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Code Forum

Jul 1, 1998 12:00 PM, By Frederic P. Hartwell, Senior Editor

Motor Circuit Terminations

How does a motor circuit affect, if at all, the use of the temperature limitations in Sec. 110-14(c) for circuit terminations?

e recently received a question relating to temperature limitations on motor circuit terminations, and how they relate to Sec. 110-14(c) limits. The question focuses on the supply side conductor terminations, but you'll find interesting wrinkles on this at both ends of these conductors. We'll discuss both ends in turn.

The supply end. Testing laboratories make routine assumptions about various worst-case conditions in testing products. How does the product (in this case, an electric conductor) perform with a potential heat source bolted to it? Suppose you're using THHN, and it actually approaches its 90 DEGRC ampacity due to high current. That represents a heat source almost at the boiling point of water. You won't damage the conductor, but the inside of mechanical devices just won't function as expected afterward.

The key here is to look at how much current will pass through the circuit with a specified conductor size. Motor circuits have overcurrent devices set far higher than the circuit load, or even the ampacity of the circuit conductors. The reason is: These devices don't normally protect against overloads, only short circuits and ground faults. Don't get derailed by this!

A small, excessively hot conductor bolted to a large circuit breaker or fused switch will be liable to make that device misbehave just as surely as a larger conductor at a similar temperature. Ask yourself: How hot will these conductors run? That question involves long-time loading, not short-time fault duty, for the purposes of this analysis.

The actual load on a particular motor branch circuit, per Sec. 220-3(c)(2), follows Sec. 430-22, or usually 125% of the full load current rating of the motor as described in the Tables in Part M of Art. 430. Running overload protective devices for motor circuits typically allows overloads up to 125% to flow for significant time periods, so allowance is appropriate.

That 125% value is what you take through the ampacity tables. Assuming the device terminals are listed for 75 DEGRC terminations, any conductor large enough to equal or exceed that ampacity in the 75 DEGRC column will do. This is true whether or not it equals the ampere rating of the overcurrent protective device, or that of the contactor. These ratings apply to the devices and not to the lugs per se. There are many contactors and switches with 90C lugs; observe the device ratings instead, which are usually 75 DEGRC.

The load end. The foregoing analysis ought to be the same for the motor terminations at the other end of these conductors, but there is a fly in this ointment for motors with termination provisions below 100A. (In this case, we would read the nameplate current rating.) The default temperature rating for these smaller terminations, in Sec. 110-14(c)(1), is still 60 DEGRC. The widely used exception that allows 75 DEGRC terminations on the lower ampere ratings only applies to equipment listed and identified for use at the higher temperature. With the exception of a relative few special purpose motors, particularly motors for use inhazardous locations, most motors aren't listed.

The reason is the testing laboratories don't want to add a listing mark to a motor when such factors as shaft loading are unknown. Testing laboratories give motors component recognition, which is not a listing. It's the assembled appliance that gets the listing. In this case, technically the motor can't comply with Sec. 110-14(c)(1) Ex. 2. Nevertheless, we urge inspectors to go ahead and allow the 75 DEGRC ratings for these terminations. The motor manufacturers have agreed this is an oversight, and the 1999 NEC will expressly allow you to do this.

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