National Electrical Installation Standards

Standards as High as Your Own

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Thursday, September 6, 2018

Question:

Hello can a 3/0 feeder be connected to a 400 amp OCPD in a series rated system? Here is the situation, there is a separate 3/0 feeder run 75 feet to a 200 amp commercial unit sub panel with a 200 amp main breaker installed within it. The distribution board only has a 400 amp OCPD available to supply the tenant space. The full load on the 3/0 will never (overload) exceed 200 amps with the installed 200 amp sub panel main breaker. If somehow there is a ground fault or short circuit (not overload) somewhere between the unit 200 amp OCPD and the distribution 400 amp OPCD that high fault current should open the 400 or even the 2000 amp building main. It would seem that the 3/0 would have overload, short circuit, and ground fault protection as required by the NEC? The 3/0 would not be considered tap conductors because they originate and end up at an OCPD. Your thoughts? Thanks Vince

A

Answer:

Hey Vince thanks for your question. Conductors must have overcurrent protection - based on their ampacities, unless other rules apply, as stated in 240.4. A 3/0 copper conductor has an ampacity of 200 using the 75 degree column of Table 310.15(B)(16). Adjustment and correction factors could change that value if those conditions exist. Overcurrent protection must be located at the supply point of the conductor, unless allowed by 240.21(A) through (H) as stated in 240.21.

You are correct, in your example the 3/0 conductors have short circuit protection (400 amperes) but that does not correspond to their ampacity. Based on the definition of Tap Conductor in 240.2 the 3/0 conductors in your example are tap conductors and would need to comply with one of the rules in 240.21(B). Unless the conductors are located outside and 240.21(B)(5) could be applied, the maximum tap conductor length is 25 feet based on 240.21(B)(2). Or change the 400 ampere overcurrent device to a 200 ampere one.

Series rating concepts are different and are based on fault currents as stated in 240.86.

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