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Service Calculation #169961
10/21/07 09:03 PM
10/21/07 09:03 PM
Sparky_1  Offline OP
Joined: Nov 2004
Posts: 14

On a residential service calculation, can the main breaker be calculated at 100% or does it have to be 80%?, code ref. please.

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Re: Service Calculation [Re: Sparky_1] #169964
10/21/07 10:31 PM
10/21/07 10:31 PM
Sparky_1  Offline OP
Joined: Nov 2004
Posts: 14

The reason I ask about the main, is because I'm installing a hot tub that has dual ratings, 30 amp and 50 amp circuit. The service size is 150 Amps, with a electric range, dryer and AC for major appliances. If I do a service calculation with the added hot tub load, I get 120 amps with the 30 amp hot tub load and 132 amp with the 50 amp hot tub load, so by using the service calculations, can I put the hot tub on the 50 amp circuit or does it have to go on the 30 amp circuit?

My inspector is telling me that the calculated load can't be more than 80 % of the main breaker, 120 Amps.

I see in the code book, on page 718 example D2c that the calculated load is 144.96 with a service
size of 150 amp.

I believe that I can add to this service as long as my service calculation are 150 amp or less.

Thanks George

Last edited by Sparky_1; 10/21/07 10:42 PM.
Re: Service Calculation [Re: Sparky_1] #169965
10/21/07 10:31 PM
10/21/07 10:31 PM
George Little  Offline
Joined: Jan 2004
Posts: 1,489
Michigan USA
And the answer is 100% The code references are: 230.42, 215.2, 210.19
The issue is "continuous load". Since residential loads are not considered to be continuous you can load a branch circit, feeder or Service to it's actual rating which is 100%. I don't think it's a good idea but the code allows it. Most inspectors would recommend a little head room and most contractors agree and provide this head room.

George Little
Re: Service Calculation [Re: George Little] #169966
10/22/07 12:44 AM
10/22/07 12:44 AM
gfretwell  Offline

Joined: Jul 2004
Posts: 9,186
Dwelling loads better be "diverse" or we will have to repeal 310.15(B)(6) wink

Greg Fretwell


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