Voltage Drop Tolerances | on ElectriciansForums

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C

Colonel Hathi

Hi
I have been asked to provide power to a building at the foot of the customer's 30 metre garden (32A ring and 6A lighting circuits).

When I calculate the required cable size in terms of voltage drop, 6mm SWA is just over limits.
I am using 6.4 mV/A/m 36A Demand and 30 metre run.
6.4*36*30=6912mV - 6.912 volts
6.912/230=3.005%
The Regs seem quite specific - but is this close enough to the 3% max?
Going to 10mm cable would bring me right down to 1.78%

Any views much appreciated.

Thanks

CH
 
Thanks very much for the quick replies.
Since I posted the question, I have spoken to the NICEIC.
2 interesting points.
1. I was using the Volt drop figure for 3 core SWA (BS7671 page 281) - because I intend to use 3 core. BUT the NICEIC explained that the 6.4 mV figure is only for 3 phase. For single phase, even if using 3-core cable, I should have used the 7.3mV figure for 2-core cable.
2. I was trying to achieve less than 3% voltage drop because of the lighting circuit. NICEIC explained that I only need to achieve less than 5% voltage drop because this is distribution circuit. The 3% would apply from the new CU at the bottom of the garden to the end of the lighting circuit.

So basically the calculation is 7.3MV/A/m * 36A* 30m = 7884mV (7.9volts)
The maximum allowed is 5% of 230v = 11.5volts - so I am well within target with the 6mm cable.

I must admit, the regs didn't seem very clear on this. It also seems odd to me that I am allowed a 5% drop on the distribution circuit and then another 3% on the lighting circuit. But I live and learn :)

Thanks again for the quick replies
 
Last edited by a moderator:
That is the problem with the NICEIC if you ended up in a court in front of the man with the white wig the court would be using the 17th edition as a reference not what some jumped up idiot from the NICEIC would say.
 
Last edited:
so what do you do then 3% form origin to last light which i fed from submain? and the submain cable would cost a lot now, i rember when 2.5 tw/e was only ÂŁ23 now double that easy.
so you need to know everything before u install sub main just incace somone els installs a lighting circuit with 6.8 v vd
and why is it that lights are 3% i just never been told
 
so what do you do then 3% form origin to last light which i fed from submain?
Yup. How you choose to split that between final and distribution circuits is up to you as the designer.
the submain cable would cost a lot now, i rember when 2.5 tw/e was only ÂŁ23 now double that easy.
I don't think anyone would argue that cable isn't expensive these days, especially for a chunky bit of SWA. The example of an outbuilding, 30m down the garden, strikes me as a bit of a luxury though.
so you need to know everything before u install sub main just incace somone els installs a lighting circuit with 6.8 v vd
If someone comes along after you and makes an addition to the installation which is not compliant that is their problem. You might want to consider future proofing your install though, and discuss the pros and cons (costs) with the customer.
and why is it that lights are 3% i just never been told
I've wondered that myself. I've got a theory, but I'd like to here from someone more knowledgeable.
 
even so i was allowing almost 1kW for lights. thats a lot of light for a shed!. i would use 6mm unless there were plans for expansion.
 

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