continuous load

[Alwayslearningelec]

when sizing feeder you use 125% of continuous load. if you have a panel that your sizing feeder for is it 125% of main breaker or you have to calculate all the branch loads and take 125% of that? thanks

 

[charlie b]

Not all the branch loads will be running at their full load for three hours at a time. You add up the ones that do, and multiply that total by 125%. You add up the others and count them at 100%. Add these two results, and that's your "calculated load." You then select the panel and its main breaker to be rated at least as high as that calculated load. Put another way, neither the panel's rating nor the main breaker's rating has to be higher than, let alone 125% of, the calculated load.

 

[Alwayslearningelec]

Not all the branch loads will be running at their full load for three hours at a time. You add up the ones that do, and multiply that total by 125%. You add up the others and count them at 100%. Add these two results, and that's your "calculated load." You then select the panel and its main breaker to be rated at least as high as that calculated load. Put another way, neither the panel's rating nor the main breaker's rating has to be higher than, let alone 125% of, the calculated load.

thanks Charlie ..so of the main breaker is already sized I can size my feeder based on that?

 

[charlie b]

No. You size the feeder on the basis of the calculated load. You use that same calculated load to discover whether the panel and its main breaker are also properly sized.

 

[Alwayslearningelec]

so once calculated loads in panel are determined what's the calculation to determine the main breaker for that panel.

on a side note when is a panel allowed to feed another panel via feed thru lugs? thank

 

[electrofelon]

Not all the branch loads will be running at their full load for three hours at a time. You add up the ones that do, and multiply that total by 125%. You add up the others and count them at 100%. Add these two results, and that's your "calculated load." You then select the panel and its main breaker to be rated at least as high as that calculated load. Put another way, neither the panel's rating nor the main breaker's rating has to be higher than, let alone 125% of, the calculated load.

I just like to point out specifically how this is structured in the code. The extra 25% for continuous loads applies to conductors (service, branch, and feeders), and OCPD's. Although there is no specific requirement that panelboards get the extra 25% for continuous loads, 408.30 requires the panelboard rating to not be less than the feeder capacity calculated for the load per 220, so since feeders get, the panelboard gets it (unless we get into 100% rated OCPD's).