So, one of the things I want to do if I ever get a chance to have a house wired to my specifications is have each room have its own lighting homerun to the panel, but multiplex these homeruns onto a smaller set of lighting circuits at the panel in a star bus topology instead of the "daisy chain" of lighting circuits run all over current houses. This makes the wiring of the house itself clearer and easier to follow logically speaking, while allowing multiplexing schemes that keep large, contiguous areas in the house from being darkened by a single breaker trip.

While 312.8 permits splices in the panel by ordinary (wire-nut or push-in connector) means provided the panel gutters do not exceed 40% wire fill and 75% overall fill, the use of twist-on or push-in connectors free-floating in the panel seems to be a somewhat...sloppy way to do this. A perhaps more neat and workmanlike approach to the problem is offered by the use of UL 1953 listed power distribution blocks, which are commonly used to provide enclosure-mounted splicing and distribution points in industrial control panels.

While the products in question, when appropriately specified for wire size, are clearly suitable for the application at hand (a block rated for 175A continuous is not going to pose a problem on a 20A residential circuit), I know of no case where they've been deployed into a panelboard cabinet to provide cabinet-mounted splice or distribution points in a neat fashion. Is this a Code-legal thing to do, provided gutter fill does not exceed 312.8 requirements? Is 312.8 even applicable to splicing points that are affixed to the cabinet (vs. floating in gutter space)? Would modifying a stock panelboard cabinet assembly in this way void the UL listing or violate some other portion of the NEC?

Although this is more frequently done in commercial and industrial gear, I have never found anything against it in the NEC nor have I ever had a AHJ say they would be against it (I'm talking about true electrical inspectors in major municipalities). So long as you meet the requirements you mentioned above. The only problem would be if you would be required to submit the calcs that show they don't violate the fill capacity and you may have to make sure the connections were not exposed to incidental contact per NFPA 70E.

The only problem I would see would be with the how UL would label it. As you know you are allowed to assemble control and terminal cabinets so long as you use "UL listed for use" material. But I have found a mixed bag of opinions from AHJ as to whether or not they want to allow that type of equipment that is not "UL approved" in their jurisdiction and the only way that I know of to get it UL approved is to submit it to UL and get them to approved it. It's obvious this is too expensive for the normal residential installation. The only other way is to find a manufacturer that has already submitted their equipment to UL for this type of modification.

Personally I am for it. It looks cleaner and is looks more workmanlike. I have actually been a part of a group that we assemble "Main Termination Boxes". for commercial installations. Instead of a big junction box with a birds nest of wires and wire nuts and we never got any blowback from AHJ's. But in your situation I would talk to my inspectors in the area just to see how they felt before I tried it.

In conclusion your design would meet NEC and UL listed requirements but it would not meet UL approved requirements.

I think you are good as long as you are under 75% fill in that space. As you said in your post.

312.8 Switch and Overcurrent Device Enclosures with Splices, Taps, and Feed-Through Conductors. The wiring space of enclosures for switches or overcurrent devices shall be permitted for conductors feeding through, spliced, or tapping off to other enclosures, switches, or overcurrent devices where all of the following conditions are met:

(1) The total of all conductors installed at any cross section of the wiring space does not exceed 40 percent of the cross-sectional area of that space.

(2) The total area of all conductors, splices, and taps in-stalled at any cross section of the wiring space does not exceed 75 percent of the cross-sectional area of that space.

(3) A warning label is applied to the enclosure that identi-fies the closest disconnecting means for any feed-through conductors.

The terminal blocks take up more space than splices. That would be my only concern. You can stagger splices. Not so with a terminal blocks. It makes a neater installation but takes up more space.

Good Luck!

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