Both the houses I deal with (which are quite far apart btw) are in need of panelboard reworks:
- One still has the original fusebox, never mind that the main disconnect is basically at the 6'7" Code maximum, rendering it unreachable by people who don't have the luxury of finding it at eye level.
- The other has a QO panel...in the master bathroom. What a waste of a QO! (Thankfully, AFAICT, the thing is still functional, but I want it out of its improper location before that statement becomes false.)
As a result, I have been making plans to rework the service and protection hardware for both houses.
The problem at hand
Considering that I want this to be the first and last time that anyone will ever have to do this to either house in question, being stingy with panelboard slots is penny-wise and ton-foolish. Also, in 2008, the NEC lifted the 42 slot limit on branch circuit (lighting and appliance) panelboards, and both AHJs have adopted the 2014 NEC, so that this limit is not a problem for me.
However, the light-duty panelboard manufacturers have been rather slow to catch up:
- Square-D HOM is only available up to 42 slots, tops; the same is true for GE THQL, as well as the GE A-Series and Eaton Pow-R-Line commercial panelboards
- Square-D QO loadcenters only go up to 42 slots as well save for a single 60 slot 200A offering (that costs almost as much as a NQ interior!); larger needs are shunted to the Square-D NQ line, which is available up to 84 slots and takes QO(B) breakers, but puts you squarely into having to order and spec a commercial panelboard (which is a fair bit more work) and also forces you to go to 20" width for upwards of 42 slots (with its attendant framing issues)
- Siemens PL and ES loadcenters both go up to 54 slots (the same is true for distributor merchandised Revised P1 panelboards btw despite bigger Revised P1 interiors being available from the factory, up to 66 slots)
- Eaton BR and CH loadcenters are both available up to 60 slots, which is the best I've seen so far from a light-duty loadcenter.
Of course, a subpanel with feeder breaker is an option for expansion, but this can be limiting as some light-duty loadcenters cannot feed a feeder as large as their main breaker/bus ampacity due to breaker availability/frame size limitations while those that can require a doublewide feeder breaker to do so:
- Eaton CH breakers only go up to 150A (there was a CH2200, but it was discontinued)
- GE THQLs have the 4-slot (apparently) TQDL line for ampacities up to at least 200A (although it's not listed in their catalog...)
- QO and HOM breakers go up to 200A but at the cost of 2 extra slots over a regular 2 pole breaker for 150-200A.
- The Eaton BR line has the BJ series double-size two pole breakers for ampacities from 125-225A
- Likewise, Siemens QPs use double-size type QN breakers for 150-200A.
Considering that breakers can't be paralleled unless it's the factory's fault (240.8) -- this means that feeding a 400A subpanel off a 400A main panel is impossible. However, subfeed lugs don't fall under 240.8 as they don't provide overcurrent protection. While a single subfeed lug set is limited to 225A for all light-duty panelboard lines save for the CH which only provides 125A, we're into fat enough wiring here that parallel wires are permitted by 310.10(H).
This allows us to effectively implement feed-through lugs using one or more sets of subfeed lugs in the first interior and feedthrough conductors connecting the first interior to the main lugs in the second, even though light-duty loadcenters don't provide a true feedthrough lug option unlike their heavier-duty panelboard brethren. With this, we can then get as many slots as we wish with the ampacity we're after, all controlled by a single main disconnect.
The bottom line
Am I correct that Code allows you to use sufficiently sized feed-through or sub-feed lugs and wiring to "daisy chain" as many panelboard interiors together as you wish to make a single super-panelboard, spread out over N cabinets that are connected via an auxiliary gutter system or via conduits? Or is there some Code clause that'd trip me up if I tried this? (I'm not concerning myself with cost or physical space at this point in the design, simply safety and Code-compliance.)
Furthermore, if that is true, where is the neutral-to-ground bond performed in such a configuration? Do you just pick a cabinet and bond it there, and then pull the bonds on the rest of the panelboards, even though they technically are not really subpanels as there is no feeder to be found in this configuration? Or should the neutral-ground bonds be left in all the panelboard interiors involved? Could I even pull all the neutral-ground bonds and install a distribution block in the auxiliary gutter to make the neutral-ground connection at?