I don't understand 3 phase

[Big Neil]

Have looked through the various posts on here mentioning the phrase and just don't understand it. I'm not too bothered about how the magic happens more the practical application.

 

In my head I'm sort of imagining it like a couple of pipes with water running through; the little pipe has a normal flow of water for a normal house where the big pipe has a big old gushb of water because there are more people in the house needing it. Single Vs 3 Phase. Is this broadly correct or am i way off the mark?

[scottishjohn]

OK  with 3 phase you have 4 or 5 cables 

3 live phases and on old systems a neutral  + an earth --but as the neutral is really an earth -then modern things use only 4 wires 

 each of the lives when paired with the nuetral give 240 vlots --approx 

so when you use all 3 lives and a nuetral on soemthing you get 440v approx

 you can split a 240 circuit from  board by just using one phase+ neutral  --so you could balance your load across the building by using the 3 phases for different circuits 

to pull even current 

 long time ago you could save money by doing this --but I am told --don,t know for certain that it don,t matter anymore cos meters are tricky

only reason for having 3 phase is to run big motors  as they use less current for same load as single phase and there is a limit on size of single phase motor 

[Big Neil]

Right. I won't even try to understand why one phase is 240 and 3 is 440, so long as the practical upshot is that 'you can run more/bigger stuff' in a property with a 3 phase supply as compared to a single phase.

 

So the wire that spurs off the mains and comes to your house is physically bigger is it, than for most people?

 

Why would you want seperate circuits? Does this protect against anything or give some sort of redundancy?

 

Is it more costly, and if so is there a rough approximation of how much in percentage terms?

 

If You are generating power on site from solar/wind, would you need 3 phase after a certain point?

[le-cerveau]

2 minutes ago, Big Neil said:

So the wire that spurs off the mains and comes to your house is physically bigger is it, than for most people?

The wire is bigger as it has 4/5 cores, instead of the usual 2/3 cores, but the cores will be similar size.

3 minutes ago, Big Neil said:

Why would you want seperate circuits? Does this protect against anything or give some sort of redundancy?

Most domestic electronics is 230v single phase.  We have 3-phase in our house but only 2 x 3-phase devices, the ASHP and lift, though both were available in single phase versions.

4 minutes ago, Big Neil said:

If You are generating power on site from solar/wind, would you need 3 phase after a certain point?

Yes you need specific 3-phase capable equipment as the harmonics between the 3 -phases have to be synchronised.  We have nearly 10kW of PV on the roof, but it is connected by enphase microinverters that can be connected 1/3-phase, ours are evenly distributed across the 3-phases buy the installation equipment (3-phase wire where each microinverter sequentially connected to the next phase).

7 minutes ago, Big Neil said:

Is it more costly, and if so is there a rough approximation of how much in percentage terms?

Yes, but the cost will vary depending on your situation, we had 3-phase directly in front of the house and we were simply spurred of the main line, the previous supply was simply one of those phases.

[LA3222]

 

26 minutes ago, scottishjohn said:

 

only reason for having 3 phase is to run big motors  as they use less current for same load as single phase and there is a limit on size of single phase motor 

This is not true.  If you have a fairly large build that is reliant solely upon electric then it is likely you will need a three phase supply.

 

My DNO could only offer me a 16kVa connection on single phase.  When I did some fag package calculations on potential usage and future proofing for potential 3ph charging of electric car I was on the edge of exceeding the capacity of the DNO fuse in the service head.

 

So it made sense to pay a couple hundred extra and get three phase put in.

 

I now have 3 x 80A fuses so more than enough. 

Edited January 30, 2019 by LA3222

[Big Neil]

3 minutes ago, le-cerveau said:

Yes you need specific 3-phase capable equipment as the harmonics between the 3 -phases have to be synchronised.  We have nearly 10kW of PV on the roof, but it is connected by enphase microinverters that can be connected 1/3-phase, ours are evenly distributed across the 3-phases buy the installation equipment (3-phase wire where each microinverter sequentially connected to the next phase).

 

Super - so what is the cut off below which you wouldn't need to think about 3 phase. Where you have 10KW, what is the maximum amount for single phase? Or have I misunderstood this?

[scottishjohn]

you will only know what the difference is by asking your supplier 

all depends how close 3 phase is and if they need to fit a larger transformer  because of other people on same line 

I think that would be my first move --find out cost difference --

one plot i was looking at -a n old farm  needed transformer replacing --£4k--so don,t guess find out 

 

[Big Neil]

6 minutes ago, LA3222 said:

This is not true.  If you have a fairly large build that is reliant solely upon electric then it is likely you will need a three phase supply.

 

My DNO could only offer me a 16kVa connection on single phase.  When I did some fag package calculations on potential usage and future proofing for potential 3ph charging of electric car I was on the edge of exceeding the capacity of the DNO fuse in the service head.

 

So it made sense to pay a couple hundred extra and get three phase put in.

 

I now have 3 x 80A fuses so more than enough. 

 

So; 3 phase required if a large amount of PV being fed back to the grid. Is it sort of like having a dual-sim phone. Two signals come into the same place but they're seperate. So If i chose to put in a 3 phase supply, i could have one for the interior of the house, so lights, sockets, appliances etc. Then I could maybe have one for the garage and lets say also the ASHP, and if I wanted could use the other for maybe charging an electric car. This way if there was a problem on any individual circuit (lets say the car one for the sake of argument), you could shut the supply down to that circuit but I could for example, just plug my car into a 3 pin socket in the garage, or if the house burns down i can still charge my car....

 

By the way, i DO know quite a lot about bran based cereal products, so any questions about them, ask away. Just get confused about the USEFUL stuff.

[LA3222]

3 phase allows you to have 12kWp of Solar without permission from the DNO as such because you can stick 4kWp over each separate phase.

 

In your domestic setting you can run 3 separate circuits across each individual phase.  AIUI best practice is to try and split the loads evenly across each phase so as not to have an unbalanced system.

 

So it takes a bit more thought than single phase but it is not insurmountable and by no means overly expensive when you consider it against your whole budget for the build.

[scottishjohn]

27 minutes ago, Big Neil said:

 

So; 3 phase required if a large amount of PV being fed back to the grid. Is it sort of like having a dual-sim phone. Two signals come into the same place but they're seperate. So If i chose to put in a 3 phase supply, i could have one for the interior of the house, so lights, sockets, appliances etc. Then I could maybe have one for the garage and lets say also the ASHP, and if I wanted could use the other for maybe charging an electric car. This way if there was a problem on any individual circuit (lets say the car one for the sake of argument), you could shut the supply down to that circuit but I could for example, just plug my car into a 3 pin socket in the garage, or if the house burns down i can still charge my car....

 

By the way, i DO know quite a lot about bran based cereal products, so any questions about them, ask away. Just get confused about the USEFUL stuff.

must be a big new build if you need a 3phase ASHP .LOL

[gravelld]

What about, in an all electric setup, having multiple instantaneous water heaters? Would've thought you'd need 3 phase for that in case two or three or running, someone turns on an electric shower and someone fancies a cuppa (or maybe I'm catastrophising).

[LA3222]

I agree.  In my build there will be two ovens, an induction hob, sauna and car charger to name but a few, all of which will be on the electric supply.

 

Yes I know that diversity can be applied.

 

However if all the DNO are willing to supply on a single phase supply is a 60A fuse then it's a bit close to the wire as to whether it blows or not should everything be on at once.

 

Installing 3ph cost me nothing extra.  My connection charge was £5500 and that involves upgrading 70m of new service cable.  Because I changed the position of where it ran to on my plot (20m closer to boundary in a kiosk) when the quote was amended it still came at £5500 so a no brainer. 

[Ed Davies]

I think it might be worth explaining the basic point of 3-phase.

 

Say you're running a 6.9 kW device (say a heat pump) off a 230 V single phase supply then that'll draw 6900 W/230 V = 30 amps. That 30 amps will run through both the  line and neutral wires incurring losses in both. You need two bits of wire into the house and to the heat pump each capable of carrying 30 amps so 60 amps worth of cable, in effect.

 

If, on the other hand, you run that same heat pump off a 3-phase 230 V supply each phase will draw 2.3 kW so 10 amps each for which you'll need 3 line wires each capable of carrying 10 amps with the associated losses. However, if the power taken on each phase is exactly balanced then there'll be no current left to flow through the neutral; when one phase is pushing electrons into the neutral the other two phases will, between them, pull exactly the same number of electrons back out so, in principle, you don't been a neutral wire at all. In reality things might not be that well balanced so you will have a neutral but it only needs to deal with the worst case where you're taking the full amount on just one phase so it only needs to be a 10 amp wire.

 

So, for three phase you'd need 3 x line wires and 1 x neutral wire each taking a maximum of 10 amps so a total of 40 amps worth of cable. Somewhat less than the 60 amps for single phase. In addition, in normal use the phases will be balanced to at least a certain extent so normally the current in the neutral will be smaller than that in the lines so losses will be less.

 

This is all pretty marginal for most domestic situations, hence the controversy. Most likely you'd want domestic three phase because the DNO can only supply 80 amps single phase and you might, in some circumstances, want to draw more than 18 kW. Or, you want to feed in more than 3.68 kW without extra regulatory hassle. The basic requirement is that you can feed in 16 amps per phase, with single phase that's 3.68 kW, with three phase that's three times that.

[Mr Punter]

Quite a few of the small tower cranes are 3 phase so you would be able to run one without a generator, but I am not keen on having a 415v site supply.

 

Our domestic supply is 100A. Amazing how skinny the cable is though.

[Onoff]

3 minutes ago, Mr Punter said:

Quite a few of the small tower cranes are 3 phase so you would be able to run one without a generator, but I am not keen on having a 415v site supply.

 

Our domestic supply is 100A. Amazing how skinny the cable is though.

 

Mine too. 100A incomer. 25mm concentric. Good for about 120A.

[LA3222]

Skinny yes - but the DNO still want 125/145mm ducting for it!! 

[Mr Punter]

4 minutes ago, LA3222 said:

Skinny yes - but the DNO still want 125/145mm ducting for it!! 

 

32mm for single phase

[Big Neil]

@Ed Davies thank you for that explanation. I understood all the words but i think over the next few days will just need to re-read it a bunch to ensure i understand what it all actually means.

 

For now irrespective the magic going on inside things, if i have a cable coming to my house supplying 3 phase, in a case where;

 

I have an electric car

I have an ASHP

I have the normal sort of household appliances and features

no gas on site

above 10KwP of solar

 

In terms of the wiring itself does it differe at all. I sort of imagine that there's a different type of sonsumer unit or some sort of spliuttter arrangment where some goes to the consumer unit for stuff maybe in the house, another consumer unit for all the rest?

[Jeremy Harris]

Looking at the loads, unless the ASHP is pretty big, then it won't be a big load on the supply.  Our 7 kW ASHP usually runs with an input power of around 0.8 kW, and the most I've ever seen it draw is about 2 kW.  The oven and hob are bigger loads.

 

Electric car charging is a good case for having three phase, if it's available and affordable.  Taking my car as an example, the maximum charge rate it will accept on single phase is 7 kW, the maximum on three phase (really two phase, as the on-board car charger is only two phase) is 11 kW.  Many of the newer generation of electric cars can charge at 22 kW from a three phase supply.  Unlike other loads in the house, electric car charge points cannot have diversity applied to them, as they can run for many hours at maximum current, and this places a hefty demand on the supply.  Putting those charge powers into context, 7 kW charging equates to charging at about 28 miles per hour, 22 kW charging equates to about 88 miles per hour (in terms of range gained per hour of charge time).

 

A large PV array is another case where it may be useful to have three phase, for the reasons given above by @Ed Davies.  You can connect three 16 A maximum (nominally 3.68 kWp) PV systems to a three phase supply without needing G59/3-1 consent from the DNO.  Our DNO was OK with giving me consent for up to 12 kWp on a single phase, but that's very much the exception, I think, and is only because we have a 95mm² supply cable running right under our meter cabinet.

 

With three phase then the usual arrangement is to split the phases so that different circuits are on different phases, trying to maintain roughly equal loading on each.  This is the normal way many houses on the continent are wired, where three phase incomers seem more common and split phase distribution boards seem normal.

[le-cerveau]

One of the main reasons I went 3-phase is that as a Passive type build we have an all electric kitchen, with 2 x 7.2kW induction hobs (Split level for wheelchair), 3 ovens drawing 3.7, 3.1 & 2.9 kW (worst case) add in 9kW for the ASHP (though the most I have seen it draw is around 4kW (recent cold)) then the standard single phase supply is at risk of tripping if you get it wrong.  I have also put in a 3-phase spur into the garage for a future car charger.