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?
Let me start by saying that the question of three-phase power in a residence is something which I’ve given a disproportionate amount of time and effort thinking about, and it’s basically a dream of mine to one day do exactly what you’re doing: refitting a home so it is unchained from the limits of standard 120/240 North American split-phase power. I will say my dreams go
majorlyslightly off the rails, as you’ll see in a bit, but I can offer some points of consideration for your questions.<background info>
To make sure we’re all on the same page, let me lay some redundant background. Single-family homes in North America are provisioned with 3-wire “split phase” service, consisting of two live conductors (“ungrounded” in NEC speak) and a neutral. At the pole transformer, the neutral is tied to a ground rod, and at the service disconnect on the house, the neutral wire is tied to the house’s ground rod(s). The live conductors provide 120 volts w.r.t. ground and neutral, and 240 volts between the two live conductors. This is “split phase” because it’s a single-phase service that taps the neutral in the middle of the transformer, which “splits” the phase in twain. A pure single-phase service would only need two wires (live and neutral), but there would be no choice of voltage; split phase allows American homes to supply appliances for single-phase 120v or single-phase 240v.
Used predominantly in light commercial buildings, 120/208v 4-wire service is a 3-phase wye with three live conductors at the ends of the “Y” and a neutral wire at the center. As most services in North American are grounded somewhere (eg TN-S-C), the neutral is tied to a ground rod when entering the building. The line-to-neutral voltage is 120v and between any two live wires is 208v, due to the relationship of the sqrt(3) and other trigonometric thingies. This way, the building can supply single-phase 120v devices in three different combos, or single-phase 208v devices in three different combos, or three-phase 208v devices. For the most part, appliances that accept 208v also accept 240v and vice-versa, but best to explicitly check.
Historically, the same benefits of supplying multiple voltages and phases to the customer was also achieved using 120/240 “high-leg” delta 4-wire 3-phase service. This also consists of four wires, but arranged as a 3-phase delta (ie triangle), with three live wires at each corner and the neutral wire situated in-between the corners labeled A and C. If this neutral seems oddly similar to split-phase’s neutral, that’s because a high-leg delta is the logical 3-phase version of 120/240 split-phase. The neutral is tied to a ground rod upon entering the building. Between any two corners is a single-phase 240v supply. Between either A-to-neutral or C-to-neutral is single-phase 120v. But between B and neutral is single-phase 208v. That’s a bonus voltage! But it’s also a curse, because sometimes people forget about this high-leg and mistakenly feed 208v to some unsuspecting 120v appliances. Big oof.
</background info>
As for the relative merits of 120/208 wye versus 120/240 high-leg delta, they both can supply 3-phase appliances, as well as be separately used for supplying single-phase appliances. The obvious difference is that 120/208 wye maxes out at 208v in either 3-phase or single-phase, whereas 120/240 high-leg delta tops out at 240v in 3-phase or single-phase. Higher voltage means the same power can be sent with less current or less copper. J1772 or J3400 electric car chargers all tolerate up to 240v single-phase, so supplying only 208v single-phase means leaving some charging capacity on the table. Likewise, a lower voltage means less resistive heat, so a domestic clothes dryer that specifies 240v single-phase will dry a bit slower when supplied with 208v single-phase, due to Ohm’s Law.
As mentioned earlier, 120/240 high-leg delta has that bonus single-phase 208v, but this may not be very advantageous then, considering all of the drawbacks about 208v I just described. 120/240 high-leg delta already provides three different ways to make 240v single-phase, so there isn’t really a compelling reason to intentionally use the single-phase 208v “bonus” that it provides. If anything, it’s just a nuisance and footgun to watch out for. But this very much turns on what sort of loads you’ll have in your home.
The prevalence of 120/240 split-phase in American homes is in-part due to its bias towards serving lots of small 120v single-phase appliances and then a handful of larger appliances that need 240v single-phase. This works out great, until you want to draw more than 1500 W from a standard NEMA 5 receptacle.
For light commercial buildings, they’re assumed to have a lot more 120v single-appliances but rather than TVs and toasters, it used to be a lot of lighting. As in, ceiling lamps. So to provide power to these businesses with less resistive losses for the utility (not discussed here), as well as balancing the utility’s 3-phase grid, these customers get 120/208 wye service. The idea is that every third equal-length string of ceiling lamps is between different phases. So if they’re 120v single-phase lamps, the wiring would be phase A to neutral, phase B to neutral, or phase C to neutral. But if they’re 208v single-phase lamps, then wiring would be phases A to B, B to C, or C to A. In either case, the service is perfectly balanced and the resistive losses are reduced for the utility by supplying the customer with 3-phase. The customer can also supply their HVAC or elevators with 3-phase 208v, which generally works fine.
The same perfectly-balanced scenario is not possible for 120/240 high-leg delta if there are substantial 120v single-phase loads. This is because – from the diagram above – the 120v single-phase only exists between neutral and either phase A or phase C. Balancing every other lamp onto the available 120v single-phase wiring combinations always results in underloading phase B. The utility won’t be happy, and because ungrounded conductor wire sizes must be identically-sized for a 3-phase high-leg delta, this means the copper/aluminum for phase B is being wasted. This is partly why 120/240 high-leg delta is somewhat rare nowadays. However, 240v single-phase lamps can be balanced with a high-leg delta.
To be more clear, the benefit of 120/208 wye is the ability to supply lots of 120v single-phase appliances without major balancing issues. The drawback is that 208 < 240. The benefit of 120/240 high-leg delta is supplying lots of 240v single-phase or three-phase appliances, but has a lower capacity for 120v single-phase appliances and a confusing 208v.
With all that said, we can now answer your questions 1 and 2 simultaneously. Since you expect to have numerous 120v loads, but want the flexibility to also feed 240v single-phase, 120/208 wye is looking pretty good. A three-phase panel would have it so that every breaker slot is fed from a different phase, so a single-phase circuit would have one hot wire from the breaker, and another wire to the common neutral bar. This setup is easy to work with and has no chance of 208v appearing w.r.t the neutral bar. You would have to contend with an electric car that charges slight slower, and a dryer that runs slightly longer.
For question 3, I’m of the opinion that balancing phases in a residence – even one with three-phase – should stop when it becomes more granular than individual breakers. Obviously, three-phase loads are self-balancing. But for single-phase 120v and single-phase 240v circuits, it’s a “best effort” where your electric car charger, dryer, and welder are placed on different phases, and your bedrooms on different phases. Anything more optimized than that is excessive and is barely noticeable by the utility anyway. They’re probably more concerned about your power factor than the phase balance anyway.
People like you are why I love Lemmy. Thank you. Even when I was looking online earlier I couldn’t find anything nearly as thorough as what you sent.
So it sounds like 240V would be better as far as sheer power goes but is much more fidly. However, my provider is also unlikely to care too much about balance as far as household consumption goes. If I have 3 phase then the heaviest consumer in my house (hvac) is going to be running on that and therefore perfectly balanced anyways. Basically every other load is going to be intermittent so I shouldn’t ever have an unbalance large enough that my provider would care.
I personally would jump on the opportunity to do 3-phase if it were offered to me. Even if my utility didn’t give me a choice in the “flavor” of 3-phase, the fact is that the appliances which need 3-phase (lathes, HVAC, large servers, certain German ovens, very very large solar systems) almost always can be rewired to either wye or delta supplies, provided the voltage is either 208v or 240v. If the appliance wants 480v 3-phase delta, then you’re truly out of luck.
But that’s why I want 3 phase so much: you can’t (efficiently) supply an appliance needing 3 phase if you don’t have it from the utility, but a utility service providing 3 phase can always be split it down to multiple single phases. And I don’t want to preclude myself from ever owning an industrial-sized lathe lol
I should note that for the 208v versus 240v power dilemma, the prevalence of 208v in commercial settings means that large equipment often already sizes their wiring for 208v. Meaning that you already get full performance with 208v, and giving it 240v is just icing on the cake that reduces the current draw a little bit. For example, a modern arc welder that is built with inverter technology usually is no different on 208v or 240v, but drastically cuts back when given 120v. Likewise, a lathe isn’t going to turn slower just because it’s given 208v rather than 240v, but may have a bit less starting torque. If it were a huge deal, industrial folks would reject 120/208 wye outright.
I work mostly with servers and networking equipment for data centers, which are single-phase appliances but the building is often supplied with 3 phase. We lump everything above 200v – 200v Japan, 208v North America, 230v Euro/Asia, and 240v British/North America – as all the same thing when it comes to calculating availabile power.
<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.
I bet it would help to know what region you live in, because I know for sure things are different per country.
Good point. Ill add it to the og post. USA:MN
Disclaimer: I’m from Europe, I only speak 240/400
About phase balance, you are right to wire stuff in a way that splits your power usage as best you can ; there is a simple way of checking it: just measure voltage between neutral and phase under load on each phase. Your risk in case of severe unbalance is to have the stressed-out phase going down, and the other going up.
How much up and down should we start to worry about? 10% is what I heard from my utility company (Belgium). Given that as a residential customer you are never alone on the grid, your chances of really twisting things up are slim.
That’s kind of what I was figuring on the phase imbalance. I’ve seen it before on industrial sites. I just wasn’t sure if a household power draw could cause the same issue or not.
