I recently relocated my HVAC system from back patio to side of house-can I convert the old system (#8 wire, etc.) to a 20 amp outlet for a nearby shop function?

  • Can you post photos of where the wiring for the old HVAC system ends? Mar 5, 2020 at 5:08

2 Answers 2


Short answer is yes, but... You might want to consider a small sub panel (outdoor rated) there if you have a 4 wire service to that location. That way, if your needs change (IE: grow), you'll have the power there. There are lots of posts here regarding connecting a sub-panel... esp. the importance of separating the neutral from the ground in a sub-panel.

If you don't want the sub-panel, you'll probably need a junction box of some sort to connect the 8 ga to 12 ga. If aluminum you'll need special connectors and noalox compound on the AL. In the main panel, you'll need to splice wires and downsize the breaker to 20 amps.

I think I got this right, but one of the "big three" might be able to improve on this answer.

  • I see you like the big three idea..lol
    – JACK
    Mar 5, 2020 at 3:16
  • LOL! Yeah. Good call on that! Mar 5, 2020 at 18:24

What you've got vs what you can do with it

Look at the wires in this circuit (besides ground, which is green or bare). White or light gray wires are special - they are reserved for neutral. (gray not to be confused with a faded black wire). All other colors are hot.

  • If you want 240V-only, any 2 wires will do. Any white wires will have to be marked with black tape. You're all set.

    • Every other option here will presume you want 120V or 120/240V.
  • If you have two hot wires, you are probably in conduit. You can add a white THHN wire to the conduit, then follow as below.

  • If you have one hot wire, you can provide 120V service. The white/gray will need to be moved to the neutral bar in your panel.
  • If you have two hot wires, you can provide 120/240V, or dual 120V.

Option 1: Change the breaker to 20A.

If you want any 120V, the white wire must go on the neutral bar. Then any hot wire(s) must land on the same breaker. If you have 2 hot wires it must be a 2-pole breaker whose handles are linked to throw together.

You can put a 240V receptacle (NEMA 6-20) on two hot wires. Or you can put a 120V receptacle (NEMA 5-20) on 1 hot and 1 neutral. If you have 2 hots they can share the neutral, but only if you used the correct breaker.

This will give you 20A per hot wire.

See George Anderson's advice for attaching #8 wire to a common recep.

Option 2: Lots more power.

Then leave the breaker at 40A and follow George Anderson's advice on setting up a subpanel.

You'll use the wires as I describe above. You you must keep neutral and ground absolutely separate. Fit an accessory ground bar in the panel, and remove the neutral-ground bonding screw.

Mind you it is totally ok to oversubscribe a subpanel. You can have a separate breaker for each of dust collector, table saw, router, planer, jointer, radial arm saw, lathe, and ShopBot. Just don't run too many at once.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.