Designing a motor branch circuit means pulling five related numbers in the right order: the full-load current, the conductor, the short-circuit and ground-fault device, the overload, and the disconnect. NEC Article 430 ties them together but spreads them across several tables and sections. This calculator reads the table values and applies each rule so you get the complete circuit summary in one step.
How it works
Full-load current comes directly from NEC Table 430.248 for single-phase motors or Table 430.250 for three-phase, indexed by horsepower and voltage. Every downstream value is derived from that FLC:
Conductor (430.22): ampacity >= 1.25 x FLC
Max OCPD (Table 430.52): multiplier x FLC, rounded up to a standard size
inverse-time breaker 250% dual-element fuse 175%
non-time-delay fuse 300% instantaneous trip 800%
Overload (430.32): 115% or 125% of nameplate FLA
Disconnect (430.110): >= 1.15 x FLCThe conductor is chosen from the 75 degree Celsius copper column of Table 310.16, and the OCPD is rounded up to the next standard size from 240.6(A) as permitted by 430.52(C)(1) Exception 1.
Example and notes
A 10 HP, three-phase, 460 V motor has a table FLC of 14 A. The conductor must carry 17.5 A, which a 12 AWG 75°C copper conductor satisfies. With an inverse-time breaker the maximum OCPD is 250 percent of 14, or 35 A, which is already a standard size. The overload sizes to roughly 16 to 17.5 A from nameplate, and the disconnect must be rated at least 16.1 A.
Remember the table FLC drives conductor, OCPD, and disconnect, while only the overload uses the nameplate FLA. When several motors share a feeder, size the feeder under 430.24 and apply ambient and conduit-fill corrections to the conductor ampacity shown here.
Why the NEC uses table FLC, not nameplate
A motor’s nameplate current (FLA) is the manufacturer’s measured running current under full load at rated voltage. The table full-load current values in 430.248/430.250 are conservative reference values set by the NEC for circuit sizing purposes. They deliberately cover motor variations, because different motors of the same horsepower and voltage can have slightly different efficiencies and power factors.
Using the table value for conductors and OCPD ensures the circuit can handle any compliant motor of that size, not just the specific unit installed today. If the motor is replaced with a higher-FLA unit of the same HP rating, the circuit still works. The overload device uses nameplate FLA because overloads are adjusted to protect the specific motor installed.
Reading the OCPD output
The maximum OCPD the calculator shows is exactly that — the maximum. You may use a smaller standard size and the motor will still start under most conditions. However, if the motor has a high starting current (high inertia load, long acceleration time), a smaller breaker or non-time-delay fuse can nuisance-trip on startup. The standard permits going up to the maximum precisely to allow for this.
If the motor still fails to start at the maximum standard size for an instantaneous-trip breaker (800%), the NEC provides an exception under 430.52(C) (2) that allows setting up to 1300% for inverse-time and higher for special cases — but this requires adjustable-trip breakers and careful coordination.
Multiple motors on one feeder
For a feeder supplying two or more motors, NEC 430.24 requires the feeder to carry at least 125% of the largest motor’s FLC plus the sum of all other motors’ FLC. Run this calculator for each motor, find the largest, and then build the feeder from those numbers. The OCPD for the feeder is sized differently from any individual branch circuit — it covers the full collection of motors under 430.62, typically sized from the largest OCPD for any single motor on the feeder.