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We've recently removed a hot tub that was originally installed in 1983. I'd like to know if it would be possible to convert the disconnect for the hot tub to a sub panel.
The breaker in the main panel feeding the disconnect is a 70A 2 pole Gould. The wire from the breaker to the disconnect is XHHW 4 CDRS 6 AL (2 hot, 1 neutral, 1 ground and all #6 aluminum), and approximately 50' in length.
I'd like to install a Square D sub panel to service at least 2 15A and 1 20A circuit, however, I'm struggling to find a panel that will work. I think need to replace the current breaker with a smaller 50A breaker based on NEC wire guidelines. Is there a sub panel solution that will allow the use of the existing wire and accommodate the load requirements?

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  • The feed breaker must be equal to or smaller than the sub panel rating and protect the wires. A 40 amp breaker for a 200 amp sub panel is okay. Today it is two 15 and one 20 amp, who knows what tomorrow brings.
    – crip659
    Commented May 6 at 23:52

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Breaker and Wire Size

According to my favorite ampacity chart, 4 AWG aluminum XHHW is good for up to 65A. In theory that would use a 65A breaker. However, since 65A breakers are not a readily available item, a 70A breaker is permitted as long as you promise not to use more than 65A. In other words: breaker/wire combination is legal.

In ye olden times, that wasn't much of an issue (who needs 70A for a hot tub subpanel), and your service/main panel Load Calculation may not allow you that much power anyway. But if it does, you can't use more than 65A (with appropriate derates, etc.) The key factor today is EV charging. Plenty of people want to add as much as possible without considering the ramifications. In this case, for example, someone might want to use 20A x 2 circuits (so 20A @ 240V) for receptacles and then 50A (derated to 40A) for EV charging - but the maximum you would actually be able to provision is 45A (65A - 20A), and since EVSE normally doesn't provide 45A (36A actual) as an option, that would mean limiting to 40A (32A actual). That may not even be a consideration here, but is important to note as similar situations crop up all the time. Fortunately, 40A (32A actual) is more than most people actually need, at least for a single EV.

Subpanel Type

You can use any size subpanel, as long as it is able to handle at least 65A (the nominal feed wire/circuit size) and is outdoor-rated. Even the smallest of subpanels these days has at least a 100A rating, often 125A or even 200A, as physically small (only a few breakers) panels are often used to distribute power to a few large circuits (e.g., shed, workshop, well, EVSE).

But you may also need a disconnect! I wouldn't reuse the existing disconnect. Instead, either use a subpanel with a backfed main breaker or get a "main" panel. The differences between a "main" panel and a "sub" panel are typically:

  • Ground bar (always included in a subpanel, not necessarily included in a main panel)
  • Neutral/Ground Bond (in theory always installed in a main panel, in theory never installed in a subpanel, in reality you always check and make sure it is correct for your use)
  • Main breaker (usually included in a main panel, not usually included in a subpanel - but if not included can always be added or use a backfed breaker (bolted down) as a "main" breaker)

A disconnect of some sort is generally needed if the subpanel is not physically in/on the same building as the main panel. But it is a good idea in general. However, I think Rule of Six may take care of it anyway, as long as you have only 6 or fewer breaker throws. But a main breaker (regular or backfed) is still nice to have, in my opinion, provided the cost is relatively low. A disconnect is required for a hot tub, with or without a subpanel, which is why you had one originally.

So a medium size (e.g., 12 space, not "6 space/12 circuit") main panel would work very well, providing room for future expansion. The included 100A or 125A main breaker works as a disconnect, and the fact that it is larger than the 70A (nominal 65A) feed breaker is absolutely irrelevant.

But if you really want to, you could install a 6 space minimal subpanel, put in a 60A backfed breaker as the disconnect and have 4 spaces - 3 used, 1 spare. But that's really penny-wise, pound foolish as you won't even have enough room to add a 240V circuit.

Since you mentioned Square D, keep in mind they have two lines - higher-end QO and lower-end Homeline. There is currently at least one Square D Homeline main panel with a 125A main breaker, ground and neutral bars, 3 20A single-pole breakers and 2 30A double-pole breakers for ~ $135 at Home Depot. Prices change all the time, but that should make it clear that "main panel" is not the same as "expensive".

GFCI

It varies a bit by jurisdiction, but all or nearly all circuits will need GFCI protection - the details depend on what version of NEC your area is using and possibly local exceptions.

For 15A or 20A 120V receptacle circuits, you can install a GFCI/receptacle as the first device on the circuit. For any 240V circuits you will need to use a GFCI/breaker.

Subpanel Working Space

This may be the biggest problem! A disconnect, a receptacle, a junction box, etc. can be almost anywhere. But a panel (main or sub) must have working space:

  • 30" wide, including the panel, but the panel does not have to be centered in this space
  • 36" deep

This space can't have any permanent stuff - appliances, plants, plumbing, etc. in it. It is also not to be used for storage. Depending on the existing disconnect location, this may be a huge problem or it may not be an issue at all. But you should figure this out before doing anything. For example, if the disconnect is positioned under an overhang with a supporting pole less than 36" in front of it then it just won't work.

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    Although I think it's a good idea, I don't think the OP presented a condition that would require a disconnect/main breaker in the subpanel. Commented May 7 at 1:30

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