Daisy Chain 120V Disconnect Box from existing 240V box

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.