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I am installing a 240v ductless mini split. My breaker box is full, and I have a seperate ( hot water ) breaker in its own box on its own meter. I see two ways I can approach the situation:

1) Add a subpanel on a breaker in the main box. Not preferred because I fear blowing the main breaker.

2) Upgrade the breaker in the hot water box to supply a new subpanel. Preferred because I wont have to be concerned with blowing the main breaker, BUT there is no shutoff between this breaker and the meter, so the meter has to be pulled.

Are there other plausable options that I am missing? Any other reasons why option 1 may be better than option 2?

Update: The main is a 100 amp Westinghouse, the house is 2000 sq ft. 4 beds 2 baths. The house is currently run on an oil furnace. + added images

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    A little more info on your existing panel and loads will be needed to provide a informed answer. The size home, the actual size of the main breaker. Gas or electric appliances even a photo of your panel would be helpful. – Ed Beal Aug 27 '18 at 19:36
  • #2 seems reasonable. If you are going to pull the meter you could also install a disconnect so this won’t happen again. – mreff555 Sep 28 '18 at 19:23
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The first thing to consider here is whether the utilization of either service leaves some of the service's capacity available for other loads.

The capacity of the service depends on the service conductors and equipment - the size and makeup of the wires going to the utility, the service equipment panel's rating, and the main breaker size.

The number of unused slots in the panel doesn't determine utilization. It's possible to have lots of open slots, but all the available capacity used by the breakers already present. If that's the case you can't add additional circuits, you'll trip the main.

It's also possible to have all the slots full but not utilizing the available capacity. If that's the case you can add a subpanel to make room for the additional circuit without tripping the main.

You can only upsize the hot water heater service breaker if the service conductors and equipment (panel, disconnect, etc.) can handle the additional ampacity.

You can determine the utilization by having an engineer or electrician do a load calculation, which approximates the utilization based on the square footage, appliances, etc., or by monitoring the actual utilization for a period of time to determine the peak utilization with your present configuration.

That utilization info will tell you whether each service would need to be upgraded to handle the additional load, and you can determine which service makes more sense for the mini-split.

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