For a motor, what percentage of FLA is used to size a time-delay fuse?

• A. 150% of FLA
• B. 175% of FLA
• C. 200% of FLA
• D. 300% of FLA

Answer: (B)

When protecting a motor with a fuse, you have to accommodate the high inrush current that occurs at startup while still keeping the motor protected from lasting overload. Time-delay fuses are designed to ride out brief overloads, so you size them larger than the motor’s running current but not so large that they won’t protect against overheating.

The standard practice is to size a time-delay fuse at 175% of the motor’s full-load current (FLA). This means multiplying the FLA by 1.75 to pick the fuse rating. The startup surge of many motors is accommodated, but if the current stays above about 175% of FLA for a sufficient time, the fuse will trip to protect the motor. For example, a motor with an FLA of 20 A would use a 35 A time-delay fuse.

Choosing 150% would risk nuisance trips on startup for many motors, while 200% or higher would reduce protection somewhat or rely on different protection schemes. The 175% figure strikes a practical balance for typical motor startup characteristics.