Minimum Kva/Amps for a new build

[Ardet R]

Hi Binky, I am no expert, but as far as i know, the dno service cable terminates at the cutout. Then you have the tails to the meter and CU. You cannot have these longer than 3 metres, so, if the CU is, say, 10 metres away, you stick some sort of OCPD at the origin, [the cutout] and then your cable to the CU is your submain. I suppose you could see it as a privately owned service cable.

Volt drop before the cutout caused by supply impedance, is not for us to worry about. It is after that point that the 5% and 3% come in. The idea of the max 3% for lighting is so that when you turn on a large load, elsewhere in the installation, say a shower, the lights do not dim excessively.

From this idea, you can see that the 3% would apply to the entire installation if it feeds lights at all, as, if it just applied to the lighting final circuit, the lights would dim regardless when the shower came on.

That is my understanding anyway!! [and i might be miles out!!]

john..

I would agree with that.  The voltage drop values are specified from the origin of the installation to the point of use.

Therefore any sub main from origin to a DB that includes lighting in its final circuits is limited to a total volt drop (sub main plus final circuit) of 3%.

I think in general design you might consider that a sub main should have a total volt drop of about 1% (depending on the installation design layout) and then the final circuits have the potential to drop 2% for lighting and 4% for power.

Of course only using the mythical calculation voltage of 230V just in case the UK ever changes to that! (because that is the voltage for which the values are given).

On that basis presumably if you were trying to minimise cable the ideal plan would be to use one sub main solely for power (which allows the full 5%) and then run another parallel sub main solely for lighting, which because the demand would be so much lower could be a smaller cable even though the volt drop is limited to 3%.  Though logistically and cost wise there is probably not much difference overall.

Like so:

one 240mm² cable or one 185mm² plus one 10mm²

 

[apprentice87]

Brilliant post!!

john..

 

[gc100]

I would agree with that.  The voltage drop values are specified from the origin of the installation to the point of use.

Therefore any sub main from origin to a DB that includes lighting in its final circuits is limited to a total volt drop (sub main plus final circuit) of 3%.

I think in general design you might consider that a sub main should have a total volt drop of about 1% (depending on the installation design layout) and then the final circuits have the potential to drop 2% for lighting and 4% for power.

Of course only using the mythical calculation voltage of 230V just in case the UK ever changes to that! (because that is the voltage for which the values are given).

On that basis presumably if you were trying to minimise cable the ideal plan would be to use one sub main solely for power (which allows the full 5%) and then run another parallel sub main solely for lighting, which because the demand would be so much lower could be a smaller cable even though the volt drop is limited to 3%.  Though logistically and cost wise there is probably not much difference overall.

Like so:

one 240mm² cable or one 185mm² plus one 10mm²

View attachment 9621

Wow thats really interesting!! Just to clarify, I have several options. Both in terms of the total KVA needed and the setup with the DNO. They are happy to put a kiosk at the edge of the land , which would then mean a 600m odd run to the barn. They are also happy to do it all the way to the barn which is what their quote is. Essentially its 10K to upgrade the transfer from 16 to 100kva, then 30K for the work to get it too the barn over that 600 meters:

Civils groundwork study
• Excavate, lay & reinstate 185mm LV waveform mains cable in unmade ground.
• Install LV 185mm waveform mains cable
• LV waveform mains pot end & installation of earth pin.
• LV Mains straight joint of a 95-185mm cable to a 120-185mm main.

They made the point that I could do this ourselves if we think we can do it cheaper and they could put a kiosk at the edge of the land. The main cost of the 30K would be the cable he said.

I'm based in Norfolk BTW.

 

[ProDave]

So the DNO calculated that 185mm would meet the volt drop requirements then?

 

[Tony S]

Have a look for independent connections to UKPN

https://www.ukpowernetworks.co.uk/internet/en/our-services/competition-in-connections/

 

[apprentice87]

So the DNO calculated that 185mm would meet the volt drop requirements then?

Hi Dave, the DNO cable size would give a volt drop of 10.5V at 60A.. Too high for BS7671, but they can do what they like. So far as i know they can go plus or minus 10% bit different to our 5% and 3%.

Thing is though, the cable impedance will be something like 0.174 ohms. Add this to the supply impedance, say 0.35, and you would have a horrid "soggy" supply.. What you think??

john..

 

[apprentice87]

They made the point that I could do this ourselves if we think we can do it cheaper and they could put a kiosk at the edge of the land. The main cost of the 30K would be the cable he said.

They are not comparing like for like.. They are either thick, or do not care.. Electricians have to work to BS 7671. This means that the max volt drop we could have would be either 3 or 5 percent. The DNO, do not have to adhere to this, which is why they think a 185mm cable is ok. It would be if they run it all the way to the house, but it will be a lettuced supply. If YOU run your own cable the 600 metres, you will need 300 or 400mm cable or it will not comply with the wiring regs. The cost would be terrible..

A 185 mm cable would be about 18k, they are charging you the rest to dig the trench then?? I would come to norfolk and dig a trench for that money!!

john..

 

[ProDave]

Hi Dave, the DNO cable size would give a volt drop of 10.5V at 60A.. Too high for BS7671, but they can do what they like. So far as i know they can go plus or minus 10% bit different to our 5% and 3%.

Thing is though, the cable impedance will be something like 0.174 ohms. Add this to the supply impedance, say 0.35, and you would have a horrid "soggy" supply.. What you think??

john..

The whole point of the volt drop limit is to prevent  lights dimming when you turn the kettle on.

Even if you had no volt drop in the internal installation, the lights are going to dim aren't they?

With all LED lighting do they still dim? I can't say ours do. So should we just ignore the volt drop? As long as it never goes below 230V at the end use what is the problem?  At 185mm2 it certainly is not going to overheat is it?

I quite often find TNCS supplies with a Ze over 0.3 ohm. Nobody seems bothered.

 

[gc100]

Have a look for independent connections to UKPN

https://www.ukpowernetworks.co.uk/internet/en/our-services/competition-in-connections/

Thanks. I did call several but they was only interested in commercial. None of them wanted anything to do with domestic (I suppose its the same for Uk Power Networks!)

 

[Ardet R]

My post was more of a technical exercise rather than practicality.

However on a practical basis, as Tony has said, the volt drop is based on the design current with diversity taken into account and not the fuse size.

The volt drop limits are 3.5% for lighting and 6.5% for power over that distance.

The volt drop will also be lower because the cable will not be being heated significantly by the low current taken so you will have about 15% increase permitted for this reason.

In general a house will take on average about 5A but this is obviously very dependent on the loads installed in a property and peaks will be very much higher.

It would be necessary to calculate a figure for the design current of the property to be able to effectively assess the practical volt drop but if all these factors are taken into account then it is possible that a 185mm² cable could suffice.

Obviously it would be critical to get an experienced person to assess this accurately in detail to ensure compliance.

You may find that on turning on the electric shower may briefly lower the voltage but this would be for 10 minutes out of every 1440 minutes, hardly significant unless you are running highly voltage dependent appliances.

Good luck.

 

[ProDave]

I would definitely design out such big loads as an electric shower. Unvented hot water tank (that can either be heated slowly with immersion heater or other heat source) would give a WAY better shower, and lower volt drop.

Likewise and LPG powered hob rather than a big induction monster.