I have an existing AC unit that hooks up to a 3 wire 240V 30amp disconnect box serviced by a 30 amp two pole breaker. I am adding a small 120V two wire mini split AC right next to it and I would like to use the existing 240v circuit. The mini split requires a minimum 15 amp circuit with a 20amp fuse.

Would it be possible to add a second disconnect box and daisy chain one of the live 120V wires and the neutral from the existing box over to the new box?

I will not use the existing AC and the mini split at the same time ever, but I do not want to replace the existing 30amp disconnect because I do run the existing AC once or twice a year for maintenance purposes. I would disconnect the existing AC the majority of the time.

Is this feasible? Is there an easier/better way?

enter image description here


3 Answers 3


One proper way to do this would be to install a small subpanel, with the 30A and 20A circuits in it. You could install other circuits (e.g., additional 15A or 20A circuits for lighting, receptacles, tools, etc.) in the same subpanel.

  • Yeah, a subpanel would make it easier for them to clean up the neutral/ground conflation that's happening in this existing disconnect May 20, 2021 at 0:50
  • 1
    So this is what I decided to do. Went with a 100 amp sub panel but only plan on using about 60 amps. I decided to get a 1-1-1-3 Aluminum SER cable so I could use the full 100 amps in case I wanted to upgrade later. My question now is how to run the the SER cable. The current cable is going from the subpanel in the garage up the wall and through the attic, then down through the exterior wall then through to outside. My question now is if the existing wires in the picture are currently going through a conduit in the wall and if not does the SER cable have to be in a conduit in the wall?
    – Rixter
    May 26, 2021 at 4:11
  • There are a bunch of variables involved. But in a nutshell, generally speaking, individual wires always need to be in conduit but cables (like your SER) only need to be in conduit when that is needed for physical protection. That protection can often be provided in other ways - e.g., by running the cable above a certain height or using nail plates, etc. Running cable through conduit is harder to do than running individual wires. Based on a quick look (but the pros know much better than I do), I think you need at least 1-1/2" conduit for 1-1-1-3. There may other specific issues with SER. May 26, 2021 at 5:24

It looks like your wires are much larger than 10 awg and you have red white and black as your feeder. If that is metal conduit you have your ground.

If plastic conduit pull a new ground and go with a sub panel.

I would upsize the breaker feeding the sub to whatever that wire size is 40,50 amp double pole. Your new outside panel needs to be rated 3R (for outside use) and you will want a minimum of 4 full sized slots 2 the old AC one for the new Mini split and 1 for a convenience outlet (required for the AC’s).

Now the trick that makes this panel legal as a disconnect!
Find some breaker lockout devices that attach to your circuit breakers. These run about 8$ +- depending on brand. If the breaker can be locked out it is a legal disconnect saving both $$ and space on the wall.

As normal we suggest always getting a bigger panel than you need a 6 or 8 slot won’t be much different price and will provide the ability to add other circuits in the future.

  • This looks to be a pullout-type disconnect May 20, 2021 at 0:50

You might be able to tap the line side to feed a fused or breaker disconnect if the combined "minimum circuit ampacity" from the labels does not exceed the 310.16 rating of the wire feeding the existing disconnect. (Air conditioning is not subject to the 240.4(D) limitation noted in the chart footnotes.) I suspect you might have some headroom because the load size wires look smaller than the line side and the MCA is often less than 2/3 of the Maximum Overcurrent Protection allowed.

It also looks like the disconnect is fed by PVC, and you are using a white as a ground. You need to pull in an additional green or bare ground wire, and repurpose the white for it's proper use as a neutral. This needs to be done anyway, a white cannot be re-identified and used as a ground.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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