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In a house built in the early 1980s in California, USA.

My kitchen and dining room are (mostly) on a single pair of 20 amp circuits - excepting the dishwasher and garbage disposal, which each get a 15A circuit, and a "Microwave" circuit which is entirely inappropriately located above the oven (maybe they planned to have a Microwave/range hood) in a spot only the range hood can utilize. The pair of circuits are handle tied, so my guess is they are most likely a multi-wire branch circuit. (And yes, I know this is nowhere near current code - it's a rental, unfortunately, so limited options for changing things.)

Unfortunately, due to the inaccessible "Microwave" circuit, I have to attach both my microwave and toaster oven to the main set of circuits (but have some choice over which outlet to use for each, as I can move them around). Is it possible to determine which outlet is on which circuit of the pair, without opening the wall or doing anything significant to the panel? And if I determine that, say, these outlets are on "5" and these outlets are on "6" (the numbering in the panel), am I correct that it would work to have the two appliances on one each, and wouldn't trip (assuming other things weren't utilizing the circuits), since each would be on a different hot leg?

Ideally I'd either temporarily un-tie the two circuits and intentionally trip one, and then see what works and what doesn't, or have some way of telling which is which using the multimeter. (I'm also considering running an extension cord to the microwave plug, as ugly as that looks, because it certainly isn't a good idea to have two 1200+W appliances that often will be run at the same time on one circuit; but hoping there's a way to have one on each hot leg.)

Here’s a pic: pic of panel

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  • A MWBC is going to be on two sequential even or odd numbers. Not simply sequential numbers like 5 and 6, unless your panel is older than I am.
    – Mazura
    Feb 15, 2022 at 3:54
  • @Mazura Maybe I am wrong? Is it on the same leg and just double stuffed?
    – Joe
    Feb 15, 2022 at 4:00
  • Double stuffed is definitely the same leg. But there wouldn't be a handle tie. Your 5&6 are the same phase/leg and should not share a neutral. 7&8 is an example of how you get the two different phases; a two pole breaker. That can share a neutral (or not even have one). - Never seen those little white things before.
    – Mazura
    Feb 15, 2022 at 4:04
  • Wait. Everything on the left is half a slot off. IDK what I'm looking at here.
    – Mazura
    Feb 15, 2022 at 4:07
  • @Mazura Yeah, IDK what's going on there either unfortunately... but perhaps that means the different phases are on track. Hopefully I'll have time to test in the next day or so and see if Greg's answer below confirms that...
    – Joe
    Feb 15, 2022 at 4:09

4 Answers 4

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You can tell by how the panel is phased.

Service panels have their 2 phases (poles, really) laid out in a very particular way.

You have a GE panel. It's a little odd, but still decipherable. Let me stripe it for you like in the link.

enter image description here

Yup, 5 and 6 are on opposite poles. Note how GE does double-stuffing a bit different. Note also the Code violations; the empty holes at breaker 0 and 6-1/4 need to be filled with GE approved blank plates. The easiest-to-find models of blank plate are THQP115 and THQP120, which are also breakers LOL.

Now, watch Technology Connections' video on how we use 120V and 240V, and particulary his measurements on how breakers are phased. I cued it up for you. Notice how breakers on opposite poles are 240V from each other. Don't open your panel; you don't own that house.

So now it's a simple affair. If you can just measure voltage from the hot terminal of the microwave from the hot terminal of any other candidate socket... if the voltage difference is around 0 volts, it's on the same pole of the MWBC. If it's around 240V, it's on the opposite pole. Mark your receptacles in some manner.

To measure at a socket, just insert the UL listed and 600V rated voltmeter probe into the hot terminal. On a NEMA 5-15 or 5-20 socket, you already know ground... neutral is the taller one (or T-shaped one). The other one is hot.

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  • That dude's awesome! But I sure would've liked to have seen the inside of this panel....
    – Mazura
    Feb 15, 2022 at 11:36
  • @Mazura No problem, see youtu.be/jMmUoZh3Hq4?t=364 Feb 15, 2022 at 18:54
  • @Harper-ReinstateMonica Thanks for that video... though I may need to post a followup question. This worked - except that I apparently have three phase current!?! In a single family home... 120V hot to neutral, either hot to either neutral, but 208V (ish) hot-5 to hot-6. Weird enough for a full answer? Gas heat, nothing interesting using both poles...
    – Joe
    Feb 15, 2022 at 19:59
  • @Joe sure, ask a question about that, and state your location as specifically as you feel comfortable. In particular if you are in metro NYC, you'll want to get down to the borough. Feb 15, 2022 at 20:18
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If these receptacles truly are wired in MWBC there's no need to remove the covers or disconnect anything. Just use your volt meter to measure AC volts between the hot/line terminal of one receptacle and that of another receptacle. Doesn't matter which meter lead (red or black) goes to which. Use an extension cord if necessary to get everything within arm's reach.

If you measure approximately 0 volts then the pair of receptacles are on the same leg of power. Mark them both with a colored tape or other tag. If, on the other hand, you measured approximately 240 volts then the two receptacles are on opposite legs of power. Mark them with differently-colored tape or other tag. By "approximately" I mean that if it's a few volts or less that's close enough to 0; if it's 220-250 or so that's close enough to 240.

Repeat the process, always measuring between one previously-marked receptacle and one unknown receptacle. When you measure 0 volts mark the unknown with the same mark as the known one; when you measure 240 apply the opposite mark.

If for some reason you still want to know which half of the handle-tied breaker each set goes to, try these steps:

  1. turn off the breaker
  2. remove the dead front from the panel
  3. disconnect the wire from one of the breakers
  4. turn the breakers back on
  5. go see which set of outlets has power
  6. turn off the breaker and restore everything as it was
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  • Hmm, voltage-meter-only does seem appealing option, and that makes sense... "hot" is the smaller of the two prongs, correct (usually on the right in ground-on-bottom orientation)?
    – Joe
    Feb 14, 2022 at 21:21
  • @Joe yes that's correct.
    – Greg Hill
    Feb 14, 2022 at 21:27
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Great question and good thinking!

  • Flip the two breakers off.
  • Open each of the outlets that are now dead.
    • Plug a lamp or something similar into each outlet before taking the cover plate off, just to be 100% sure. You don't want to end up dead from this little experiment.
  • You should note that some of the outlets are wired to the black wire and some to the red wire.
    • This presumes that the house was wired with NM-B cable, not conduit in which case the hots could be any color but green, white and grey.
    • If you are on conduit, then you'll find some outlets on "Color A" and others on "Color B".
  • It's possible that you'll see a red wire on the top outlet and a black on the bottom outlet of the same device.
    • This means the top outlet is one one breaker and the bottom on the other. Perfectly legal and acceptable.
    • Make 100% certain that the metal tab is broken off between the screws. If not, that means that the two circuits will both be energized when either breaker is on (should the handle tie ever be removed).

The ones on the black wire are on one of the handle tied breakers and the ones on the red wire are on the other breaker. It doesn't really matter which is which for your purposes, you just need to know "A" and "B" circuits so you can distribute your loads appropriately.

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  • Thanks, next time I have a chance I'll do this. Am I correct in my belief that, if I can identify "A" and "B", putting the microwave on one and the toaster oven on the other is both ideal (to the extent any of this is) and will balance the load such that neither circuit will be overloaded (if, say, both use 15A of current at the same time)?
    – Joe
    Feb 14, 2022 at 19:29
  • That would be a good follow up question to use the "Ask Question" button for.
    – FreeMan
    Feb 14, 2022 at 22:25
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FreeMan's answer is the standard way to do this. There is one other option, which involves removing the front panel of the breaker panel (which you may or may not feel comfortable doing) but does not involve disconnecting any wires.

  • Remove the front panel. This is usually pretty easy as it needs to be done any time a breaker is added or changed. **Be careful. Keep metal (rings, watches, tools) away and stay away from the feeder (usually at the top or bottom, and usually connected directly to a big breaker).
  • Get a clamp meter:

Klein CL120

  • Get a helper
  • Unplug everything from all 5/6 receptacles.
  • Have the helper plug a reasonably high powered device (e.g., a lamp with incandescent bulbs - remember those?) into each receptacle
  • Measure the current on the hot wire going into each breaker (5 & 6) to determine which receptacles are on each breaker
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  • Thanks - I do have a clamp meter (that one, I think!) and might do that if FreeMan's solution doesn't work or we run into any issues with still-energized circuits. (For what it's worth, mostly the electrical here was done "minimum code" but still code, so odds are it'll be okay... but still obviously being very careful!)
    – Joe
    Feb 14, 2022 at 20:32
  • This definitely sounds like it would work, though I wonder what circumstances would require you to use this approach versus the other ones suggested? (Great suggestion btw, just curious myself)
    – maples
    Feb 14, 2022 at 21:24
  • 1
    @maples An advantage of this method is that it will work whether you have conduit/wires or cables. It will also work even if the circuit is split at some point - i.e., from a /3 cable to two separate /2 cables. Feb 14, 2022 at 21:38
  • 1
    With incandescents going the way of the dodo, the typical ubiquitous high power portable (= not a kettle) testing device could be a hairdryer.
    – TooTea
    Feb 16, 2022 at 8:59

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