Load calculation

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ccbieg51

Member
If you have a load calculation of 216 in your panel can
you go down to 200 or do you have to go up? All machines
will not be on at once.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
All machines will not be on at once.
Did you factor that into your calculated load? If not, then I suspect you could redo the calculation and come up with a value under 200.

But to answer your specific question, the rating of the service must be at least as high as the results of the load calculation. You can get that from several paragraphs within article 230, including several that address the conductor sizes and one that addresses the disconnecting means. Article 408 has a paragraph that addresses panelboards. All of these say essentially the same thing: each component must be rated to handle the calculated load.

Welcome to the forum.

 

StephenSDH

Senior Member
Location
Allentown, PA
It depends on the equipment and when it runs. You need to do the math and figure out the load for the maximum scenario expected, and give yourself a buffer.

Be upfront with the company. Alot of industrial plants won't even bat an eye at over sizing panels for future expansion.
 

ccbieg51

Member
Inspector wants info for panel after the fact panels are in place. I have done several of these through out the states and never had an inspector
want load calculcation. Service in place and plant running...they will not have all machines on at once...immpossible..system 250v 3phase delta system. Duty cycles are brief like 30 seconds to 2 minutes..then they move out of that area...never see full load..60% at max..where in nec does it refrence derating of f.l.a. Based on duty cycles...i just need nec article that supports that the 200a panels are properly sized for intended loads.
Can anybody help me?
 

topgone

Senior Member
NEC Reference

NEC Reference

Please find below a provision from NEC 2008:
"430.26 Feeder Demand Factor. Where reduced heating of
the conductors results from motors operating on duty-cycle,
intermittently, or from all motors not operating at one time,
the authority having jurisdiction may grant permission for
feeder conductors to have an ampacity less than specified in
430.24, provided the conductors have sufficient ampacity
for the maximum load determined in accordance with the
sizes and number of motors supplied and the character of
their loads and duties.
FPN: Demand factors determined in the design of new facilities
can often be validated against actual historical experience
from similar installations. Refer to ANSI/IEEE
Std. 141, IEEE Recommended Practice for Electric Power
Distribution for Industrial Plants, and ANSI/IEEE Std.
241, Recommended Practice for Electric Power Systems in
Commercial Buildings, for information on the calculation
of loads and demand factor.
Hope this helps.
 
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