EDIT to answer replies:

@3phaseeel 30-amp wires are 6 AWG aluminum, 50-amp wires are 4 AWG aluminum. The sheathed wires run along the garage roof truss overhead (see picture), exposed for about 28 feet in length, then in-between insulated joists and down a wall, then exposed again in the enclosed crawlspace. I added a picture of the panel door, not sure if the sticker from 1982 adds valuable details.

@manassehkatz Subpanel will be used for dedicated circuits to bathroom heaters, not for heavy-duty usage like welding.

Original question:

I have an unused electric furnace that is serviced by one 30-amp double pole breaker and two 50-amp single pole breakers. I'm thinking of:

(1) reusing the existing wires from the 30-amp circuit to service a new heat pump with a dedicated 25-amp replacement double-pole breaker

(2) reusing the wires from the two 50-amp circuit to service a 100-amp subpanel to serve future wiring projects

Does that sound like a reasonable plan? Or is this plan not as simple as it seems?


We switched to using a mini-split heat pump two years ago, which cut our electric bill in half, even when used in combination with a few space heaters. Having had good experience with the mini-split, I now want to install another unit to completely replace the space heaters and retire the unused, noisy, leaky, 38-year old furnace. I am a DIY homeowner with advanced beginner to intermediate level of know-how and tools.

furnace circuit breakers

wiring along garage overhead

panel door stickers

whole panel covered

panel uncovered

50-amp cable label

left inside panel

right inside panel

right label, inside panel

  • 1
    What size are the wires going to the 50A breakers, and what sort of cable or conduit are they run in? Also, where were you planning on putting the new subpanel, and can you post a photo of the labeling on the inside of your breaker box's door please? Nov 29 '20 at 2:32
  • When you say "100A subpanel" is that "I want to put in a 100A panel because they're nice and big, but I will only have a few circuits in and nothing really big" (in which case, assuming the old wires were to code, etc. they are fine for a 50A feed into 100A (or larger!) subpanel) or do you mean "I want to put in a subpanel that can push out 100A of current to stuff at any given time - heater and welder and car charger and tools...." Big difference. More details will help. Nov 29 '20 at 3:30
  • 1
    That is a good answer! 100A (and sometimes larger) are much more cost-effective than smaller panels because you have room to expand if you need it. So if you can stick to 50A then you're golden. But checking the limits...you may even be able to upsize the breaker a bit, but there are a bunch of factors relating to wire/cable type, connections, etc. so I'll leave that to the experts. Nov 29 '20 at 4:43
  • 1
    Replacing the 4/2 SE cable is only an option if I feel confident enough to redo/rerun the wires. Otherwise, it is not an option if I cannot overcome my effort-conserving tendencies, especially trying to avoid overthinking it. EDIT: but please let me know what you think, especially if you see issues with keeping the 4/2 SE cable. I want to do things right, or not do it myself at all.
    – Edgar
    Nov 29 '20 at 5:14
  • 1
    That piece of tape is not a legal blank cover. Just get a CH120 breaker and stick it there. Eaton makes a flimsy blank cover that's hard to find, but it costs near as much as a breaker lol. Nov 29 '20 at 5:31

Those two 50A single breakers need to be handle-tied, or replaced with a 50A 2-pole breaker. You can't have two singles on a 240V load like that.

The empty breaker space at bottom left should be filled with a proper UL-listed thing. They make blank filler plates, but I find them flimsy and expensive. I just use actual breakers, a CH120 is around $5.

(1) reusing the existing (6 AWG aluminum) wires from the 30-amp circuit to service a new heat pump with a dedicated 25-amp replacement double-pole breaker

Yeah, that plan is fine. Use "MAC Block connectors" to splice from the #6 aluminum to whatever other wires (Al or Cu) you'd continue with.

Note that since the #6 aluminum run does not have a neutral, they must be 240V-only heat pumps. Cannot use bare as a neutral.

You may find it challenging to land #6 wire on a 25A breaker.

(2) reusing the wires from the two 50-amp circuit to service a 100-amp subpanel to serve future wiring projects


OK, you see that dual 30A and you immediately get "This is a 30A 240V circuit".

But then you see those two 50A singles and for some reason you think that isn't the same exact thing. It is the same exact thing, but somebody left off the mandatory handle tie so it looks weird.

So think of it as a 50A 2-pole breaker.

The problem is, the existing #4Al cable has only 3 wires and cannot supply a 120/240V subpanel.

  • You need the bare wire for safety ground. This isn't 1963, you need a ground wire.
  • Because it's #4 or larger, you can get neutral by re-marking one of the black wires white with tape.
  • That leaves you 1 wire left to use for "hot", and that means the far panel can only be 120V.

The good news is, the SE/XHHW cable is allowed 75C temperature, so it can be provisioned to 65A and use a 70A feed breaker. There is no such thing as a 70A/1-pole breaker, so you will need to either re-use a 50A, or obtain a 70A/2-pole and use only one pole of it.

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