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Single Motor Tap Calculation per the NEC

jar546

jar546

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I know of three people on this forum that can easily do this. Let's see if I am right. Here is the question for those who want to give it a try. Everything you need to calculate the answer is in the question and in the NEC.

A 25-hp, 230-V, 3-phase squirrel-cage motor is to be supplied from a branch circuit that has 2/0 AWG THW copper conductors, which have an ampacity of 175 A. See Table 310.16. Determine the correct size for the tap conductors less than 25 ft.
 
2020 NEC: Per 430.28 exception: feeder taps under that condition will be at 125% of FLC found in table 430.250 (68 amps x 125% = 85 amps.) Conductor size will be #4 copper THW at 75 degrees Celsius.
 
Minor quibble: the OP describes a branch circuit tap, not a feeder tap, so the applicable NEC section is 430.53(D). This assumes the branch circuit as a whole meets the requirements of 430.53(B) or (C).

Cheers, Wayne
 
wwhitney said:
Minor quibble: the OP describes a branch circuit tap, not a feeder tap, so the applicable NEC section is 430.53(D). This assumes the branch circuit as a whole meets the requirements of 430.53(B) or (C).

Cheers, Wayne
Click to expand...
One-third of 175 A is 58 A. The tap conductor normally would be sized at 6 AWG THW copper (65 A). But a 6 AWG tap conductor does not meet the requirements of 430.22, that is, 125 percent of the full-load current of the motor (68 A from Table 430.250), or 85 A. Therefore, the branch-circuit tap conductors are not permitted to be smaller than 4 AWG THW copper, with an ampacity of 85 A. See Table 310.16.
 
jar546 said:
One-third of 175 A is 58 A. The tap conductor normally would be sized at 6 AWG THW copper (65 A). But a 6 AWG tap conductor does not meet the requirements of 430.22, that is, 125 percent of the full-load current of the motor (68 A from Table 430.250), or 85 A. Therefore, the branch-circuit tap conductors are not permitted to be smaller than 4 AWG THW copper, with an ampacity of 85 A. See Table 310.16.
Click to expand...
I agree with your computation, which also matches post #2. But it has no particular bearing on my post, so I'm not sure why you quoted me.

Cheers, Wayne
 
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