sub panel

[walt]

Hi I was wondering if you could tell me if there is requirement for sizing sub panels. I have a 100 amp service feeding a 100amp panel. I want to install a 100 amp breaker and feed a sub panel in another part of the house. Is there a calculation for determining the maximum size subpanel I can feed off the main 100 amp panel.

[Ron]

There are two things to consider.
There is a calculation to establish the minimum size of your service/main panel. See the link above titled "Demand Load Calc."
The main panel will have a limitation regarding the maximum size branch breakers that can be properly installed in the panel, see the paper stuck to the backside of the door of the main panel. Very often it will permit only certain model breakers, which inherently have a max size associated with that model branch breaker.

[suemarkp]

Assuming you can implement a 100A feeder from your panel, you can do a demand load calculation for the devices and floor area served only by that subpanel. Whatever value this comes to, say 80 amps, add this value into the service calculation for your main service with no adjustment factors for diversity (i.e. it counts 100%).

In addition to what Ron mentioned, you may have other constraints for putting a 100A feeder into a 100A panel -- bending space rules, gutter fill limitations, available knockouts, etc. Something else to look at is does your main panel have feed-through lugs or can they be added? If so, then feed your subpanel from those lugs and the feeder will be protected by your 100A main. You must have a 100A main breaker in the main panel in order to do this.

A 60A feeder makes more sense on a 100A panel, and just about any panel should be able to support that size. This could be done with 6-3 NM cable, which is substantially cheaper than the cable you'll need for a 100A feeder.