As a home inspector, I was recently helping a fellow inspector on a large house with two main panels side by side with a 150A main in each. State SOP says as inspectors we inform the buyer of the "Amperage and voltage rating of electrical service". He stated while looking at the two mains, "two 150 breakers, total 300A". I told him no, it's 200A based on the SE cable size at the meter which in this case was 200A cable. We went back and forth and agreed to disagree. What I'm looking for is any definitive answer on how to calculate Service Size when you have multiple mains and educate him, unless I'm wrong. NEC doesn't say anything about adding breaker sizes for multiple panels... just 230.2 "A building... shall be supplied by only one service..." Is there something else I'm missing? Thanks and stay safe!
Determining Service size for house with two main panels
- Thread starter HiSpeed68
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
You can have as many panels as you want. You cannot figure out what the service size is from the panel rating.
I would go look at the meter. That is more definitive as far as service size goes.
There might be an issue with having two panelboards with 150 A mains as the protection for a 200 A rated cable, but it is also possible there is a 200 A fuse or CB upstream which would make it OK..
It is also possible someone did it wrong when it was installed.
I would go look at the meter. That is more definitive as far as service size goes.
There might be an issue with having two panelboards with 150 A mains as the protection for a 200 A rated cable, but it is also possible there is a 200 A fuse or CB upstream which would make it OK..
It is also possible someone did it wrong when it was installed.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
That’s a better drawing than we ever got from HH Sting
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
It is also possible that one panel feeds the other via feed thru lugs. That would be OK too.
I
I don't do it this way but he's "more experienced" due to his time as an inspector and was absolutely certain! I was taught to look at the service cable size feeding the main panel... in this case both panels and go by the size of the cable, not add the breakers. I just want to get straight or make sure he's straight!
@petersonra - Thanks. They are each fed from the sealed meter panel on the exterior wall, not one from the other to make a sub. We don't cut seals on meter cans to verify they are connected at lugs but given there was no exterior main, I go by the size of the conductor feeding the panel. My main question is "do you add the two 150A mains to determine total amperage in the home?"
I don't do it this way but he's "more experienced" due to his time as an inspector and was absolutely certain! I was taught to look at the service cable size feeding the main panel... in this case both panels and go by the size of the cable, not add the breakers. I just want to get straight or make sure he's straight!
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Since the 2 150 amp breakers are there you are limited to a total of 300 amps-- 150 on each panel. If those breakers were changed to 200 amps then you could have 400 amps- 200 amps at each panel however the wire sizes must be the appropriate size.
But if you have a 200A meter you can't get 400A out of it... it's only limited to what the meter and cable can handle. That's my understanding... am I missing something after you split past the meter? Was my training all wrong... I don't think so considering physics.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
So there is only one meter and the SEC's to that meter are 200 amps? Then the service size is 200 amps.
Thanks! So, as usual, you don't add the breakers after the meter? Was my reference to the NEC correct as well in my original post or there another reference for determining service size for multiple mains?
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
@HiSpeed68
I endorse going by the service conductor size or the meter or the meter socket rating, whichever of those you can find. Your colleague was wrong (at least in principle; judging the rating of the cable can be complicated with 310.12 involved).
230.90(A) Exception 3 allows two 150A breakers on a 200A service so long as the load calc is 200A or less. I see almost no one doing proper load calcs for residential but I also see many oversized residential services so it's anyone's guess if that arrangement is legit or not.
I endorse going by the service conductor size or the meter or the meter socket rating, whichever of those you can find. Your colleague was wrong (at least in principle; judging the rating of the cable can be complicated with 310.12 involved).
230.90(A) Exception 3 allows two 150A breakers on a 200A service so long as the load calc is 200A or less. I see almost no one doing proper load calcs for residential but I also see many oversized residential services so it's anyone's guess if that arrangement is legit or not.
- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
I can almost guarantee the meter is a 320A/400A. It will have dual lugs for feeds to two panels. I don't know why they used 150A panels. It could be choice or availability. As is, the service is 300A with capability of being 400A by changing the panels, or main breakers, to 200A and the conductors to the appropriate size.
In this case you would. Coming directly from a meter with nothing limiting the current until the two 150 amp breakers would in fact allow 300 amps. The rating of the meter and SE conductor size would not change the possibility of 300 amps being available unless of course one of the two failed.
In as much as you have a single meter and a single set of conductors supplying it you have one service
I think this whole "what is my service size" is sort of a quandry. As an home inspector, I'd indicate what the current safe service size is, and maybe add what it could be. The service size to me is the weakest link in the system:
In a typical 320/400A service, the SECs are paralleled, so you can add them. Yours seems to be a tap. But I'd specify the service size as the size permitted by those SECs if it is less than the handle ratings of all the downstream disconnects being fed. If the breaker sum is smaller, then specify that. Conductors could be increased for voltage drop or just what they had available so that isn't always the limit.
I would ignore the wire from the poco feeding the mast or the meter feed if underground. These are smaller than NEC sizing but to me still play a role. I have a 250' feed from the street, mixed overhead and underground. The overhead is #2 Al and the underground is 1/0 Al. My main breaker was 200A, but I reduced it to 150A. I thought it a bad idea to have something think they can effectively pull 200A through that without burning it up or experiencing excessive voltage drop. It also makes adding solar easier as I have a 200A panel bus with a 150A main now.
- Meter socket rating (although most 200A meter sockets have a continuous rating, so a 200A one may actually be good for 240A non-continuous).
- The size of the service entrance conductors. Need to know if these are parallel or tapped, and if tapped is there a larger conductor feeding the tap block. Sounds like maybe a 2/0 is feeding the taps so that's the limit.
- Sum of panel main breaker ratings.
In a typical 320/400A service, the SECs are paralleled, so you can add them. Yours seems to be a tap. But I'd specify the service size as the size permitted by those SECs if it is less than the handle ratings of all the downstream disconnects being fed. If the breaker sum is smaller, then specify that. Conductors could be increased for voltage drop or just what they had available so that isn't always the limit.
I would ignore the wire from the poco feeding the mast or the meter feed if underground. These are smaller than NEC sizing but to me still play a role. I have a 250' feed from the street, mixed overhead and underground. The overhead is #2 Al and the underground is 1/0 Al. My main breaker was 200A, but I reduced it to 150A. I thought it a bad idea to have something think they can effectively pull 200A through that without burning it up or experiencing excessive voltage drop. It also makes adding solar easier as I have a 200A panel bus with a 150A main now.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
I agree. A service with multiple disconnects illustrates the difficulty of simply saying the size of the service by the main breaker size as in a single main service. Another example is 3, 150 discos on a 400 amp socket. Perfectly compliant as long as the load calc is 400 or less. Done all the time.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
@1/2retired -- check your conversations in the upper right hand corner
growler
Senior Member
- Location
- Atlanta,GA
I think you are right.
They probably used the 150 amp panels so they could use the 4/0 AL SE cable to feed the panels.
I would call this a 300 amp service.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
When you have multiple service disconnecting means, any "common" supply side conductor(s) is only required to be sized to the calculated load. This can result in ampacity being less than the sum of the overcurrent devices.
A single service disconnect must have supply conductors that correspond with the overcurrent protection, this includes the individual disconnects when there is multiple disconnects involved.
A six tenant meter center might have six 100 amp tenant breakers as the service disconnecting means. If load calculation for that building is only 150 amps you can supply that meter center with 150 amp conductor even though you have 600 amps sum of the overcurrent devices
- Location
- New Jersey
- Occupation
- Journeyman Electrician
So regarding the OPs question in your example the service size would be 150 amps.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Maybe. From realistic perspective that is all that can be delivered for long term loading.
I can almost guarantee if I were filing for a permit for that and only put down 150 on the application that most the inspectors here would end up sending out a correction of fees after final inspection and want the fee associated with 600 amps vs the fee associated with 150 amps.
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