Question on 2020 NEC 705.12(B)(3)(6)

[pv_n00b]

This section covers panelboards with feed through lugs and it has this which I find confusing:

Where an overcurrent device is installed at the supply end of the feed-through conductors, the busbar in the supplying panelboard shall be permitted to be sized in accordance with 705.12(B) (3) (1) through 705.12(B) (3) (3).

If there is an OCPD on the supply end of the feed-through conductors isn't that just like any other conductors from the panelboard with an OPCD? My idea of a feed-through is conductors attached directly to the supplying panelboard bus with no OCPD and feeding another panelboard.

 

[wwhitney]

That section is very poorly written and I have no idea what it is intended to mean.

Is it referring to coming off the feed-through lugs and hitting an immediate OCPD before supplying another panelboard?

Is it possibly referring to the far end as the "supply" end of the feed-through conductors? That would make some sense for the case that an alternate power source is interconnected there, it would be the supply end with respect to that alternate power source. But then the rest of the sentence uses the phrase "supplying panelboard" which would be very confusing if that is meant to refer to the panelboard with the feed-through lugs.

Edit: I think your best best is to read the 2023 First Draft version and assume that is what was intended by the 2020 word salad.

Cheers, Wayne

 

[jaggedben]

I think what they are saying is that if the feed through conductors have an overcurrent device near the panel board -- e.g. fused disco in a separate enclosure -- then the circuit can be treated like another breaker in the panelboard. I assume they mean you to count the aforementioned overcurrent device in sum of overcurrent devices for the case of (B)(3)(3). I think this was intended to close the loophole where you don't count the load on the feed throughs because they don't have an overcurrent device.

I don't like the way it's written either. The 'supply end' part is vague and shouldn't matter.

 

[wwhitney]

FWIW, the 2023 First Draft Report version:

Connections shall be permitted on busbars of panelboards that supply lugs connected to feed-through conductors, or are supplied by feed-through conductors. The feed-through conductors shall be sized in accordance with 705.12(A). Where an overcurrent device is installed at either end of the feed-through conductors, panelboard busbars on either side of the feed-through conductors shall be permitted to be sized in accordance with (1) through (3).

Cheers, Wayne

 

[pv_n00b]

The 2023 first draft does make it more clear. Basically, it says if there is no OCPD on the feedthrough conductors then treat a connection like a feeder tap. If there is an OCPD at either end of the feedthrough conductors treat a connection like a busbar. It makes sense, a feedthrough conductor with no OCPD is just an extension of the busbar from one panel to another. With an OCPD on the supply end of the feedthrough conductors, it's the same as if the load panel was supplied by any one of the CBs in the supply panel. I'm not convinced that with an OCPD on the load end of the feedthrough, in a main breaker load panel for instance, that it should be treated like a busbar. It's much more like a feeder tap in that case.

 

[jaggedben]

The 2023 first draft does make it more clear. Basically, it says if there is no OCPD on the feedthrough conductors then treat a connection like a feeder tap. If there is an OCPD at either end of the feedthrough conductors treat a connection like a busbar. It makes sense, a feedthrough conductor with no OCPD is just an extension of the busbar from one panel to another. With an OCPD on the supply end of the feedthrough conductors, it's the same as if the load panel was supplied by any one of the CBs in the supply panel. I'm not convinced that with an OCPD on the load end of the feedthrough, in a main breaker load panel for instance, that it should be treated like a busbar. It's much more like a feeder tap in that case.

What difference would the location of the OCPD make?

 

[pv_n00b]

What difference would the location of the OCPD make?

If the OCPD is at the source end it protects the feedthrough conductor from short circuit and overload, if it is on the load end it only protects the feedthrough conductor from overload.

 

[jaggedben]

If the OCPD is at the source end it protects the feedthrough conductor from short circuit and overload, if it is on the load end it only protects the feedthrough conductor from overload.

Right, but it's really only overload we're worried about here, right? We were discussing 705 qualification of the upstream busbar. I don't see how short circuit protection of the feed through conductors figueres into that.

 

[pv_n00b]

It's because the feedthrough conductor could be tapped. It's much less likely that a busbar will be tapped.