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All devices are 240V, United States.

I want to add the "NEW" device and NEW wiring. Both devices will never be used at the same time, for they would draw up to 65 Amps which will trip the 50 Amp breaker.

The NEW device has this "current device" in the plug, but I don't think this is the same as a circuit breaker, so I would need to add an inline 15 Amp breaker ??

Is this going to work and be safe? (It seems to me it will work: I am an experienced engineer and I think I understand physics of electricity, but I am not a licensed electrician.)

Added after discussion: Thank you everybody! I learnt a lot. From all the comments, and reading lots of stuff online, I need 20Amp cable and breaker, not 15, and a subpanel with just one feeder cable from the main panel , easily accessible.

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  • What is this "new device", and what is the existing device for that matter? – ThreePhaseEel Mar 17 at 2:59
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    existing device is range, up to 50 amps if I used all the burners and oven at once. New device is window air conditioner, described in manual as 15 amps continuous, maybe surging more on startup – Mark Galeck Mar 17 at 3:11
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    @ThreePhaseEel Existing kW rating of the range is 11.9KW @ 240V, 8.9KW @ 208V, works out to 50 Amps @ 240V. Yes lots of space behind the range. The existing electrical box is there, to connect the correct receptacle for the range plug. I don't want to route the new cable inside the wall, I will just attach it to the wall. Are you saying that to be safe, I need something more complicated than a Polaris terminal tap? – Mark Galeck Mar 17 at 3:32
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    @MarkGaleck normally, if it's a party wall in a condo, it'll be constructed so there's a solid barrier of some sort mid-wall that serves as the demarcation between units -- this may either be masonry/concrete, or a double layer of extra-thick drywall (shaftliner) – ThreePhaseEel Mar 17 at 4:08
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    15 amps continuous, maybe surging more on startup - Startup is a separate issue. But if the air conditioner actually uses 15A continuous then it will need a 20A circuit, not 15A. Which makes little difference to the overall answer of "run a whole new circuit or install a subpanel". – manassehkatz-Moving 2 Codidact Mar 17 at 4:10
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Do you really need that large a unit?

Based on your friend's report, it sounds like, if anything, your unit will be too large. You're doing it for medical reasons not comfort, so you say.

Given the ever-rising SEER numbers (efficiency) we're seeing from consumer tier units, you may be better off simply seeking a 120V unit.

Those SEER numbers reflect how much you will be paying for electricity so it's a very big deal.

Definitely do not take a "hand-me-down" unit without looking closely at SEER numbers. Many "gift / side-of-road / hand-me-down" A/C units are older, and did not benefit from the very rapid gains in A/C efficiency made in recent years. A typical 8000+ BTU unit these days only needs about 5 amps @ 120V. You'd have no trouble running 2 on a single 120V/20A circuit.

240V units are largely gone by the wayside; they don't need the energy anymore... the ones that remain mostly support 240V because they are needed for installations already wired for 240V.

Lastly, if you consider a "portable" air conditioner (the kind with a hose that goes out a window, it's very important that you consider a 2-hose one. 2 hoses seems like an anti-feature until you learn how they work.



From a comment in chat:

I know the whole thing is arithmetically wrong, I have 50 coming in, and I want 50+20 out, so that is why I don't want to run both devices at the same time, at least not at capacity

... and that's the biggest worry you have? Not

the comical cavalcade of Code violations...

It seems like you want to "freestyle" safety based on your lay impressions of electrical safety. The entire point of Code is to pass down the paid-for-in-blood experience of a century of electrical use, so you don't have to pay in blood for a lesson already learned.

That is especially relevant in a condo, when your electrical fire could kill innocents in other units. I guarantee there's a city ordinance, or HOA rule, that says you must use a licensed electrician for work in the unit. Weren't we just talking about new-air-conditioner finances, and "good money after bad"?

The use of a circuit breaker is a good sentiment, but you can't just use some random loose circuit breaker in some random enclosure. You must use approved equipment and enclosures (NEC 110.2) and install it according to its instructions (NEC 110.3) in a neat and workmanlike manner (NEC 110.12) and with appropriate working space (NEC 110.26). And this is just the first few pages of NEC.

The oven/range circuit must be dedicated to oven/range. You are only allowed to have one load/outlet on a 50A breaker (NEC 210.23) unless it's one of the listed exceptions (ranges/ovens covered in NEC 210.23(C)). An air conditioner is not cooking equipment and is not allowed under the NEC 210.23(C) exception.

Further, your range is already too large for a 50A circuit. It's only on one because of a Code exception (NEC 220.55) which factors for the fact that not all burners are at 100% duty cycle all the time. When you add non-range equipment to the circuit, you void that discount and now your circuit is way too small.

Your logic, "all my burners are not 100% all the time" is correct but Code has already factored for that - there's no headroom there.

A 50A circuit can only have 50A receptacles on it. NEC 210.21(B)(3). So there's no such thing as tapping a 50A circuit to a 15/20A receptacle.

Even when you're in industrial spaces where you can use the tap rules to full effect, you're still talking about hardwired loads.

And I haven't even started into Article 440, which is the section applicable to air conditioners. I'm sure plenty of violations are there too, but no point beating a dead horse.

You're gonna spend more on Polaris connectors than a subpanel would cost!

So it's simple enough. Use a subpanel.

You currently have a 50A branch circuit from main panel to range. Route that to a subpanel, at which point it is a 50A feeder. You then install 2 breakers on the subpanel, a 50A and a 15A.

Now we sidestep all the above Code violations, because we're no longer torturing a branch circuit.

We now have 2 individual branch circuits, each following its own rules - the range keeps its favorable derate, and the 15A A/C circuit is definitely allowed a 15A receptacle.

As far as the "50A+15A > 50A" concern you have, the 50A feeder supply breaker will take care of that.

Do you really need a 50A breaker in the panel when the panel is fed from a 50A breaker? Yes. The supply line is feeder. You can't put loads on a feeder, and a breaker is ten bucks, so who cares?

Now we just need to work out the Article 110 issues about approved equipment and working space. Use the smallest subpanel you can find - a 2-space panel will suffice with a 15/50 quadplex breaker in it. Run of the mill Eaton, Siemens etc. stuff from the hardware store will satisfy NEC 110.2. Install it competently, and we tick off 110.3(B).

The real nut is the working space - you need 30" wide x 36" deep x 78" high working space in front of the panel. Surely there are either kitchen cabinets next to this oven. I would hack a kitchen cabinet's shelves so there is room for the subpanel to sit right at the front of the cabinet facing outward. It will block some of the cabinet's space, making it inconvenient to put things on the shelves, but this is the best we can do. So you open the cabinet door and voilà, there is the face of the subpanel right there. Presumably you walk around that area a lot, so there is 30x36 working space floor to ceiling, should be all set.

The panel might be inset maybe an inch from the face of the cabinet doors... an inspector might flag that, but I doubt it. Putting subpanels behind cabinet doors for aesthetics is perfectly common.

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  • Wow this is cool (pun intended), this is awesome. Thank you so much! As for the BTU, well, various "calculators" estimate I need around 11000 BTU. I tried a couple of those last summer. with dual hoses of course, single hose is nonsense. Not a chance, did not cool one degree down. So that's why I am going for a larger window unit. – Mark Galeck Mar 18 at 2:07
  • I will put a subpanel right on the wall. It'll be aesthetically pleasing - I'll choose the nicest looking one :) Just one question, I am not completely clear about that: Right now, I have the branch circuit, from the main panel, behind the drywall, to an electrical box in a hole in the drywall, behind the range. I can extend that with 4 appropriate twist-on connectors, using a cable that is rated for 50Amps (6 gauge), to the subpanel, with the cable running on the wall, attached to the wall with some solid plastic widgets. Is that OK by the code? – Mark Galeck Mar 18 at 2:15
  • Or does the code require that the cable from the main panel to the subpanel be uninterrupted, without connectors, and/or behind the drywall? – Mark Galeck Mar 18 at 2:18
  • The Code requires any splices be inside a junction box. @MarkGaleck #6 requires 5 cubic inches per wire, so I count 35 cubes (grounds are 4 for the price of 1). A "4-11/16" square deep junction box should do it, with appropriate cable clamps for whatever your cable is. – Harper - Reinstate Monica Mar 18 at 8:20
  • Monica thank you! yes I knew that any splices must be in a metal box with wiggle room, but I did not realize there are specific volume requirements. – Mark Galeck Mar 18 at 20:54
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There is something called a tap rule. More specifically you can tap a 50 amp circuit with 12awg wire normally reserved for cooking equipment.
See NEC 210.19.A.3 exception #. 1
A good picture of this in exhibit 210.24 notice the tap is in conduit no circuit breaker required.

The problem with a multi family dwelling they may not allow this for the AC

If you install a small breaker panel to service the ac unit it would be code compliant.

The wiring method commonly used in multi family units is metal clad not type nm wire.

A small 60 amp panel 4 space panel can be found for under 50$ To feed from the range circuit to the AC.
Make any junctions in the panel to feed the panel.

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  • thank you! Of course they don't allow AC on the outside where it is visible, as it an eyesore. But I have an "inside" sort of window, where it will not be visible, so I will "assume" it is OK :) because doc said I need AC. – Mark Galeck Mar 17 at 18:35
  • Recently a family member had me wire a 240v 2 hose portable ac unit (18k btu) it could be moved around but had 2 hoses that fit in the window. It was a nice unit and the 3” hoses really are not visible from outside, condo HOA rules. She plans on selling in a year and wants to take it with her instead of a mini split that is allowed. – Ed Beal Mar 17 at 18:52
  • Yes I need 18k btu min, but I could not find a portable one of that size. Well, there are some "industrial" ones, very expensive and I assume noisy. But not a home portable appliance of that strength. What is that unit you talking about, can you share a name or link? – Mark Galeck Mar 17 at 19:02
  • I will have to ask him I did not look at it very close I was surprised it was that large it required 240v 20 amp , it was fairly well insulated but the fans on high were really noisy but it cooled his unit down quickly. – Ed Beal Mar 17 at 19:11
  • Mark, she just texted back it was a commercial vexor. She said with the outside air manifolds it cost more than the mini split price I told her I could get (probably 1400.00 or so I don’t remember) so it was expensive but it cooled her 1 bedroom unit in 5 minutes after turning it on. And she can take it with her when she moves. – Ed Beal Mar 17 at 19:32
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It looks like your new 15 A wire for the new device needs a 15 A max breaker to protect it, and you've got a 50A breaker protecting both. Unless the "polaris" device has a separate breaker for the 15 amp line, you are not properly protecting the new circuit. The 15 A breaker need to be before the wires, so as to protect them. (When you say "15A wires" I'm assuming you ran 14 awg instead of 12 awg...)

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  • can you elaborate about the "before the wires", this is what I don't understand. This is parallel circuit, there are two resistances, one range, and two AC. They both draw current = voltage/resistance. If the AC is operating, it is drawing current 15 Amp, if it is off, it is drawing the current of 0 because resistance is infinite. In no case, there would be any current of more than more than 15Amp (maybe 20Amp if surging), through the part of the 12 awg wire between the tap and the breaker. Where am I wrong with this ? – Mark Galeck Mar 17 at 18:33
  • So the breaker is needed on the source side of the 15A circuit. For example if your device experiences a short of 40Amps, the tiny15A rated wires will melt and potentially start a fire. The breaker is there to protect the wires from the maximum current they are rated for. In your case it appears you've run "15 Amp conductors" (14 awg copper wires). I am not familiar with the Polaris device it it is possible it has breaker protection for your new circuit, but not likely. Breakers are not for when things are nominal, but for faults in equipment to keep us from burning down our homes... – mark f Mar 18 at 16:48
  • re-read Harpers too-long post on safety. He's wordy but it comes from his years of experience ;) – mark f Mar 18 at 16:50

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