Downsizing Sub-Panel Breaker

[miimura]

I have a relative whose house has 400A service from PG&E. The main panel has large bus bars going from the meter socket to individual 200A main breakers. One main feeds the breaker bus immediately below and the other feeds 4/0 aluminum wire that goes to a sub-panel on the other side of the house.

There are a couple situations where this is not desirable - for example a Powerwall installation where you want the whole house backed up through a single 200A Gateway.

So, my question is, what are the considerations and limitations to feeding the sub-panel from a smaller breaker, say 125A, installed on the main breaker bus, obviating the need for the second main breaker?

In the specific situation cited at the beginning of this post, I cannot imagine tripping the smaller breaker because the house was built with a spare 60A feed from the sub-panel for possible pool and hot tub that were never installed.

 

[mongo]

I have a relative whose house has 400A service from PG&E. The main panel has large bus bars going from the meter socket to individual 200A main breakers. One main feeds the breaker bus immediately below and the other feeds 4/0 aluminum wire that goes to a sub-panel on the other side of the house.

There are a couple situations where this is not desirable - for example a Powerwall installation where you want the whole house backed up through a single 200A Gateway.

So, my question is, what are the considerations and limitations to feeding the sub-panel from a smaller breaker, say 125A, installed on the main breaker bus, obviating the need for the second main breaker?

In the specific situation cited at the beginning of this post, I cannot imagine tripping the smaller breaker because the house was built with a spare 60A feed from the sub-panel for possible pool and hot tub that were never installed.

Depends what the house load calculation says and if there are loads that could move to an unbacked up panel. The lack of pool and hot tub could make it feasible.

 

[Vines]

Most locations and certainly Los Altos will not let you feed the distribution bus other than as shown in the sticker on the door, without calling UL field Evaluation services out to approve the modifications.

They used to allow this as long as it was done safely. Recently we did it but had to call out UL and it adds a layer of complexity that isn't awesome.

It takes a separate panel, but our solution today is to move these loads elsewhere, and then land a 100-200A breaker on the distribution bus to feed that new panel. Most 400A service panels will take a 200A breaker on the distribution bus.

 

[wwhitney]

Most locations and certainly Los Altos will not let you feed the distribution bus other than as shown in the sticker on the door, without calling UL field Evaluation services out to approve the modifications.

That makes sense to me when the main breaker has one end bolted to the distribution bus. But how about the case where the distribution bus has factory mounted main lugs, and factory installed conductors from a separate main breaker, like in the photo below?

Seems like that's just an enclosure with two main breakers and an MLO panelboard, and you could field rewire it as desired. I mean, other than the factor installed weirdly bent conductors, there's no difference I can see between the top and bottom main breaker locations.

[This might not be relevant to the OP; I took it to mean moving the feeder that is supplied from the upper breaker to be supplied by a smaller breaker on the lower distribution bus. In which case the only issue is doing the load calc.]

Cheers, Wayne

 

[Vines]

That makes sense to me when the main breaker has one end bolted to the distribution bus. But how about the case where the distribution bus has factory mounted main lugs, and factory installed conductors from a separate main breaker, like in the photo below?

Seems like that's just an enclosure with two main breakers and an MLO panelboard, and you could field rewire it as desired. I mean, other than the factor installed weirdly bent conductors, there's no difference I can see between the top and bottom main breaker locations.

[This might not be relevant to the OP; I took it to mean moving the feeder that is supplied from the upper breaker to be supplied by a smaller breaker on the lower distribution bus. In which case the only issue is doing the load calc.]

Cheers, Wayne

What you said sounds great and I argued this up to the CBO.

They said it needs to be just like the installation manual, or listed to prove the new configuration was safe. This was a Millbank version of exactly the same panel setup, except the wires were straight down to the main bus instead of question mark shaped. All we wanted to do was route wires into that distribution bus other than from the main breaker tied to the utility. It was a multi week process and in the end we called in UL because 1 inspector wanted to be hardline about "Used per the manufacturers installation instructions"

 

[wwhitney]

What you said sounds great and I argued this up to the CBO.

FWIW, the response you got was total BS. 3/0 Cu is 3/0 Cu. If you can reroute the conductors in a different, useful way, without having to bend a conductor through a space with less than the required bending space, there's zero real problems. Just made up ones.

I assume the UL field rep was like "of course that's fine"?

Cheers, Wayne

 

[Vines]

FWIW, the response you got was total BS. 3/0 Cu is 3/0 Cu. If you can reroute the conductors in a different, useful way, without having to bend a conductor through a space with less than the required bending space, there's zero real problems. Just made up ones.

I assume the UL field rep was like "of course that's fine"?

Cheers, Wayne

The UL rep was like sure I'll take your money. Maybe I should hire you to win that fight through code next time. You can submit the AMMR lol.

The AHJ said that since those wires are factory wires, that was a modification of the panel to remove them and feed that bus from elsewhere.

 

[wwhitney]

The AHJ said that since those wires are factory wires, that was a modification of the panel to remove them and feed that bus from elsewhere.

Not to relitigate your issue (too much), but did you get detailed info from Square D on the listing(s) of that CSED? My understanding is that the bus assembly supplied by those factory wires is itself a listed panelboard, and you can mix and match and put it in any enclosure with adequate bending space. The enclosure is subject to Article 312 for a typical cabinet with a door, but the panelboard listing isn't specific to a certain enclosure.

So I would think the distribution side of that CSED is just a convenient cabinet, with 2 listed main breakers with their mounts, a listed panelboard, some (listed?) neutral/ground buses, and some convenient factory wires.

Cheers, Wayne

 

[Vines]

Not to relitigate your issue, but did you get detailed info from Square D on the listing(s) of that CSED? My understanding is that the bus assembly supplied by those factory wires is itself a listed panelboard, and you can mix and match and put it in any enclosure with adequate bending space. The enclosure is subject to Article 312 for a typical cabinet with a door, but the panelboard listing isn't specific to a certain enclosure.

So I would think the distribution side of that CSED is just a convenient cabinet, with 2 listed main breakers with their mounts, a listed panelboard, some (listed?) neutral/ground buses, and some convenient factory wires.

Cheers, Wayne

It was Millbank and in the end they didn't offer anything to counter the inspector.

The inspector's position was that factory wires were removed so we were modifying it from the service equipment it was. That modification was not evident on the sticker as an option so the answer was no without UL verification. I am not saying it makes sense, but San Jose did the same thing recently. We decided that we are no longer backing up those distribution busses and instead have to put more equipment on the wall.

The manufacturers will happily sell you a new service panel but getting timely backup information with a code fight is harder. I would seriously pay you 2x what we paid UL to get the inspectors to accept this solution because it's such a big win. Whole home backup with a 2x200 is messy regardless of the tech, but feeding that distribution bus from elsewhere is a big win.

I got sick of the fight as each iteration was taking 2 weeks to get a new answer of no, while the customer is pressuring us asking why it is taking so long. This service in question was right next to a gas meter, so relocating loads wasn't a great option.

We would have had to install all AFCI breakers on a house never designed with them because of the distance we would have had to relocate the breakers from the MSP. Likely those would have nuisance tripped and there wouldn't be any real resolution to that problem that didn't involve surgery.

 

[CrazyRabbit]

I think your best option is to do loads calculation for all backed up loads and then use the main panel for non backed up loads, evse chargers, etc. add a new panel that can accept 200 amp breakers, probably will be a 400 amp panel, on opposite side of wall if possible and feed the house.

 

[miimura]

The situation is a Milbank panel with two vertical 200A breakers, but we're not touching the original wiring between the main panel bus and "Main Breaker A". We are wanting to re-route the field installed 4/0 Aluminum feeder that goes to the sub panel to land it with a new breaker on the main bus instead of on "Main Breaker B".

Let's say the load calculation says you should use a 150A breaker to feed the loads on the sub-panel. The feeder is already sized for 200A. Is there any reason you can't use a 125A breaker to feed that sub-panel?

 

[wwhitney]

Let's say the load calculation says you should use a 150A breaker to feed the loads on the sub-panel. The feeder is already sized for 200A. Is there any reason you can't use a 125A breaker to feed that sub-panel?

Yes, NEC-wise. If the load calc is over 125A, then a 125A breaker is too small. [In practice, it might work fine indefinitely, as the NEC load calc is very conservative.]

So you'd either need to find a 4 position breaker rated 150A, 175A, 200A that has been listed to work on that bus, or you'd need to find some feed thru lugs that are listed to use with that bus.

Or if those don't exist, you'd have to separate the loads in the subpanel into groups of no more than 125A, and use more than one breaker and more than one feeder to supply those loads.

Cheers, Wayne

 

[miimura]

Yes, NEC-wise. If the load calc is over 125A, then a 125A breaker is too small. [In practice, it might work fine indefinitely, as the NEC load calc is very conservative.]

So you'd either need to find a 4 position breaker rated 150A, 175A, 200A that has been listed to work on that bus, or you'd need to find some feed thru lugs that are listed to use with that bus.

Or if those don't exist, you'd have to separate the loads in the subpanel into groups of no more than 125A, and use more than one breaker and more than one feeder to supply those loads.

Cheers, Wayne

So, this then begs the question about the main breaker size. If I add a 150A 4 position breaker on the main bus, the chance that the total load calculation would recommend a main breaker over 200A is pretty strong. Is an undersized main breaker allowed? I have heard of down-sizing the main to accommodate the 120% rule for solar, but this is just more loads that could theoretically overload the main.

 

[wwhitney]

So, this then begs the question about the main breaker size. If I add a 150A 4 position breaker on the main bus, the chance that the total load calculation would recommend a main breaker over 200A is pretty strong.

The size of the breaker for the relocated feeder doesn't change the result any load calculations. The breaker just has to be at least as large as the load calculation for the loads supplied. [As do the conductors, and the conductors have to be large enough to be properly protected by the breaker.]

If the total load calculation ends up over 200A, then you can't do what you propose, as a 200A service would be too small for your loads. You can split out some of the loads and make them non-backed up and supply them from your second 200A service breaker.

So step one is to do a tight (precise) optional load calc for the whole residential service, and see what result you get.

Cheers, Wayne