How many 125A ckts in a 500A panel?

[bill@usps06492]

I want to know if my load calculation is correct,I have 4-125A circuits that I want to install in a 500A panel, but I think that the max I can do is only 3-125A ckts(everything is 3phase 208/120V). 125Ax208Vx1.73=44980VA 44980VAx4=179,920VA 500Ax208Vx1.73=179,980VA 179,980x80%=143,936VA 143,936VA/208Vx1.73=400A 44,980VAx3=134,940VA. The load being servered is in an industrial environment,so do I have to be concerned with loading the panel over 80%,can I rate it at 100%? All the equipment will run at the same time, and the maximum run time is about two hours of steady running, and the actual Amp draw when running(minus start in rush) is less than 80% of the 125A ocpd.

 

[charlie b]

Re: How many 125A ckts in a 500A panel?

You are making the math way too hard. You don?t need to use the 208 or the 1.732, in order to discover that 80% of 500 is 400.

But more importantly, you do not use the breaker size in any type of calculation. The only thing that matters is the current drawn by the load. You say that it is less than 80% of the 125 amp OCPD. If that is true, you can use all four breakers without doing any math at all.

 

[bphgravity]

Re: How many 125A ckts in a 500A panel?

Panelboards are required to have a rating not less than the feeder capacity required for the load as calculated by Article 220.

Assuming the load is indeed 125A and not continuous, you have a load of 500A. This load is permitted to be served by a 500A panelboard.

 

[bill@usps06492]

Re: How many 125A ckts in a 500A panel?

Charlie, how do I calculate the max load for this 500A panel? These machines draw between 80A to 90A when fully loaded, the manufacture requires 125A ocpd. I did all of the math because I wanted you to understand what I was thinking. also the feed from this panel is from a 150kVA transformer, it has a 225A breaker on the 480V primary, and a 500A main on the 208/120 secondary.

 

[charlie b]

Re: How many 125A ckts in a 500A panel?

The ?max load? is 500 amps, as Bryan had said. But I don?t think that was your intended question. It sounds like you want to know how to get a ?calculated load,? using the processes in NEC article 220 (and the articles to which it sends you). For that, you need the horsepower of each motor and the amps (or VA) of any non-motor loads. You do not get to use any current value that you might be able to measure, nor do you use the current rating that might appear on the nameplate. You look up the motor amps in Table 430.150, multiply by 208 and by 1.732 to get VA, and add 25% of the largest motor. Then you add in the non-motor loads, including an extra 25% for any that are continuous loads. Finally, you divide by 208 and again by 1.732. That result is the value you compare against the 500 amp rating of the panel.