Maximum voltage drop on SWA runs to outbuildings, am I over engineering the solution

Whilst it could be argued that modern LED lights are more tolerant of reduced supply voltages than are incandescent lamps, that does not tell the whole story.

These days we should be considering the energy cost of voltage volt drop, especially for large and long hour loads such as EV chargers. Consider a typical 7 kw charger used for 1000 hours a year. A voltage drop of 5% represents the loss of 350 watts. 350 watts for 1000 hours a year is 350 kwh. At current prices of about 30 pence a unit, that is over £1,000 a year in wasted energy.

Doubling the cable size to halve the loss to about £500 a year is likely to be worthwhile. Cable of twice the size costs about twice as much, but the trench or other installation overheads are about the same.

I would aim for a voltage drop of no more than 2.5% in a submain to an outbuilding that includes an EV charger. That allows 0.5% for the presumably short conductors between the service cut out and the origin of the sub main, and between the load end of the submain and the EV charger. If these connections are not short, then calculate properly for all parts of the install.

So whilst the light fitting should be fine at say 200 volts, the 3% LIMIT IS SENSIBLE for energy saving rather than for lighting effectiveness.

Broadgage has said exactly what I was going to say, for things like an EV charger, which is using a lot of current for a long time, cable losses could be a significant cost, so going up one or 2 cable sizes could actually save money over the life of the charger. The only downside is trying to fit, say, a 50mm cable into typical domestic style consumer units and switch fuses. You need to size up the enclosures to get those cables in.

Or run it in T & E or singles to an adjacent metal box and gland off the SWA there, leaving the consumer unit untouched. Provided you have space to mount such a box. 

Yes the volt drop is something you have to watch and the way fuel prices have gone recently then more so than ever. If we had a crystal ball and could reasonably predict the  expected prices of KWhrs in future then we might rethink our allowable losses too

Getting 25 mm² cable into domestic stuff is bad enough, but you'd only be going up from 4 mm² to 10 mm² or perhaps even 6 mm² to 16 mm² if there are other significant loads.

alanblaby said:

The only downside is trying to fit, say, a 50mm cable into typical domestic style consumer units and switch fuses.

Getting 25 mm² cable into domestic stuff is bad enough, but you'd only be going up from 4 mm² to 10 mm² or perhaps even 6 mm² to 16 mm² if there are other significant loads.

alanblaby said:

The only downside is trying to fit, say, a 50mm cable into typical domestic style consumer units and switch fuses.