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Motor Calculations Part 2: Feeders By Mike Holt, NEC Consultant May 1, 2004 12:00 PM What’s the correct way to size motor feeders and related overcurrent protection?

Part 1 of this two-part series explained how to size overload protection devices and short-circuit and ground-fault protection for motor branch circuits. Understanding the key point of that article, which was that motor overloadprotection requires separate calculations from short-circuit and ground-fault protection, clears up a common source of confusion and a point of error. But another source of confusion arises when it comes to sizing short-circuit and ground-fault protection for a feeder that supplies more than one motor. Let's look again at branch-circuit calculations and then resolve the feeder issues so yourcalculations will always be correct. Branch-circuit conductors and protection devices. Per 430.6(A), branch-circuit conductors to a single motor must have an ampacity of not less than 125% of the motor full load current (FLC) as listed in Tables 430.147 through 430.150. To illustrate this, let's size the branch-circuit conductors (THHN) and short-circuit ground-fault protection device for a 3-hp, 115V,single-phase motor. The motor FLA is 31A, and dual-element fuses for short-circuit and ground-fault protection are in use (Fig. 1).
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Per Table 430.148, the FLC current is 34A. 34A×125%=43A. Per Table 310.16 (60°C terminals [110.14(C)(1)(a)]), the conductor must be a 6 AWG THHN rated 55A.

Per the motor FLC listed in Table 430.52, size the branch-circuit short-circuit and ground-faultprotection devices by using multiplication factors based on the type of motor and protection device. When the protection device values determined from Table 430.52 don't correspond with the standard rating of overcurrent protection devices listed in 240.6(A), you must use the next higher overcurrent protection device. To illustrate this, let's use the same motor as in the previous example.
• •Per 240.6(A), multiply 34A×175% You need a 60A dual-element fuse.

To explore this example further, see Example No. D8 in Annex D of the 2002 NEC. Once you've sized the motor overloads, branch-circuit conductors, and branch-circuit protective devices, you're ready to move on to the next step. Motor feeder conductor calculations. From 430.24, you can see that conductors that supply several motorsmust have an ampacity not less than:
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125% of the highest-rated motor FLC [430.17], plus The sum of the FLCs of the other motors (on the same phase), as determined by 430.6(A), plus The ampacity required to supply the other loads on that feeder.

Fig. 1. Don’t make the mistake of using a motor’s FLA nameplate rating when using the short-circuit and ground-fault protection devices. Youmust use the FLC rating given in Table 430.148.

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Use Fig. 2 and solve the following problem. Example No. 1. For what ampacity must you size the feeder conductor if it supplies the following two motors? The terminals are rated for 75°C.
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One 7.5-hp, 230V (40A), single-phase motor One 5-hp, 230V (28A), single-phase motor

(a) 50A (b) 60A (c) 70A (d) 80A Let's walk through the solution.• • •

The largest motor is 40A. 40A×1.25+28A=78A. 80A is the closest selection that's at least 78A.

Fig. 2. Motor feeder conductors shall be sized not less than 125% of the largest motor FLC plus the sum of the FLCs of the other motors on the same phase.

What size conductor would give us this ampacity? (a) 2 AWG (b) 4 AWG (c) 6 AWG (d) 8 AWG Per Table 310.16, a 6 AWG conductor rated at75°C provides 65A of ampacity, so it's too small. However, a 4 AWG conductor provides 85A of ampacity, which will accommodate the necessary 78A. Therefore, you need to size this feeder conductor at 4 AWG. Next, we have to determine what size overcurrent protection device (OCPD) we must provide for a given feeder. Example No. 2. Using a slightly more complex example, try sizing the feeder...