Motor Calculations — Part 1

Motor Calculations — Part 1

Calculations for motor circuits are different from those of other circuits because of how motor circuits must handle overcurrent.

When motors start across the line, they typically draw five times the running current. This temporary “inrush” is an overload condition. To handle inrush while still providing adequate overcurrent protection, we divide overcurrent protection into two parts (Fig. 1):

1. Short circuit and ground fault protection. Typically, this is via a fuse or circuit breaker sized large enough to let the motor start. But if it’s large enough to let the motor start, i

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