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# Power Panel Question(s)

#1
02-28-07, 10:08 AM
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Join Date: May 2001
Location: USA
Posts: 715
Power Panel Question(s)

Starting with basic terms and calculations:

How do you figure the load handling capabilities of a power panel?

Say you have 120 Amp service panel; Does this mean the breakers below the main breaker should add up to 120 Amps? Or is there 120 Amps available on each leg?

What is the best way to determine the right size breaker for each circuit in the panel?

Now that I have asked the questions, let me explain (confuse) what I am trying to do. The neighbors had their panel upgraded to a 120A service panel. Now she wants to be able to figure out the load of each circuit? I would think it would be easy enough...If there is a 20A breaker on that circuit, that is your load! I understand that on outlet circuits it is going to depend on what you have plugged into the outlet; a microwave is going to provide more of a load (R) than a nightlight. The higher the "R" the higher the "I" (current).

I have some experience in house wiring and a strong background in electronics, so figured with a little additional information, I should be able to determine the overall and circuit capabilities. I know there is a name for this, but I can't think of it right now....."load budget, loss budget?"

If you have an idea of what I am talking about and know the correct term(s), help me out here.

Thanks,

#2
02-28-07, 10:21 AM
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,620
I doubt that your neighbors service was upgraded to 120 amps. This is not a standard size. Perhaps it was 100, 150 or 200 amps? These are the typical sizes.

There are load calculations that can be done to properly size a service. Allowances are made for loads that will not be used at the same times, like air conditioning and heating. The sum of the breaker handle means nothing in regards to the service size. A typical 200 amp service could have breakers totaling over 400 amps.

The ampacity of the service means that so many amps are available per leg.

Breaker sizes are determined by the wire size run to the loads. The size of the load will determine the required wire sizes.

#3
02-28-07, 10:42 AM
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Join Date: May 2001
Location: USA
Posts: 715
Originally Posted by pcboss
There are load calculations that can be done to properly size a service. Allowances are made for loads that will not be used at the same times, like air conditioning and heating. The sum of the breaker handle means nothing in regards to the service size. A typical 200 amp service could have breakers totaling over 400 amps.

The ampacity of the service means that so many amps are available per leg.

Breaker sizes are determined by the wire size run to the loads. The size of the load will determine the required wire sizes.
Okay, that cleared some of it up.
So if they have a 150 Amp service....that means there is 150A available on each leg?

Breaker sizes: Is there a guideline that is followed for different circuits? ie. lights normally 15A, outlets normally 20A with X number of outlets on circuit...

#4
02-28-07, 03:16 PM
Member
Join Date: Sep 2004
Location: Saint Louis
Posts: 259
In resi there is no limit on how many receptacles per circuit, however the number should be based on an load calculation. In today's bedrooms, I can expect a computer (maybe more) per room. As I'm wiring my bedrooms, each gets a 20 amp recept circuit. It is a bit of overkill, but I know it will suffice for my lifetime. Now you could also put the lighting on that circuit as well, but I'd like the lights to stay on if there was a serious problem. So I have a few bedrooms sharing the same 15 amp lighting circuit, along with the smokes. 14 gauge is much easier to work with in lighting boxes, where you have dumb things like three ways and dimmers that take up too much room.

Some of this is subjective, but there are some good books. I like "Wiring a House, for Pros by Pros" and the standard "Wiring Simplified", which is a must have.

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