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# sub panel

#1
08-02-05, 08:43 PM
New Member
Join Date: Mar 2003
Location: PA
Posts: 405
sub panel

What formula is used to detirmine what size a sub panel that I can run? for instance what size should my main service be to run a 60 or 100 amp sub panel to my garage?
Thanks

#2
08-03-05, 04:10 AM
Member
Join Date: Sep 2003
Location: Central New York State
Posts: 13,970
The issue is not what size the main panel is, or even what size the sub panel is. The issue is what your load is on your main panel and what you intend the load to be on the sub panel.

#3
08-03-05, 04:22 AM
Member
Join Date: Nov 2004
Location: USA
Posts: 973
There isn't a formula to determine max sub panel size based on main panel size. If you have a 100A main, you can have a 100A sub. Of course, that doesn't mean it's practical.

Keep in mind when determining your needs for your garage that if you wanted, say, five 20A 110 circuits, that would NOT require a 100A sub panel. For one thing, the subpanel is 100A at *220*, so, theoretically, you would have to have *10* 20A 110 circuits (five on each "leg") all drawing their max load before you would reach 100A @ 220. The other thing to realize is that very few 20A circuits actually get maxed out frequently, and certainly you won't max them all out at once (unless you have lots of equipment all running at once). That's why adding the rating of circuit breakers together does not reveal a lot about a panel boxes load.

I suppose the best way to use formulas to determine for your specific situation, what the max sub panel size to put in, would be to do a load calculation of your house. Subtract your result from the size of the main. The difference is the draw that your service should be able to provide the sub panel. That number wouldn't necessarily be the cap for a sub panel size, but it will give you a realistic look at what is reasonable.

#4
08-03-05, 06:40 AM
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
Received 1 Vote on 1 Post
The subpanel should be sized to the load to be drawn from the subpanel. If the load will increase in the future, then I suggest you size the subpanel for the greatest future need, even if the existing main panel is currently insufficient to handle that future load. Then when the future arrives, you can upgrade the main panel to handle it without redoing the feeder or subpanel.

Theoretically, it possible to install a 200-amp subpanel off of a 100-amp main panel. Of course you would still limited to 100 amps total until the main panel is upgraded.

As with anything, there are cost tradeoffs too. We often don't get exactly what we want because it would cost too much so we adjust our desires.