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32V or ~13%.

In the US, only residential areas have split-phase 240V (240/120 single-phase). In the commercial world, 3-phase goes everywhere and that means 208V line-to-line 3-phase transformers are used to run 120V outlets since 208V line-to-line means 120V line-to-neutral in a three-phase system. That means, if you want to run something other than 120V, you have three choices - 208V or the next steps up, 480V/277V (line-to-line and line-to-neutral on a 480V three-phase system). This is why many lighting systems in commercial buildings run on 277V.

If our car chargers could handle 277V, that would have been a reasonable thing to run to L2 chargers. Since they can't, 208V is the highest voltage available at a commercial establishment that doesn't exceed the voltage rating of our chargers.

 

I guess I didn't answer the question - why do we use 3-phase?

The answer is simple optimization. Three phase transmits the most power through a given cross-sectional area of conductor of any phase-count. Further, you can't generate a rotating magnetic field with single-phase but three-phase does this well. You need a rotating magnetic field to drive a motor.

So, commercial and industrial applications use 3-phase for those reasons.

 

yes there is a reason. Commercial and business districts are already wired for it. IE it's handy.
EDIT
Oops, I see LeeJay already touched on it.

not to pick nits, but to be more precise, between single and three-phase - yes 3 is optimal, but if we wanted to spend more money, we could have run higher infrastructure phases - 5 ... 10, Etc.

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One additional item is that in single phase, power is pulsed -- 120 pulses per second on our 60Hz systems. But 3-phase ((*) or any balanced multi-phase system) has flat smooth continuous power. While each individual phase is pulsed, the shapes and offsets are such that when added together, the sum has no pulses, an advantage on rotating machinery, i.e. motors and generators.

(*) one of my old college profs worked on a legacy industrial 2-phase system in his early years. 90 degree phase difference. This had the same smooth power flow as 3-phase, but not its minimum-wire-mass advantage.

 

A higher phase-count is less optimal than 3-phase. As I said, for a given cross-section of conductor, 3-phase is the best split of that cross section (if you ignore the skin effect). If you include the skin effect, DC can be better over a longer distance.

 

Commercial and industrial use 3 phase power as standard off the tap.
The reason for 208 is because the company could care less about 110 or 220, they just need the proper 3 phase power and 208 is an incidental afterthought that requires no extra equipment to produce and is used solely for low end equipment, anything mission critical is on the 3 phase which they DO care about,
208 is cheap to produce and accidentally available if someone needs it for some reason.


Around here every light pole has 277VAC, too bad no EVSE/charger supports it (Another accidental voltage from existing 3 phase power)

 

The opposite is the case.

As I said above, the reason for 208 is 120. They need 120 for convenience outlets and the best way to get tons and tons of it from 3-phase is to step the 3-phase down from whatever it starts at (which can be many things) to 208 line-to-line. This gives them 120 line-to-neutral times three (three phases). That's why 208 is available in buildings - because of the need for 120. And the 208 is always 3-phase, it's just that we have only one phase to each charger.

 

My guess it allows battery heating with lower reduction in charge time.
Plus, less options means lower costs for the smaller market.

 

one of the Pioneers in power transmission say you WILL get more - nearly double the power going 3 to 6 phase, only it's more complicated / expensive at the substations -

We used to have a 3 phase Bridgeport Mill in our residential garage. No problemo ... rotary phase converter motor ... surprisingly - very little loss going from a modern 240v single phase converter to 3 phase. And, even if it wasn't ... some need it in the residence, as we did. Some have used this configuration to run DIY - DC QC projects - both Chademo & SAE.
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Twice as much power. But also twice as many wires, circuit breakers, bus bars, etc. And no hint that any of them could be made smaller. He gave no indication that the power per hardware unit might improve.

Well, maybe for the transmission towers. But many of them already have 6 wires (or wire bundles, for those that use multiwire bundles to reduce inductance), for two separate 3-phase circuits.

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With 3-phase and an appropriate transformer bank, you can build 6- or 12-phase. More broadly, from any balanced multiphase system, one should be able to produce any other. Though these may be approximations due to the constraint that transformer turns counts should be integers.

 

He agreed with what I said. You use double the conductors and you get ALMOST double the power. What I said was that, with a given cross section of conductor, you'll get maximum power with 3-phases. Same thing. To get more *for the same cross section of conductor* by switching to 6 phases with twice as many conductors, you'd need to more than double the power, not less.

We did this in college - you can prove (with simplifying assumptions like no skin effect) that 3 phase is the way to get the most power through a given amount of conductor cross section.

It's pretty easy to see why 3-phase is better than 2-phase (which is the same as single-phase:

P (two-phase) = V * I/2 where I is the amount of current you can get through a single conductor of the total cross section of the two you're using here.
Power (three-phase) = sqrt(3) * V * I/3

1/2 = 0.5
sqrt(3)/3 = 0.577 <--- More than 0.5 so more power from the same cross section.

 

True 2-phase is quite different than the normal household split-phase, which really is single-phase.

2-phase has two hot wires 90 degrees out of phase, plus a mandatory neutral. It produces a true rotating field in motors, and smooth non-pulsed power flow, just like 3-phase and higher orders. But it doesn't save on conductor area the way 3-phase does, so it vanished long ago.

Whereas balanced 3-phase (and even balanced split-phase) can dispense with the neutral wire, true 2-phase can never be used without its neutral. In fact, its neutral must be larger than its hot lines, as it is carrying sqrt(2) more current, thus no conductor savings. 3-phase transformer banks can be configured as Wyes or Deltas, but 2-phase has no Delta equivalent.

One of my profs had direct first-hand experience with 2-phase, before it was put to bed. More recent students are unlikely to have had teachers from that era.

 

Now I see the advantage of 6-(or more) phases compared to 3-phase: it allows more power to be carried in the same physical space, such as the limited width of a right-of way. It doesn't save parts count or hardware mass, only space:

"High-phase-order (HPO) power transmission has been frequently proposed as a way to increase transmission capacity within a limited-width right of way. The required conductor spacing is determined by the phase-to-phase voltages, and six-phase power has the same voltage between adjacent phases as between phase and neutral. However voltages between non-adjacent phase conductors increases as the difference increases between phase angles of the conductors. Conductors can be arranged so that non-adjacent phases are spaced farther apart than adjacent phases."