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In my 200A ITE main panel there's a feed to a 100-amp sub-panel that’s elsewhere in the basement, via a 100-amp breaker at the bottom of the main panel. The photo shows the wire (#1 AL, I think) and the connection to the breaker. As you can see, the neutral for the subpanel feed -- circled in red -- is not connected to the neutral bar in the main panel but to a lug on the enclosure. Seems to me this is a poor idea because the neutral current from any unbalanced load in the subpanel is now being carried by the enclosure. It's been working this way since before we moved in 20 years ago but I only noticed it this year.

My thought to address this is to move the subpanel neutral to a lug attached to the neutral bus. I do have the right lug (Siemens brand) and it will accommodate the wire. I have to make sure the lug fits in the physical space between the neutral bus and the panel cover, and that I can bend the wire up there, but I'm pretty sure it will work. Does this sound like the right approach? And is it worth the trouble, or is the whole issue of neutral current flowing in the enclosure not as big a deal as it seems to me?

Related question, as you can see this subpanel feed is at the very bottom of the main panel, which is fairly large (14 spaces on each side). Should it be higher up so it doesn't load up as much of the bus? Or is that not important?

Thanks!

(Edited to add photo of 100A feed entering subpanel)

100A feed to subpanel

100A feed where it enters subpanel

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2 Answers 2

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You have a couple of questions here. Let's start with the most important one:

  1. It is completely unacceptable to use the panel itself as an intended conductor for neutral current. Hate to say it but that's a real hack job. Your idea to move it to the neutral bus bar is perfect. You said you have the needed lug and as long as you have space on the bus bar, you're good to go. I always say this, but when working with AL wire, don't forget about the goop! (NoAlox), it's important to prevent corrosion later.

  2. Regarding moving the feed breaker for the sub-panel closer to the main breaker, not really that important. It's best practice to have high current breakers close to the main, but not critical. What does the sub-panel run? A shop? A/C?

  3. One more important thought: Assuming that wire connection circled in red is indeed the neutral for the sub (see my question below). But assuming it is, You probably have a 3 wire feed to the sub which means no way to properly isolate the ground from the neutral, which is required in sub-panels.

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  • Thanks George. Exactly what I thought. I will move the wire and be sure to use the goop. I am not used to dealing with wire this size so bending it and fishing it up to the bar will be the interesting part, but there is plenty of length. That will be done with the main off. The sub runs my shop (60A sub there, but draw is very small most of the time), heat pump (35A circuit), range (40A), and 20+ lighting and receptacle circuits. It's a lot -- probably should get a clamp ammeter and see the actual load sometime.
    – trawson
    Commented Sep 4, 2022 at 17:02
  • Definitely not the only hack job in this house. Spent a lot of time and $$$ a few years ago dealing with some old square recessed lights, wired through several hidden junction boxes, some of which had hot / neutral swaps. Net result was that on most of the lights the outer ring with the threads for the bulb was hot even if the lights were switched off, so you could easily get hit with 120V from a small slip when changing a bulb. Fortunately caught by an electrician with good detective skills who was working on something else, not by me the hard way!
    – trawson
    Commented Sep 4, 2022 at 17:10
  • Thinking more about this: Where is the big white wire (cable?) going? Could that have been the neutral for the sub and the wire connected to the casing of the panel itself be the ground for the sub? Next: I'm not quite following all of this, you originally said 100 amp sub in the basement, is there yet another sub panel for your shop (you said 60 amps)?
    – George Anderson
    Commented Sep 4, 2022 at 17:29
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    Sorry for the confusion on the subs. There's a 200A main, 100A sub, in different parts of the basement about 30' apart. Then there's a 60A sub out in the (detached) shop that runs off the 100A panel. The big white wire is unrelated -- part of a 6/3 feed for an old 50A basement cooktop circuit, for some reason they looped it around to the left side neutral bar instead of keeping it on the right where it comes in. The 100A sub is wired through EMT and afaik that provides the ground. I can get a better picture showing the wires to the sub as they enter the box if you want.
    – trawson
    Commented Sep 4, 2022 at 18:00
  • Re your #3, in addition to the EMT serving as the ground, I know the ground and neutral connections are on separate bus bars in the 100A sub, and the ground bar is mounted to the enclosure. I'm not quite sure if there's an easy way to verify that the neutral bar is not bonded to the enclosure, just because it's hard to see with all the wires, but I can take a look. I know for sure it is all correct in the 60A sub because I installed it.
    – trawson
    Commented Sep 4, 2022 at 18:18
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The neutral goes on the neutral bus

Tying it like that is worthy of a ground. So it needs to move to the neutral bar. They make splitter lugs designed to land 1 fat wire on 2 neutral screws.

A ground can be added, in which case that lug would become useful as that is a legitimate way to attach a ground ... IF... the screw is not a panel mounting screw, but in fact is a screw dedicated to holding the ground lug, and with a thread pitch of 8-32 or 10-32 NF. (-32 or finer, and #8 or larger).

If that's #2 aluminum, that's only 90A actually.

I can't quite tell, but #2 is a very common size of wire. If that wire is indeed #2, then the supply-end breaker needs to be 90A. This panel is not rated for 90C thermal, which means you must use the 75C thermal rating of the wire.

Some people grab for 310.15(B)(7) as justification for #2@100A, but they need to actually read the accompanying rule.

If you want honest 100A and are replacing the cable, you can use #1 aluminum. Nothing wrong with aluminum at these large sizes; copper is no upgrade not least because it's going onto aluminum lugs!

Respect stab limits

Related question, as you can see this subpanel feed is at the very bottom of the main panel, which is fairly large (14 spaces on each side). Should it be higher up so it doesn't load up as much of the bus? Or is that not important?

No, the bus bars are enormous and are not burdened by carrying the amps a few more inches. In fact, one of the stupidest "stupid pet tricks" people do with panels is bunch all the largest breakers at the top. Because of an actual rule called "Stab limits".

You notice that the breaker shares a bus stab with the breaker across from it. Those stabs have limits. They should be indicated on the panel labeling, but 125A is typical. So if you follow the "bunch toward top" logic, you end up with the 100A across from the 60A. And the stab is carrying what? Whoops!

It really doesn't matter that much (except for solar), but if you need a "rule to blindly follow", it'd be "Large breakers down one side largest to smallest, small breakers down the other side smallest to largest*, but offset toward the top". That would guarantee 15's across from your 100A, giving 115A stab load. However a more rational balancing would think about mandatory locations of things like solar, surge suppressors, generator interlocks and other stuff I can't think of now.

  • and for this purpose tandems count as the sum of their throws, e.g. a 20/20 tandem is a 40.
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  • Harper, I hardly ever disagree with you, but in this case, you may have missed a couple of things: 1) We don't know if there is a true neutral from the sub to the main panel. The fact that it looks like there's some dirty white tape around the wire in question suggests that it is indeed a neutral, which of course would be really bad. 2) Regarding stab limits, I would think there is enough of a safety margin regarding exactly where breakers are placed. Also, given the fact that probably all the higher power breakers are oversized for the anticipated load, makes it even less of an issue.
    – George Anderson
    Commented Sep 4, 2022 at 22:06
  • There are 3 wires from the main to the sub, all the same size. When I try to photograph the terminations on the breaker and the lug attached to the enclosure the breaker wires look larger because they're closer to the camera. I can't find a way to safely get a camera angle that is equidistant from all 3 and has nothing in the way without turning off the main, and too many family members will object to that at the moment :). However, I do have a photo of those wires entering the subpanel and you can see that they're all the same size and one is a neutral. I'll add it to the post in a moment.
    – trawson
    Commented Sep 4, 2022 at 23:54
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    @GeorgeAnderson the rule of thumb you use doesn't really matter, but the stab limits rules are in black and white. On the neutral, I am now educated. I should have been paying more attention to the photo of the subpanel.
    – Harper - Reinstate Monica
    Commented Sep 5, 2022 at 18:02
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    @Harper-ReinstateMonica Thanks for the resp. I'd never heard of "stab limits" before and I've even had electricians say you should put the heavier loads nearest to the main breaker. Now I'm educated (as you said) but will research it on the NEC to make sure I fully understand it. Next: The OP said in a comment that there were a lot of other electrical problems in the house he'd spent many $$$ fixing, so it wouldn't surprise me if some hack guy just used the panel itself as a neutral conductor. Terrible idea of course. But as they say, "just because it works doesn't mean it's safe!"
    – George Anderson
    Commented Sep 5, 2022 at 19:08
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    @George well now that I understand the neutral is the same size, it has 2 defects: attached in the wrong place and not compliant with post-2008 requirements for separate ground wire. I suspect the work pre-dates 2008 and is grandfathered on the latter.
    – Harper - Reinstate Monica
    Commented Sep 5, 2022 at 19:09

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