PD drop over tails to CU

[davelittlewood]

House build in mid 80's
Single Phase 240V installation with main supply board in exterior wall enclosure.
CU mounted in downstairs toilet in the middle of the house. Main tails length in excess of 7m.

1) During the installation of the house alarm the alarm fitter (Qualified Electrician) commented that the main tails to the CU were small.
2) If the cooker and extractor fan are running and then the kettle is turned on then the fan speed noticeably drops.

I have measured the drop in pd at the CU (21:30 tonight).
With almost no load = 242V
With ~4kW load = 237V.

(I guess that grid pd is up now as it's catching up with Sunday lunch load!)

Is a drop in 5V across the tails something that needs attention?

I'm well aware of Part P so I'd get a Qualified Electrician to actually replace the tails if needed.
Just looking for some advice.

With a drop of 5V across the tails, the power consumed is about 85W.

Any help appreciated.

Cheers,

Dave

 

[Marvo]

Why do you suspect the 5V is being dropped across the tails?

 

[davelittlewood]

I measured the pd at the main switch in the CU with and without load.
I guess it could be in the single phase supply to the house.
I'll do the same test at the main supply board at the side of the house

 

[Marvo]

You're not measuring the volt drop across the tails by doing the tests you describe. The test would kinda moot, the wiring manufacturers publish volt drop figures for the wiring. If the tails are undersized just get them replaced.

 

[Octopus]

Hum...tails too small? What actual size are they?

 

[davelittlewood]

The tails are actually T+E, can't get it in to see the writing on the sheath or measure directly but the overall width of the outer is about 20mm so from various tables I think that it's 16mm^2.

The house originally had 2 x 8kw showers, 7kW cooker + usual lighting and ring main circuits.

 

[davesparks]

Are you sure that T&E is the tails? Is it not fed from - fuse or mcb in the main supply board you described? If it is fed from another board then that changes the situation

 

[davelittlewood]

As far as I can tell it's T+E from the mcb on the main supply board all the way to the CU.
I can see the T+E leaving the mcb and coming into the CU.

It comes in from the top, curls round behind the main switch and then pops back up again.

 

[davesparks]

In which case, that isn't the tails. That is what is called a sub-main or distribution circuit.
Tails are the cables which connect between the meter and the main switch or first fuseboard.

A little bit of voltage drop when loads are applied is not uncommon in the suppliers distribution cables, especially on long cable runs.

Where I live it is quite normal to see the lights dim when the kettle is switched on as we are in a rural location on an overhead line from a small pole mount transformer.

 

[Dave OCD]

That looks like 16mm to me, you mention an MCB at the main supply end, what is the current rating ? I'd be more used to seeing a switch fuse set up than a MCB at the source for a sub-main.