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I have a 240V subpanel on one side of my house which is fed from a main panel on the opposite side of the house. It is on a 100A breaker. This subpanel is full, with 2 50A breakers and a 30A breaker to an oven and 2 A/C condensers. I would like to somehow add a 20A breaker to this circuit to power a small 240V mini split A/C unit for my office, if possible.

The questions: Can the existing 100A circuit feeding this subpanel handle another 20A circuit? If so, would I need to add an additional subpanel fed by this one? Not to sound stupid, but is there a way to safely add it to the existing subpanel with some kind of tandem breaker or something? Maybe this: HOMT220230CP?

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Edit: Spec plates (what I could find anyway...)

Oven: Oven

A/C 1: A/C 1 I think that model # is N4H348AKF200 - I wasn't able to get a good angle as it's too close to the adjacent A/C unit.

A/C 2: A/C 2

The mini-split hasn't arrived yet but here is a link to the listing which has some technical specs: SENL-12CD/220/X

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    The pics are very helpful. You'll get a good answer soon.
    – FreeMan
    Sep 20 at 16:34
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    That is 2 AWG (33.6 mm2) aluminum AA-8000 feeder. It is only good for 90 amps. If the supply breaker is 100A, that's a common blunder people make because of a common error misreading NEC. Sep 20 at 20:58
  • Can you get us photos of, or the Minimum Circuit Ampacity numbers from, the nameplates on your existing air conditioners, as well as the kW rating of the oven, and the make/model of the new minisplit? Sep 21 at 0:51
  • I just added photos of the spec plates I could access. Also a link to the mini-split listing since it hasn't arrived yet. Thank you all for your help!
    – B S
    Sep 22 at 16:16
  • Those are happy numbers, though. Sep 22 at 18:00
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That 2-2-2-4 is just a trifle too small for your plans...

Were this an actual 100A subpanel, fed by 1AWG hot and neutral wires, you'd be able to do this without much issue. However, since you're dealing with 2AWG Al here, your feeder is only good for 90A due to the fact the 83% service/feeder rule for entire dwelling units doesn't apply to feeders that serve parts of dwelling units unless your service is the same size as the feeder, which I seriously doubt's the case in your situation. This hasn't become an issue yet since the feeder's NEC calculated load is under 90A as it stands at the moment; however, adding the minisplit is the proverbial straw that breaks the camel's back, pushing you to 93.5A (and a bit of change).

As a result (the load calcuation comes out to 1.25 * the wattage of the largest air conditioner + the wattage of the remaining air conditioners as computed by their rated amperage times 230V + the watt rating of the oven), you'll need to upgrade the SE cable from a 2-2-2-4 to the correct cable, namely a 1-1-1-3, before you proceed further with this.

...but the good news is that the breaker you need is a thing

Once that's out of the way, we can then fit the quadplex ("quad tandem" in Square-D lingo) Homeline breaker you will need, namely a HOMT220230, in place of the existing HOM230 serving the smaller of the two air conditioners. From there, it's just a matter of running the flex whip from the new outdoor unit to the breaker box and landing the wiring appropriately.

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The feeder needs to be correctly breakered at the supply end.

That is 2 AWG (33.6 mm2) aluminum AA-8000 feeder. It is only good for 90 amps.* If the supply breaker is 100A, that's a common blunder due to misreading NEC. Change the breaker to 90A.

Determining whether you have the amps to spare

And adding a 20A to this subpanel is fine given that it's behind a 100A circuit breaker that already has 130A worth of breakers on it, so 150A total? I'm guessing you can't really just add up the amps of each circuit breaker to determine that, correct?

That's correct. Loads must be appropriate for a panel. That is done by doing a Load Calculation on all the circuits in the panel. Straightforward work for an all-240V panel with all large loads like this one.

The calculations for an oven can get a little bit loopy, but they tend to come out better than you'd expect.






* NEC Table 310.15(B)(16), aluminum 75C column. Some people read 100A out of the wrong table. This happened so often, that the NFPA deleted the other table altogether in the 2014 edition. Here is the correct table.

enter image description here

You are limited to 75C thermal because the lugs on the supply breaker and subpanel are limited to 75C. Certain wire types can run 90C, but they would need to be pigtailed using 90C rated splices - that would be Polaris connectors x 6 at $20/pop.

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  • Thanks for this info, very helpful.
    – B S
    Sep 22 at 15:54
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The datasheet for your panel says it is a 6 slot, 12 circuit panel, which means it should be compatible with tandem breakers. A tandem breaker is two small breakers that take up the same slot as one full sized breaker.

But you have all double-pole breakers... Well, they make a solution for that too which is two tandem breakers joined together. The middle two handles will be a double pole breaker and the outer handles will be two separate single pole breakers.

Edit: I missed that you wanted to add another 240v (2 pole) circuit. The breaker you linked to is indeed made for that purpose.

tandem double pole breaker

Sample product listing

They come in different sizes, but based on whats in your box now, the one I included a picture of should be perfect for you.

So I guess technically you can't add one 20A breaker, but you can add two.

(I know your question included a reference to these, but figured I would explain it for future readers.)

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  • And adding a 20A to this subpanel is fine given that it's behind a 100A circuit breaker that already has 130A worth of breakers on it, so 150A total? I'm guessing you can't really just add up the amps of each circuit breaker to determine that, correct?
    – B S
    Sep 20 at 17:15
  • What size is the wires leading from main panel...looks like 6 or maybe a 4. That's my main question. But whatever is done that thick white wire looping over to the second pole should be indicated as hot? With black or red tape on it? Sep 20 at 17:27
  • @LimoDRIVER No! There are big black and red - on the sides - and white is for neutral, going into the big lug on the neutral bar. There is a smaller white neutral in the neutral bar on top. Sep 20 at 17:59
  • @BS, Yea, you can't really just add the breakers and make a determination that way. You really have to look at what would reasonably be used at the same time. Ovens don't use 30A, and AC units don't use 50A, that's just the breakers they are rated for based on the manufacturer. They don't constantly use that amount of power.
    – JPhi1618
    Sep 20 at 18:28
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    If you post pictures of the electrical specs from the labels on the chassis of both your A/Cs, oven, and new mini split, it will be possible to determine if they can all be fed by a 100A circuit.
    – jay613
    Sep 20 at 19:04
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Without nameplate data it's hard to give you an accurate answer. But 100 amps is enough to power most average size households so I'm guessing you'll be fine adding the additional circuit.

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