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Comment by nickff | Hacker News Reader

>"[W]e basically always have at least one phase providing power at any time."

This isn't really how three-phase power works.

In household settings, you're usually getting only one phase, so all your power comes from that phase. Different neighborhoods get different phases, so they're roughly balanced loads.

Industrial (and some consumer) users get two (208V) or three phases (480V), and they get power from the difference between the phases (for their higher-voltage loads).

Now, if you've ever looked at an oven or a dryer (in North America at least), you'll know that they are higher voltage, and look like they are two-phase... But it's really more like one phase that's been mirrored.

Here's a nice diagram: https://electronics.stackexchange.com/questions/146420/obtai...

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Typical residential service in the US is 240v single phase. The transformer on the pole is putting out 240v, but there's a center tap on the transformer that's bonded to ground. That's your neutral. The other two legs are essentially +120v and -120v relative to ground/neutral. Your dryer hooks up to both legs and sees the full 240v. Don't ask me how grounding works for 240v appliances. I have no idea.

I am not an electrician or EE. I just spent a bunch of time looking into this when I got my 240v table saw and had to hook it up to something.

As proof that I'm not totally bullshitting you: until recently our house had a 120v outlet on a 240v breaker for a baseboard radiator. The electrician was here to fix an unrelated issue, and rejiggered things in the panel to hook it up to a 120v breaker.

As far as I understand it (not very) there is no such thing as two-phase power.

The 240V that you're talking about is what I mentioned as 'mirroring'. There definitely is such a thing as two-phase power, my electric car charger is 208V from two industrial phases, and our reflow oven is 208V, from two similar phases.

That's two phases of a three-phase supply.

From your car's or oven's perspective, it's still just single-phase power that happens to be floating w.r.t. ground.

Well, it would be the same as single-phase power (to the device) if the devices were not connected to the neutral. The oven actually takes one of the phases (along with the neutral), and uses it to power some accessories.

With the massive caveat that I don't have a first principles understanding of any of this: I don't thing any individual load in your oven takes both legs and the neutral to do work. They're all either 120v or 208v single phase.

Aside: a lot of motors can run on either 240v single phase or 208v single phase (or, presumably, anything in between?). Take a look at Grainger or McMaster-Carr.

That's still single phase as the power you're seeing is the vector sum of the two phases.

> Don't ask me how grounding works for 240v appliances. I have no idea.

Ground is 'the same' for 240v and 120v on split phase. A usually bare copper wire is run from the ground pin to the service panel, in the main service panel ground and neutral are connected together and to the utility neutral and the grounding rod.

Most 240v outlets are hot-hot-ground, or hot-hot-neutral-ground; some older outlets were hot-hot-neutral and the appliance was supposed to be wired separately to ground (and we can all suppose how often that was done properly, resulting in that outlet type becoming disfavored).

I'm currently rejiggering a 240v wired box for 120v outlets, after finding the wires in the panel and confirming they don't power any other 240v outlets, I'm connecting the white wire to the neutral bus bar, and the black wire to a spare 120v breaker, and then it's normal wiring of the outlet.

Enjoy https://youtu.be/jMmUoZh3Hq4 .

> As far as I understand it (not very) there is no such thing as two-phase power.

Because it's obsolete (but still exists in some places like center city PA). Two phase has two windings in the generator 90 degrees apart. Service was delivered via 3, 4, or 5 wires. Three phase beat it for a number of technical and economical reasons.

At least in Sweden, all houses have a 3-phase, 400V supply. This is very different from the US which just supplies one phase to each house even if the supply itself is 3-phase. This is why you commonly see 200+A main breakers on a house in the US whereas in Sweden, 25A is a pretty common main breaker size.

(It's as if the US grid characteristics were decided by the copper vendors... ;-)

Well, you have to consider that the USA (and Canada) were on the bleeding edge of technology when they adopted electrical grids. They were left with a... legacy, including the type-A plug and 110V single-phase power to residences.

In your case, I believe each prong of your two-prong plug gets a different phase, so you have 3 possible pairs of phases to power each circuit from. This has the advantage of requiring smaller wires, but the disadvantage of increasing the risk if one is electrocuted.

> 110V single-phase power to residences.

The US is 120V, or split-phase 240 V. Depending where you are, it's allowed to be ±5% at the meter. NEC guidelines say no more than 3% voltage drop from there.

Yeah, I got the number wrong, 120/220 always messes me up; will leave previous comment as-is so that yours makes sense.

You might have 208V and not 240V if you have two 120V phases 120 degree apart.

No, one prong gets a phase and one prong gets the neutral, resulting in 230 V. Only equipment designed for 400 V gets two (or three) phases.

House-wide RCDs are also standard so if any current is not returned through neutral the circuit will be broken.

> but the disadvantage of increasing the risk if one is electrocuted.

Avoiding this was one of the design parameters of the "Schucho" plug design that is reasonably standard across Europe. You'll note the three prong plug has a longer ground that makes a connection before any hot one does. The US three prong is supposed to work that way too but the ground pin isn't long enough.

I continue to be amazed how many devices still ship with only two prong plugs (everywhere: Europe, North America, Australia...) even if those prongs are polarized.

> I continue to be amazed how many devices still ship with only two prong plugs (everywhere: Europe, North America, Australia...) even if those prongs are polarized

Because most devices these days have plastic cases (Class II double insulated), so there's nothing metal to touch to get shocked even in the case of a fault. That coupled with how GFCI/RCDs are being widely adopted as well leads to grounding not being as important.

I'm (mostly) with you on the plastic casing, but in general (in the US at least) you only see GFCI in kitchens and bathrooms.

Meanwhile I'm sitting 2 feet away from a nearly completely metal lamp with a two-prong plug.

Find this very scary not having RCD on some plugs even with 120 Volts. Sure 240v with RCD will give you a massive shock but still will very unlikely kill you. No RCD on the other hand can be fatal on 120V: https://edition.cnn.com/2017/07/18/health/teen-bathtub-elect...

Its Schuko, short for SCHutzKOntak, "contacts and ground".

If we're being pedantic, it stands for "Protection contact"

>It's as if the US grid characteristics were decided by the copper vendors...

It was designed without an issue with copper usage. Europe had very little development and quit a bit of rationing while building out the grid post-WW2. The US had no such issue and spent no time working within those constraints.

Not just Sweden. In Netherland, 3x25A is also pretty standard. To our surprise, our house turned out to have 1x35A, so when we got a big induction cooker, we had to upgrade to 3x35A.

ConEd standard* service in NYC provides 120/208 three or four wire service from a wye connected transformer bank. With the density of the city it's easier to just run three phase all over and let homes pull alternatingly off the hots. So homes connect A-B, B-C, A-C and so on.

Three wire service means you get two hots and a neutral and is considered a single phase "open wye" service. BUT you can reconstruct the third phase from the two half vectors in each phase using transformers.

Four wire gives you full three phase service so you can run big motors, machinery, lots of lighting and such.

*Though plenty of neighborhoods are fed 120/240 or mixed. I'm on 120/240 yet the next block is 120/208. Sucks because I want my three phase.

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