Meter Tail Size on long run.

223vmax · 07-08-2012 10:05AM #1

I'm currently building an extension and have the existing ESB supply coming to the front gable. Since the extension now joins into this gable I want to move the ESB supply entry point. The current meter is situated in the main hall of the house.
I have decided to install an exterior meter box at the front of the house in the extension and now need the meter relocating along with the ESB supply entry point moving from the gable to the meter box via an undergraound cable. I have trenched out and installed the red ESB duct from pole and up a hockey stick into the new meter enclosure. The ESB pole is approx 10M from the house.
The distance between the new meter box location and the existing CU is approx 23M. Part of the run of this cable is through some attic space which often in summer gets quite warm above 35C. The supply is a domestic 80A supply. Would 25mm2 cable be sufficient for the consumer tails to the new meter considering the conditions? Any help or pointers appreciated.

M cebee · 07-08-2012 12:26PM

when you say 80amp do you have an enhanced 16kva supply or do you mean the esb fuse is 80amp?

223vmax · 07-08-2012 01:22PM

Sorry M cebee, the main ESB fuse is 80A and the CU RCD is 63A.

07-08-2012 04:45PM

A 25 sq. will be able to safley and indefinitley carry that load.

What you have to do now is ensure that the maximum permissable volt drop is not exceeded on the far end of any final circuit.

As far as I can remember the maximum allowable volt drop is 4%.
I would be normal to allow 1 to 1.5 % volt drop to the distribution board and no more than 2.5 to 3% on the final circuit.

For this you will need to know the design current and mV per amp meter for each of the cables.

M cebee · 07-08-2012 05:17PM

yes it's 4% max installation volt drop from esb meter to socket or light etc.
and obviously cable needs to be sized correctly according to main ocpd
(63amp mcb in cabinet in this case)

what design current would you use for the tails on a 12kva supply to calculate volt drop?
would you put in 63amp to be on the safe side?

he definitely won't have any issue with 25's anyway

and 16's may even do

07-08-2012 06:26PM

Yes, I would use 63A as the design current.
It would not be possible to use anything else unless you can provide a very good reason.

223vmax · 08-08-2012 03:57PM

Thanks for the replies. Anyone know where I can source some 25mm2 cable. All my local suppliers dont go any bigger than 16mm....I'm in East county Galway.

08-08-2012 04:06PM

It may be cheaper for you to buy 3 x 25 sq. mm SWA than drive around the country or pay for delivery. Just a thought.

223vmax · 09-08-2012 05:23PM

M cebee wrote: »

and 16's may even do

Would 16mm2 do on this job? Im being quoted high prices for this cable.:(

.G. · 09-08-2012 06:34PM

It depends on the method of installation,the ambient temp,the length of run and the design load.

On the info you've given even at temps of 40c in your attic the CCC of 16sq is still 81amps.

Volt drop won't be an issue on a run of 23m at a design load of 63amps either as its well within the 4% allowed.

If the cable is mounted in free air, ie not run through or under any thermal insulation then I think your fine with 16sq.

223vmax · 10-08-2012 09:36AM

thanks, what Ci value would you use for a 10M length of this cable running through duct in the cavity of a wall filled with pumped bead insultaion? The insulation does not come into contact with the beads at any point.

M cebee · 10-08-2012 12:09PM

superg wrote: »

It depends on the method of installation,the ambient temp,the length of run and the design load.

On the info you've given even at temps of 40c in your attic the CCC of 16sq is still 81amps.

Volt drop won't be an issue on a run of 23m at a design load of 63amps either as its well within the 4% allowed.

If the cable is mounted in free air, ie not run through or under any thermal insulation then I think your fine with 16sq.

4% volt drop is for the tails and the 'final circuit'

when i checked it i think it was just over the 1.5% figure that 2011 quoted

i think 16's might just do anyhow

the other option besides swa is 25mm pvc/pvc singles and a 16sq pvc earth

they can be got online from meteor

10-08-2012 12:19PM

M cebee wrote: »

4% volt drop is for the tails and the 'final circuit'

Correct.

when i checked it i think it was just over the 1.5% figure that 2011 quoted

1.5% is a guide only. That figure is set by the designer. Once you do not exceed the 4% mentioned above you comply with ET101.
A 3% would be permissable, but then you may have to increase the size of cable for final ciruits to keep the overall volt drop < or = 4%.

i think 16's might just do anyhow

Yes, but see above.

I would simply work out the VD on 16sq. cables at 63A and take it form there.

the other option besides swa is 25mm pvc/pvc singles and a 16sq pvc earth

they can be got online from meteor

That would work.

M cebee · 10-08-2012 01:14PM

2011 wrote: »

Correct.

1.5% is a guide only. That figure is set by the designer. Once you do not exceed the 4% mentioned above you comply with ET101.
A 3% would be permissable, but then you may have to increase the size of cable for final ciruits to keep the overall volt drop < or = 4%.

Yes, but see above.

I would simply work out the VD on 16sq. cables at 63A and take it form there.

That would work.

i doubt it would be practical to have 3% for domestic tails

10-08-2012 01:41PM

M cebee wrote: »

i doubt it would be practical to have 3% for domestic tails

Probebly not, but my point is it is permitted. This is why 1 to 1.5% is generally accepted as a good guide.

223vmax · 10-08-2012 05:31PM

M cebee wrote: »

the other option besides swa is 25mm pvc/pvc singles and a 16sq pvc earth

they can be got online from meteor

Thanks M cebee. I looked on their website and I can't find 25mm they have the 16mm pvc/pvc alright but I must be looking in the wrong place.Can you send me the link please?

M cebee · 11-08-2012 10:00AM

Doesn't seem to be in Stock
I've always bought 25 pvc./pvc off them before for 80amp domestic enhanced supplies

I don't know if 3*25 swa is stocked here at all
You may get by with 16t&e

.G. · 12-08-2012 01:27PM

M cebee wrote: »

4% volt drop is for the tails and the 'final circuit'

Fair enough, not what we were told in college but there you go.

He'll have to do a few calculations then,one for the 16sq and then one for each of his final circuits and add them up.

Only way to be sure or go with 25's to be certain

M cebee · 12-08-2012 03:18PM

ya

i don't recall anything much from college-mostly checking up rules

getting 25's for single phase is a hassle if i recall-i always used meteor for the singles

is 3*25 swa with single phase colours available?doubt it

223vmax · 15-08-2012 07:35PM

Ok I managed to source some SWA 3 core for my job. Next question "Do I need to put ferrels on the ends of the L,N & E for connection into the ESB meter?

M cebee · 15-08-2012 08:21PM

no

the swa will gland into the enclosure containing the 63amp main ocpd
not the meter
assuming you're fitting one

Bruthal · 15-08-2012 08:37PM

I think he was more asking about using ferules on the ends of the conductors.

In answer to that, if that was being asked, they are not needed for swa cores, and would not be a good idea.

Ferules would be used on flexible cores which would be in silflex cables as an example, or single core flex used for linking different busbars and terminals in DB`s.

As these flex cores have lots of very thin strands, lots of the strands tend to go around the sides of terminal screws if ferules are not used, and so the connection would not be great.

Meter Tail Size on long run.

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