I want to add a subpanel that is supplied by multiple hots

Question

Overview:

I have an existing woodshop space located in a rented commercial building that I share with other businesses. My shop is currently supplied by 2 subpanels (total 10 circuits) that are located over 100' away. These circuits run through other rental spaces (12/2 Romex and solid MC). Accessing the subpanels and running new wire is inconvenient and tedious and also I'd like to have control of my circuits for safety concerns. For these reasons I want to add a new subpanel of the same breaker sizes and load characteristics

My question(s):

Is is possible/safe/legal to supply the new sub by bringing all 10 of the hots together at the new sub breaker bus? The new box will have 20A breakers that I can use for both fault protection and isolation. The neutrals will 'pass-thru' and not be shared. The grounds will land at the box and then pass-thru to the branches as normal.

Answer 1

If you are limited by your landlord to 10 x 120V 20A circuits, and if the goal is isolation so you can safely work on the wiring in your space, you can achieve that by running the ten circuits through light switches. Say, 10 light switches installed in two 5-gang boxes. (Which is a lot more expensive than using 5 2-gang boxes, so it's up to you). This will allow you to easily turn circuits off to change the wiring of your shop as long as you stay within the limitations of 10 20A circuits.

For added safety you can use switches with keys, lockouts, covers, or other "non tamper" features. You don't need a breaker panel or to join circuits up on a bus.

If you want to do anything else, like add 220V circuits or 30A or higher circuits, the other answer is correct: you need a real feed from your landlord. If you pay for the install, the landlord ought to love it!

Answer 2

TL;DR No. Run a new cable (or conduit).

Sorry, you can't do that. As I understand the question, you would end up with everything together on the bus and then split again to the circuits, which would be paralleling which is not allowed except in very limited circumstances (that absolutely don't apply here). For the "why" (aside from "because Code says so"), imagine the following possible scenario:

• 10 circuits connected initially
• 5 circuits go bad (e.g., one of the subpanels lost power)
• The 10 circuits stay live and are pulling 10 x 10A = 100A total, which is permitted by code.
• The current is going back to the subpanel on 5 wires = 20A each = total breaker value but above the 16A each permitted for continuous loads.

The 20A won't trip the breakers quickly. Possibly not ever. But the wires may overheat as the continuous load should only be 16A each.

The problem can actually be much worse, particularly if there are any larger (e.g., 30A or 40A) circuits, because the electrons don't know which wires they are "supposed to use".

The real solution is a new subpanel feed direct to your shop. This can be a properly rated cable. Or even better is conduit, which may actually be required in a commercial building anyway (jurisdiction dependent). With a large-enough conduit, the real pain (and expense) is a one-time job, but then you are done forever. Running new, larger, wires as needed (e.g., if you later upgrade from 60A to 100A or whatever) is a relatively small task.

Even if you don't need more circuits now, get a nice big panel. It won't cost much more and will make life easier the next time you need to add a circuit. The bare-minimum would be a 12 space panel, but 24 or larger makes sense. Also get one with a main breaker. You don't (most likely) need a main breaker, but you will need a shutoff (possibly by code, but practically since the feed panel is not easily accessible) and a main breaker in your panel is the easiest way to do that. That might be a 100A or larger breaker even if your feed is much smaller - that's OK. For example, I easily found a 100 Amp, 24 space panel at Home Depot (and you may do better elsewhere) for ~ $ 90. That includes 3 x 20A single breakers and 2 x 30A double breakers, so you'll need to add more breakers and a ground bar, but all the hardware won't amount to much.

Your big cost will be running cable and/or conduit/wires, and you must get an electrician to do the initial installation (install subpanel, run cable/conduit, connect existing circuits to your subpanel) or else you put yourself, your landlord and the other tenants at serious risk.

Answer 3

Your idea is no-go.

You seem to understand the importance of keeping neutrals separate. Well, the same applies to hots. Anytime hot wires are cross-connected you are inviting all sorts of problems.

Also I don't think you realize the hot wires are on 2 different phases (or possibly 3). Hooking them to each other would cause bang-boom!

If that last paragraph is news to you, watch this video. Understanding multiple phases is really imporatant for "the sizing conversation" we'll have later. I can't have you panicking and thinking you need 200A feeder for 10x 20A circuits.

However, let me do a bit of a "code review" on your setup at large.

The 120V circuits are too long.

My general rule with voltage drop is I don't even crunch the numbers until 170' or so (because default wire sizes are usually adequate). However, that's for 240V circuits. With 120V circuits, voltage drop (by percentage) is double because voltage is half. So 120V circuits need to start thinking about voltage drop at 80' or so.

Also the fact that you have 10 circuits tells me you are adding circuits more often than you'd like.

So you have 10 circuits. Suppose one is maxed at 20A, and the others are quiet - no loads. That one circuit is getting mauled by voltage drop, while the other copper is just sitting there useless. *And I think you kinda know that, and are hoping to "parallel" all that otherwise wasted copper". Nope, paralleling like that is a no-no. causes many problems.

Also you are spending a fortune on wires.

No more #12 wire

I gather you know what a PITA it is to add these circuits, because you have been the one adding them. Well, we're not going to do that anymore. Next wire will be a big fat one that will replace all those #12 runs and solve your voltage drop problem too.

And it'll be laughably cheap. Might even get to use that sub panel of yours!

Run a fat aluminum feeder from the larger of the two sub panels, to a subpagel in your workshop area. #2 will more than support your ten 20A circuits with room for more, depending on usage. (Remember what I said about understanding split-phase? That is important here. Further, if your power is 3-phase, then 1 additional hot wire will give a third phase).

Also we get to significantly oversubscribe circuits that won't be running maxed out all the time, so for instance ten 20A household circuits only need 40-50A of 2-pole/2-phase power, since they are never used maxed out all at once. (if yours are, well, you have to think about that).

• #2 aluminum is 90A per phase.
• #1 aluminum is 100A per phase.
• #1/0 aluminum is 120A per phase.

If you need to pull out of both subpanels, then fine, add another #2 feeder and another sub panel. (they're cheap).

Aluminum feeder is cheap. The going rate for 2-2-2-4 AL feeder (pre-COVID) was about $1.30/foot. Note that if your origin panels are 3-phase, you'll need 5-wire feeder, which may need to be individual wires in conduit.

But you only have to install it once.

Once it's in, you'll be able to support any number of circuits, and have local control of those circuits.

It makes no difference whatsoever, to load, whether you use this feeder+subpanel arrangement, or use individual #12 runs. So you cannot say this will increase burden on what is being done now.

And this will solve the "voltage drop" problem for good, since if one 20A circuit is maxed while the others are idle, the 20A circuit gets the #2 or #1 or 1/0 cable all to itself.