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rcstevensonaz 05-24-2012 12:43 AM

Run two AC systems on 30A circuit (adding mini-split)
I want to add a mini-split air condition for our master bedroom and would like to tap into same power line that is used for 2 ton roof "package" AC system. The disconnect box for the existing unit is very close to where the additional mini-split will be installed, so I'm hoping I can avoid having to pull a whole new run.

Here is the current configuration (including some details that may be irrelevant)

400A main service panel.
125A sub-panel from the main service.
30A 208V breaker that runs to AC disconnect box.
Aluminum 8 wire between subpanel breaker and disconnect box.
From disconnect box, wire runs to a 20A fuse box mounted to the existing 2 Ton AC

The new inverter mini-split requires a 15A 208V. Amp load is 13A.

My question is: can I change the existing disconnect box into a subpanel with a 20A 208V breaker feeding the old AC and a 15A 208V breaker feeding the new AC? Since the wiring is aluminum 8, I believe the breaker back as the feeding subpanel has to remain 30A.

Other question: does this new subpanel need to be physically grounded or can it pull it's ground from the existing subpanel? All of this wiring is part of the existing physical structure.

Other question: do I really need a sub-panel or can I just use some form of a junction box since there will be a fuse box at each AC and there is already the overall 30A breaker back at the subpanel?

Or, do I just need to give up and pull a separate line (which means running new exterior conduit).

I appreciate your insight. I have had several AC contractors provide quotes and all of them have proposed variations of the above with no clear consensus on what I should do (or am allowed to do).


biggles 05-24-2012 06:17 AM

seperate breakers on each system. if they go to start near each other then they will trip the breakers

AllanJ 05-24-2012 06:24 AM

]IGo through the load calculation (use applicable code such as National Electric Code or any city supplement) to see if the two AC units can coexist on the 30 amp 8 gauge aluminum circuit.

There is (or should be) a nameplate or sticker inside or on the back of each AC unit that specifies the power requirements and power draw.

If the calculation comes out okay then you may put a sub-subpanel in place of the first AC unit's disconnect.

This sub-subpanel needs only the ground wire accompanying the 8 gauge feed since it is in the same building as the main panel. (A subpanel in a separate building from its feeding panel needs ground rods or other appropriate grounding electrodes as well as a ground wire accompanying the feed.)

YOu may be able to use a slow-trip 30 amp breaker at the 125 amp subpanel so if both AC units start at the same time you don't get a trip. A slow trip breaker might not be disallowed by code but it is not an outright substitute for failing the load calculation.

(A "sub-subpanel" is wired in the same fashion as a subpanel including with grounds and neutrals separated, and with grounding electrode(s) if it is the first panel in a separate building. "Sub-subpanel" is not official electrical terminology.)

kbsparky 05-24-2012 06:46 AM

Post the nameplate ratings from both units:

Minimum circuit ampacity
Maximum overcurrent protection.

The sum of the minimum circuit ampacities must be lower than the rating of the feeder cable.

Billy_Bob 05-24-2012 08:27 AM

A/C systems can draw more electricity and trip a breaker when they begin to fail. That is the way you know something is wrong with the unit.

With that said, I would not want a grossly oversized circuit to an A/C unit, rather a "just right" sized separate circuit. Then if some bearings or whatever began to fail (and draw more electricity), a breaker would trip and I would know the unit needed service.

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