Location: USA:MN
To preface, I’m a refrigeration mechanic, so I only know just enough about three phase power to get into trouble hook things up and make sure they work.
I’m working on a large remodeling project in my home durring which I want to future proof as much as I can (because foam insulation makes changing things later a bitch). I’m going the full 9 yards running conduit and everything. As part of that future proofing I am planning on upgrading my service from 100A to 200A. However, since I’m upgrading my service anyways, I am also strongly considering getting a three phase service. If I ever wanted to stick an electric car charger or other big piece of equipment in then a three phase connection would be handy to have. It also seems like the kind of upgrade I want to get done while the house is mostly gutted rather than trying to shoehorn it in later. So my questions are as follows.
- Do I go with a 120/208V 4 wire service or a 120/240V 4 wire service? My provider offers both to residential customers in my area. There are additional restriction on the service drop for the 240V option. None of those appear to apply in my case but it might make 240V a bit more of a pain to get.
- Do I need to worry about phase balance? Since this is for a single family home most of my power draw is going to still be 120V between a single phase and neutral. Obviously I want to split circuits up between the 3 phases to try to draw on them evenly, but it’s never going to be split perfectly evenly. Is drawing on the phases unevenly going to cause any sort of issue?
- Are there any other footguns to watch out for here? For example, is having three phase power going to mess with my home insurance rates or anything like that?
<extra section; something I can’t do but maybe you can>
If you’ve made it this far, I congratulate you in keeping pace with my rambling lol. But since you mentioned future proofing, I wanted to mention something which I’ve envisioned but was rendered impractical with NEC 2017 and later. Perhaps your jurisdiction is on an older version and you can pull this off. That you’re running conduit might make this less attractive, but maybe it’s food for thought.
Imagine if at every general-purpose 120v outlet in your house could be rewired – without re-running the circuit – at the junction box to instead provide 240v through a NEMA 6 outlet, all while the other devices on the same circuit still see 120v. Or maybe instead, you want to double the capacity of every duplex outlet in your house, so the upper receptacle can use all 20 Amps while the lower receptacle has its own 20 Amps. All this is possible, by running only an additional wire as part of your circuits.
A multi-wire branch circuit (MWBC) is when three or more conductors are part of a circuit. The classic form in a home is two live wires and a neutral. That way, the receptacle can be wired A-to-neutral or B-to-neutral for 120v, or A-to-B for 240v. Or even both: have the upper outlet be A-to-neutral and the lower outlet be B-to-neutral. This is basically bringing the split phase supply all the way to every junction box. With this in place, if you wanted to use a welder, space header, or cryptocurrency mining rig in your living room, you only need to replace the NEMA 5 receptacle with a NEMA 6 receptacle suitable for 240v. The current limit remains the same, so doubling the voltage means doubling the power.
In my dreams, every general-purpose circuit in my house would be an MWBC, and every outlet would be a 2-gang junction box, with NEMA 5 120v receptacles on the left (split so phase A is on top and phase B is on bottom) and with NEMA 6 240v receptacles on the right. These junction boxes would need 12/3 wire (ie 3 conductors plus some sort of ground), so the extra cost to implement an MWBC is from that third conductor, compared to a standard circuit which might use 12/2 wire. From thes outlets, I could attach two 120v space heaters OR one industrial 240v space heater, and not trip the circuit. I would retain full choice.
With 120/208v three-phase supply, it is trivial to use double-pole breakers to supply the two hot wires for every MWBC, and just by placing them one after another in the panel, the same balance is achieved across the phases, at least at the circuit level. The reason NEC 2017 killed my dreams is because AFCI protection is a requirement on general circuits, but some special circuits also require GFCI as well. There do exist two-pole 20-amp AFCI breakers, and there do exist two-pole 20-amp GFCI breakers, but I’ve never found a combination AFCI+GFCI 20-amp two-pole breaker. Meaning NEC 2017 and beyond does not allow MWBC for these special circuits, which includes bathrooms, kitchen counters, garages, and outdoors. And no doubt that future NEC versions have more requirements.
But if your area doesn’t have NEC 2017 yet, and MWBC seems enticing to you, it’s certainly an option. It would give you a choice to be a mostly-120v house, a mostly-240v house (similar to European 230v), or anywhere in the middle. But since you’re running conduit, I suppose re-running a circuit might not be too difficult.
I wish you the best of luck with this interesting project, and I hope you’ll keep us up-to-date on what you choose.