Residential Breaker Sizing
- Thread starter luckyuky
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- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Residential Breaker Sizing
What do you mean when you say you have a ?demand? of 140 amps? Is this a new (not yet built) facility, and is that figure the result of a load calculation, per article 220? If so, then you would have had to include a 125% factor as part of that calculation process, wherever it is appropriate. This would have taken care of the 80% breaker rating. So a 150 amp breaker would be acceptable.
But if this is an existing facility, and if it is undergoing a remodel project, and if the 140 amps is a measured value, then you do have to ?biggie size? the load (or equivalently, downsize the breaker rating). So a 175 amp breaker would be needed.
What do you mean when you say you have a ?demand? of 140 amps? Is this a new (not yet built) facility, and is that figure the result of a load calculation, per article 220? If so, then you would have had to include a 125% factor as part of that calculation process, wherever it is appropriate. This would have taken care of the 80% breaker rating. So a 150 amp breaker would be acceptable.
But if this is an existing facility, and if it is undergoing a remodel project, and if the 140 amps is a measured value, then you do have to ?biggie size? the load (or equivalently, downsize the breaker rating). So a 175 amp breaker would be needed.
Re: Residential Breaker Sizing
This is a new construction. The units are very large (5,000 sq ft range) with a 3-phase 208/120V service. When I mentioned demand I was referring to load calculations performed using the optional method of article 220. Where in this article does the 125% factor come from?
This is a new construction. The units are very large (5,000 sq ft range) with a 3-phase 208/120V service. When I mentioned demand I was referring to load calculations performed using the optional method of article 220. Where in this article does the 125% factor come from?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Residential Breaker Sizing
The 125% factor does not appear as a line item within the optional method. But I think it is there anyway, though some will likely disagree. What I mean is that the purpose of including an occasional 125% factor is fulfilled in another way by the process used in the optional calculation. Put it this way: If (just for the sake of this discussion) there were never a need for a 125% factor, then the optional method might have been written to say that you take 100% of the first 12KVA (instead of 10), and 42% of the rest (instead of 40%). The bottom line is that method is what the method is. It does not tell us to include a 125% factor, and it leads us to a calculated load. I think you are OK with a 150 amp breaker.
The 125% factor does not appear as a line item within the optional method. But I think it is there anyway, though some will likely disagree. What I mean is that the purpose of including an occasional 125% factor is fulfilled in another way by the process used in the optional calculation. Put it this way: If (just for the sake of this discussion) there were never a need for a 125% factor, then the optional method might have been written to say that you take 100% of the first 12KVA (instead of 10), and 42% of the rest (instead of 40%). The bottom line is that method is what the method is. It does not tell us to include a 125% factor, and it leads us to a calculated load. I think you are OK with a 150 amp breaker.
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