When to use: Use this calculator when designing motor branch circuits for HVAC equipment, pumps, fans, compressors, and industrial machinery. NEC Article 430 has specific requirements for motors that differ from standard branch circuits — conductors must be sized at 125% of FLA (NEC 430.22), overcurrent protection at 150–300% of FLA (NEC 430.52), and overload protection at 115–125% (NEC 430.32). Using standard branch circuit rules for motors is a common — and dangerous — mistake.
This free motor branch-circuit sizing calculator applies NEC Article 430 to a single motor load — sizing the branch-circuit conductors, the short-circuit and ground-fault overcurrent protective device (OCPD), and the overload relay, plus pointing you to the disconnect and controller requirements. Pick the motor's phase, voltage, and horsepower and it pulls the table full-load current (FLC), then applies the correct percentages so your design matches the code instead of standard branch-circuit rules. It's built for electricians, electrical engineers and designers, plan reviewers, maintenance techs, and exam candidates working with HVAC equipment, pumps, fans, compressors, and industrial machinery.
Motor circuits use four coordinated devices, each sized off a different basis.
1. Conductors (NEC 430.22): branch-circuit conductors for a single continuous-duty motor must carry at least 125% of the motor full-load current (FLC), and the FLC comes from NEC Tables 430.247–430.250 — not the motor nameplate.
2. Short-circuit / ground-fault OCPD (NEC 430.52 and Table 430.52): the branch-circuit protective device protects against faults and starting inrush, so it is sized well above FLC. The maximum is a percentage of FLC by device type — for example an inverse-time (thermal-magnetic) breaker up to 250%, a non-time-delay fuse up to 300%, a dual-element (time-delay) fuse up to 175%, and an instantaneous-trip breaker up to 800%. You take the next standard size where needed.
3. Overload (NEC 430.32): separate running-overload protection is set at 115% to 125% of the motor nameplate full-load amps (FLA), based on service factor (1.15 or greater) and temperature rise.
4. Disconnect and controller (NEC 430 Parts VII & IX): the disconnecting means must be rated at least 115% of the motor FLC (430.110), and the controller must be sized for the motor horsepower.
Worked example — 10 HP, 460 V, 3-phase motor: table FLC = 14 A. Conductors: 1.25 × 14 = 17.5 A, so a #12 AWG copper conductor (rated above 17.5 A) is the minimum. Inverse-time breaker: 2.5 × 14 = 35 A (a standard size, so 35 A). Overload (SF ≥ 1.15): up to 1.25 × nameplate FLA.
1. Select the motor phase (single- or three-phase). 2. Choose the system voltage (e.g. 230 V, 460 V, 575 V). 3. Pick the motor horsepower — the calculator reads the FLC from NEC Table 430.248 (1-phase) or 430.250 (3-phase). 4. Set the service factor so the overload calculation uses 115% or 125%. 5. Choose the OCPD type (inverse-time breaker, dual-element fuse, etc.) to apply the correct Table 430.52 multiplier. 6. Read the minimum conductor ampacity and size, the next standard OCPD rating, and the maximum overload setting.
FLC (full-load current) is the standardized table value from NEC Tables 430.247–430.250 for a given HP and voltage. FLA (full-load amps) is the actual current stamped on the motor nameplate. NEC 430.6(A) requires using the table FLC for sizing the conductors, the OCPD, and the disconnect — even if the nameplate reads differently — so equipment is sized consistently. The nameplate FLA is used only for the overload protection (430.32), because overload is protecting that specific motor. Mixing these up is a common and code-violating mistake.
Per NEC 430.22, conductors for a single continuous-duty motor must have an ampacity of at least 125% of the motor full-load current (FLC) taken from NEC Tables 430.247–430.250 — the table value, not the nameplate amps.
FLC (full-load current) is the standardized value from NEC Tables 430.247–430.250 used to size conductors, the OCPD, and the disconnect (NEC 430.6). FLA (full-load amps) is the actual current on the motor nameplate, used to size the overload protection. They are often different, and using the wrong one is a code violation.
Running-overload protection is sized off the motor nameplate full-load current per NEC 430.32 — generally up to 125% of nameplate FLA for motors with a service factor of 1.15 or greater (or a marked temperature rise of 40°C or less), and up to 115% for other motors.
The branch-circuit short-circuit and ground-fault device is sized from Table 430.52 as a percentage of FLC by device type — up to 250% of FLC for an inverse-time breaker, 300% for a non-time-delay fuse, 175% for a dual-element fuse, and 800% for an instantaneous-trip breaker — rounded to the next standard size where the rule allows.
Motors draw a large inrush current at startup (often 6–8× running current), so the short-circuit/ground-fault device must be set high enough not to nuisance-trip — which means it cannot also protect against overload. NEC Article 430 therefore splits the job: oversized branch-circuit protection for faults and inrush, plus separate overload protection sized close to nameplate current.
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