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I purchased a house that had baseboard electrical heating (240 volt baseboard heaters) that were connected to multiple double pole circuit breaker inside the electrical panel. The electrical heating system was throughout the entire house and it was in good working order. I had a professional even upgrade the panel as the old panel was non-functional.

Shortly after having the panel upgraded/ changed, I converted the whole house to a natural gas baseboard system and all the heating wiring was capped off and left in the walls and the thermostats wiring was capped off with the thermostats were removed from the wall in each room.

I would like to add more 120v circuits within the house and was wondering if I can re-purpose the wiring cables for the baseboard heating to create new 120v circuits without having to running new cable back to the panel. I do have access to the cable bundles from below in several access points. Suggestions?

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    Are the old thermostat sites covered with blank junction box covers? Or did you bury the former box site in the walls? Did power visit the heat first, or just the thermostat first? How many cables at each heater location? – Harper - Reinstate Monica Oct 25 '19 at 10:41
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    Can you post photos of the breaker panel and of the box locations for the heaters and thermostats? – ThreePhaseEel Oct 25 '19 at 11:34
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MWBC

There are two ways to turn a 240V circuit into a 120V circuit:

  • Downgrade Use one of the previously-hot wires as hot and use the other previously-hot wire as neutral, or if you already have a separate (3rd) wire for neutral, use one of the hot wires plus neutral. Super easy but you get only get to use 1/2 the circuit - i.e., 1/2 the power.

  • Multi-Wire Branch Circuit An MWBC uses 3 wires - 2 hots + neutral - to provide 2 full 120V circuits. This can work provided the hots are on separate poles with common shutoff (which you already have) and you have a 3rd wire for neutral (see below). It saves a wire compared to two separate 120V circuits because the neutral only carries the difference between the current in the two hots instead of adding them together.

I'm not going to try and explain the details of MWBC. Search elsewhere in DIY SE and you can find plenty of information. But a few things to watch out for are AFCI and GFCI - now that you are redoing circuits, one or the other is likely to become a requirement and you can do that in the panel (easy) or elsewhere (gets complicated with MWBC).

Neutral?

The key question is whether you have neutral in the cables or not. 240V heaters do not necessarily need neutral. By comparison, clothes dryers and ovens typically do require neutral in order to provide 120V to controls & lights.

The problem is that neutral must be white or gray. There are certain exceptions (due to the way standard cables are produced) where a white wire can be used as a hot or switched hot but not, as far as I know, the other way around.

There are 4 possibilities:

  • Wires in conduit If you actually have wires in conduit instead of cables then this is super simple - just add a white neutral wire (if you don't already have one) and then wire up an MWBC.

  • Black/White cable You can't add a 3rd wire (you would have to replace the entire cable), so move the white wire at the breaker to the neutral bar, replace the double breaker with a single breaker (you get the other space back for a new circuit) and you have a standard 120V circuit.

  • Black/Red/White cable Black & Red on the two poles of the breaker (as originally installed), white on neutral bar and you have the perfect setup for an MWBC.

  • Some other cable If you have two non-white color wires in a cable (e.g., black/red) and no white wire then I believe you are stuck. You can't turn a colored wire into neutral and you can't add another wire without replacing the entire cable. You could use the cable to power many LED lights (they typically have a wide voltage range) but not ordinary 120V receptacles.

Ground

If your existing circuits have ground (either a ground wire or metal conduit) then you are all set. If not then you may or may not be able to reuse the existing wire without retrofitting a ground (basically connecting a ground wire from the new receptacles to some existing ground elsewhere in the building).

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    Your last paragraph actually is retrofitting a ground. One of the options for retrofitting ground is going to a nearby outlet. – Harper - Reinstate Monica Oct 25 '19 at 15:01
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    @Harper - yeah, not 100% clear - can't take the time to fix it now. My longer point would be that sometimes you can do GFCI instead of ground, sometimes you need to retrofit - and you know all the details where I need to actually look them up. Feel free to edit (or to write your own more detailed answer). – manassehkatz-Moving 2 Codidact Oct 25 '19 at 15:16
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You must make all splices inside junction boxes

So it depends how your wires are run. For instance if power came first to a thermostat site, then onward to the heater, and you want power at the heater, then those wires would need to be spliced in a junction box.

And the junction box covers must remain accessible without tools

The box can't be buried behind drywall or anything that would force you to demolish or even use tools to access it. I once dealt with a "removable" plywood panel that covered up a junction box, and half the drywall screws were cammed out.

So it depends whether you're willing to reinstall junction boxes, tolerate blank cover plates on walls, or have other places to put a junction box that can remain accessible.

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