3 phase vs single phase wiring

[WWilts]

My understanding as a layperson is that 3 phase electrical wiring is much more complicated and expensive than single phase wiring.

 

However, we managed to get a quote for 100A 3 phase supply (46kVA) at reasonable cost. Intention was mainly to future proof for electric vehicle (EV) charging.

 

Question: Can we still supply the house with simple single phase?

Eventually we will have EV charging, near the external meter box if necessary. That is the time when 3 phases can be used.

 

Are we on the right track? We don't want to incur huge costs unnecessarily (eg 3 phase wiring to house which might be overkill).

[TonyT]

You are on the right track!

 

make the most of it and try and get 3phases worth of PV!

 

 

[ProDave]

The only bit about 3 phase that is significantly more expensive is a 3 phase distribution board costs more than single phase consumer units, and less choice.

 

All the actual wiring will be identical single phase circuits same as if you had a single phase supply. 

 

If you choose a 4 phase PV inverter that will obviously be a 3 phase circuit.

[Thorfun]

30 minutes ago, TonyT said:

make the most of it and try and get 3phases worth of PV!

 

we've actually gone the other way (DNO approval pending) and have decided to run our 10.2kWp PV array via a single phase inverter even though we have a 3-phase electricity supply. we are running the house on a single phase and didn't want the other 2 phases from the PV inverter just being 'lost' to the grid when we can use it in the house. A 3-phase inverter would've meant only a 3rd of the generated electricity being usable most of the time. At the moment we don't drive enough in our EV to make a 3-phase charger necessary and so hardly any of the PV generated 3-phase electricity will be used.

 

if we have a need for 3-phase in the future we can make changes then but for now just running a single phase throughout the house and with our PV systems seems logical. the 3-phase supply was a trivial amount more than a single phase connection so it was a no-brainer to future proof.

[Dan F]

12 minutes ago, Thorfun said:

 

we've actually gone the other way (DNO approval pending) and have decided to run our 10.2kWp PV array via a single phase inverter even though we have a 3-phase electricity supply. we are running the house on a single phase and didn't want the other 2 phases from the PV inverter just being 'lost' to the grid when we can use it in the house. A 3-phase inverter would've meant only a 3rd of the generated electricity being usable most of the time. At the moment we don't drive enough in our EV to make a 3-phase charger necessary and so hardly any of the PV generated 3-phase electricity will be used.

 

if we have a need for 3-phase in the future we can make changes then but for now just running a single phase throughout the house and with our PV systems seems logical. the 3-phase supply was a trivial amount more than a single phase connection so it was a no-brainer to future proof.

 

Few things:

- Have also future-proofed by taking 3-phases to main consumer unit?  I would if I were you, else future 3-phase car charger(s) could impact how many amps you left on your single house phase.  If main consumer unit has at least enough cores cabled, then you can easily switch to using multiple phases in the house if you need to.

- If you use a 3-phase inverter you wouldn't be throwing away anything due to vector-sum metering.  If you are generating 2kW on each phase, and consuming 6kW on phase 1 then that is does not count as import nor export.  A lot of people, including some solar installers, don't actually understand how this works!

- Only disadvanage with using 3-phase inverter is should you loose power, you'd need 3 batteries (one on each phase) to using your PV off-grid, rather than just one.  But given power if never off for any extended time in the U.K. this wasn't an issue for me really.

 

 

Edited July 26, 2021 by Dan F

[Thorfun]

1 minute ago, Dan F said:

 

Few things:

- Have also future-proofed by taking 3-phases to main consumer unit?  I would if I were you, else future 3-phase car charger(s) could impact how many amps you left on your single house phase.  If main consumer unit has at least enough cores cabled, then you can easily switch to using multiple phases in the house if you need to.

- If you use a 3-phase inverter you wouldn't be throwing away anything due to vector-sum metering.  If you are generating 2kW on each phase, and consuming 6kW on phase 1 then that is does not count as import nor export.  A lot of people, including some solar installers, don't actually understand how this works!

- Only disadvanage with using 3-phase inverter is should you loose power, you'd need 3 batteries (one on each phase) to using your PV off-grid, rather than just one.  But given power if never off for any extended time in the U.K. this wasn't an issue for me really.

 

 

all good points Dan. pretty sure we'll be taking 3-phases to main consumer unit, I see no reason not to.

 

yeah, I've read about vector-sum metering before and it seems logical but as we're having a Powerwall I'd rather the generated power ALL went to the battery and then any excess is 'sold' back to the grid for a paltry amount. just seems more logical and more simple that way.

 

I presume that if I wanted to change to a 3-phase Solar PV system in the future I could swap out the single-phase inverter for a 3-phase inverter? if so, think I'll just stick with the simple setup for now.

 

obviously, if the DNO says no to the PV array size on the G99 then I'll go for a 3-phase inverter.

[Dan F]

1 minute ago, Thorfun said:

as we're having a Powerwall I'd rather the generated power ALL went to the battery and then any excess is 'sold' back to the grid for a paltry amount. just seems more logical and more simple that way.

I need to check to confirm this,  but I'm 95% certain the Tesla Gateway levages the vector-sum metering to effectively charge the battery from all three phases!

 

1 minute ago, Thorfun said:

I presume that if I wanted to change to a 3-phase Solar PV system in the future I could swap out the single-phase inverter for a 3-phase inverter? if so, think I'll just stick with the simple setup for now.

Potentially, but you'd need to make sure that whatever strings you've used on the roof will also work with the three-phase. If you might want to switch you should plan your strings to allow for both options. We did this and actually have three seperate strings on the roof, but currently two of these are connected in series (in the garage next to the inverter) for 3-phase inverter.

 

 

[Thorfun]

1 minute ago, Dan F said:

I need to check to confirm this,  but I'm 95% certain the Tesla Gateway levages the vector-sum metering to effectively charge the battery from all three phases!

 

Potentially, but you'd need to make sure that whatever strings you've used on the roof will also work with the three-phase. If you might want to switch you should plan your strings to allow for both options. We did this and actually have three seperate strings on the roof, but currently two of these are connected in series (in the garage next to the inverter) for 3-phase inverter.

 

 

I've got the install guides for the Powerwall somewhere so will read up on the Gateway.

 

all our PV panels are on Solaredge Optimisers due to shade. would that make the strings irrelevant? 

 

our PV is being designed by Enhabit who, iirc, who used as well? I'll also speak to them again about this whole subject.

[Dan F]

1 minute ago, Thorfun said:

all our PV panels are on Solaredge Optimisers due to shade. would that make the strings irrelevant? 

We used SolarEdge too and have a 12.5kW three-phase inverter.  We didn't use enhabit for PV.

 

I told them what strings to used after working out a scheme (using solaredge designer web tool) that I knew would work for both options.

[WWilts]

58 minutes ago, ProDave said:

The only bit about 3 phase that is significantly more expensive is a 3 phase distribution board costs more than single phase consumer units

It seems 3 phase distribution boards are available for under £200.

 

If the distribution board is the main cost difference, and wiring costs are about the same, then it seems 3 phase to the house is sensible instead of single phase.

 

Does the distance from the distribution board to the EV charger(s) matter much in electrical terms? 

[Dan F]

10 minutes ago, WWilts said:

Does the distance from the distribution board to the EV charger(s) matter much in electrical terms? 

The further it is the thicker the cable you need.  As does type of charger(s) and how cable gets to charger.  See: https://www.elandcables.com/cable-calculator

 

We have 2 x 3-phase boards:

- One in garage for PV, Batteries, Car Chargers, Garden, Outhouse, Front Gate etc.

- One in plant room for house.

 

Having the board in the garage is practical as Supply/PV/Batteries/Chargers are all close.

 

[Dan F]

@Thorfun. I'm told that Telsa Gateway also leverages vector summation when charging.

 

Yes, I have a 3 phase supply and a single powerwall on L1, let say I’m generating 9kw of solar (so 3kw per phase) the powerwall will charge at 5kw.

 

[joth]

On 26/07/2021 at 23:25, Dan F said:

The further it is the thicker the cable you need.  As does type of charger(s) and how cable gets to charger.  See: https://www.elandcables.com/cable-calculator

 

We have 2 x 3-phase boards:

- One in garage for PV, Batteries, Car Chargers, Garden, Outhouse, Front Gate etc.

- One in plant room for house.

 

Having the board in the garage is practical as Supply/PV/Batteries/Chargers are all close.

 

This also makes sense to plan this way as you only need one penetration through the airtight layer for a single 3 phase supply, and all loads (however many phases they need) can come from the appropriate distribution board, outside Vs inside

[Dan F]

2 hours ago, joth said:

This also makes sense to plan this way as you only need one penetration through the airtight layer for a single 3 phase supply, and all loads (however many phases they need) can come from the appropriate distribution board, outside Vs inside

 

We have a 110mm duct that will bring the supply into the garage (once we move the meter) and next to the meter is the main 3-phase board for PV, batteries, garden room, garden lighting, garage and front gate.  Then from garage there is a 63mm duct through the EPS to the plant room to secondary 3-phase board (which I'd intitially thought would be single phase). 

 

One word of warning with ducts through EPS though is that given the space is insulated, you need a thicker cable to meet regs.  Our electrician said he couldn't get a 5-core 25mm cable through the 53mm duct, so has used 16mm instead.  The run is 12m so this gives a maximum of 60A/phase.  This is sufficient, but it will mean we'll need to split the house load across phases rather than use just one (and think about what we want to backup).

 

We will reuse the 25mm 3-phase cable currently used to bring 3-phase supply from site kiosk into the garage for future car charger(s).  Rather than put car charger next to the garage as you typically see, we plan to but them elsewhere on the drive where you could easily charge two cars without getting in the way of the garage.