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Discussion Starter #1
I have a 100 amp service panel and i am trying to find out if i can install a 100 amp breaker in the exsisting 100 amp panel to suppy another mlo panel(sub). I can't find any thing in the nec. I guess i really need to do a load calculation first of all i want to put on it to see if the initial 100 amp service will be able to handle the circuits being added. Enlighten me on this.
 

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Licensed Electrician
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A load calc would be appropriate, but there is nothing in the NEC prohibiting what you would like to do.
 
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If the load calcs work out, check if the panel you have will accept a feed through lug kit.

Otherwise, check the stab rating on the panel. I doubt it's rated for more than an 80A breaker.

Another option is to use taps. Post some detailed pictures of the existing panel's internals. And one picture of the whole wall. Let's take a look.

BTW, do you know the load calc formula?
 

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This is not the official load calc formula. But it's a quick and dirty calculator to get you in the ballpark. For amperage, take the answer in watts and divide by 240.

http://www.generatorsales.com/wattage-calculator.asp
So you are saying to take all the items on the panel, add up the wattage and divide by 240V?
Even for a "down and dirty number", that's not how a load calculation works. :no:
 
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So you are saying to take all the items on the panel, add up the wattage and divide by 240V?
Even for a "down and dirty number", that's not how a load calculation works. :no:
You're right. Thanks for catching that, Speedy Petey. That was a lousy answer, I'll try to make amends.

Either of these will give an approximation for a house main panel.

http://www.constructionmonkey.com/calculations/electrical/ResidentialOptional.aspx

http://www.nojolt.com/load_calculations.shtml

I trust this one the most. But it took me some time, and trial and error, to get every field to accept the correct input.

http://www.docstoc.com/docs/97696973/Residential-Load-Calculations---Mike-Holt-Enterprises

As for the generator calc from my previous post, it would be ok for a quick and dirty ballpark on a sub panel or feeder panel, only. Not a main or service panel.
 

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Discussion Starter #8
Sorry,we can only take off 80% of the main rating.

So bob you are saying that we are only allowed to take 80% of the service to another panel for more space and /or more circuits. so i could only take off 80amps to another panel? I think my best thing would be to upgrade my service because i am looking at installing an electrical heating service as back up. I have been told my service would have to be upgraded to do so but i have no idea what a system such as a heat pump would pull. I do know at best with what i have now i never pull 60 amps on the current service. I put this to the test to see with a electrician friend and with more than what we would ever pull at a given time together it was under the 60 amps.
I have learned in school on how to do a load calculation and have found a site with a calculator that seems to work well after inputting the info. Thanks to you all on your replies. taking the experience of pros in the field help me to understand more of what i am looking at.
 
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