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]

It's not that simple with lighting circuits because the load is distributed.

The load decreases as you get towards the end of the circuit i.e. the length of cable to the first light in the circuit will be carrying the full load, the length of cable from light 1 to light 2 will be carrying the load for lights 2,3,4,5,6,7 and 8 and so on and so on.

As the load decreases after each light so does the VD.

Have a look at this thread....

http://www.electriciansforums.net/e...1-voltage-drop-outside-lights.html#post505686

 

[ianbrookes]

use LED's

 

[Sparky83]

It's not that simple with lighting circuits because the load is distributed.

The load decreases as you get towards the end of the circuit i.e. the length of cable to the first light in the circuit will be carrying the full load, the length of cable from light 1 to light 2 will be carrying the load for lights 2,3,4,5,6,7 and 8 and so on and so on.

As the load decreases after each light so does the VD.

Have a look at this thread....

http://www.electriciansforums.net/e...1-voltage-drop-outside-lights.html#post505686

Thanks, I'll take a look

use LED's

Can't unfortunately, have to use them ones on the drawing which are 150w high pressure sodium wall lights

 

[Sparky83]

It's not that simple with lighting circuits because the load is distributed.

The load decreases as you get towards the end of the circuit i.e. the length of cable to the first light in the circuit will be carrying the full load, the length of cable from light 1 to light 2 will be carrying the load for lights 2,3,4,5,6,7 and 8 and so on and so on.

As the load decreases after each light so does the VD.

Have a look at this thread....

http://www.electriciansforums.net/e...1-voltage-drop-outside-lights.html#post505686

Just had a read through that post, thanks again, that has helped no end, also one of the comments on there was about not using diversity due too them being all on at the same time, which i had forgotten about.

Another question related to the lights within the building, in the function room i planed on using a Klik distribution panel above the dealing feeding 8 lights. When measuring the total circuit length do i need to add each leg to the fittings themselves? or just to the panel? what about the switch cable?
My understanding of these is you take your supply to the panel above the ceiling, where each light is plugged in, and a cable from the switch also goes to the panel.
So if the supply cable was 10m, each light was 1m and the switch cable was 5m what would the total circuit length be?

 

[truckster]

As I see it, you would be looking at the max circuit length, supply 10m, switch 5m and distance to furthest light

10+5+1 = 16m

 

[Guest123]

What type of lamp are you using? as you may need to factor in the multiplier for discharge lighting....if they are infact discharge types.

 

[Sparky83]

In my original question they are...

150w High Pressure Sodium Wall Mounted Up-Lights

Am i correct in doing the following?

150w / 230v x 1.8 = 1.17a per fitting?

 

[truckster]

In my original question they are...

150w High Pressure Sodium Wall Mounted Up-Lights

Am i correct in doing the following?

150w / 230v x 1.8 = 1.17a per fitting?

Thats the way I'd do it

 

[Guest123]

Yes, the 1.8 multiplier would need to be used with sodium type lamps, as you have shown.

 

[Sparky83]

Thats the way I'd do it

Yes, the 1.8 multiplier would need to be used with sodium type lamps, as you have shown.

Thanks guys, much appreciated!

It surprising all the stuff you don't think off when doing "normal" installs, and not many of them either.

 

[alanl]

You are making it far too complicated.No need for a ring lighting circuit, you just make the cables bigger. You are the Designer, and you justify the larger size in your calculation 1.5mm is toosmall for th current/length, so you have to go to the next size up.They are not looking for clever ways to reduce the installation cost, they are looking for safe practice, which, by giving the correct cable size, you are doing.If you get time, work out the cost of a 1.5mm ring, and 2.5mm radial for that long circuit. I'd bet there is little difference in the cable cost.Alan.

 

[RISElectrical]

4mm cable job done haha

 

[Sparky83]

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?

 

[Richard Burns]

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.