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Code Forum

Oct 1, 1999 12:00 PM, By Fred Hartwell, Hartwell Electrical Services, Inc.

Sizing wye-delta conductors to a hermetic refrigerant compressor don't follow the same rules as regular motor conductors in similar use — an error in the NEC.

We received an interesting series of questions about sizing conductors on the load side of a wye-delta starter to a hermetic refrigerant motor-compressor. The compressor in question has a rated load current (RLC) of 751A. The submitter wants to size these conductors at 125% of (751A x 0.58) = 545A per Sec. 430-22(a). However, he wonders if Sec. 440-32 amends the calculation, thereby requiring 125% of 751A = 939A?

He notes that with respect to branch circuit, short-circuit protection per Sec. 440-22(a), you must size the overcurrent protection less than 175% of RLC. He wonders if he can use a 100% rated, 800A breaker, assuming it were capable of carrying the starting current of the motor? If he did this, would it affect the allowable conductor selection; that is, would he still need to go to 125% of 751A with an 800A 100% breaker?

Finally, per Sec. 90-3, Chapters 1, 2, 3, and 4 apply generally. How do the exceptions to Sec. 210-19(a) and Sec. 215-3 affect Chapter 4?

The response. Starting with the last question first, the first four chapters should internally correlate, since they have equal precedence under Sec. 90-3. One of the more fertile grounds for proposals has always been ferreting out these correlation problems and bringing them to a Code making panel's attention. So, the quick answer is they don't affect Chapter 4; you have to hope the Code language avoids a conflict. I don't think there's a conflict with Chapter 2, but there may be a technical problem in Chapter 4.

This question relates to a large branch circuit. Art. 440 only relates to refrigeration equipment (see Sec. 440-1) and the branch circuits supplying it. Sec. 440-3(a) describes the rules of Art. 440 as being "in addition to or amendatory of" the rules in Art. 430. Therefore, properly applied rules in Art. 440 by logical extension fall within the exclusion of Art. 210 provisions to Art. 430 applications by Sec. 210-1. This application certainly isn't a combination load. Therefore Sec. 210-19(a) Ex. doesn't apply, and since we're not dealing with feeders, Sec. 215-3 doesn't apply either.

If using an 800A circuit breaker (100% rated), don't confuse the 25% bonus rule for continuous loading of an overcurrent device with the 25% bonus rule for motor circuit ampacity. Although the two factors are identical, they have nothing to do with each other.

The bonus rule for conductor sizing reflects other motor protection rules that capitalize on one of the virtues of electric motors versus internal combustion motors: Electric motors can run for substantial periods of time in excess of their rated horsepower. Internal combustion engines only reach their rated horsepower at close to their maximum speed. Therefore, motor running overload protective devices may not trip until an overload approaches 125% of RLC, and the motor conductors must be sized accordingly.

Continuing to take the questions in reverse order, since Sec. 440-22(a) is a not-to-exceed rule, you're free to use any overcurrent device you choose, as long as it starts the motor and doesn't exceed 175% of the RLC. Assuming the submitter's stipulations are correct, there isn't a problem.

Finally, as to conductor size, I agree with the technical merits of using the 58% allowance for this application. The problem is the literal text of Sec. 440-32 hasn't caught up with Sec. 430-22(a). In this case Sec. 440-3(a) works against you, since it calls for the rules in Art. 440 to supersede those in Art. 430 in a case like this one. I suggest making a case to the local inspector of equivalent safety under Sec. 90-4. This question has the distinction of being one of those I can count on one hand where I reached such a conclusion over the last ten years. These two sections need technical correlation around wye-delta starting for the 2002 NEC.

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