US vs. EU Power Supply: Understanding the Differences for Your Home

[Guineafowl]

Hi all,

I've just watched a video about the problems of back-feeding your home with an inverter/battery source, most of which relate to the split-phase system in the US.

It got me thinking about the differences between this and the UK (and EU) mains supply, which is single-phase 230V. This seems better because:
1. Simpler wiring in and up to the home
2. Smaller wires (cheaper)
3. High-power appliances, like our standard 3 kW kettle and 2.6 kW vacuum cleaner, can be plugged in anywhere without needing (as I understand it) a special double-tap arrangement.

Other than greater safety from lower voltage, is there an advantage to the US system that I can't see? Or is the system there for historical reasons and there'd be too much upheaval to change it? I wouldn't be surprised if it's the latter; there are PLENTY of systems in the UK that fall under this heading.

 

[BvU]

Did you google 'Mains voltage history' ? Pretty clear story !

 

[Guineafowl]

Did you google 'Mains voltage history' ? Pretty clear story !

Yes, I see the reasons are historical, but I really wondered what the experienced bods across the pond thought of it, and whether they've found any particular advantages.

 

[BvU]

I read up on the split-phase system and it seems the US only have 2 phases, whereas EU has three (still less copper required).

The only advantage I can think off in favour of the US system is that the voltage is lower. However, that lower risk of serious accidents is more than offset by the psychological effect that people are less afraid of 117 V than they are of 230 V. Both are deadly.

 

[Windadct]

Outside of the whole risk 230 vs 120 V issue(dead horse) - I do not see there being any relation to back-feeding and split phase wiring vs 3 phase.

 

[russ_watters]

I would think the higher capacity branch circuits would result in larger feeders going into each house, unless a more significant diversity factor is allowed in the UK than in the US.

 

[russ_watters]

It got me thinking about the differences between this and the UK (and EU) mains supply, which is single-phase 230V. This seems better because:
1. Simpler wiring in and up to the home
2. Smaller wires (cheaper)
3. High-power appliances, like our standard 3 kW kettle and 2.6 kW vacuum cleaner, can be plugged in anywhere without needing (as I understand it) a special double-tap arrangement.

None of those is necessarily true, at least as stated. From the bottom-up:
3. The US doesn't have such high power appliances. I suppose that is a benefit for those in the UK, but the circuiting issues haven't changed: you wouldn't want to run the kettle and vacuum cleaner on the same circuit. And higher-power, permanently installed appliances like water heaters and stoves/ovens still need their own dedicated, higher power/amperage circuits.

1&2 are the same, but I don't think they are true -- or, at least, a compromise would need to be made elsewhere in order to make them true. A 3phase circuit of equal voltage to a single phase circuit carries 1.73x as much power, but a single phase 240V carries twice as much as a 120V. So either the incoming feeders need to be larger in the UK or the number of circuits in the house needs to be reduced (increasing the risk of overloading individual circuits) or the diversity assumption needs to be more aggressive. Otherwise, if you use the same math and layout, you could overload the incoming main in the UK house by 15%.

Other than greater safety from lower voltage, is there an advantage to the US system that I can't see?

Flexibility would be the main one I see, as you get to choose a voltage and circuit size more appropriate for your load.

 

[Guineafowl]

Outside of the whole risk 230 vs 120 V issue(dead horse) - I do not see there being any relation to back-feeding and split phase wiring vs 3 phase.

A standard UK home supply is just one of those three phases, and all appliances run off it. A single 230V inverter, if correctly set up, could easily be plugged into a socket and back-feed into the house. The US-based video covered the inability to run 220V appliances, even with two inverters (not easy to synchronise them 180deg apart), problems with neutrals common to both phases and their interference with GFCIs, etc.

 

[Windadct]

240V Outets are pretty common in the US, but backfeeding through an outlet is deadly dangerous and should NEVER be done.

 

[Guineafowl]

None of those is necessarily true, at least as stated. From the bottom-up:
3. The US doesn't have such high power appliances. I suppose that is a benefit for those in the UK, but the circuiting issues haven't changed: you wouldn't want to run the kettle and vacuum cleaner on the same circuit. And higher-power, permanently installed appliances like water heaters and stoves/ovens still need their own dedicated, higher power/amperage circuits.

1&2 are the same, but I don't think they are true -- or, at least, a compromise would need to be made elsewhere in order to make them true. A 3phase circuit of equal voltage to a single phase circuit carries 1.73x as much power, but a single phase 240V carries twice as much as a 120V. So either the incoming feeders need to be larger in the UK or the number of circuits in the house needs to be reduced or the diversity assumption needs to be more aggressive. Otherwise, if you use the same math and layout, you would overload the incoming main in the UK house by 15%.

Flexibility would be the main one I see, as you get to choose a voltage most appropriate for your load.

We in the UK would be lost without our kettles! And on a standard UK ring main, capacity 32A at 230V, you can certainly run the vac and kettle on the same circuit. Yes, a 6 kW oven does have its own spur, but I was really talking about portable appliances.

On a point related to Russ's comment, a UK home does not have 3 phases, just one. 3 phase power is supplied to commercial premises and farms, and I think the rms value is 400V.

For interest, my home is wired fairly typically and has 8 circuits:
1. Upstairs ring main 32A
2. Downstairs ring main 32A
3. Upstairs lights 6A
4. Downstairs lights 6A
5. Shower 40A
6. Immersion heater 16A
7. Borehole pump 16A
8. Workshop 16A

 

[russ_watters]

A standard UK home supply is just one of those three phases, and all appliances run off it.

Oh, I was going to suggest that was an additional advantage for the UK. It would allow two different voltages and single or three phase. Residentially, you get that in apartment buildings in the US: 120V, 208V and 277V single phase and 208V and 480V three phase all in one building.

But wait, does this mean that a house in the UK gets only one "hot" wire? That negates some of what was said previously about UK houses benefiting from smaller incoming service size...

And how many wires would they get? A hot, neutral and ground?

 

[Guineafowl]

Oh, I was going to suggest that was an additional advantage for the UK. It would allow two different voltages and single or three phase. Residentially, you get that in apartment buildings in the US: 120V, 208V and 277V single phase and 208V and 480V three phase all in one building.

But wait, does this mean that a house in the UK gets only one "hot" wire? That negates some of what was said previously about UK houses benefiting from smaller incoming service size...

And how many wires would they get? A hot, neutral and ground?

Yes, only one hot, called Live or Line, and only one neutral. My incoming feed cables to the consumer unit (breaker panel) are 25mm^2. The ground (called Earth here!) type depends - in rural areas you have a simple rod in the ground, but in more urban areas the Earth is split from the neutral just before entering the house.

 

[russ_watters]

We in the UK would be lost without our kettles! And on a standard UK ring main, capacity 32A at 230V, you can certainly run the vac and kettle on the same circuit. Yes, a 6 kW oven does have its own spur, but I was really talking about portable appliances.

On a point related to Russ's comment, a UK home does not have 3 phases, just one. 3 phase power is supplied to commercial premises and farms, and I think the rms value is 400V.

For interest, my home is wired fairly typically and has 8 circuits:
1. Upstairs ring main 32A
2. Downstairs ring main 32A
3. Upstairs lights 6A
4. Downstairs lights 6A
5. Shower 40A
6. Immersion heater 16A
7. Borehole pump 16A
8. Workshop 16A

In that case, I suppose you have different wire sizes throughout? Counter-intuitively, most of your circuits are equal or greater amperage from a US house!

In a fuel heated house in the US, you would have many (perhaps a dozen) 15A, 120V circuits for lighting and general power. You would have dedicated 120V, 15-20A circuits for small permanently installed appliance loads (refrigerator, garbage disposal, furnace, dryer) and dedicated 240V circuits for larger ones (washer, A/C), up to 60A. A non-fuel house would have additional 240V 40-60A dedicated circuits for the stove, water heater and dryer.

 

[Guineafowl]

In that case, I suppose you have different wire sizes throughout? Counter-intuitively, most of your circuits are equal or greater amperage from a US house!

In a fuel heated house in the US, you would have many (perhaps a dozen) 15A, 120V circuits for lighting and general power. You would have dedicated 120V, 15-20A circuits for small permanently installed appliance loads (refrigerator, garbage disposal, furnace, dryer) and dedicated 240V circuits for larger ones (washer, A/C), up to 60A. A non-fuel house would have additional 240V 40-60A dedicated circuits for the stove, water heater and dryer.

Sorry for the metric sizes -

1. 32A ring and 16A single - 2.5mm^2 3 core cable
2. 6A lights - 1 mm^2
3. 40A shower - 10mm^2 real beefy stuff

So the US system is more complicated but perhaps more flexible? But then that flexibility is only needed because of the complications? Maybe, maybe not. I don't know, but this is far more interesting than reading a Wiki article...

 

[Guineafowl]

I must say I prefer the US-style plugs - so much sleeker than the bulky, ugly UK ones. They're designed to minimise shock risk when pulling them out, and all have to be fused, so that's good I suppose, but if you've ever trodden on an upturned one at night...

Also, we're pretty much banned from having power sockets in bathrooms. Odd that it's considered OK in the US but not UK.

 

[russ_watters]

Sorry for the metric sizes -

1. 32A ring and 16A single - 2.5mm^2 3 core cable
2. 6A lights - 1 mm^2
3. 40A shower - 10mm^2 real beefy stuff

Standard 15A circuits in the US are 12 or even 14ga (12ga is 2.05mm). So your wires are even bigger than ours for the same amperage. And I was going to ask if the ring main allowed you to cut the wire sizes in half... What is the point of the ring main otherwise?

So the US system is more complicated...

I'm not sure I agree! But maybe it is just what we are used to...

 

[Guineafowl]

Standard 15A circuits in the US are 12 or even 14ga (12ga is 2.05mm). So your wires are even bigger than ours for the same amperage. And I was going to ask if the ring main allowed you to cut the wire sizes in half... What is the point of the ring main otherwise?

I'm not sure I agree! But maybe it is just what we are used to...

I suspect there's some overkill in the UK regs - a 2.5mm2 cable is rated to carry 16A, but if arranged in a ring main, is allowed 32A. This ring main can then supply an unlimited number of sockets, as long as the area covered is less than 100m2.

EDIT: Oh, and 2.5mm2 cable is actually 1.8mm in diameter.

 

[mheslep]

I read up on the split-phase system and it seems the US only have 2 phases, whereas EU has three (still less copper required).
...

In the US, split phase is commonly used close to the load, for residential and light commercial distribution. Higher power transmission up to those points is still three-phase.

 

[Averagesupernova]

https://www.physicsforums.com/threads/electrical-services.755036/page-5

https://www.physicsforums.com/threads/where-lies-ac-efficiency.849199/page-5#post-5331608

There may be more threads. These are the ones I came up with doing a quick search on my phone. Mobile is nice but certainly a compromise. I prefer a real computer to navigate PF.