Voltage Drop Tolerances

[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

 

[armbands0]

Does the client need a 32A ring?

 

[rryles]

Can you not apply diversity to the lighting?

 

[Colonel Hathi]

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

 

[rryles]

Like so many things, that may be NICEICs opinion, but it isn't shared.

Q & A of the Day – What voltage drop for sub-main feeding a dis-board? - Voltimum UK - Electrical Installation Products and Contracting

http://www.electriciansforums.net/e...nal-circuits-voltage-drop-table-question.html

Why should feeding from a submain allow a larger voltage drop? What if you have a submain feeding a submain? Are you now allowed a 13% drop for your lighting?

 

[ian.settle1]

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.

 

[redface]

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 £23 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

 

[rryles]

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 £23 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.

 

[Chr!s]

I've wondered that myself. I've got a theory, but I'd like to here from someone more knowledgeable.

Probably to do with the increase in current and the fact that control gear wont function correctly at lower voltages on some type of lighting

 

[redface]

But what about just domestic lights then ?

 

[rryles]

All fluorescent lighting has control gear of some sort, including CFL lamps.

 

[telectrix]

applying diversity, i would use 26A as my load on the ring final and 4A for the lights. 30A total. so vd = 7.3 x 30 x 30 /1000 = 6.57v. or 2.8%

 

[rryles]

That's just in the distribution circuit though. That only leaves 0.2% for the lighting circuit, which is rather tight.

 

[telectrix]

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.

 

[ian.settle1]

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.

Maybe 1 or 2 500w TH floodlight may be fitted to illuminate the garden should also possibly taken into account?

 

[Engineer54]

I doubt very much your going to see anything like a 26A loading at this shed. As mentioned many time's here, diversity, and commonsense often go hand in hand. Too many ''what ifs and ''maybes'' always tend to cloud the waters.

 

[rryles]

Maybe 1 or 2 500w TH floodlight may be fitted to illuminate the garden should also possibly taken into account?

In theory shouldn't happen. At least in England and Wales. 500W TH floods are very unlikely to be deemed to satisfy building regs part L.

 

[telectrix]

stuff part L . when all the government ( local and national ) leave all their office lights on all night. anyway, i ain't finished reading parts j and k yet. i got sidetracked by part pee.