National Electrical Installation Standards

Standards as High as Your Own

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Wednesday, January 9, 2019

Question:

The transformer in the switchgear  has an output at 480 volts of 1203 amperes.

The main breaker in the gear downstream of the transformer is a 1600 Amp. With the 80% rating inherent in breakers, this 1600 amp breaker will carry 1280 amperes for continuous loads. Code defines continuous as 3 hours or more, which would apply to the machine operation we are installing. Thus the 1600 amp breaker will have a max rating of 1280 amperes. We are installing a 1000 kva transformer that max load is 1204.

By code do I have to run feeders large enough for the 1600 amp breaker or can I run feeders for the 1204 amp draw. Piping directly into my transformer. With no disconnecting means at transformer. Which is 110' away in another room. Thanks for all the great stuff I learned from you. Brett

A

Answer:

Hey Brett thanks for your question and you are welcome. Feeder conductors must be suitable for the load as stated in 215.2 and if the load is entirely continuous, they must be sized at 125% of the load. The 1600 ampere overcurrent device in your example still has a rating of 1600 it must also be sized at 125% for continuous loads as stated in 215.3.

You don't say what the continuous load is but that is where we need to start. Let's use 1200 amperes as an example. The conductors would need to have an ampacity of at least 1500 amperes (1200 X 1.25%). The overcurrent device must also have a minimum rating of 1500 (same math) based on 215.2 and 215.3.

The 110 ft secondary conductors are another problem because overcurrent devices need to be provided where conductors receive their supply as stated in 240.21 unless we can find another rule that allows something different. The rules in 240.21(C) that are applicable to your situation generally limit the length to be 25 feet along with other conditions. Also, don't forget about transformer protection as stated in 450.3.

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