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I am adding an addition to my house, and running a new 100A subpanel over to the main part of the house.

In the house I have the main 200A panel, plus an existing 100A subpanel, and the two are right next to each other and connected via a short conduit.

The situation is such that for the new subpanel I am planning on coming up through the wall (with a 2-2-2-4 cable protected in a conduit in the wall) INTO the existing subpanel, where I will then run the wires over through the large shared conduit to the main panel, and then to a 100A breaker.

My question - and this is NOT about the neutral and the ground bonding – is whether I can attach the new ground to the subpanel's ground bar, and the new neutral to the subpanel's neutral bar, while running my loads through to the main panel to their breaker.

The purpose of my question is because the main panel's neutral and ground bars have lugs that are used to connect to the first subpanel, and the main panel is otherwise really full, while the subpanel is not. I'm not actually sure if I can run a lug "sideways" off the main panel's bars to connect the #2 neutral and #4 ground (although I found a lug connector that does seem to allow for this, but it's not optimal).

EDIT: picture of main and subpanel: will come up through floor into subpanel, then go through to main panel

MAIN PANEL

EXISING SUBPANEL

PANELLABELS

BETWEEN BOXES

  • Why are you using a cable inside conduit instead of individual wires? Also, what make and model are the existing panels in question? – ThreePhaseEel May 12 at 0:18
  • I put a pic in main post. I'm using 2-2-2-4 Aluminum SE-R between the panels. Using the cable across the two crawlspaces. The conduit is just a way to protect from the panels down the wall into the crawlspaces (with proper connector/bushings) (conduit not shown in picture). The existing panels are Siemens (not sure model, put in in 1998). – Mike Osswald May 12 at 0:25
  • What's that below the existing subpanel? Is that simply a void space? – ThreePhaseEel May 12 at 0:35
  • Also, can you post a photo showing the main panel square-on? Might help if we got a better view of it. Furthermore, what size is the neutral between the existing panels? – ThreePhaseEel May 12 at 0:41
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    Eel, yes there is -- I actually have a neighborhood-sized power box next to my house that is "all mine" because I'm far back from the road. Then I have the meter, and next to the meter outside, there is actually a small box with a 200A shut off / breaker, so that goes to my "main". I didn't put the cable in, but I have it ready to go. – Mike Osswald May 12 at 17:14
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I'd simply run the new neutral back to the "main" panel

While adding accessory ground bars to your "main" panel will be trickier than it looks, since you'd have to field-fabricate a replacement for the neutral tie strap that originally came with said panel in addition to unbonding the left-hand bar and moving all its ground wires onto a newly installed ground bar, you do have enough neutral space on the right-hand bar for an ECLK1-2 lug to fit there, and you can do the same with the left-hand bar for your new grounding wire.

From there, you can run the wires from the new cable through the subpanel and existing nipple to reach the "main" panel, as there's no shortage of space in that nipple for more wiring. You will need to label on the subpanel's directory that there are feed-through conductors present and their disconnecting means is in the "main" panel next to said subpanel, as well, as this is required by NEC 312.8(A) point 3.

The alternative plan you propose of landing the neutral in the subpanel and continuing the hots off to the main panel is legal, under a little-used Code provision found in NEC 215.4:

215.4 Feeders with Common Neutral Conductor.

(A) Feeders with Common Neutral. Up to three sets of 3-wire feeders or two sets of 4-wire or 5-wire feeders shall be permitted to utilize a common neutral.

(B) In Metal Raceway or Enclosure. Where installed in a metal raceway or other metal enclosure, all conductors of all feeders using a common neutral conductor shall be enclosed within the same raceway or other enclosure as required in 300.20.

But, given the short distance of the shared run and the fact that there is space in the neutral bar for another add-a-lug, there's really not that much point in finessing things that tightly (and risking confusing an inspector who doesn't see that sort of thing on a regular basis at all).

HOWEVER: you'll need either a 90A breaker or 1-1-1-3 cable

There is a problem with your plan, though: namely, that 2-2-2-4 Al SER cable you are planning on using is only rated for 90A at 75°C (which you are allowed to use when running from panel to panel). So, you'll either need to get a Q290 for your feeder breaker instead of a Q2100, or run 1-1-1-3 Al SER instead of the 2-2-2-4.

While you're in there, you'll want to swap the two breakers in the far bottom left of the "main" for the correct Siemens breakers, since the existing breakers there are foreign to your panel, and thus won't fit quite right. In particular, the BR250 in the far bottom left position of that panel needs to be swapped out for a Q250, and the mystery 40A breaker above it needs to be swapped out for a Q240.

Other notes

It would be a good idea to replace the existing subpanel with something with more spaces; given the room available in that bay, using a 200A, main lug Siemens panel with 40, 42, or even 54 spaces for the replacement subpanel would not be at all out of place. I would also use a larger subpanel than your existing 20-space panel for the new addition; a 30-space, 125A, main lug subpanel would not be out of place there, either.

Also, as with any panel work, you'll want to use an inch-pound torque wrench or torque screwdriver when tightening the lug setscrews on your connections. Not only is this a NEC requirement as of 2017, found in 110.14(D), it's a very good idea anyway, lest your electrical system lose you the race!

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  • Now, if 2 feeders share a neutral per 215.4(A), does that neutral need to be upsized to accommodate the sum of the feeders or some sensible fraction thereof? – Harper - Reinstate Monica May 12 at 20:00
  • @Harper-ReinstateMonica -- I'd expect a shared neutral feeder situation to use a neutral sized to handle the maximum expected imbalance from both feeders (although that sometimes can be harder to predict than it looks) – ThreePhaseEel May 12 at 20:02
  • THANK YOU EEL! I'm going to get those lugs and use 90A, and replace those breakers, so everything is all good to go. – Mike Osswald May 13 at 0:35
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Inspector cat says No No No No No... Edit: Apparently Code says otherwise.

Returning neutral in the wrong place is a firestarter

Edit: Apparently this is allowed per ThreePhaseEel's reference, but still, I'd be happier if neutral were upsized to accommodate both sets of loads, for the below reasons.

A lot of people start out noticing that ground and neutral go to the same place. And they start thinking of ground and neutral "as the same thing". Certainly not. Ground is a safety shield. Neutral is the normal current return: the other half of the loop. By all rights, neutrals should have breakers... but as a matter of economy, they don't, on the logic that neutral can't possibly carry more current than the hot. Note that this logic utterly depends on the neutrals returning current ONLY for their own partner hot.

Your plan is to have the neutral you pictured return current for this 100A subpanel, and also, another 100A subpanel. That sounds like 200A to me, on a wire rated for 115 amps (and breakerable to 125A). Whoopsadaisy!

There are other reasons, too, which relate to AC characteristics. Basically if you don't keep hots and neutral together, you're creating some sort of "circle" the power is running around. The middle of that circle becomes the core of a transformer, with large magnetic fields being thrown. The least of this will be that your panel will be warm and buzz loudly. But the vibration can also cause wire damage, shorts, or opens. And an open neutral to a subpanel creates a real mess!

Your idea of sourcing it out of the subpanel is the right idea, but I'd rather source all of it there. First we must detour to address a couple of serious codevio's.

Alien breakers!

You must use a type of breaker that is UL listed for your same type of panel. Same-manufacturer breakers are (usually) a safe bet. Certain specific competitor breakers are UL-Classified (functionally equivalent to Listed), but they are rare (and usually Cutler-Hammer/Eaton). The issue is the bus stabs: each stab is a slightly different shape for patent reasons, and that means the wrong breaker makes corner contact, instead of surface-on-surface contact, and that matters most on high current draw breakers. Like a 50A.

Look closely at the main panel breaker, lower left. Your pictures are excellent but I can't quite read it. Look closely. If they say CL250, nevermind - that's the UL-CLassified breaker that's fine. But I really doubt that. I gather it says BR250 / Type BR-Type C. That breaker can't be in this panel. Goodbye! Replace it with a Siemens/Murray (or an Eaton CL obviously). The correct breaker is $10-ish.

But we'll use that breaker as a navigation aid. So pretend it's still there :)

Now, the breaker above it, is an I-don't-know-what-that-is. It's old. It may be a Square D HOMeline, it may be a GE. Definitely not a Siemens/Murray, they've been doing the beveled ON/OFF thing for 40 years. Byebye.

Bus stab overload

Edit: Apparently the Siemens panel doesn't state any bus stab limits. That is unusual, but believable; some of their 150-200A breakers actually lay across 2 breaker widths (i.e. 2 poles and the 2 poles across from it, precluding any use of the opposite side breaker). So maybe they can accommodate 200A.

Now, this panel doesn't have any double-stuff breakers, which makes this easier. You see the metal tabs which the breakers clip onto: those are called bus stabs. Notice that breakers on both sides clip onto the same stab. Stabs have their limits. Most stabs are 125A.

On the bottom 2 stabs, you have a 50A and a 100A both sharing. That's concerning.

Mr. Snippy didn't give us much wire length to work with, but it looks like you have the slack to exchange two breakers: a) the one above the Eaton, and b) the one to the right of the Eaton. That'll put 90A on the bus stabs in question, and allow 125A on the stabs above it, since nothing is across from that breaker. That's not too bad.

I know I've just relentlessly picked apart your panel -- but I gotta say, compared to most panels I see, this panel is very well done, with detail and panache. Even the stuff I hate (runting off the wire length; using neutral bar for grounds) is only arguable, and was done with a competent hand. And I have a feeling the alien breakers did not come from the original installer.

On to our subpanel challenge.

Simplest/hardest: Feed from the subpanel.

Edit: Since you're using #2 Al, you must use 90A.

Slap a 100A 90A breaker in the existing subpanel. Land both hots, neutral and ground there. And Bob's your uncle. The issue is landing neutral and ground since I don't see thru-lugs, but I infer that you already have a plan for that.

We don't need a 100A breaker here. The only reason to use it is to give us lugs to attach to, since this panel doesn't have thru lugs.

The downside of this is that both subpanels together are limited to 100A. But we can do something about that.

Bump this subpanel.

The #2 Cu wire in use between main and old sub is actually rated for 115A. You're allowed to "round up" to the next available breaker size. So Since the busing in the existing sub allows 125A, you can change the main panel breaker to 125A just like that. Since our rearrangement, this breaker has nothing across from it, so it can pull the full 125A from the bus stabs without worry.

Well, one nitpick, I know you'll hate this, but the existing cables do not have wire markings. It was probably shucked out of cable and the markings were on the sheath. Code sees no difference between unmarked wires and vines you hacked out of your garden. At some point think about changing this stuff to #1 Cu or 1/0 Cu THHN wires, for a reason I'll get into later. Safety ground is not a big deal since the panel chassis are connected by metal conduit, and that's a valid ground path.

Now, since you're using #2 Cu Al to the new subpanel, yes, you can breaker that at 125A also you must breaker at 90A. But still, this has these 2 panels sharing 125A.

Hardest, but bestest: Bump existing subpanel to 150A

This would require changing the existing subpanel, but this buys two very nice things for us: higher capacity, and thru lugs. Also, it solves the earlier bus-stab overload problem, because we are using a breaker specifically designed to tap 150A.

Note that in Murray, a 150A feed breaker is actually a pretty obscure beast. Don't try to buy this at Home Depot; you'll leave with the wrong thing. Ask a proper Siemens dealer.

The old mains-sub feeder wires must be replaced with #1 Cu (130A; round up) or ideally 1/0 Cu (150A honest). I don't worry about copper costs for such a short wire run, and copper does save space in the conduit. But make sure to use the antioxidant goop on the lugs!

For the new panel, we specify at least a 30-space for space reasons, 200A internal busing, and also thru lugs - which will make extending neutral and ground a cinch.

To feed to the new sub, for #2 Cu Al wire, you use a 100A 90A breaker. (it's good for 115A 90A, round up to the next available size, 125A 90A).

In principle, you can take this main-first sub feeder even further, and bump to 2/0Cu and run a 175A breaker. Or 3/0 Cu and a 200A breaker or simply subfeed lugs.

Better still: bump the new feeder wire; use the thru-lugs for hots

In this scenario, we do the above 150A scenario with a thru-lug panel, but also bump your onward wire 1 wire size so it too can be breakered 150A. As a result, it doesn't need a breaker in the "old" sub. The hots can be taken right off the "thru lugs". Easy as that! I love thru-lugs.

And all it takes is a wire size bump from #2 Cu Al to 2/0 Al.

The larger wire size requires 150A breaker protection. It has it -- back in the main panel. That's perfectly fine. When daisy-chaining subs, there's no statutory requirement for a breaker in every sub.

Cable in conduit

You mentioned you planned to run conduit to the other end of the house. If that conduit will be continuous all the way, then dump the cable - it's a bugbear of a pull. Use individual THHN wires. They will pull 10 times easier than a cable, and that will make it much easier to DIY.

If your conduit is non-flexible metal (e.g. EMT), the conduit itself is the ground path. No separate ground wire is necessary.

Somebody filled your neutral bars with grounds!

Much as I love the workmanship in these 2 panels, I do not agree with the strategy of breaking the neutral bars in half and using one bar for ground. Accessory ground bars are like 7 bucks, good grief! This panel foresaw GFCI and AFCI breakers. The neutral bars are there specifically so you can land the neutral right where the hot is landed. That will make conversion to GFCI very easy. You're not in a neutral crunch (they gave you 2 neutral lugs per breaker space, in anticipation of "double-stuff" breakers), but if you ever are, look at accessory ground bars and convert the left neutral bar back to neutral bars.

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  • Just curious, shouldn't his neutral buss in the main be connected to the panel? The other buss used for grounds is. – JACK May 12 at 12:21
  • Harper, this was 20 years ago, so I'm not messing with anything in these two panels. I have room in the main panel to add the #2 neutral lug. I also have room on the ground bar in the main to add a lug. But as I simply observe that the current subpanel has a ton of space to easily connect these lugs to those bars (and those bars are connected to the main with the same size wires), my primary question was simply if I can use those, or if i have to put them on the main bars. – Mike Osswald May 12 at 15:09
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    Prior to the 99 code change isolation was not required and this looks like a nice job from that date. But the ground and neutral need to come from the panel it is fed from. – Ed Beal May 12 at 15:39
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    @MikeOsswald OK, I see what you're saying. Harper can be slow on the uptake sometimes, but when I do get it, I get it. Yes, I see where my suggestions seem trivial and unnecessary. I will amend to talk about the critical ones. – Harper - Reinstate Monica May 12 at 16:14
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    @Harper-ReinstateMonica -- also, the OP could do what they describe legally (NEC 215.4), but there just isn't that much point given that there is enough room for an add-a-lug in the proper place. If their neutral bar was full to the brim, I'd consider it, though – ThreePhaseEel May 12 at 19:18

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