Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Why Does the U.S. Have Three Electrical Grids?

spectrum.ieee.org

Post reply on HN
Print view View original

31–40 of 155 posts

Re: Why Does the U.S. Have Three Electrical Grids?

#31

Post by ghouse »

nickff wrote: post #26
lutorm wrote: post #16

Earlier quoted context omitted.

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 smal…

> 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.

Re: Why Does the U.S. Have Three Electrical Grids?

#32

Post by mauvehaus »

nickff wrote: post #6
Pfhreak wrote: post #4

> Everything within those separate zones is synched up. So you’ve got your 60 hertz AC wave; 60 times a second the AC power flow is changing direction. And all of the generators, all of the power consumption within each zone is doing that synchronously. But the east is doing it on its own. The West is on a different phase. Same for Texas. Folks may not realize that power is transmitted in three phases in the US. AC p…

>"[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 differe…

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.

Re: Why Does the U.S. Have Three Electrical Grids?

#33

Post by ghouse »

crmrc114 wrote: post #14
Pfhreak wrote: post #4

> Everything within those separate zones is synched up. So you’ve got your 60 hertz AC wave; 60 times a second the AC power flow is changing direction. And all of the generators, all of the power consumption within each zone is doing that synchronously. But the east is doing it on its own. The West is on a different phase. Same for Texas. Folks may not realize that power is transmitted in three phases in the US. AC p…

Close, Three phase systems actually are a carry over from the days of early induction motors. Three phases can easily be connected to create a rotating magnetic field. We don't just use one phase at a time on the grid. All three are active. Phases A and B and C are all 120 degrees out of phase with each other. (That has nothing to do with the 120volts you know of in your house- it just happens to be another 120 numbe…

> Both 120 degrees out of phase. So if you remember elementary school math the abs() of phase A at its peak of +120 and phase B at it's valley of -120 = 240.

I think you mean 180 degrees out of phase.

Re: Why Does the U.S. Have Three Electrical Grids?

#34

Post by labcomputer »

crmrc114 wrote: post #14
Pfhreak wrote: post #4

> Everything within those separate zones is synched up. So you’ve got your 60 hertz AC wave; 60 times a second the AC power flow is changing direction. And all of the generators, all of the power consumption within each zone is doing that synchronously. But the east is doing it on its own. The West is on a different phase. Same for Texas. Folks may not realize that power is transmitted in three phases in the US. AC p…

Close, Three phase systems actually are a carry over from the days of early induction motors. Three phases can easily be connected to create a rotating magnetic field. We don't just use one phase at a time on the grid. All three are active. Phases A and B and C are all 120 degrees out of phase with each other. (That has nothing to do with the 120volts you know of in your house- it just happens to be another 120 numbe…

> Close, Three phase systems actually are a carry over from the days of early induction motors.

It's not just some historical accident. We continue to use multi-phase power because it enables continuous delivery of power (unlike single-phase, where the delivered power drops to zero at least twice per cycle). And two phases require just as many wires as three phases, with more power delivered for a given mass of copper and insulation, so... we use three.

> If the current on phase a is 20 amps and the current on phase B is 20 amps what is the return current on my neutral?

That depends on the (complex) impedance in each branch. In the pathological case, the current on the neutral could be as high as 40 Amps.

Re: Why Does the U.S. Have Three Electrical Grids?

#35

Post by selectodude »

lutorm wrote: post #16
nickff wrote: post #6

Earlier quoted context omitted.

>"[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 differe…

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... ;-)

>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.

Re: Why Does the U.S. Have Three Electrical Grids?

#36

Post by nickff »

mauvehaus wrote: post #32
nickff wrote: post #6

Earlier quoted context omitted.

>"[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 differe…

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 ele…

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.

Re: Why Does the U.S. Have Three Electrical Grids?

#37

Post by mauvehaus »

lutorm wrote: post #21
gerdesj wrote: post #7

Earlier quoted context omitted.

"Folks may not realize that power is transmitted in three phases in the US" My office in the UK has three phase power (I own it.) The phase shift thing is to enable putting more power into a place and nothing to do with "so that we basically always have at least one phase providing power at any time." whatever that means. My home only has one phase because I don't need to draw that much power here (yet.)

"so that we basically always have at least one phase providing power at any time" is sort of what makes things like AC motors run better on 3 phases.

They're also self starting and trivially reversible. Single phase motors require a separate starting arrangement.

Re: Why Does the U.S. Have Three Electrical Grids?

#38

Post by labcomputer »

ghouse wrote: post #33
crmrc114 wrote: post #14

Earlier quoted context omitted.

Close, Three phase systems actually are a carry over from the days of early induction motors. Three phases can easily be connected to create a rotating magnetic field. We don't just use one phase at a time on the grid. All three are active. Phases A and B and C are all 120 degrees out of phase with each other. (That has nothing to do with the 120volts you know of in your house- it just happens to be another 120 numbe…

> Both 120 degrees out of phase. So if you remember elementary school math the abs() of phase A at its peak of +120 and phase B at it's valley of -120 = 240. I think you mean 180 degrees out of phase.

It's not clear what he meant (because this part doesn't really have anything to do with 3-phase), but:

Most houses in the US have 240V service for large loads like A/C, ovens, stoves, dryers, EV chargers, etc. The way you get 240V (in the US) is that you take a single phase, and apply it to a transformer. The transformer's secondary has a grounded center tap, so you get +/- 120V, 180 degrees out of phase (this is called "split phase").

Re: Why Does the U.S. Have Three Electrical Grids?

#39

Post by nickff »

ghouse wrote: post #31
nickff wrote: post #26

Earlier quoted context omitted.

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 smal…

> 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.

Re: Why Does the U.S. Have Three Electrical Grids?

#40

Post by gizmo385 »

danielheath wrote: post #24
SECProto wrote: post #17

Earlier quoted context omitted.

I see a lot more natural electricity (lightning, monthly) than I do natural fire (wildfires? Never)

Almost as though methods to prevent wildfires are one of the earliest human inventions.

Given the record fire seasons in Colorado and California, not sure how well this comment reads.
Post reply on HN