Volt drop on spurred lighting

[adam.h]

I have a run of garden lights to wire up, roughly 1000w so about 4.3A. If they are on an rcd spur from the ring main, is the volt drop allowed for a lighting circuit (3ish %) or for ring main (7% is it)? As 100m at 4.3A will require 4mm if my calcs are correct for a 3% Vd.

 

[MarkieSparkie]

Adam, The allowed voltage drop is a maximum of 3% for lighting and 5% for other uses, when supplied directly from a public low voltage distribution system. For every metre your cable run exceeds 100m you may increase the voltage drop by 0.005% /metre up to a maximum of 0.5%. You may also apply diversity to your garden lights at 66% of total current demand. The voltage drop can be calculated from the current demand of the lights having applied diversity, or from the design current of the circuit as appropriate. Remember to calculate the maximum voltage drop for the whole final circuit and NOT just the spur.

 

[JUD]

I have a run of garden lights to wire up, roughly 1000w so about 4.3A. If they are on an rcd spur from the ring main, is the volt drop allowed for a lighting circuit (3ish %) or for ring main (7% is it)? As 100m at 4.3A will require 4mm if my calcs are correct for a 3% Vd.

Your voltage drop should be calculated from the origin of the supply to the end of the circuit.

As you are spurring off a ring circuit then the voltage drop of the ring circuit must be taken into consideration.

So the sum of the voltage drop of the ring circuit and the voltage drop of your lighting circuit needs to be 3% max.

Hope that helps.

 

[andyb]

You may also apply diversity to your garden lights at 66% of total current demand.

As these are garden lights I presume they would all be on together, so you shouldn't apply any diversity.

And as Jud says, you should take the load of the circuit you are spuring off into account.

 

[malcolmsanford]

Not a good answer lol

 

[adam.h]

Yes, they would all come on at the same time so diversity would not be taken into account. I just got to check that the nearest socket is not a spur itself!

If the lights were wired up in a ring configuration (as they will be going all around the garden anyway) theroatically this would reduce the Vd, would you work out the Vd then with half the length (so 50m instead of 100m) or best to keep the lights wired in a series / radial configuration, so to speak.

 

[pushrod]

Think i would be looking at putting these lights on their own circuit.

 

[adam.h]

Think i would be looking at putting these lights on their own circuit.

Then the supply would need to be either split to a separate cu or a cu change. Which I don't mind doing but they won't lol

 

[pushrod]

Then the supply would need to be either split to a separate cu or a cu change. Which I don't mind doing but they won't lol

Still think i would go with something like a shower CU not going to be much different in price to a rcd fcu.



If you go for the radial spur option then the volt drop calc is a whole series of calcs up to the individual lighting points as the current at each point will decrease. So full current up to first point, then the short section up to the next point with slightly smaller current and so on. Vd on individual sections then added up to give total Vd.
The Vd on the ring then also has to be considered and this is even more complex as it would depend on the load on that circuit and its length.

As 100m at 4.3A will require 4mm if my calcs are correct for a 3% Vd

.

Not a single calc, as said above, as by the time you get to the end of the circuit there will not be 4.3 A

 

[adam.h]

Yea I had completely forgot about that. It was late though

Going to separate the supply as suggested as they are also thinking about having a work shop at the end of the garden.