the same power delivered at higher voltage draws less current. Protection type only affects Max OCPD size, choosing between the 250% breaker multiplier and the 175% fuse multiplier -- it has no effect on Motor FLA (amps) itself, which is set entirely by horsepower, phase, and voltage.

For horsepower values that fall between the NEC table's standard listed sizes, the calculator rounds up to the next larger listed horsepower rather than interpolating or estimating -- so an in-between horsepower value always gets the FLA (and every downstream breaker, conductor, and overload figure) of the next standard motor size up, never a smaller or intermediate one. That protects against under-sizing within the tables' real coverage range, but the breaker and conductor tables are finite: once required OCPD or conductor ampacity exceeds their largest listed entry, the loop has nowhere left to round up to. Exceeds Standard Sizing (1=Yes) flags that condition -- for a 460V three-phase motor on a standard inverse-time breaker this first happens just above 250 HP (verified: a 300 HP motor needs 451A of conductor ampacity, but the largest listed conductor, 500 kcmil, is only rated for 380A), and it can trigger at much lower horsepower for single-phase motors, whose FLA table only lists sizes up to 10 HP before falling back to a linear extrapolation (verified: a 230V single-phase motor trips the flag above roughly 38.5 HP). When the flag reads 1, the displayed conductor and breaker figures are only the largest table entries shown for reference -- not a safe recommendation -- and the circuit needs a custom or paralleled-conductor design from an electrical engineer.