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I'm upgrading a pool service room.

Currently three 6-gauge copper wires run from the main service to the pool service room sub-panel. We've added another load to the sub-panel and that results in a 60 amp draw at peak so I need to upgrade to 4-gauge copper (or equivalent aluminum). The lines carry 120v. One of the three separate wires is bare copper.

Do I have to use 4 gauge for all three supplies or can I reuse the 6 gauge ground wire?

Relatedly, there's no way I'm going to be able to pull the cable through the existing conduit. Can I run two new wires in new conduit and continue to use the existing copper ground as is?

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    You've only got 3 wires and one of them is neutral not ground? I've presumed that these are 240v pumps, but it seems that they're 120v instead, please edit to clarify and, even better, add pics of both pump's info plates. Additionally, is the wire feeding a small breaker panel or is it directly feeding the pumps?
    – FreeMan
    Dec 21, 2023 at 15:50
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    @AdamO 60A subpanel. Almost certainly that's 240V. Dec 22, 2023 at 5:22
  • Excellent edits! I voted to reopen. Only one question: Just to be sure, are you only supplying this subpanel with 120v power or is it supplied with 240v even though all the loads are 120v? Again, assuming a 3-wire feed, I presume the supply is only 120v, but want to make sure.
    – FreeMan
    Dec 22, 2023 at 14:27
  • 120v @freeman thanks
    – Ram
    Dec 22, 2023 at 14:32
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    You don't (usually) pull cable through conduit. You pull individual wires. So we need more information. Really helpful would be: Picture of the main panel showing the breaker(s) for the subpanel feed. Picture of the subpanel with the front cover off so we can see the breakers and the wires. Picture of the conduit so we can see what type it is. Size of conduit if you can measure it. Dec 22, 2023 at 15:35

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There are two separate issues here: Neutral and ground.

You actually asked about neutral, so we start with that. And the answer is that at typical residential sizes, neutral must be the same size as the hot wires. Yes, in a good balanced system (lots of 240V loads and/or 120V loads evenly balanced), neutral doesn't carry nearly as much current as the hot wires. But even at full residential service feeder sizes, the neutral is normally required to be the same size as the hot wires. That's why you commonly see cables such as aluminum 2-2-2-4 - that's hot, hot, neutral all 2 AWG and ground 4 AWG.

The second part is ground. You didn't mention ground at all. Since you are talking about replacing 2 out of 3 wires, maybe your wires are individual wires inside metal conduit and the conduit is the ground. If that's true, great. If not, even if your original installation pre-dated subpanel feeder separate ground wire requirements, you need to put one in now. The ground can be smaller than the hots and neutrals. See this post for details.

But the real question is cost. The way to save on subpanel feeder cost is not to try and cheat the system on neutral. It is to use aluminum instead of copper. Almost all jurisdictions allow feeders to be aluminum, although some have certain minimums - basically to avoid using aluminum for branch circuit wiring. But for 60A and larger subpanels, aluminum is the gold standard. The cost varies by vendor and size, but at typical feeder sizes, aluminum will probably be half the cost, or less, of copper, even after accounting for the larger size required for aluminum (e.g., 4 AWG copper vs. 2 AWG aluminum).

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  • IIRC, pool wiring is one case that requires a green insulated ground wire, even for metallic conduit. Not bare, and not use the conduit as ground. It's a special case due to the intersection of corrosion potential and life safety.
    – Ecnerwal
    Dec 23, 2023 at 21:37
  • @Ecnerwal Is that for wiring to the pool equipment itself? Or is it also for the subpanel feed, which is the main topic here. Dec 23, 2023 at 22:57

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