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Topic Title: Volt drop query
Topic Summary: Combined sub main / final circuit volt drop
Created On: 18 February 2014 08:39 PM
Status: Read Only
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 18 February 2014 08:39 PM
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Posts: 32
Joined: 29 October 2007


Can someone please help me with the following.

I'm trying to work out the max cable length allowed for a sub main supplying a flat within a block with regards to volt drop.
As the flat obviously has lights and power the max vd will be 3% but do I have to apply that from the origin of the sub main or the flat's consumer unit?

If it's from the origin of the sub main is the following correct?

Calc the max volt drop for a typical lighting circuit: 20m x 1.5mm t&e, 2.17A(500w) load

20 x 29(mv/a/m)x 2.17/1000= 1.26v

3% of 230v = 6.9v

6.9 - 1.26 = 5.64

Therefore I have 5.64v left for the sub main.

The sub main is 16mm 90 deg thermosetting cable; mv/a/m is 2.9, the max demand load of the flat is assumed 40A.

So ..... 5.64/(2.9x40) x 1000 = 48.6m

Don't often have to do this so I'd appreciate your input

Many Thanks
 19 February 2014 06:43 AM
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Joined: 07 August 2007

I would say no more than 3% in total.
The idea is to avoid displeasing voltage drop that causes dimming or flickering of lamps, and to ensure proper functioning of loads.

The lamp or other load does not "know" if the voltage drop occured in the final subcircuit or in the submain.

I would aim for about 2% in the submain and about 1% in lighting subcircuits, and about 3% in non lighting subcircuits.

The accepted figures are recomendations and not absolute requirements, but I would stronlgy advise following them for a standard domestic install.

In some specialist applications much greater voltage drop might be fine, but not for a typical flat or other home.

Edited: 19 February 2014 at 04:00 PM by broadgage

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