430.52
Motor Branch-Circuit Short-Circuit and Ground-Fault Protection
On a normal branch circuit, you match the breaker to the conductor ampacity. But motor circuits are different. If you put a 14-amp breaker on a motor that draws 14 amps at full load, the breaker would trip every time the motor starts because the startup inrush is way beyond 14 amps. Table 430.52 solves this by allowing much larger overcurrent devices specifically for motors. Depending on the type of device, you can go up to 150 percent, 175 percent, 250 percent, or even 300 percent of the FLC. For example, with an inverse-time circuit breaker, the standard maximum is 250 percent of the FLC. If that calculated value does not land on a standard breaker size from 240.6, you are allowed to round up to the next standard size. For hard-to-start motors, an exception lets you go even higher — up to 400 percent for inverse-time breakers on motors of 100 amps or less. The key thing to remember is that this device is for short-circuit and ground-fault protection only. The motor overload relay handles the overload protection separately.
When You Need This
- Selecting the breaker or fuse for any new motor branch circuit installation
- Troubleshooting nuisance trips on an existing motor circuit — the OCPD may be undersized for the inrush
- Designing a motor control center or a motor branch panel with multiple circuits
- Preparing for an exam — motor OCPD sizing is one of the most common calculation topics
Key Points
Common Mistakes
Using the nameplate FLA instead of the NEC table FLC to calculate the OCPD size
Rounding down to the next standard breaker size instead of rounding up — the code specifically permits the next size up
Confusing the branch-circuit short-circuit protection device with the overload relay — they serve different purposes and are sized by different rules
Applying the 125-percent continuous-load rule on top of the Table 430.52 percentage — motor circuits do not stack these two factors
Forgetting to check the exception for hard-starting motors before telling a customer you need a larger conductor
Exam Tip
Know the standard percentages cold: 150 percent for non-time-delay fuses, 175 percent for dual-element fuses, 250 percent for inverse-time breakers. When the calculated value does not land on a standard size, round UP. Many exam questions hinge on whether you round up or down.
Frequently Asked Questions
Motor startup inrush current is typically 6 to 8 times the full-load running current. A standard-sized breaker would trip immediately on startup. The larger OCPD lets the motor start, and the separate overload relay protects against prolonged overcurrent during normal operation.
The NEC provides exceptions for hard-starting motors. You can increase the OCPD size above the standard Table 430.52 value — for example, up to 400 percent for inverse-time breakers on motors 100 A or less. If that still trips, the motor may have a mechanical or electrical problem that needs repair.
Standard inverse-time circuit breakers are commonly used and allowed. Motor-circuit protectors (instantaneous-trip breakers) are a specialized option used mainly with combination motor starters that include overload relays. The type you use determines which percentage column in Table 430.52 applies.
Inline Tools
Motor FLC Calculator
Look up full-load current for any motor HP and voltage
Related Code Sections
This is an educational summary, not the official code text. The NEC® is a registered trademark and copyright © National Fire Protection Association (NFPA). The CEC is copyright © CSA Group. For official code text, visit nfpa.org or csagroup.org. SparkShift is not affiliated with NFPA or CSA Group.