Service feeder sizing

M

Masterhack

Member
Location
New Jersey
Occupation
Electrician
Boss man had me installing a 3 bay meter stack unit with 300 amp feeders which feeds 3 100 amp breakers. 2 of them are for a 2nd and 3rd floor apartment one of them being for a convenience store on the first floor.just curious for my own knowledge but I’m assuming the service feeders feeding the stack unit would need to be rated for the full 300 amp load as the building is technically not a multi family dwelling and feeder size doesn’t fall under the 83% of load rule because of the one unit being a storefront.
 
You're correct you cannot use the 83 % rule but the service conductors to the meter stack are not required to be sized at 300 amps if the load calculation comes up with a lesser ampacity.

Welcome to the Forum. :)
 
If the service has a capacity of 300 amps why wouldnt the feeders need to be sized at 300 amps just because I can prove the load is less now couldn’t somebody add more stuff in the future assuming a 300 amp service can handle it.


Thanks for the welcome
 
Masterhack said:
If the service has a capacity of 300 amps why wouldnt the feeders need to be sized at 300 amps just because I can prove the load is less now couldn’t somebody add more stuff in the future assuming a 300 amp service can handle it.


Thanks for the welcome
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Yes someone could add a load but the NEC does not care about that. If your total calculated load is 200 amps then the conductors are not required to be larger than 200 amps.
 
infinity said:
Yes someone could add a load but the NEC does not care about that. If your total calculated load is 200 amps then the conductors are not required to be larger than 200 amps.
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200A of conductor on a 300A OCPD main?

Or are you just saying that your load calc tells you how to size the service (conductors +OCPD) or alluding to exception 3 in 230.90?
 
infinity said:
Where is there a 300 amp
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infinity said:
Yes, when there is no main then you can apply Exception #3 in 230.90(A) where the sum of the OCPD devices within the multiple service disconnects can exceed the size of the service conductors.
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If there was a 300 amp main in line would the service feeders now need to be rated for 300amp? I would size them for 300 amp regardless but am just curious.
 
Masterhack said:
If the service has a capacity of 300 amps why wouldnt the feeders need to be sized at 300 amps just because I can prove the load is less now couldn’t somebody add more stuff in the future assuming a 300 amp service can handle it.
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You're certainly allowed to if you want.
 
Masterhack said:
Makes sense.whoever comes along next should be knowledgeable enough to know that however that isn’t always the case unfortunately.thanks for the info
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I am a firm believer that codes should not be based around unqualified and unknowledgable people coming in down the road later. Where does it stop if you try to write codes to keep things safe in those instances? It's impossible to try to brother-in-law proof things with codes


Masterhack said:
If there was a 300 amp main in line would the service feeders now need to be rated for 300amp? I would size them for 300 amp regardless but am just curious.
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I don't mean to be a complainer, but "service feeder" is a confusing term as a service conductor and a feeder are mutually exclusive and different, so if you use them together there is a danger the fabric of space will rip apart and destroy the universe. 😉 Perhaps read over the article 100 definitions of service conductor and feeder 😇 📖
 
electrofelon said:
I am a firm believer that codes should not be based around unqualified and unknowledgable people coming in down the road later. Where does it stop if you try to write codes to keep things safe in those instances? It's impossible to try to brother-in-law proof things with codes




I don't mean to be a complainer, but "service feeder" is a confusing term as a service conductor and a feeder are mutually exclusive and different, so if you use them together there is a danger the fabric of space will rip apart and destroy the universe. 😉 Perhaps read over the article 100 definitions of service conductor and feeder 😇 📖
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Thanks for the tip your right I was talking about the service conductors. I’m new on the forum trying to get more involved on the technical side of things so all info is helpful.
 
Usually when you do a load calc with 3 separate areas and an overall service, the service calc will be smaller than the sum of the separate areas combined. This is because the more things you can add to your calc, the greater the odds of getting to use some diversity factor. Say 3 electric ranges instead of 1, or having some number of fixed appliances such as 4 or more allowing a .75 factor to be applied, or crossing some KW threshold where a diversity factor gets applied.

So I would expect a service that feeds 3 areas of 100A each to be about 250A and maybe even 200A. No problem with going larger on the service though. The 300A service would most likely allow the feeders to be larger if needed, like 125A each. But it all depends on the calc you do for the overall service and each area as to what the minimums are.
 
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