Three phase, split phase, or single phase when no rotating machines why three phase?

[ericmark]

I have seen reference to having a three phase supply to charge a car, but why, would not a 200 amp supply be better than a three phase 60 amp supply? For the user there is no worry about balancing the load, I remember a mill being converted into a house, and three consumer units were fitted, which were fed from a 160 amp three phase switched fuse box with three 60 amp fuses, all fed from the same phase, done that way as it was uncertain at the time what the supply would be, but for the user unless he has motors why would they want three phase, they could just as easy have two or three consumer units all supplied from the same phase.

Have I missed some thing, what advantage is three phase unless running motors?

 

[bhm1712]

To answer in the most simplest way, 3 phase means you get more power, at a lower current and therefore less conductor material required (smaller cables) and so costs are lower. There are technical reasons why 3 phase has advantages over single phase, but for the purpose of discussing EV charging or home useage, I don’t think it necessary to go into that.

 

[flameport]

Have I missed some thing, what advantage is three phase unless running motors?

For most end users, none.

It's provided like that because that's how it's generated and distributed.
Switching to high current single phase supplies would require significant changes to existing infrastructure.

 

[plugwash]

Ultimately unless you want to pay through the nose you have to deal with what the supplier will give you.

The supplier/distributor would normally rather give you a three phase 60A supply than a single phase 200A one for several reasons.

1. It allows them to use less copper/aluminium while maintaining acceptable volt drop/cable heating.
2. It helps them keep the network balanced on their side.
3. Standard UK "direct metering" equipment doesn't support 200A. So a 200A supply would require either a new type of meter or CT metering.

 

[ebee]

one reason, if you consider a number of properties on the same estate. to supply a single phase to them all would require two conductors plus earth perhaps. a 3 phase supply would need 4 conductors. a 2 phase supply would require 3 conductors. These conductors 2, 3 or 4 would be running near to each property then the last small portion need only be 2 conductors for single phase usage. The 3 phase conductors need only carry one third of the current that the equivalent single phase conductors would. so less cable material used.

For industrial power to machines you might liken it to the difference of a single cylinder motor bike and a 3 (ok then 4) cylinder car engine. Gives it more umphh.

you could run any number of phases you want and the cables you need are number of phases plus one for the N conductor. However 3 phase is easy peasy to reverse rotation be simply transposing 2 of those phases.

Note - in practise the N conductor might be reduced in size if the loads are, to some extent, balanced

 

[ericmark]

The normal domestic supply varies between 60 and 100 amp, or 13.8 kW to 23 kW, after 23 kW some where some how the supply does need splitting as you can't get a consumer unit over 100 amp, and to be in the control of an ordinary person we want consumer units not distribution units.

So the supply will need multi fuses over 23 kW however also you don't want 400 volt or 460 volt (three phase or split phase) in any one room, so to use three phase or split phase the home will likely need rewiring. And we will have two or three consumer units, and two or three fuses.

The problem is then likely one phase will power only the car charger, one upstairs, and one down stairs, so the power is not even distributed, and the whole thing we are looking for is for the three phases to be even loaded.

So one would get a more even load with 200 amp supply to three homes, with two fuses in each home and a single phase to each home, than allowing the home to split up what is used on each phase, as one could have three car chargers on one phase, three down stairs on another phase, and three upstairs on the last phase. Which could lead to a massive imbalance.

Having a single phase split into two one fuse for car charger and one for rest of house, would allow the DNO to use smart controls so in an emergency they can just turn off supply to car charger without effecting the main house. In other words just like the old system with storage radiators, with the old white meter.

Also no chance in any home of getting 400 or 460 volt.

I can see the point in three phase with rotating machinery, but for everything else, would be better to have two supplies from same phase rather than two or three supplies from different phases. From a single phase it must be safer.

 

[ebee]

eric. around here some end terrace house properties - think the good old corner shop - have 2 phases from a 3 phase system coming in. one such i had 2 of the old electric board lads to move the supply head to the cellar, thereby shortening the PILC. one chap exclaimed "oh there are two reds and one black, i will join the reds together for more current!" , bedfore i could shout "WHAT!" his pal stopped him and checked with his tester, two phases of a three phase system.

we dont practice methods of keeping adjacent phases separate as much as we used to not many years ago

 

[brig001]

Car chargers (and most non-linear loads e.g. inverters etc.) have a harmonic current that doesn't cancel in the neutral

A three-phase car charger (or inverter etc.) eliminates this problem as there is no neutral connection

Neutral Sizing in Harmonic Rich Installations

Published by Prof Jan Desmet, Hogeschool West-Vlaanderen, Email: [email protected] & Prof Angelo Baggini, Università di Bergamo, Email: [email protected], June 2003 Source: Leonardo Po…

powerquality.blog

Brian

 

[JohnW2]

..... as you can't get a consumer unit over 100 amp, and to be in the control of an ordinary person we want consumer units not distribution units.

You often say that, but I'm not sure where it comes from.

The only sort-of relevant reg of which I am aware says that IF an installation is single-phase and IF it is no more than 100A and IF is is 'under the control of an ordinary person', then the DB may be a ('type tested') CU - which affords some relaxation of the requirement for current-breaking capacity of devices in it.

However, I am not aware of anything which says that if the installation is 'under the control of an ordinary person' it cannot have a DB which does not qualify as a CU (and therefore does not enjoy the relaxation in afforded to CUs).

Am I missing something?

Kind Regards, John

 

[ericmark]

So why do we have consumer units? Likely it is not BS 7671 but some other ruling, maybe building regulations, since I did not question when told, I don't know what regulation, I know I questioned the size of tip on multi-meter test leads, and I was directed to GS 38 which again has nothing to do with BS 7671.

 

[JohnW2]

So why do we have consumer units?

As I said, as far as BS7671 is concerned, the only 'advantage' of having a CU (rather than a 'non-CU' DB') I can think of is that it will usually allow the use of 6kA devices within it, whereas a non-CU DB may well require (per BS7671) devices with higher breaking capacity. In any event, as you know, BS7671 is not 'mandatory'.

Likely it is not BS 7671 but some other ruling,

I am unaware of any other rules or regulations ('mandatory' or not) or legislation which relate to details of domestic electrical installations, are you?

maybe building regulations,

As you are aware, the only part of the Building Regulations which relates to electrical matters ('Part P') is just one sentence, which certainly says nothing about CUs or DBs.

I may be wrong, but it seems to me that the alleged requirement that DBs 'under the control of ordinary persons' have to be CUs (as defined by BS7671) is probably something you have imagined or 'made up' - which I would say makes it a little misleading that you repeatedly make the assertion.

Kind Regards, John

 

[ericmark]

As you are aware, the only part of the Building Regulations which relates to electrical matters ('Part P') is just one sentence, which certainly says nothing about CUs or DBs.

Not really, as you read through the other parts we find reference to height of sockets for example, and conserving energy, each regulation seems to refer to others within them, IP rating for example.

I do see the problem, I remember being told not allowed a socket within a meter of the sink, and it seems back with the 14th Edition (Not a British Standard back then) the wiring regulations was like the guide to the wiring regulations and regulations combined, and there was a reference.

We have seen it with bonding in bathrooms, and earthing of lights, and when @flameport kindly showed us what the old editions actually said, it was not as cut and dried as we had been lead to believe, with "mounted at such a height that they cannot readily be touched and are out of reach of earthed metal." so it seems wall lights should have been earthed?

There is the question is a home a place of work? Be it a social worker, ambulance man, police, or some one working from home, likely at some point every home will be a place of work for some one, so electricity at work act becomes valid. However I trained as an electrician not a lawyer, and I have taken on trust what my collage lecturers have told me, I agree there is a lot of miss information given out, but the common sense approach has to be, if we can use a commercial distribution unit, why was the consumer unit invented?

We know a consumer unit is a type tested distribution unit, but why are they type tested? I have been lead to understand they were type tested to ensure they were safe to be in the control of an ordinary person, if that's not the case, why do we have them?

 

[ebee]

well they certainly make it easier than a dist board and separate main switch for the ordinary person

 

[JohnW2]

Not really, as you read through the other parts we find reference to height of sockets for example, and conserving energy, each regulation seems to refer to others within them, IP rating for example.

I think you're scraping the barrel. Few, if any, of the Parts of the BUilding REgs contain any detail at all. For example, given that you mention sockets height, all that Part M says is ...

Despite what you suggested, I am sure that there is no Part of the Building Regs which says anything specific about CUs or DBs.

We know a consumer unit is a type tested distribution unit, but why are they type tested? I have been lead to understand they were type tested to ensure they were safe to be in the control of an ordinary person, if that's not the case, why do we have them?

As I keep saying, the only answer I am aware of to "why do they have them" is that they are generally allowed to contain 6kA devices, whereas that may well not be true of non-type-tested DBs.

Kind Regards, John

 

[JohnW2]

well they certainly make it easier than a dist board and separate main switch for the ordinary person

There surely nothing about a non-type-tested D that says that it can't include a 'main switch' / incomer ?

Indeed what was otherwise a ('type tested') CU, with a Main Switch, could cease to be a "CU" just because it contained one 'non-approved' device.

In any event, domestic installations ('under the control of ordinary persons') with more than one CU (which is far from uncommon) should (per BS7671) have a 'separate' Main Switch / isolator ("single point of isolation).

Kind Regards, John