Motor Load Calculations, Part 1

Part II of Article 220 provides the requirements for branch-circuit load calculations. Inside Part 11, we find Part B of Subsection 220.14; it lists many different types of loads you must address individually in your calculations. Table 220.3 contains three lines, each referring you to a Subsection of Article 430. And 220.14(C) tells you where to find Article 430’s requirements for outlets supplying motors.

Just a few references, so it’s simple—right? Unfortunately, the words “simple” and “motor calculations” just do not go together.

A big stumbling block for wrapping your mind around motor calculations is understanding that the requirements for sizing motor conductors and overcurrent protection devices don’t follow the normal logic. There are motor calculations, and then there’s everything else.

With motor circuits, the overload protective functions are split such that overload protection is handled separately from ground-fault and short-circuit current protection. This arrangement exists to accommodate two issues with motors: they have significant inrush current upon starting, and they are subject to overload when running.

Absent this split, nuisance trips would be likely during normal operation. Assuming you could even start the motor.

Motor load calculations can result in some seemingly strange numbers. For example, consider a feeder that supplies a motor along with other loads. When you address this per the requirements of 430.63, it’s possible that your short-circuit device settings will be higher than the ampacity of the conductor. This situation would never arise on any circuit that doesn’t contain a motor.

Part 2 » | Source: Mark Lamendola | Mindconnection

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