Volt drop at the other end of a SUB main.

[Kyran]

Hello people Just so i get this right i thought i would post my maths here for people to comment on.


Garden lights on a sub main. Sub main is 20meters from the main CU on a 6mm armoured cable. the last light is 50 meters from the sub main. meaning end to end 70 odd meters.
So my maths are right yes?


cable to sub main
6mm = 7.3 mv/a/m on a 20 amp breaker, so
7.3 x 20 x 20 = 2.92v


cable from sub main to last light.
4mm = 11 mv/a/m on a 6 amp braker (4 core armoured, for two sets, switches of lights)
11 x 6 x 50 = 3.3


Add to to together and you end up with 6.22v drop at the end of 70 meters of cable. which is just in side the 6.9v allowed.


Sound good.


NEXT QUESTION unless i am being proper think.


How would you wire the switching as just released that that 70meters does not account for the switch lines. which would mean getting even larger CSA if not just for the sub main supply.


ops almost buggered it up.

 

[Simonslimline]

What is the actual Ib of each circuit?

 

[Kyran]

What is the actual Ib of each circuit?

For the sub main its only 10.4amps as its a little waterproof garage CU
and 0.45 amp for the garden lighting, it low as there are all led totalling a 104watts

 

[Chr!s]

For the sub main its only 10.4amps as its a little waterproof garage CU
and 0.45 amp for the garden lighting, it low as there are all led totalling a 104watts

There lies your answer, redo with Ib

Chris

 

[Simonslimline]

As above, you have used In instead of Ib giving you a greater volt drop value.

AVD = (mV/A/m)xIbxL divided by 1000.

 

[telectrix]

what he said and him before him etc. use Ib as your current.

 

[Kyran]

Hi people, "what a plonker Rodney", ya the writings on the wall with that one. i know that there was something wrong he he. Any way thank you everyone for point out the obvious.

 

[D Skelton]

You also haven't taken account of the staggered load on the garden lighting circuit. The volt drop on a circuit with staggered loads will never actually be anywhere near what you calculate using the length of the entire circuit and the total load of the entire circuit. Not that it matters anyway in your situation, but it is something that most people don't take into account when designing.

 

[Kyran]

You also haven't taken account of the staggered load on the garden lighting circuit. The volt drop on a circuit with staggered loads will never actually be anywhere near what you calculate using the length of the entire circuit and the total load of the entire circuit. Not that it matters anyway in your situation, but it is something that most people don't take into account when designing.

Ya yes good point, i work out circuits before adding that in but for got to do it this time. as you said its not one to worry about with this. but thank for reminding me. Thats why i like to put stuff up here people are always very helpful and not judge mental.