Feed Thru Lugs

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fst442

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Vancouver, WA, USA
My apprentice and I are having a debate. We have a 240V, 3 ph, center tapped delta(high leg) system in an industrial setting. A 200A fused disconnect supplies the existing 200A, 3ph panelboard. This panelboard supplies a 100A, 1 ph panelboard that is located right next to it and supplies 120V lighting loads. The panels are aging and need to be replaced to ensure selective coordination, proper AIC ratings, etc. The high leg makes the 1phase panel kind of necessary to keep the installation clean in my opinion. We are now gearing up to replace the panels and this question has come to mind. Any help would be greatly appreciated. Thanks.

1: Is it acceptable to feed the new 1 ph panelboard with 2 of the 3 feed thru lugs on the bottom of the new 3ph panelboard? Assuming, obviously, that we size the feeders so that they are protected and the second panel is of the same voltage rating as the first panel. I've never done it this way. My apprentice believes this would be an acceptable installation, however, I worry that this would be a poor installion.
 
Why not use a 2 pole breaker that is connected to only phase A & C. This way your panel has over-current protection, and your 2nd panel can be MLO.

Also, if I understand your question and situation correctly, your new panel for the 120v loads is required to be rated at 200A, not 100A if this is a feed thru installation.
 
Npstewart said:
Why not use a 2 pole breaker that is connected to only phase A & C. This way your panel has over-current protection, and your 2nd panel can be MLO.

Also, if I understand your question and situation correctly, your new panel for the 120v loads is required to be rated at 200A, not 100A if this is a feed thru installation.
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Why would the 100A panel need to be changed to a 200A panel unless it is MLO?
 
I guess he didn't really clarify the proposed panel amp rating. The proposed panel would have to be rated for at least 200A in my opinion if it is feed thru.

The existing 100A panel installed next to the 200A panel is fine.

Like I said though, it would be a better installation to provide a 2 pole breaker for the new panel and feed it from phases A&C.
 
Npstewart said:
I guess he didn't really clarify the proposed panel amp rating. The proposed panel would have to be rated for at least 200A in my opinion if it is feed thru.

The existing 100A panel installed next to the 200A panel is fine.

Like I said though, it would be a better installation to provide a 2 pole breaker for the new panel and feed it from phases A&C.
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Does it really matter? One case you have a 2 pole breaker on the supply side of feeder, other case you have 2 pole breaker on load side of a feeder tap. Some cases a panel with factory main costs less than adding a 100 amp branch breaker and buying main lug panel. If cost is issue one should also compare cost of 200 amp main lug panel - a main breaker would be needed at all.
 
You mentioned selective coordination.

If that's a goal, I would avoid the 100A breaker. It may be tough to coordinate a 200A, 100A, 20A series of breakers.

Make both panels 200 amps, and I don't see anything wrong with using (2) subfeed lugs.
 
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