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Old 10-05-2012, 10:09 AM   #1
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Question for electricians about service sizing and calculated load


Hey guys, I have a quick question. This is for curiosity only.

Last year I replaced my entire electrical service, I used Table 310.15(B)(7) to size the conductors that I used for the new service. I used a 200A main panel and 2/0 copper conductors inside of 2" PVC.

My question is whether I could have used smaller conductors if the calculated load was smaller? From the code that I am reading, that seems to be true.

So if my house's calculated load was 65A, I could have used 6AWG conductors while still using a 200A main breaker?

That just doesn't seem right. What am I missing?

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Old 10-05-2012, 10:32 AM   #2
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Question for electricians about service sizing and calculated load


Your missing the fact that conductors have to be protected as per they're rating. There are base values used for square footage. Also #6 is only good for a 60a service. I highly doubt your calculated load is anywhere near 60a unless you live in an 1200 square foot home with gas appliances and no ac

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Old 10-05-2012, 11:42 AM   #3
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Originally Posted by andrew79 View Post
Your missing the fact that conductors have to be protected as per they're rating. There are base values used for square footage. Also #6 is only good for a 60a service.
What about 230.42? I've seen discussion on other electrical forums about only having to size the service conductors to the calculated load. Those forums are for electricians only so that is why I wasn't able to ask this question there.

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I highly doubt your calculated load is anywhere near 60a unless you live in an 1200 square foot home with gas appliances and no ac
This is a fictitious question for curiosity only, so let's pretend that the calculated load is 65A as stated.
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Old 10-05-2012, 11:52 AM   #4
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Question for electricians about service sizing and calculated load


What you actually do is to calculate the service size. Then you protect the conductors with the proper size over current protection device (breaker).

So if your calculated need is 60 amps, you install 60 amp conductors and a 60 amp breaker. Other rules apply, I am just generalizing to give an example.

Of course most locations require a minimum 100 amp service for a residence so this doesn't matter.

But if you go in the other direction and choose the breaker first, then you use the proper size conductors for that breaker.
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Old 10-05-2012, 12:25 PM   #5
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Question for electricians about service sizing and calculated load


The min. service for a resident is 100 amps.
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Old 10-05-2012, 12:29 PM   #6
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Question for electricians about service sizing and calculated load


I see what I did.

Last edited by Trane; 10-05-2012 at 12:53 PM.
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Old 10-05-2012, 12:58 PM   #7
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Question for electricians about service sizing and calculated load


You cannot fuse conductors at a higher amperage than what they are rated for. You can fuse them at a lower amperage.

Keep reading 230.42...look at part (B).

Now read 230.79 (C)

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