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I want to add three mini split systems to my house (two 18k, one 9k). I have a 100 amp panel that is full. I did a load calculation and with the additional mini splits I will be at 89 amps total (I have oil heating and gas dryer and stove).

Someone suggested I could upgrade some of the lower amp breakers to higher amps (like switch out a 15 amp breaker to a 30 amp) and pull new wire (12AWG) from one upgraded breaker to one mini split system (3 upgraded breakers total). The logic was that since my total load is under 100 amps it won’t trip the main breaker, and since the mini split is not running on an existing wire then it won’t be overloading that wire (most likely 14AWG). Is this right or completely wrong?

Photos of panel (I tried to take a clear photo, but it is an old panel and the labels are already fading)labels panel inside label 1 enter image description here

Links to mini splits that are to be installed: Master bedroom unit https://minisplitsforless.com/collections/single-zone-wall-mount-ductless-mini-split-ac-heat-pump/products/cooper-hunter-9-000-btu-230v-ductless-mini-split-air-conditioner-ch-09dkt230vi-o?variant=39556122968256

Living/dining room unit https://minisplitsforless.com/products/ch-ng18mia230vi-o-cooper-hunter-18-000-btu-230v-ductless-mini-split-air-conditioner-wifi-ready-heat-pump-19-seer?variant=39549631332544

2 small bedrooms unit https://minisplitsforless.com/collections/2-zone-dual-zone-ductless-mini-split-ac-heat-pump-system/products/2-zone-6k-6k-wall-mount-ductless-mini-split-a-c-and-heat-pump-with-25ft-installation-kits?variant=42782337794293

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    You can't have 12AWG or 14AWG wire running to a 30A breaker.
    – brhans
    Oct 25, 2022 at 1:14
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    Also, you didn't ask this, but two 18k and a 12k BTU mini split is a lot of heating/cooling capacity. Have you had a manual J done or is this just a guess based on an installer's gut feeling? Unless you have a big house or live in a rather cold climate it's might be too much heat pump.
    – KMJ
    Oct 25, 2022 at 14:41
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    @GaryH I'm in Zone 4 and a 12k unit was never at full capacity heating the 950sf of my older poorly insulated house. If you're primarily focused on heating, look at the specs of the units because the heating number is often a fair bit higher than the cooling number, at least for units that can handle cold ambient temperatures. The Mitsubishi H2i unit I had was capable of 17k BTU of heating at 17F outside, which is about as cold as it ever gets here. These low ambient heating units are great, very much a buy-once cry-once sort of situation.
    – KMJ
    Oct 26, 2022 at 23:04
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    I'm going to say you get what you pay for here. The Mia NY 18k unit you linked only has a max 18,800btu heating capacity. At 17F it's down to 12k maximum heat output. Meanwhile a Mitsubishi H2i unit will put out 11k BTU at -13F. Other brands make units with similarly good cold weather performance. If you want good heat output in cold weather and the lowest total operational cost, you probably need to spend a bit more up front. Spend some time digging through data sheets.
    – KMJ
    Oct 27, 2022 at 1:31
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    @GaryH depends on the brand. Most of them have submittal sheets with a rating at low temperature. For instance hvacdirect.com/media/hvac/pdf/… is for a 24k Daikin unit, which still heats quite well at 5F by the specs. a1ac1dcb67cc9f847a73-0b6da349d0197cd2922796e57d5f1d84.ssl.cf5.r… is a sheet that shows max heat at low outdoor for LG in one of the spec grids. You have to a hunt a bit to find the figures sometimes.
    – KMJ
    Oct 30, 2022 at 18:01

2 Answers 2

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Replace the breaker for the master bedroom A/C with the feeder breaker for your new subpanel

What I'd do in this situation, since the master bedroom's window unit is getting decommissioned as part of this, is take out the breaker that fed that socket. This frees up enough space in your panel that you can fit a new two-pole QO breaker of the appropriate size to feed a NEMA 3R main lug subpanel at where the new outdoor units are going. This setup also neatly provides local disconnecting means for the AC units, and does not require a particularly large subpanel, either. (I'd go with an 8 space, 100A unit for this, presuming that you're dedicating this feeder to air conditioning that is.)

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Sounds possible. But there are a few other ways to deal with this, depending on the specifics of your particular panel:

  • Subpanel

This is the "best practices" total solution. You replace a pair of breakers with a (typically) 60A double-breaker and use that to supply a subpanel which will contain the removed breakers plus breakers for the new mini-splits, etc.

The cost is actually not that big a deal. Except that you need to have a proper working space in front of the subpanel. You may have that available next to your main panel, or you may not. Even if the old panel might be grandfathered, you can't get away with that for a new subpanel. Space (30" x 36") is critical.

  • Combine Circuits

Certain circuits (2 x kitchen countertop, 1 x bathroom, 1 x laundry, dedicated circuits such as oven, HVAC, etc.) can't be combined. However, there are often several general receptacle/lighting circuits that can be combined, as long as you don't overload the circuits.

  • Half-size Circuit Breakers

Many panels allow for a limited number of half-size breakers. Essentially, replace a single 15A or 20A breaker with a pair of breakers. There are some limitations. Most important is the panel specifications - you can't just put these anywhere/everywhere. But in addition there are other constraints - half-size breakers don't have GFCI or AFCI options, so if you need those then you are out of luck. Plus any Multi-Wire Branch Circuits or other circuits that require two hots 240V apart need to be handled properly. And there are some other potential issues. But if you don't already have a bunch of half-size breakers, that might be enough to solve your problem.

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  • My electrician once spoke of the legendary “2-pole quad-tandem” breaker, which squeezed 2 240V circuits into 2 breaker slots. I see one at the orange store for $40. Oct 25, 2022 at 2:44
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    The catch is that the 2-pole quad-tandem, like all breakers, is going to be brand-specific. If orange has a GE but you have a Square-D panel (or vice versa) it does no good. But in theory, yes that is a great solution. Oct 25, 2022 at 2:58
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    Eaton does make their 'Classified' breakers which work for General Electric, Thomas & Betts, ITE/Siemens, Murray, Crouse-Hinds, and Square D. It's an exception to the rule though, and of course no quad-tandems in that group.
    – KMJ
    Oct 26, 2022 at 16:15
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    Looks like a subpanel would be the best solution (aside from getting a new panel). Thanks!
    – Gary H
    Oct 27, 2022 at 0:16

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