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Motor Disconnects, Part 5

Motor Disconnects, Part 5Motor disconnects (if rated 1Kv or less) must have an ampere rating of at least 115% of the Full Load Current (FLC) of the motor [430.110]. Of course, you can go larger; it’s not like an OCPD needs to open automatically. It just needs to be stout enough to handle the current. So if you want to simplify the design and standardize on a few sizes, each covering a range of possible FLCs, the NEC won’t stop you from doing that.

An exception to this ampere rating requirement applies to listed unfused motor-circuit switches. If such a switch has a horsepower rating equal to (or greater than) that of the motor, it can have an ampere rating less than 115% of the motor FLC.

While not called out as an exception, the rule bends for torque motors. For these, you use the motor nameplate current instead of the FLC [430.110(B)].

Combination loads present some additional complexity when sizing the disconnect; see the requirements in 430.110(C).

For example, you have a single disconnect for several motors. This arrangement can be desirable in situations where you want to disconnect all functionally related (to the process) motors simultaneously. That would make maintenance much easier rather than having to identify and lock out / tag out each one individually. If the maintenance tech happens to miss one motor for a particular conveyor system, someone could get hurt. So you put them all on one disconnect.

« Part 4 | Part 6 » | Source: Mark Lamendola | Mindconnection