anything against using the main lug as subfeed lug?! from a backfed mainbreaker load center

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al in wi

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Is there anything against using the main-lug (of an MLO panel with backfed main) as the feeder lug to a downstream panel?
Am i violiating anything?


Picture this....
Using Square D's QO312L125G. which is a main lug 3phase panel.
I would use a QO3100 branch breaker with a holding kit PK3MB to be used as a backfed main.
(The reason I don't want to use the QO327M100, because I want the smallest box)
Then, since the panel's main lug is unused...I would use that to feed a 2nd panel, which will have a backfed main as well.

This is for an enclosed generator. Intention is one panel is on the genset, the other on the enclosure.
I want to separate the loads between the two panels (genset loads: ie block heater, battery charger versus enclosure loads: lights, space heaters)

Either panel feeds the other, depending on where the site electrician lands the building feeder.

Hope that makes sense and get a conversation going.

Thanks in advance.
 
It's generally fine. The only pitfall is if you have a solar system breaker in the panel it will likely cause a violation of 705.12(B)(1). (2020 NEC reference.)
 
electrofelon said:
Only if their ampacity is less than that of the ocpd back feeding the panel.

I also don't see an issue with using them for a sub feed
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I am not sure i am "picking up what you are putting down"... :)

The (unused MLO terminals) would always be equal or larger than the backfed OCPD.
In this example, the panel's main rating is 125A.
The OCPD main breaker in branch position would always be equal to or less than the main lug rating.
The MLO ampacity would never be "less than that of the OCPD backfeeding the panel" .
Branch OCPD (whether branch or used as main), is always less than or equal to the bus and lugs capacity

And that also seems counter intuitive to me. Don't we want that main lug terminal larger, than the back-fed main?
Otherwise that would not be a protected tap according to tap rule (if that is the applicable code section)
 
al in wi said:
I am not sure i am "picking up what you are putting down"... :)
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I think the point is that as long as your aren't trying to feed something like a 200A downstream panel......

Edit: Or more specifically, a downstream panel that might have more load than 125 amps.
 
al in wi said:
I am not sure i am "picking up what you are putting down"... :)
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When you connect conductors to the unused main lugs, they are protected at the OCPD rating of the backfed "main". Post #2 commented that the conductors would be considered tap conductors. Post #3 clarified that is only true when the conductor ampacity is less than that OCPD rating.

Cheers, Wayne
 
al in wi said:
I am not sure i am "picking up what you are putting down"... :)

The (unused MLO terminals) would always be equal or larger than the backfed OCPD. ...
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But the wires you connect to those lugs may or may not be rated as high.

If the wires you connect to the lugs are protected by the backfed breaker then you're done.

You can connect smaller wires, but then it's a tap and you need an overcurrent device downstream in compliance with 240.21(B).
 
wwhitney said:
When you connect conductors to the unused main lugs, they are protected at the OCPD rating of the backfed "main". Post #2 commented that the conductors would be considered tap conductors. Post #3 clarified that is only true when the conductor ampacity is less than that OCPD rating.

Cheers, Wayne
Click to expand...
got it...thanks. In this case I would make the conductors with same (or larger) rating than the main lugs, allowing it to be outside the privvy of tap rule.

thanks everyone.
 
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