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200 amp service.

We have a service disconnect upstream of our main panel, which effectively means our neutrals and grounds are still isolated in our main panel (as one does when setting up a sub panel).

As such our Siemens panel has two neutral bars connected with a “neutral tie strap” (insulated copper bar that connects both neutral busbars.

Question is: when setting up a second “daisy chained” subpanel (125amp), where should that neutral (2awg) connect on the main panel’s neutral busbars?

Is that copper “neutral tie strap” rated to handle a situation where excessive current on a left neutral bar travels through it to the mains neutral on the right busbar (and back upstream to the service disconnect)?

Image for reference. Our panel has the green bonding screw removed. siemens load center with neutral tie strap

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    It's presumably good to the rating of the panel, even if you'd only expect it to see half the rating of the panel on it. OTOH, putting the neutral to the next panel near the neutral from the disconnect would minimize any issues of bus or strap resistance. Presumably "excessive neutral current" would be followed shortly by the 125A feed breaker tripping, or are you doing unfused feed-through (one of those things I'm not versed in and don't care for philosophically which leads to me not becoming versed in it.)
    – Ecnerwal
    Feb 19, 2021 at 16:56
  • Subpanel is behind a 125a breaker so it would possibly trip, although realistically the subpanel is set up to charge two cars (someday) at 240v. So the neutral really wouldn’t even be used in that scenario, and the remaining slots @120v aren’t likely to come anywhere near tripping the breaker. Really glad to hear that the strap should be rated for the whole panel.
    – drew covi
    Feb 19, 2021 at 17:33
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    When you're making the panel you should design it so the other guy cannot screw up. The strap should be able to handle 200 Amps. When you're using the panel you should design it so the other guy cannot screw up. Don't rely on the strap to handle 125 Amps when all you have to do is connect here instead of there. I think it's a great question where the "correct" answer ("200" is the correct answer) is not the one you should choose.
    – jay613
    Feb 19, 2021 at 17:34
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    Typically someplace the panel cover will say to use an adapter such as an ECLK2. If there are restrictions where to mount that info should be on the cover also.
    – NoSparksPlease
    Feb 19, 2021 at 17:44
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    If no restrictions are on cover then it shouldn't matter, but every connection is a potential failure point, so I would attach to the same side as feed.
    – NoSparksPlease
    Feb 19, 2021 at 18:17

1 Answer 1

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The bonding strap will be fine for as long as the breaker on the live side feeding the current needs to trip (unless otherwise specified in the documentation of the panel).

To connect higher gauge wires that are too large for the neutral bar holes you can use a neutral lug which is a little metal block that screws into one of the neutral bar screw holes and provides a place to land the larger wire. And yes it's better to put that block on the same bar that the neutral lands on.

Also when installing a subpanel always go for more spaces than you think you will need. The small premium you are paying now will be much less than the cost of yet another subpanel in the future. As long as the wires feeding the subpanel are appropriately fused the rating of the main breaker in the subpanel doesn't matter.

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  • Right now the subpanel neutral is tied in on the top of the left (via lug as you mentioned). So current would run through the strap in the (unusual) circumstance that I add excessive 120v circuits. Right now I’ve gone with 125 amps to support two cars with overhead. But since that would be 240 + ground, neutral would be largely unaffected.
    – drew covi
    Feb 19, 2021 at 17:38

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