Max volt drop for 25mm or 35mm feed to sub CU

[n180]

How does max volt drop for 25mm or 35mm feeds to sub CU work.

Do we for example allow only 1% drop on the 25/35mm so we can keep 2% drop for lighting circuits and 4% for power circuits, or is there any where else in the regs that gives specific figures on maximum allowed volt drop? Any difference for single phase or 3 phase supplies?

Thanks.

 

[westward10]

Single phase is no different to three phase in terms of vd but you are on the right track. 2% is often an unwritten rule of thumb for submains but this is flexible depending on your vd for final circuits at the served distribution board.

 

[pc1966]

Depending on the cable length trying to keep to 1% could be rather expensive! Ideally you would work through acceptable combinations of feeder size and final circuit sizing to see what is best overall value for money.

Three phase is more tolerant simply as you have a higher voltage (or are assuming cancellation of current on the neutral) but the same idea as westward10 said.