Motor Full-Load Current and Circuit Protection Reference
Look up motor table full-load current, continuous-duty branch conductor ampacity, and general short-circuit and ground-fault protection limits.
Nameplate and table selection
The branch-conductor ampacity reference assumes a continuous-duty motor. Other duty cycles use different sizing rules.
Use the motor's design and rotor marking. Select the high-efficiency option only when its listed design category applies.
Choose the motor's rated voltage, not a nearby system-voltage label.
Motor current and protection references
| Branch-circuit short-circuit/ground-fault device | Table multiplier | Calculated maximum rating or setting |
|---|---|---|
| Non-time-delay fuse | 300% of table FLC | 42 A |
| Dual-element time-delay fuse | 175% of table FLC | 24.5 A |
| Instantaneous-trip circuit breaker | 800% of table FLC | 112 A |
| Inverse-time circuit breaker | 250% of table FLC | 35 A |
Protection values are the calculated table limits before permitted next-standard-size allowances, exceptions, equipment markings, or local amendments. This calculator does not select a fuse or breaker.
About this calculator
Choose a listed motor phase, type, horsepower, and rated voltage. The calculator looks up full-load current from Table 430.248 or Table 430.250, calculates the continuous-duty branch-conductor ampacity reference, and shows general branch-circuit short-circuit and ground-fault protection limits.
It reports current references and table limits. It does not choose a conductor, fuse, breaker, overload relay, starter, or complete motor circuit.
Method and assumptions
For listed single-phase motors, full-load current comes from Table 430.248. For listed three-phase induction motors, it comes from Table 430.250. Synchronous motor values use the unity-power-factor table current, adjusted by 1.10 at 90% power factor or 1.25 at 80% power factor.
For a continuous-duty motor, the branch-circuit conductor ampacity reference is 125% of table full-load current. This result is a minimum ampacity value, not an AWG selection. Verify conductor ampacity under the actual insulation, terminal, ambient, conductor-count, raceway, and wiring-method conditions. If a motor is marked Design BE or CE, also verify the additional conductor thermal requirement in 430.22(H); this tool does not calculate that check.
The four protection limits use the general multipliers in Table 430.52(C)(1). The 1100% instantaneous-trip multiplier applies only to the specific Design B energy-efficient/premium and BE/CE categories shown in the table; a general marketing claim of high efficiency is not enough to select this row. Wound-rotor motors use their separate row. These are maximum table calculations before permitted next-standard-size adjustments or other exceptions.
For overload protection, use motor nameplate current and the applicable service-factor or temperature-rise rules. Table full-load current does not replace nameplate FLA for overload-device sizing.
Example
A 10 HP, 460 V, three-phase standard squirrel-cage motor has a 14 A table full-load current. For continuous duty, the conductor ampacity reference is 17.5 A. The general inverse-time circuit-breaker limit is 250% of table current, or 35 A; the instantaneous-trip circuit-breaker limit is 800%, or 112 A.
Those calculated limits do not identify the installed device rating. Review motor starting, the exact device and starter, permitted standard-size allowances, conductor protection, equipment markings, and locally adopted requirements.
Interpret the results
Use the table full-load current for the branch-circuit calculations that call for table current. Use the listed conductor ampacity as an input to a separate conductor check. Compare protection limits with the actual device only after confirming the motor classification and applicable allowances.
Use the nameplate and overload-device instructions for overload protection. Special motors, duty cycles, installations, and starting conditions can change the required method.
FAQs
Why does the table current differ from the motor nameplate?
The NEC motor tables provide standard full-load current values used for branch-circuit conductor and short-circuit/ground-fault protection calculations, subject to applicable exceptions. The nameplate current is used for overload protection and for other checks that specifically require nameplate data.
Does this give me a conductor size?
No. It calculates the minimum ampacity reference for a continuous-duty motor. The conductor size also depends on insulation, terminal ratings, ambient temperature, current-carrying conductor count, wiring method, and other installation requirements.
Are the protection values the breaker sizes to install?
No. They are general maximum ratings or settings calculated from the table full-load current and the selected motor type. The tool does not apply next-standard-size provisions, starting conditions, equipment markings, permitted exceptions, or coordination requirements.
Does this calculate overload protection?
No. Use the Motor Overload Protection Setting Reference with the nameplate current and applicable service-factor or temperature-rise markings. Confirm the actual overload device setting with its manufacturer instructions.
Which motor types are included?
The reference includes single-phase AC, three-phase squirrel-cage standard, the listed Design B energy-efficient/premium and BE/CE squirrel-cage categories, wound-rotor, and synchronous motors in supported horsepower and voltage combinations. It does not cover multi-speed, low-speed/high-torque, DC, hermetic refrigerant-compressor, or other special motors.
Does the Design BE/CE option include every conductor check?
No. For a motor marked Design BE or CE, verify the additional conductor thermal requirement in 430.22(H). This tool does not calculate that check.
Why does a synchronous motor ask for power factor?
Table 430.250 lists synchronous-motor current at unity power factor. The calculator applies the table adjustment for 90% or 80% power factor when selected.
Sources
- NFPA 70, National Electrical Code, 2026 edition — Tables 430.248, 430.250, and 430.52(C)(1); Sections 430.6(A)(1), 430.22(A), 430.22(H), and 430.32. Confirm the adopted edition, final code text, and local amendments.
- Verify motor classification, ratings, overload data, and equipment instructions from the motor and starter nameplates and manufacturers.