National Electrical Installation Standards

Standards as High as Your Own

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Monday, July 27, 2020

Question:

Hello and thanks for this forum,

My question is about sizing overcurrent protection for a 40HP 3 phase motor. I will be using a breaker. I do the following calculations: 40 hp motor- Table 430.250 full load current 52-amps. According to table 430.52 If I use an Inverse time breaker, I multiply that 52-amps by 250% which is 130-amps and according to the exceptions to non-standard sizes, I can go to the next up standard size. That would be a 150-amp breaker. I have two motor slide charts, one from Square D and one from Schneider and they both give different answers. The Square D slide table says a 40 HP three phase motor is to be protected by a 90-amp breaker and the Schneider slide chart states that a 40 hp is to be protected by a 100-amp breaker. What is the correct answer? How are my calculations wrong?

Thank you,

Jeff Sparks
A

Answer:

Hi Jeff,

Thanks for participating and for your question. You didn't mention the voltage in this question but based on the ampere value provided from Table 430.250, it’s presumed at 460-480 volts. Your methodology is correct, but remember you are using the NEC to calculate what the maximum rating of the branch circuit, short circuit and ground fault protective device would be required not to exceed. The values provided in the slide charts are more conservative and specify breakers that will work to carry locked rotor currents during starting conditions and provide the necessary branch circuit short circuit, ground fault protection as well. Remember to address the overload protection as required in Part III of Article 430.

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