The main service panel in my house has eight 2 pole breakers in it. It breaks down as:
2 40A breakers for the AC units
2 50A breakers for the furnaces
1 40A breaker for the stove
1 30A breaker for the dryer
2 30A breakers that are unlabeled, but have 10ga wire landed on both.

We have a gas water heater, so no need for 240V power there.

My question is this: Any idea what the two unlabeled breakers could be for? I cannot think of any other 240V loads. We do not have a subpanel anywhere, no hot tub, no RV plug, generator backfeed... I have them turned off now, but have not noticed a change in anything. I would like to repurpose one of them to feed a subpanel in my shed, but don't want to drop something in the house. Thanks in advance for the help.

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    Follow the wire. Given it's a typical water heater size, see if there's a junction box near the water heater location with capped off 10Ga wires in it, but other than that, follow the wire. – Ecnerwal Nov 23 '20 at 14:51
  • I followed it as far as I could-- which is to say, where it exited the main service. I plan to get up in the attic (where the water heater is) and checking this evening. Any suggestion on how to trace out the circuit? I have a fox and hound for outlets, but that is not going to help me much if I don't know where the other end of the line is. – Michael Nov 23 '20 at 15:06
  • Do you have a separate oven or fancy built in microwave? Any special tools or EV charger in the garage or shop? If you don't notice anything amiss for several days with the breakers off I would feel pretty confident you are ok to repurpose them... – bigchief Nov 23 '20 at 15:07
  • I don't think there's any way anyone who does not have access to your house (either you or an electrician you hire) could possibly answer this question. The only way to know is to physically follow the wire. You could also look for anything that looks like it used to be an outlet, but now has a blank face plate on it, because it may lead there. – FreeMan Nov 23 '20 at 15:07
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    If you are comfortable working in the panel, you can connect your Fox to the wires that connect to the breakers, and use your Hound to trace the wire. – Ecnerwal Nov 23 '20 at 15:13

2 x 50A for furnace implies electric heat. (If it was a heat pump, that would be combined with the 2 x 40A AC). That all points to "everything electric" because in most places if you have gas available then you use that for heat rather than electric. Plus "2x" indicates this is a large house. While you have a gas water heater now, my hunch is that your house originally had electric water heater(s) and, due to the size of the house, was wired up for 3 water heaters. That also fits with the (typical, but not universal) slower recovery time for residential electric water heaters compared to gas. One large gas water heater could easily produce as much hot water (between faster recovery and larger capacity) as 2 electric water heaters. Then at some point (my parents did this years ago, but they added gas heat at the same time), someone got gas piped into the house and replaced the electric water heaters with gas.

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    Great answer, thank you. A little more information-- it is a larger two story house, AC and heat broken up between to units, one for upstairs and one for downstairs. Yes, it is electric heat-- the only gas loads are the water heater and the fire place. I will be in the attic tonight seeing if that is where there cables terminate. – Michael Nov 23 '20 at 15:33
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    Replacing electric appliances with gas is pretty common, when possible. When our previous home was built (1989ish), there was a moratorium on new gas hook ups, so everything was electric. Around 10 years after we moved in, the moratorium lifted. The gas company would run the line into the neighborhood and up to your house for free, providing you connected one major appliance up to the gas, which most people jumped at. – SteveSh Nov 23 '20 at 15:37
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    Run the numbers for your local prices, but usually gas is SO MUCH CHEAPER than electric that replacing an electric furnace with gas is going to pay back VERY quickly. – Ecnerwal Nov 23 '20 at 15:48
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    I don't want to get political here, but unfortunately many cities (esp. in California) are banning natural gas for heating and cooking. Now they want everything electric. CA has trouble keeping power supplied during heat waves for A/C and cuts power during hot, high wind events to minimize the chance of wildfire. What's going to happen to emissions when everybody fires up a generator during power outages. And what's powering the electric? peaker plants, nukes, coal, etc. If this comment violates rules here at SE, I'm sorry, but it has to be said. Delete if admins see fit. – George Anderson Nov 23 '20 at 20:53
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    @GeorgeAnderson Certain people want to: ban fossil fuels because of originally pollution and now global warming (both noble goals, but...) and then ban nukes because of Fukushima, etc. (again, real problems, but workable) and because of the other kind of nukes (and again, there are ways to keep one from connecting to the other) and nuclear waste (which can be managed, but NIMBY so it never does) so "all electric" but "not enough electric". Seems some people didn't learn in kindergarten that you can't have everything you want all the time, that you have to compromise. But I digress... – manassehkatz-Moving 2 Codidact Nov 23 '20 at 20:59

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