voltage drop over sub mains

[vde-electrician]

I am going to be installing a sub main from a house db (at meter position) to shed(1), then from that shed to shed(2), then from that to shed(3).

I am aware that lighting is max 3% drop (6.9v of 230v)
non lighting is max 5% drop (11.5v of 230v)

voltage under no load at supply is 240v. To comply with regs do i disregard this and work on basis of 230v?

In sheds there are only going to be 13a sockets
But sub mains will vary in length and cable size.

So does this mean that i can lose max of 11.5v to the shed 3 socket outlet from meter (house db) ?

Would i go about it like this: , 230v at origin, 1st submain is say 5v loss,
then for the second submain: calculate the next volt drop from 225v?
and then just ensure that through calculation no more than 11.5v has been lost at the furthest point of furthest
circuit, ie a socket outlet of shed 3?

Also what would the ambient temperature typically be if the swa cable is installed 2 feet underground.

thank you.

 

[telectrix]

for VD calcs. i use the actual voltage at source, not the made-up 230. your ambient underground temp. should be around 10 -15deg. ...less in winter.

 

[Richard Burns]

In calculating volt drop for design you should use the nominal 230V to ensure consistent calculations across installations, however if you have 240V then this might give you some leeway (but not according to the regulations).

Calculate the voltage drop for each section and add them together.
Take account of the csa of each section and the total load carried by each section.
For example if you had a design current of 10 A for each shed then:
the sub main from origin to shed 1 would be carrying 30 A;
the one from shed 1 to shed 2 would be carrying 20 A and;
the last one carrying 10 A;
for the final circuit in shed 3 the easiest way would be to assume the last socket in the circuit would be carrying all the 10 A load and calculate the volt drop for that section as the length of the final circuit carrying 10A (this is an approximation as you could calculate for each length to every socket, but it would not make a lot of difference compared to the total volt drop).
The maximum voltage drop will be at the end of the socket circuit in shed 3 (unless you have very very long circuits in the other sheds) so you can ignore the final circuits in the other sheds.
Pictorial reference: