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Helping my dad with a walk-in tub installation and need dedicated 20amp line. In effort to avoid cutting up a major part of the basement ceiling to access main house panel, I'd like to use an abandoned line of 10-2 plus ground sending 240 from a dual 30amp breaker in main house panel to where an old furnace used to be. Hoping I can connect a subpanel even though the wire is 10-2. Is a "jumper" OK since I only have one HOT line coming in? Are two separate HOT lines absolutely necessary?

The end result will be a subpanel, one circuit feeding the 20amp tub and a second 20-30amp circuit feeding a small wall heater. Nothing else will be powered by this subpanel. Can abandon the wall heater if needed, just need to get 240 down to 120 for tub, so assumed a subpanel would be best option.

Thank you...

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  • If you only need 120v now, can probably remove the white(second hot) from the breaker, and place it on the neutral bus. If you need 240v and 120v will need a three wire plus ground cable. Two hots and a neutral. Edit the voltage and amps needed for the tub and heater to your question.
    – crip659
    Commented Dec 28, 2022 at 18:20
  • Thank you for the reply. This is the route I will be taking. Took off the panel last night, saw the white wire that needs to be moved. Appears there's enough slack. Never messed with breaker boxes before but have a basic understanding now. I should remove the 30amp double-pole and replace with a single 20amp, correct? Since my 12-2 wire is rated for no higher than 20amp.
    – Kevin in WA
    Commented Dec 29, 2022 at 18:17
  • Would turn off the main breaker before touching breakers/wires to replace/move.
    – crip659
    Commented Dec 29, 2022 at 19:02
  • Got it! Didn't expect to see the neutral and ground blocks connected, at least that's how it appears. Read that it's OK since it's the main service panel. Just FYI...It's an older home, built in 1972. Murray panel.
    – Kevin in WA
    Commented Dec 29, 2022 at 20:09

2 Answers 2

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If you just want 120 volts 20 amps for the tub, you don’t need a sub panel. Just replace the 30 amp dual breaker with a 20 amp single in your main panel. I don’t know if a GFCI breaker is required but I’d recommend it for safety. Add a blank front or an unused single breaker to block the opening.

If you need to extend the cable, you can use 12 gauge wire. Having 10 gauge earlier in the run doesn’t matter. (Although I would label the cable in the breaker box so a future homeowner doesn’t try to put in a larger breaker.)

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  • Well...after the old furnace was replaced, the 10-2 line that remained was used for a small 240v basement wall heater. Was hoping to be able to keep it wire where it is. But willing to sacrifice it. Tub is priority. :-)
    – Kevin in WA
    Commented Dec 28, 2022 at 18:45
  • Will be wiring a GFCI receptacle at the tub location, so that should be good enough? It's all that was mentioned in the installation booklet.
    – Kevin in WA
    Commented Dec 28, 2022 at 19:01
  • @KevininWA If the tub is plug-in, the GFCI outlet instead of the breaker is just fine.
    – DoxyLover
    Commented Dec 28, 2022 at 21:04
  • So splicing (wire nut) 10 gauge to 12 gauge is OK? I already have 12-2 in place from tub area to basement where the 10-2 line terminates.
    – Kevin in WA
    Commented Dec 29, 2022 at 0:17
  • @KevininWA Yes, that's fine. The same size as for two #12 (normally red) should be fine. As always, the splice must be in a junction box with a cover but otherwise accessible.
    – DoxyLover
    Commented Dec 29, 2022 at 1:13
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The 10 AWG wire is not a problem - you can use that any place 12 AWG wire is needed with no concerns. (Well, you couldn't jab 10 AWG wire into the backstabs on a receptacle, but you should use the screws even with 12 AWG, so not an issue.)

However, 2 wires is a real issue. With 2 wires (+ ground, but ground must always and only be ground), you can either get 240V with no neutral, or 120V with a neutral. You can't get both. If you wire up a subpanel using the black wire to both hots and white to neutral then you'll get lots of 120V circuits but you won't get any 240V circuits. If you wire it up with two hots and no neutral then you'll get lots of 240V (double breaker) circuits but no 120V circuits.

Yes, it is possible to work around this with transformers, but that gets really sketchy, in my opinion, for inside a house.

So either run two new cables (12/2 each - one for the 240V circuit and one for the 120V circuit) or run a 10 AWG or larger /3 cable to a subpanel and run individual circuits from there.

Remember that a subpanel needs 30" wide by 36" deep working space kept empty in front of it. Easy in some places. Nearly impossible in others.

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  • Thank you for the reply. I feel best option then is to wire the subpanel with a jumper, and forego the 240. The tub is priority and running a new 3-wire line to main house panel would be a major undertaking. Just getting the tub into his 1970-built home was a bigger chore than planned. Will seek other options for the rarely used 240v wall heater.
    – Kevin in WA
    Commented Dec 28, 2022 at 23:03
  • I was assuming (hate to use that word)...that even with a jumper, I could install a two-pole circuit in the subpanel and get 240v that way..... ???? not possible? Fire hazard?
    – Kevin in WA
    Commented Dec 28, 2022 at 23:05
  • Sorry, doesn't work that way. Think of it as if it were DC (AC is not, it is different in every way, but for illustration): Hots= -120, +120, neutral = 0. So if you use one hot, wire it to both hot lugs on the subpanel, plus neutral - you get either -120/0/-120 or +120/0/+120 - so if you put a breaker across the two hots on the subpanel, the difference between them is 0 and you get nothing at all.
    – manassehkatz-Moving 2 Codidact
    Commented Dec 28, 2022 at 23:10
  • If you're going to only end up wiring the one 120V circuit, skip the subpanel and just just an ordinary junction box and extend with 12 AWG. The subpanel does make sense if you'll end up pulling a 2nd 120V circuit out of it - which realistically you can do as long as you don't expect usage on the two 120V circuits together to go over 30A (or over 24A continuous).
    – manassehkatz-Moving 2 Codidact
    Commented Dec 28, 2022 at 23:12
  • But the 10-2 line coming to the location, from the main house box, is 240 (dual-pole 30amp circuit). How can I get that down to 120? Remove the dual-pole breaker from main box and replace with a single as another on here mentioned?
    – Kevin in WA
    Commented Dec 28, 2022 at 23:40

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