I need a better understanding of NEC Code Section 240.4B. When installing the feed conductors from the utility transformer to the 400A 3-phase building main service, I often see 500 kcmil copper being used. The rating of the 500kcmil 75 deg. chart is 380A. Since there is no overcurrent protection and the ampacity of the service is based on the calculated load, this is acceptable. If we now go to a 200A 3-phase breaker within the service to feed a 200A sub-panel, I see estimators using 4/0 aluminum wire based on 240.4B. The wire at 75 degrees is good for 180A, the next size breaker is 200A. I do not agree with this. Since the known load of the electrical panel is not known, the conductor sizing needs to be rated for the breaker protection and the temperature rating of the lugs. 210.3 requires 15A thru 50A to have conductors and brekaer amperages to match, above that is it allowable to use the next smaller size wire? I would like some input.
UNDERSTANDING 240.4B
- Thread starter mhuskey
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: UNDERSTANDING 240.4B
It's a fair question. Here is my view.
I think that neither situation is acceptable, as you have described the two situations. The conductor must be sized to handle the load. You cannot use a 500 MCM copper, if you have a calculated load of 390 amps. You cannot use a 4/0 aluminum, if the calculated load is 190 amps. If you do not do any calculations, then you are being remiss in your duties.
It's a fair question. Here is my view.
I think that neither situation is acceptable, as you have described the two situations. The conductor must be sized to handle the load. You cannot use a 500 MCM copper, if you have a calculated load of 390 amps. You cannot use a 4/0 aluminum, if the calculated load is 190 amps. If you do not do any calculations, then you are being remiss in your duties.
Re: UNDERSTANDING 240.4B
The calculated loads of the service are not 390A and are not 200A. The calculated load of a building my be 240A, requiring a 400A service. The 200A feed to an electrical panel is based on the 200A brekaer, no calculated load for the panel is known. This is my problem, if the calculated laod is not known, how can you use the next smaller size wire and rely on the next larger breaker size?
The calculated loads of the service are not 390A and are not 200A. The calculated load of a building my be 240A, requiring a 400A service. The 200A feed to an electrical panel is based on the 200A brekaer, no calculated load for the panel is known. This is my problem, if the calculated laod is not known, how can you use the next smaller size wire and rely on the next larger breaker size?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: UNDERSTANDING 240.4B
I agree with you. You can't use the smaller wire.
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