Main Breaker for Multiple Motors

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Leo1

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Per Art. 220.18 (A) three 17 amps motors can be feed from one circuit breaker (CB). Size of CB is 1.25 times largest motor load + other motors or 1.25 x 17 +17+17 = 55 amps or 60 amp CB in this example. However the VFD equipment manufacturer I am working with says the three panel VFD requires an 80 amp CB according to UL508? Does anyone know why UL508 is requiring a larger CB?

Should I assume UL508 will supersede NFPA 70 on this subject?
 
Leo1 said:
Per Art. 220.18 (A) three 17 amps motors can be feed from one circuit breaker (CB). Size of CB is 1.25 times largest motor load + other motors or 1.25 x 17 +17+17 = 55 amps or 60 amp CB in this example. However the VFD equipment manufacturer I am working with says the three panel VFD requires an 80 amp CB according to UL508? Does anyone know why UL508 is requiring a larger CB?

Should I assume UL508 will supersede NFPA 70 on this subject?
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The breaker size ahead of a VFD is typically something that the drive mfr submitted to UL when the drive was listed, so once the drive is listed with that breaker, it has to be that breaker. Smarter mfrs submit to UL with a MAXIMUM breaker size, so anything smaller is fine. Yours must not have done that, or it might be that someone there does not really understand. Got a make and model?

Also, is it one VFD running all 3 motors, or 3 VFDs?
 
Leo1 said:
Per Art. 220.18 (A) three 17 amps motors can be feed from one circuit breaker (CB). Size of CB is 1.25 times largest motor load + other motors or 1.25 x 17 +17+17 = 55 amps or 60 amp CB in this example. However the VFD equipment manufacturer I am working with says the three panel VFD requires an 80 amp CB according to UL508? Does anyone know why UL508 is requiring a larger CB?

Should I assume UL508 will supersede NFPA 70 on this subject?
Click to expand...

Once you add a VFD you need to look in article 430 for part X. Adjustable-Speed Drive Systems

In particular 430.122 and 430.130
 
iwire said:
Once you add a VFD you need to look in article 430 for part X. Adjustable-Speed Drive Systems

In particular 430.122 and 430.130
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Yes, although 430.122 only relates to the conductor size, not the CB size. I made that mistake for years, then someone in here pointed that out to me.

430.130 was only added in 2014, so if you are still not using 2014 yet (like us here in Calif.), there is no 130, although it might be a good idea to follow it anyway. 430.130 (A) 2. is basically what I was saying; you have to go by what the mfr says.
 
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