Cable calcs?

[Sparky83]

Hi,

Im doing some work on my 2400 Design and verification course (or whatever its called now) and i have drawn my circuits onto my building plan, but having worked out the outside lights they are way over for VD, i increased the cable size to 2.5mm from 1.5mm this helped, but not enough.

As i've already drawn it, separating into 2 separate circuits will make the drawing look ruff, so i was thinking of turning it into a ring circuit instead.

What do i need to change in my calc for the fact it is a ring?? i know that my cable length will increase but that alone makes it worse, what else do i need to do??

Circuit 8 - Outside Lights
Length of circuit - 65.5mts
There is 11 light fittings, each with 1x 150w lamp per fitting.
150w ÷ 230v = 0.65A per fitting
0.65A x 11 (fittings) = 7.15A (Ib)
After 90% Diversity is applied...
7.15A ÷ 100 = 0.0715
0.0715 x 90 = 6.435A (Ib)
(In) needs to be equal to or greater than (Ib), so (In) = 10A
Using table 4D4A from BS7671...
1.5mm Armoured cable using reference method D6 has a capacity of 22A
Volt Drop,
This is (mV/A/m) x Ib x Length of circuit ÷ 1000, so from table 4D4B...
1.5mm = 29 (mV/A/M)
29 x 6.435 x 65.5 ÷ 1000 = 12.22V
As this is a lighting circuit, it must be under 3% or 6.9V to be satisfactory. This fails.


Any thoughts???

 

[JUD]

I have another question....

Although this is a colleague of mine who is also doing this course. He wants to use trunking, he has a lighting circuit that he can put on a 6A MCB, but when doing the correction factors calculation, the only one that has any relevance to him is the grouping factor.

So he would do In / Cg

Looking in the regs, his Cg is 0.03 as he has 30 circuits in this trunking.

6 / 0.03 = 200A
10 / 0.03 =333A

Is this correct?

Is his only option to use a separate run of trunking and split up the circuits?

Grouping factors in the regs only go up to 20 circuits. But extrapolating to a horrendous 30 circuits would only give you a grouping factor of about 0.3 not 0.03 which would require you to use a cable having CCC of 20A so you would be looking at 2.5mm2 and some large trunking.

It's also worth keeping in mind that circuits carrying less than 30% of their grouped current-carrying-capacity can be ignored when calculating the rating factor for the rest of the group. (Note 9 under table 6C in the red OSG)

 

[i=p/u]

equaly loaded circuits