Sub-Feed Lugs

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lkrueger

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Portland, OR
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Electrical Engineer
I'm having trouble understanding the difference between sub-feed lugs and feed-thru lugs and when would be appropriate to utilize each. Any light y'all can shed on this would be much appreciated. Thanks!
 
In regards to 'panel boards' typically 'feed thru' lugs go at the opposite end of the main breaker (or main lugs) and the current feeds thru the panels bus to the lugs.
'Sub feed' is a doubble 'main lug' that allows you to daisy chain from one panel to another.

Say for example you have a Eaton PL1a 400 Amp panel, and you need to feed a 150A fused disconnect, they do not make a '150A breaker' that bolts in a normal breaker position, so you could install 'feed thru' lugs and use a tap rule to feed the disconnect.

Or say for example you wanted to put two 400A panelboards off a 400A feeder you could pipe into one panel and nipple over to another, then use 'Sub feed' lugs on the first to feed the second.

At least thats my take.
 
It means Article 312(6)(B) in the 2017 edition of the NEC, which is entitled: Wire-Bending Space at Terminals.
Which has nothing to do with how short of a bend you can physically make with the conductor itself, it is about having room at the terminal to insert the conductor(s) into the terminal without putting too much strain on the terminal or conductor.
 
I don't know if the names vary with manufacturers but with Square D feed thru lugs is mounted directly to the bus bars, sub feed lugs is a plug on/bolt on assembly that mounts in place of a breaker but has no switch or overcurrent device, just a place to land outgoing conductors. Either case conductors will need to be sized to be protected by the feeder OCPD or comply with feeder tap rules.
 
I don't know if the names vary with manufacturers but with Square D feed thru lugs is mounted directly to the bus bars, sub feed lugs is a plug on/bolt on assembly that mounts in place of a breaker but has no switch or overcurrent device, just a place to land outgoing conductors. Either case conductors will need to be sized to be protected by the feeder OCPD or comply with feeder tap rules.
Not really.

For many decades the Square D factory installed standard was, feed thru lugs are connected such that the current flows through the panel bussing and any main device. These lugs can be plug-on or mounted direct on the bus. Sub-feed lugs are placed at the incoming line terminals.

Using a main breaker as an example, sub-feed lugs would be on the line side of the main breaker and would be protected the by the feeder to the panel. Feed-through lugs would be mounted on the load side of the main breaker and would be protected by the main.

For main lug only panels, sub-feed lugs would be mounted at the same point as the incoming lugs. Feed-through lugs would be mounted at the opposite end of the panel bussing or as a branch device.

However, some confusion occurs because all of their field installed branch style lug kits are called sub-feed lugs.
 
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How common are sub feed lugs with main breaker panel boards? I always thought of them on MLO and never really thought about the option on MB.
For standard "loadcenters" not many at all have these sort of options.

For real "panelboards" it is an option on nearly all of them.

If asking about having additional lugs on line side of a main breaker - I would think it would depend on what lug options are available for the breaker in question. Again not a likely option with very many "loadcenters".
 
If asking about having additional lugs on line side of a main breaker - I would think it would depend on what lug options are available for the breaker in question. Again not a likely option with very many "loadcenters".

You usually only get that option, if multiple wires in parallel are a likely possibility for the amps of the main breaker. And the likely capacity, is only going to be slightly greater than what you typically need for that main breaker.

For instance, you might have a 400A breaker that has "snowman" lugs on it, rated for either 1 wire up to 750 kcmil, or rated for 2 wires up to 250 kcmil. You would not have enough capacity to connect 400A to the breaker, and then daisy-chain another 400A to an identical panelboard, since you couldn't fit 2x 600 kcmil in this particular example. You could connect the 400A to this breaker via 600 kcmil, and then make a tap to an adjacent 200A panelboard via 3/0.
 
You usually only get that option, if multiple wires in parallel are a likely possibility for the amps of the main breaker. And the likely capacity, is only going to be slightly greater than what you typically need for that main breaker.

For instance, you might have a 400A breaker that has "snowman" lugs on it, rated for either 1 wire up to 750 kcmil, or rated for 2 wires up to 250 kcmil. You would not have enough capacity to connect 400A to the breaker, and then daisy-chain another 400A to an identical panelboard, since you couldn't fit 2x 600 kcmil in this particular example. You could connect the 400A to this breaker via 600 kcmil, and then make a tap to an adjacent 200A panelboard via 3/0.

Yeah that was what I was getting at, that you would kinda need some bussing on the line side of the breaker going to a separate lug set to have any meaningful sub freed lugs.
 
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