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I am installing a 240v ductless mini split. My breaker box is full, and I have a separate (hot water) breaker in its own box on its own meter. 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.

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 won't 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 plausible options that I am missing?
  • Any other reasons why option 1 may be better than option 2?

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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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    First thing you need to answer is why is the hot water on a separate meter? Is it billed at a different rate? Some utilities have special incentives for certain appliances which means they're billed at a lower rate on a separate meter (my utility does this for EV chargers, for example), and if that's the case here, your utility may forbid you adding other loads to that meter.
    – Nate S.
    Aug 28 '20 at 20:03
  • Did you ever get this resolved? If so, please give a check-mark to the answer or write up your own answer explaining what you did to get it fixed and give yourself a check mark. That will help others with this kind of problem know that this has a resolution and is a good place to look for their answer.
    – FreeMan
    Sep 28 '20 at 12:05
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Option 2 is probably not an option at all - that setup is almost always for a special-rate storage electric water heating tariff and is only on for a few hours when demand is low overnight (which is why it's a special lower rate) or else it's switched off whenever loads are high (a different scheme, similar concept.)

For my utility, you can keep them if you have them from decades ago (when they had a nuke plant idling away all night), but you can't get a new one done this way. You certainly cannot add to it, (for my utility) you don't own the water-heater associated with it - it's part of the overall deal, they own it.

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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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