Sum of mains vs. rating

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Zorch

Member
Is there a code section that says the sum of the mains cannot exceed the rating of the service equipment? I could have sworn there was, but I seem to be missing it.

I've got an apartment-style service enclosure. It has three meters each with its own 200-amp disconnect. The termination cabinet and the bus are rated at 600 amps. I need to add a fourth 200-amp main/meter.

I know I can replace the whole thing with an 800-amp set-up. I could also gather load calcs and possibly replace all of the main breakers with 150-amp units - the building loads are quite small. Obviously the easiest and least expensive is to just add a fourth meter/main combo to the 600-amp equipment, but I'm confident that this is a code violation. I'm just missing it in the book.

Can someone point me in the right direction?

- Zorch
 

iwire

Moderator
Staff member
Location
Massachusetts
Take a look at exception 2 or 3 of 230.90 it may let you add the forth meter if the calculations stay under 600 amps.
 

Zorch

Member
Bob,

Yeah, that was going to be one of the least desirable options as the job is four hours away. I have prints on two of the three units - no problem with those loads, but no one can find prints on the third. It would mean a long day of driving.

I get reports on what's in the third unit, but I don't engineer off of other people's observations - that makes me nervous. Besides, not being able to find that code passage makes me crazy.

- Zorch
 

Jim W in Tampa

Senior Member
Location
Tampa Florida
200 amps is rather high for an apartment. Just what are you adding as far as a new load ? Should be someone able to give you basic size of the unknown unit and what is in it. Takes a lot to need 200 amps. Make the trip and if 600 don't cut it your looking at a lot more work.
Gotta just figure in them driving hours. I have been sent on 2 hour trips to do a 15 minute ufer bond. Easy day the way i see it.
 

Zorch

Member
Actually it's a multi-tenant commercial strip mall. And you're right ... 200 amps is indeed a lot of juice, especially when it's three-phase. But replacing the breakers would cost more than upgrading the whole shebang to an 800-amp unit.

This customer has been very, very good to me. I want to take care of him. If I can save him several thousand dollars then I want to do that. That brings us right back to putting four 200-amp mains on a 600-amp rated cabinet.

That's what I'm trying to establish the legality of.

- Jake
 

Jim W in Tampa

Senior Member
Location
Tampa Florida
Then plan on it working out ok and when you get there do the load calculations for the 4 of them. Chances are you will be ok. If not even the drop may not be large enough. Could you tap this extra 200 in on the line side of the 600 ? Here again a picture would really help.
And yes 3 new breakers on 3 phase is not cheap. Maybe make a Sunday trip with the wife or GF or both and look at the job and have lunch. Charge him for gas and meal.
 

Cavie

Senior Member
Location
SW Florida
Is there a code section that says the sum of the mains cannot exceed the rating of the service equipment? I could have sworn there was, but I seem to be missing it.

- Zorch

No. The rule is the sum of the load cannot exceed the rating of the service equipment.
 

Zorch

Member
Cavie,

That's how I'm seeing it too. No matter how many times I go back through the Code I just can find no other requirement. Perhaps I'm remembering a line from many, many editions past.
 

Zorch

Member
John,

You do indeed understand perfectly. And this is a good possibility.

Part (1)
I'm trying to get the demand numbers from the utility company since we don't have that 30-day period. Unfortunately, they may not accept that figure as I've just recently built out two of the spaces for new customers. There isn't really accurate demand for a 1-year period.

Part (2)
I suspect we'll meet this section.

Part (3)
The service feeders are actually bus. The multi-tenant meter base has meters and accompanying circuit breakers attached to bus so we should be good here too.

It's just "Part (1)" that may shoot this idea down. But this would seem to indicate that it is quite acceptable (as we think) for the sum of the mains to exceed the rating of the service equipment - again, it appears to all be based on connected load.

Thanks for pointing this one out.
 
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