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I recently moved into a new house and I've been mapping my panelboard. I've got pretty much everything mapped except for a couple circuits. I know there are some experienced folks on here so I thought I'd see if anyone could weigh in on what I'm looking at here or had any advice.

Some context, I live in the northeast of the US and many of the appliances are gas powered (stove top, gas boiler, water heater, and dryer). The largest electrical loads on the panel from what I can tell are the condenser fan/compressor unit on the outside of the house and the AC/blower unit in the attic. Each of those are on their own 240V 20A circuit. Everything else, except for one circuit (next paragraph), is a single pole 20A or 15A circuit.

I've mapped each breaker by opening it and then checking the voltage at the terminals and then checking the lights/outlets throughout the house. What I'm having some trouble with is the following:

  1. There is a 240V 30A circuit and I have no idea what it is connected to. Even if I were to open it, how would I know what to look for if I don't have any idea what it might be powering? I'm confused because I'm not sure what other load in the house would require a circuit that size. There are no major electrical heating elements that I know of (its all hydronic) and the only large motor loads are the compressor/condenser fan motor and attic unit.
  2. I think the last two breakers might be bad. I get 300-700 mV readings when I open them and check the voltage across either of their terminals. Thus even when I open them I can't tell what is on them. I plan to replace both of them soon.
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    Don't forget to leave a fair copy of your documentation in a conspicuous place, e.g. taped to the inside of the service panel door, so that if I buy your house I don't have to do it all again. Thank you.
    – A. I. Breveleri
    Aug 26, 2019 at 2:12
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    Are you rural? I'd wonder if the big circuit is for a well pump.
    – Criggie
    Aug 26, 2019 at 10:53
  • Code requires that you label all breakers, and do so in a sufficiently generic way (northwest bedroom NOT Suzie's room).
    – Harper - Reinstate Monica
    Aug 26, 2019 at 18:38
  • Note that there may be a 240v electric outlet behind the stove.
    – Hot Licks
    Aug 26, 2019 at 21:57

3 Answers 3

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240v / 30 amp would typically be for a clothes dryer outlet. As people live in homes, and time goes by, they change things. I'm guessing, of course but possibly the dryer is now a gas dryer and there behind that dryer sits an outlet that isn't being used.

Other stuff : A outlet in the garage for a "welder" that somebody had installed. And possibly hidden behind something.

Again, I'm guessing.

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    You beat me to it with the dryer. Maybe a sprinkler pump on the other one
    – JACK
    Aug 25, 2019 at 1:21
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    You got it - I just found it. It was a blank junction panel in the back corner of the garage. Maybe the dryer used to be the in garage? Or maybe it was a welding receptacle?
    – Nukesub
    Aug 25, 2019 at 1:32
  • Also not uncommon in houses with gas fired hot water to still have legacy 2-pole 20A breakers to feed what used to be an electric water heater. Always safest just to leave these off, particularly if wires leave the panel and have ended up chopped and/or buried in a wall or junction box somewhere where you haven't yet found them.
    – J...
    Aug 26, 2019 at 16:16
  • Now you are all set to buy an electric vehicle :)
    – jalynn2
    Aug 26, 2019 at 17:00
  • The only other thing not listed here is an electric range/oven, and many of them have even higher amperages than that.
    – Monty Harder
    Aug 26, 2019 at 17:11
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Ah yes, the old "identify circuits" task. Dollar-store incandescent night lights are your friend!

First, if you don't know what they are, turn them off. Something will stop working... or nothing will. At that point, take anything that doesn't work very seriously. Dead socket, mystery do-nothing switch, dead doorbell, etc. You may also find you knocked out your thermostat or Radon pump!

A 30A 2-pole breaker is the most common large breaker size. It can be any of:

  • Standard electric dryer (look for a dryer receptacle somewhere that makes sense)
  • Standard electric water heater (again search where the plumbing is compatible with a water heater being there)
  • Air conditioning equipment (this is the standard size)
  • Range or oven, but this is an uncommon (not rare) size.
  • Whole-house surge suppressor.
  • Air compressor or workshop equipment, typically in garage
  • Mid-size RV connection (typically somewhere that would make sense)
  • Small RV connection (30A/120V) wired onto half the breaker. 30A/120V breakers are readily available, but silly to buy since they're useless for anything else).
  • Electric car charger (electric cars have been a thing since the mid-1990s)
  • Circuit out to shed or garage to a subpanel there
  • Solar panels on the roof, no longer allowed to be this simple since NEC 2017
  • A smallish pool, hot tub or spa which perhaps they took with them
  • Generator backfeed receptacle - this would be very obvious if there was the required and necessary generator interlock to prevent backfeeding the grid, but this is often skipped.
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    Thanks, those are all good suggestions. I ended up discovering it ran over to a little junction box in the corner of the garage with a blank faceplate. Guessing either the dryer used to be in the garage or more likely the guy who used to live here used the garage as a workspace and had a welder connected there. Maybe an air compressor like you say.
    – Nukesub
    Aug 25, 2019 at 14:39
  • Not likely a pool. Even a large in-ground pool will not typically need a pump larger than 15A or 20A, single pole (1-1.5HP). A small pool or spa electric heater would be possible, as these start around 5kW and go up from there.
    – J...
    Aug 26, 2019 at 16:22
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I think the last two breakers might be bad. I get 300-700 mV readings when I open them and check the voltage across either of their terminals. Thus even when I open them I can't tell what is on them. I plan to replace both of them soon.

Getting small (a few volts at maximum) readings when breaker is open is normal, if the cable runs in parallel with a powered cable for a long distance.

The AC voltage can capacitively induce a small voltage in the other cable, which your multimeter then detects. The voltage is small enough that it won't be dangerous and will appear as "off" for any appliances.

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  • Thanks - I didn't know that. I'll open them again today and see if I can tell what is off/no longer works.
    – Nukesub
    Aug 25, 2019 at 14:36
  • Actually found what those were connected to as well. There is a whole house surge protector to the left of the main panelboard. Those two breakers are connected to it.
    – Nukesub
    Aug 25, 2019 at 15:43

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