volt drop

[Jackflash]

Hi,

I need to take a Supply from a house CU to an outbuilding which has a 20A radial socket circuit and 6A lighting. The cable run to the outbuilding is 20M then the lighting circuit inside the outbuilding is approx 10M in length and the socket cicuit 15M in length.

I'm trying to calculate a suitable size cable to be feed via a 32A mcb in the house.

Taking volt drop into account so that the full 32A can be utilised i have calculated that.

6mm feed 7.3 x 32 x 20 / 1000 = 4.67v
---------

1.5mm lighting circuit 29 x 6 x 10 / 1000 = 1.74v ------- > 1.74v + 4.67v = 6.41v < 3% 6.9v (ok)
-------- ---------
2.5mm socket circuit 18 x 20 x 15 / 1000 = 5.4v -------- > 5.4v + 4.67v = 10v < 5% 11.5v (ok)
------- ---------

so a 6mm 2 core swa would be suitable to supply this installation

Does this look good to you?

Thanks

 

[telectrix]

without double checking your sums, that looks spot on to me, although i don't think you'll pull more than a couple of amps on the lighting, meaning that you're on the right side.

 

[Guitarist]

What is your house earth supply type, and are there any ECP's in the outbuilding?

 

[Jackflash]

Excellent,

The house earth type is tn-s, there's potentially a 2 steel roof brace that could be earthed. I was planning on installing an Earth rod to make it a TT system. what do you think?

 

[telectrix]

use the TN-S. 3 core SWA and earth the armour both ends. as for the roof brace. determine if it's extraneous by testing to earth on 500V IR. if the reading is above 22k, ignore it. is there a water service into the outbuilding?

 

[Jackflash]

ok good thanks! From what i can see there used to be a service from a pvc pipe that's got a tap on the end, i presume it's been turned off at a stop cock before entering the building as nothing comes out the tap. There's no internal plumbing and no plans for it in the near future (being used as an office) so i guess i'm safe with exporting tn-s from the house.

 

[yellowvanman]

With the 2 circuits out in the garage you can only pull 26A anyway(20A + 6A).

 

[Jackflash]

Yea that's right although I'd like to design the sub feed in a way where the full 32A can be utilised in the circumstance of another circuit being needed.

 

[Guitarist]

Unless there are plans to do something heavy, I've always found that a 20A supply is more than adequate for a garage or shed.