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I am upgrading a sub-panel from 60A to 90A. The original feeders were #8, which I will replace with #4 copper. Can I leave the #8 neutral and ground that bond the main and sub-panel in place, or should I also upgrade the neutral to #4? Other posts on this forum seem to indicate that the neutral wire size can be smaller than the feeders. The sub-panel circuits are mostly 120V single-phase, but it also handles the 240V AC circuit (which is why it kept getting overloaded). Is there any overload risk on that neutral bond?

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    How far from the main panel to the subpanel? If more than a few feet then 2 awg aluminum will save a lot of money. Aug 19, 2022 at 22:49
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    Quite sure(100%) that neutral needs to be same size as feeders or on any circuit. Think ground can be a size(maybe two) smaller(but want expert to confirm).
    – crip659
    Aug 19, 2022 at 22:54
  • Listed wire gauge is the minimum size allowed, larger gauges are can be used.
    – crip659
    Aug 19, 2022 at 23:41
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    #8 is only 40-50A. Any reason you want copper? It's very costly and only 85A wire (70A if NM). Nothing wrong with aluminum at these large sizes. #2 is an extremely common size and is honest 90A. Aug 19, 2022 at 23:51
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    Is the 240V load an electric vehicle charger? If so, we gotta talk as there are other options. Aug 20, 2022 at 0:18

2 Answers 2

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No, the neutral must be the same size as the hots. The only exception I know of is the main feed where you can have 4/0 hots and 2/0 neutral from the meter or full disconnect.

In your case, at 90 amps, you are between 8 ga and 6 ga if using AL according to section 250.122 of the NEC for the Equipment Grounding Conductor (EGC). So you should go with 6 ga AL for the EGC.

I agree with others here, AL is way more cost effective than copper right now. The only reason I'd go with copper is if it's a very short run because copper is easier to work with. I did that when I wired my son's house. It went from the meter to the gen xfer switch to the main panel, just a few feet. BTW, the inspector called out that it was 3/0 not 4/0, I had to point out that it was copper and rated for 200 amps. The copper is easier to wrestle with than AL.

GEC wire sizing

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  • No middle point, it's 8 ga copper or 6 ga. Aluminum. Can be bare if copper. Aug 20, 2022 at 0:31
  • @Harper-ReinstateMonica well, they are in the middle of the range, but I did call out 6 ga AL I think pretty clearly. Aug 20, 2022 at 0:34
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    oh yes, I see. Sorry. Aug 20, 2022 at 0:38
  • Like you, George, I used copper because its a very short run, and I already have enough for the feeders. I'll just have to buy a few feet for the neutral. The connecting conduit is 1", so 3 #4 should fit. Thank you for your reply.
    – AKG
    Aug 20, 2022 at 23:40
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Neutral can't be downsized on residential feeders, no. In commercial you can do if you can show the engineering data showing higher neutral current is not possible. However in your case the panel is entirely 120V loads except for one 240V load, and it's tripping a 60A breaker which is already too big for the wire. So absolutely not.

It's even possible the subpanel is poorly balanced, i.e. have too many many 120V loads stacked onto one pole/phase and not enough on the other. And if that's true then the neutral is working quite hard.

#4 copper is only good to 85A.
For 90A you need #3 copper, which is also good to 100A.
#8 copper is only 40A in NM or UF, and 50A in other wire types. So you have been overloading that already.

It sounds like you have a conduit. The size of the conduit is critical because of conduit fill limits. I gather that's the nature of your problem? 4x#8 requires a 3/4” conduit, and maybe they were cheap and installed exactly that. Now you're trying to fit a larger feeder in there. I see where 2x#4 and 1x#8 just fits in schedule 40 PVC. So should I assume that's why you're asking?

Assuming that's your issue... for 60-100A your ground wire size can be #8 copper and should be bare to maximize conduit fill.

The neutral needs to match the hots, so 3x#4 just won't fit. Best you can do is 3x#6, which is 65A wire. Since you're tripping a 60A breaker already, I take it 65A isn't going to cut the mustard, since you're supposed to design with 20% headroom.

You can try rebalancing your 120V loads or at least thinking about how they are phased. I bet you have a bunch of high-draw 120V loads stacked on the same phase.

Other than that, you are stuck with new larger conduit, but that will let you go #2 aluminum as everyone is advising. #6 aluminum for the ground.

Honestly, the aluminum wire plus all new conduit will be cheaper than the copper.

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  • So, digesting all the good advice - I am going to save my 15 ft. of #4 for another day, and use some #2 I have in my stash to handle the 90A, which will mean upgrading the conduit and L-box to 1-1/4. I am not going to fit (2) #2 feeders + the #2 neutral through the 1" conduit. Yes, copper is more expensive, but I have some and it makes the job easier, at least for me. Once upgraded, I'm going to look at re-balancing the sub-panel, and maybe move a circuit or two over to it for when its on the generator. Thanks again to everyone who commented.
    – AKG
    Aug 21, 2022 at 22:33

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