Sizing Branch fusing for multiple loads

M

Thread Starter

Mike Hubbell

Does anyone know how to calculate the size of branch fusing and branch wire size for a branch having multiple loads (i.e. a branch having one PLC, computer, printer, 2 DC drives, touchscreen, and miscellaneous instrumentation) Most of these loads have maximum current or FLA ratings which these devices will probably never see and there are no typical current ratings for most of the devices. If all of the FLA currents are used to size the fuse then the fuse size will be very large and seemingly unnecessary. Loads that are not ON at the same time has already been accounted for. What is the industry accepted or standard practice for sizing these type of branch fuses??
 
Mike,

Ask a dozen "experts" and you will get a dozen different answers. I put a clamp-on on a cabinet today that had a PLC, Motion Controller,
PanelMate, 7ea AC Drives (all running motors with normal operating loads), some miscellaneous contactors, relays, etc. etc. By the book
(NEC) Incoming Circuit Breaker / Disconnect is sized at 100 Amps @ 480VAC . . . . the clamp on gave me a reading of a whopping 11 Amps with
all systems running in production . . . . hmmmm

Ken
 
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Mike,

If you are in the U.S.A. you need to use the National Electrical Code as a standard. However, this can be difficult to use, and feeders to machines do seem to be excessively large sometimes. The N.E.C. sizes everything to the maximum possible current load. Typically, many machines never pull that much current, but some do. Either way, I just follow the N.E.C. and I suggest the same to anyone else in the U.S.A.

Gerry
 
Mike,

If you are in the U.S.A. you need to use the National Electrical Code as a standard. However, this can be difficult to use, and feeders to machines do seem to be excessively large sometimes. The N.E.C. sizes everything to the maximum possible current load. Typically, many machines never pull that much current, but some do. Either way, I just follow the N.E.C. and I suggest the same to anyone else in the U.S.A.

Gerry
 
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