Motors & Loads calculator
Motor Protection Calculator
Use this page after the base motor current has already been identified. A 10 HP, 460 V, three-phase Design B motor screens at 14 A FLC, 16.1 A overload, 35 A inverse-time breaker, and 17.5 A conductor ampacity before listed-equipment review. The Full Load Current Calculator handles NEC Table 430.248/430.250 lookup, the Motor Current Calculator handles formula running-current comparison, and the Motor Starting Current Calculator handles inrush and locked-rotor screening.
Updated July 10, 2026
Example Calculations
How to Use
Motor Protection Calculator for NEC Article 430 Workflows
This page is for protection review, not base-current discovery. Start with the correct current basis, then use the protection result as a preliminary screen before selecting listed equipment and final settings.
1. Choose the correct current basis
| Protection task | Current basis | Next step |
|---|---|---|
| Overload protection | Actual motor nameplate FLA | Apply the service-factor and temperature-rise rule for the installed motor. |
| Branch-circuit short-circuit and ground-fault protection | NEC table FLC from the applicable motor table | Apply the device percentage and standard-size rules for the selected device type. |
| Starting-current review | Locked-rotor or starting-current data | Use the starting-current workflow when nuisance tripping or source stiffness is the question. |
2. Review overload protection separately
Overload protection protects the motor from sustained overcurrent and overheating. For common continuous-duty motors, the overload setting is based on the actual nameplate FLA. Motors with a service factor of 1.15 or higher, or a qualifying temperature rise, can commonly be reviewed at 125% of nameplate FLA; other motors are commonly reviewed at 115%. Confirm the final device selection against the adopted NEC edition and the overload-device instructions.
3. Review branch-circuit fault protection separately
Branch-circuit short-circuit and ground-fault protection has a different job. It must clear high-current faults while still carrying normal motor starting current. For this part of the workflow, use the NEC table full-load current from the Full Load Current Calculator, then apply the selected device type. A common inverse-time breaker screen uses 250% of NEC table FLC before standard-size review.
4. Do not use one current value for every decision
A motor circuit can involve formula running current, nameplate FLA, NEC table FLC, locked-rotor current, and measured operating current. This page uses those values for protection review after the current basis is already known.
For the current basis, use the Full Load Current Calculator when the job is NEC table FLC lookup, the Motor Current Calculator when the job is formula running-current comparison, and the Motor Starting Current Calculator when the concern is inrush or locked-rotor current. Then return here for overload, branch-circuit, and conductor-protection review.
Common Applications
More applications. Open to review 3 additional use cases.
Frequently Asked Questions
Should motor overload protection use nameplate FLA or NEC table FLC?
Use actual motor nameplate FLA for overload protection. NEC table FLC is used for branch-circuit conductor sizing and branch-circuit short-circuit and ground-fault protection, so it should not be treated as the overload-current basis.
When should I use the Full Load Current Calculator instead of this page?
Use the Full Load Current Calculator when the task is the NEC Table 430.248 or 430.250 lookup by horsepower, voltage, and phase. Return to this page after that value is known and the task becomes overload or branch-circuit protection review.
What is the difference between overload protection and short-circuit protection?
Overload protection responds to sustained overcurrent that can overheat the motor. Branch-circuit short-circuit and ground-fault protection responds to high-current faults and must also tolerate normal motor starting current. The two functions use different sizing logic.
Does this page select the final breaker, fuse, starter, or relay?
No. It provides a preliminary calculation and current-basis check. Final selection depends on the adopted NEC edition, listed equipment, manufacturer instructions, available fault current, time-current coordination, and site conditions.
Where does starting current fit in the protection workflow?
Starting current matters when reviewing nuisance tripping, source stiffness, soft-starter settings, VFD behavior, or locked-rotor conditions. Use the Motor Starting Current Calculator for that workflow, then return here only if the question is protection-device review.
Related Calculators
Full Load Current Calculator
Owns NEC Table 430.248/430.250 FLC lookup by HP, voltage, and phase, including 10 HP and 200 HP at 460V field checks.
Motor Current Calculator
Estimate formula running current from HP or kW, voltage, efficiency, and power factor, then compare against NEC table FLC when needed.
Motor Starting Current Calculator
Compare induction-motor DOL, star-delta, soft-starter, VFD, and autotransformer starting current screens.
Motor Branch Protection Calculator
Screen NEC Article 430 motor branch-circuit short-circuit and ground-fault protective device sizing.
Motor Cable Size Calculator
Screen NEC table motor conductor size with ampacity derating, termination limits, and branch-circuit voltage drop.
Wire Size Calculator
Handle conductor gauge selection from load current, standard OCPD, and voltage drop after ampacity and raceway constraints are known.