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Motors, Part 19

Motors, Part 19In non-motor branch circuits, the jobs of protecting against overcurrent, faults, and overloads are done by the branch circuit overcurrent protection device (OCPD).

In motor circuits, the branch OCPD protects against overcurrent and faults, but not against overloads. This is because motors have a high inrush current. If the branch circuit OCPD were sized to protect against overload, you wouldn’t be able to start the motor. Special devices are used for overload protection, and they allow for a temporary overload.

From the preceding points, you can conclude that the branch circuit OCPD must be sized to carry the starting current of the motor [430.52]. But how exactly do you determine how big is big enough, without being too big? The NEC provides Table 430.52 “Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Devices.”

This table is very helpful and would, at first glance, seem to make the selection process clear. However, the limitations of this table can mean your motor can’t start. That’s why there are two exceptions to it [430.52(C)(1)].

For example, you have an inverse time breaker rated at 250% of the Full-Load Current (FLC). The motor won’t start; every attempt trips that breaker. You try the next size up, and you’re able to start the motor. The NEC permits this. But what if that doesn’t work? You can keep trying, until you get up to 400% of FLC (currents of 100A or less) or 300% of FLC (currents over 100A).

« Part 18 | Part 20 » | Source: Mark Lamendola | Mindconnection