UK New electric supply connection

[wwilts]

New 46kVA electric supply to new backland dwelling (37m from footpath). Mains cable runs along the footpath at the road access.
Which option would you choose and why?

Option 1

Meter box near footpath. Own fuse and armoured cable (>30m distance) underground to new dwelling.

Option 2

Meter box 25 m from footpath. Own fuse and armoured cable (<15m distance) underground to new dwelling.

 

[pc1966]

I'm guess the two-phase is common in rural places when they just have twin HV cables to pull the supply off, and prefer to offer 230-0-230 at 100A rather than 230 at 200A for obvious reasons.

Usually I prefer the name "split phase" but it really is two phases as for a N-phase system each line is 360/N degrees different, and so when balance there is no neutral current.

The issue with single and 2 phase is you can't generate a rotating field or other phase sets from it, so motors need capacitors to get going and all of the sort of problem. In these days of VFD for motors it might be less of an issue, but there is still loads of industrial kit (and probably farming equipment) that assumes a 3 phase supply is present.

With 3-phase (or higher) you can use weird transformer arrangements to get any other phase arrangements you want, probably the most common has been the Scott-T to get 4-phase (as 2 * single phase at 90 deg to each other) or the ones using both star and delta windings, etc, for 6-phase supplies common for DC generation in the olden days of mercury arc rectifiers. Still probably used today, as it lowered the ripple on the DC supply (so less of a smoothing choke needed) and dropped the 3rd harmonic seen on the incoming HV side, but it has been years since I studied that sort of thing.

 

[telectrix]

agree with 3 phase option. you might want 3 phase equipment at some time in the future. and meter box as close to the dwelling as possible.

I'm guess the two-phase is common in rural places when they just have twin HV cables to pull the supply off, and prefer to offer 230-0-230 at 100A rather than 230 at 200A for obvious reasons.

Usually I prefer the name "split phase" but it really is two phases as for a N-phase system each line is 360/N degrees different, and so when balance there is no neutral current.

The issue with single and 2 phase is you can't generate a rotating field or other phase sets from it, so motors need capacitors to get going and all of the sort of problem. In these days of VFD for motors it might be less of an issue, but there is still loads of industrial kit (and probably farming equipment) that assumes a 3 phase supply is present.

With 3-phase (or higher) you can use weird transformer arrangements to get any other phase arrangements you want, probably the most common has been the Scott-T to get 4-phase (as 2 * single phase at 90 deg to each other) or the ones using both star and delta windings, etc, for 6-phase supplies common for DC generation in the olden days of mercury arc rectifiers. Still probably used today, as it lowered the ripple on the DC supply (so less of a smoothing choke needed) and dropped the 3rd harmonic seen on the incoming HV side, but it has been years since I studied that sort of thing.

think when i was at college, that part of studies clashed with a 2 week beer fest. no contest.

 

[brianmoooore]

It's commonly known as split phase around here, but technically it's two phase.

 

[plugsandsparks]

If it is 2 phase, might be worth re-visiting your EV charger options as the 22KW ones (AC) i know of are 3 phase and ditto for rapid chargers

 

[brianmoooore]

I'm guess the two-phase is common in rural places when they just have twin HV cables to pull the supply off and there is still loads of industrial kit (and probably farming equipment) that assumes a 3 phase supply present.

Tell me about it! I've spent many hours of my life trying to get suppliers to understand that I require equipment to be supplied with 480 volt single phase motors, not 440 volt motors.
It was common for them to deny such motors existed.

 

[wwilts]

If it is 2 phase, might be worth re-visiting your EV charger options as the 22KW ones (AC) i know of are 3 phase and ditto for rapid chargers

I was a bit confused by apparently differing messages following the DNO's quotation. The design states dual phase. The staff who then made a site visit to plan connection dates etc. said "3 phase". Perhaps it is 46kVA 3phase, for all I know (which is not much).

 

[plugsandsparks]

Post up the quote and we can advise

 

[wwilts]

Post up the quote and we can advise

It seems to be 100A 3p, just as you all advised. Apologies for the confusion.
Their drawing said "dual phase", but that is not in their detailed quote.
100A 3p it will be, I believe.

100A 3p 46kVA will suffice for EV charging, I imagine. For the dwelling itself (no heat pump / no AC), how many kVA would you allow?
And what underground cable would do from the meter box to the dwelling? T&E in duct or SWA? If SWA, 16mm2 or 25 mm2? 3 core?

Distance from meter box to dwelling<15m, of which <4m will pass under a driveway turning area.

 

[Rockingit]

100/3 is a ‘standard’ 3ph supply, you don’t really go downwards from that.

Why the fixation to have the meter separate from the dwelling?? Because, straight away in order to get 100/3 from an external point to the dwelling itself you’ve probably gained £1000+ of expensive and probably unnecessary works, and that’s before groundwork costs and hassle. You’d actually be downgrading your supply before you’d even used it for reasons to do with technical regs that I won’t go into here as I don’t want to confuse.

WWilts - if your user name is a reference to geography then drop me a private message, I may not be that far from you!

 

[JD6400]

New 46kVA electric supply to new backland dwelling (37m from footpath). Mains cable runs along the footpath at the road access.
Which option would you choose and why?

Option 1

Meter box near footpath. Own fuse and armoured cable (>30m distance) underground to new dwelling.

Option 2

Meter box 25 m from footpath. Own fuse and armoured cable (<15m distance) underground to new dwelling.

Normally I would say , get a quote from your DNO for their side of the work , for either option and then you will see if you have a larger enough price difference between the two in order to make it economically viable to do the majority of the work yourself. ( Saved one client £36k by doing this)
But now that you have shown your quote I would say that the DNO should the whole length , as it is not worth the pennies you'll save.

Also as you will see yourself if you follow the forum , Rockingit knows his stuff and you won't go far wrong with his help.
I am not trying to to put your knowledge down , but the fact you asked if running twin and earth in a duct was acceptable , to me would indicate you could do with some external paid help.

Good luck with your project

 

[wwilts]

100/3 is a ‘standard’ 3ph supply, you don’t really go downwards from that.

Why the fixation to have the meter separate from the dwelling?? Because, straight away in order to get 100/3 from an external point to the dwelling itself you’ve probably gained £1000+ of expensive and probably unnecessary works, and that’s before groundwork costs and hassle. You’d actually be downgrading your supply before you’d even used it for reasons to do with technical regs that I won’t go into here as I don’t want to confuse.

WWilts - if your user name is a reference to geography then drop me a private message, I may not be that far from you!

An electrician building their own house did this:
"a switch fuse in my meter box with an 80A fuse in it to discriminate from the suppliers 100A fuse. Meter In external kiosk. Can be connected and finished before the house is built so you have a site supply without having to move it later. No need for a meter box on the outside wall of your house. Run your own SWA able from this kiosk to the house when you are ready with no DNO involvement."

The new dwelling will come later, just trying to get supply to site. And to avoid unnecessary costs.
Will PM

 

[wwilts]

Normally I would say , get a quote from your DNO for their side of the work , for either option and then you will see if you have a larger enough price difference between the two in order to make it economically viable to do the majority of the work yourself. ( Saved one client £36k by doing this)
But now that you have shown your quote I would say that the DNO should the whole length , as it is not worth the pennies you'll save.

Also as you will see yourself if you follow the forum , Rockingit knows his stuff and you won't go far wrong with his help.
I am not trying to to put your knowledge down , but the fact you asked if running twin and earth in a duct was acceptable , to me would indicate you could do with some external paid help.

Good luck with your project

No pretence of knowledge here. Just trying to understand enough to make intelligent suggestions to builder.

 

[Strima]

I'd get 3ph all the way to the property then distribute from there.

 

[pc1966]

An electrician building their own house did this:
"a switch fuse in my meter box with an 80A fuse in it to discriminate from the suppliers 100A fuse.

You won't get meaningful selectivity between 80A and 100A as they are both subject to something like +/-10% tolerance, etc, and the total let-through I2t for the 80A clearing is less than the pre-arcing I2t of the 100A.

Typically you need to have a 1.6:1 ration for BS88 and other European fuses (i.e. two sizes, so 63A & 100A or 50A & 80A, etc) for that.

But really selectivity is only needed if you expect to need it. If you are putting in a switch-fuse for the long tails it is largely down to the regulations that require both a means of isolation (the switch) and OCPD that is matching the sub-main characteristics (i.e. 5s disconnect for fault in or at the end of the cable).

The regulations do allow the DNO fuse provided they agree to it being used. Realistically it is easier to fit a switched-fuse!

Where you need selectivity (and so fusing down a bit from the DNO) is when you have several circuits coming off the DNO fuse. And by that I mean several sub-mains, for example at a Ryefield board or where tails have been split to allow two CU or some massive load to be added and you reasonably expect a fault on one sub-main not to take out the lot.

For the typical case of long meter tails / sub-main you really ought to have it designed so that it won't be overloading the DNO supply anyway (diversity allowing) so having your own choice of fuse is down to it independently providing protection to your choice of cable and supply Ze, etc.

In that case you might be putting in 80A, even though on a hard fault it probably won't save the DNO fuse, because it then meets your 5s requirement and is enough for the planned loads.

 

[brianmoooore]

I've been involved with builds in the middle of nowhere on many occasions, and on some, I've got over the problem of a site supply by getting the builders to build just a small triangular section of the outer house wall to build the meter box into. I've then fixed my board (used on many sites) with an IP65 small RCD protected consumer unit and two IP55 double switch sockets on it, to the reverse side of the wall section, in what eventually will be the cavity.