I'm looking to add a 30A 240V circuit for solar.. but my main panel is almost full (pix below). Is there any way to add the circuit without adding a subpanel?


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    IMO your panel (even though it has one empty slot) is already overfull, considering all those tandem breakers. I also have a suspicion that it may be incorrectly wired and unsafe - if those 4 tandem breakers each with a red and a black wire are powering Multi-Wire-Branch-Circuits then it's likely that the neutrals there are being overloaded... – brhans Oct 14 '20 at 18:29
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    As much money as you are likely spending for the solar installation, you should upgrade this as well. You didn't say what the capacity of your proposed solar installation is but it's likely that even another breaker could be squeezed in here, you'd still have an overloaded panel. – jwh20 Oct 14 '20 at 18:32
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    @brhans A properly wired MWBC cannot overload the neutral. Suppose it was a kitchen circuit and you had a toaster running at 1,000 watts and a coffee maker also running on the other leg of the MWBC also at 1,000, there would be zero load on the neutral. Another example: If the toaster was running at 1,000 watts and another appliance on the other leg running at 500 watts, the neutral would be carrying 500 watts. – George Anderson Oct 14 '20 at 18:40
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    @GeorgeAnderson There's a tandem at the top-left which feeds a red and a black conductor. It's a single-pole breaker, no? We can see those conductors loop around the panel and exit together at the bottom right. They can't serve a 240V load because they're fed from the same leg, and we can't identify the neutral wire that goes along with them (but can speculate it's the same gauge since the whole panel seems to feed NM cables). This may be what gives rise to brhans' miswired-MWBC concern. There are three more tandems in the middle on the right side set up the same way. – Greg Hill Oct 14 '20 at 19:24
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    @GeorgeAnderson my point is that those, if they are MWBCs, are not properly wired because both wires are on the same leg. – brhans Oct 14 '20 at 19:32

Aside from the general "overfull panel" problem as already noted by others in comments, there is a much bigger concern. Adding 30A @ 240V for solar power is not the same as adding an electric dryer or other 30A @ 240V appliance. In one sense it is much easier - this is something supplying power rather than consuming power. But in fact there can be a bunch of other issues that really complicate the situation. Similar to (but not quite the same) the way that adding a generator is not as simple as adding a feed from the generator and remembering to turn off the utility power before turning on the generator (and vice versa). For example, if there is a utility power outage, what prevents the solar power from backfeeding into the utility feed and zapping the linemen?

I recommend discussing this with the technical department of the solar power company, and possibly with your regular utility as well. There may be a number of different things to be done, beside simply provisioning a 30A 240V circuit in the panel, to make sure that the solar installation will be both safe & effective.

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    It's not code legal to add a generator feed to a panel without a mechanical interlock that prevents the main breaker and the generator breaker being on at the same time for the reasons you mentioned. Also, solar inverters connected in a net metering fashion will not supply power unless utility power is available, so no worries about back feeding the grid with a properly installed solar system. – George Anderson Oct 14 '20 at 18:54
  • Good to know, I will have a net meter before interconnecting. – ScottP Oct 14 '20 at 19:30

Hey Scott: I hate to say it, but you have a mess on your hands. As others have pointed out, you have multi wire branch circuits (MWBC) that are not wired correctly. brhans correctly noticed (I missed it) that the MWBCs on the double stuff (tandem) breakers can overload the neutral and not trip....up to 40 amps on a wire with 20 amp capacity (12 ga). In detail, because both 20 amp breakers are on the same hot leg, if fully loaded, they could send 40 amps down a wire with 20 amp capacity. The breakers wouldn't trip because they only measure the amperage going out on the hot. It's clearly a dangerous and very code illegal install.

This whole panel should be replaced with a panel that has more capacity for breakers. A sub-panel might be a less expensive alternative to get you the breaker capacity needed and correct how the MWBCs are installed. At an absolute minimum the MWBCs need to be re-wired so that both hots are on opposite legs.

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    Thank you all for the detailed responses, I really appreciate it. I'm in escrow on the house with this panel, but I know a little about the history. House was built in 2006, and in 2007 the original owners put in a pool and spa (they used all 3 bottom taps), so that might be when things got jacked. I saw they pulled permits for the install, but it seems the city inspector, and the 2 home inspectors missed what many of you found. Thank you again, I'm appreciative and impressed. I will sort this out ASAP. – ScottP Oct 15 '20 at 1:24
  • A very rational and reasonable response, @ScottP! Most people don't want to hear that they're going to have to spend even more money. I'm sure you don't either, but you've got enough sense to realize that money spent now is less than money and inconvenience later when the house burns down. – FreeMan Oct 15 '20 at 15:22

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