Electrical question

Business Man · 7 Dec 2015 at 17:43

Is it OK to put 1.00mm twin earth pvc cable in 20mm pvc round conduit above a plasterboard ceiling? This would be a distance of 2 meters from the centre load bearing wall to the light fitting above living room ceiling.

There is no insulation in the ceiling and the lighting circuit is on 6a.

Am I being over cautious in thinking the cable may over heat in pvc conduit with reduced air circulation

Samueltastic · 7 Dec 2015 at 17:46

Why are you putting it in conduit? but no it would be fine

Mynight · 7 Dec 2015 at 17:48

you mean 1.5mm?

Can you not just run the cable where the previous one is? I'm not seeing the need for pvc unless youre running it at a weird angle(even then I'm not sure why).

JohnnyG · 7 Dec 2015 at 17:56

There are calculations when running cable in conduit or trunking to offset the reduced current carrying capacity and the increased thermal effects however on a run that small with a max load of 5/6a & so much free air still in the conduit the effects will be negligible.
As mentioned above though I'm not sure why you'd actually want to anyway.

Samueltastic · 7 Dec 2015 at 18:01

Mynight said:
you mean 1.5mm?

Can you not just run the cable where the previous one is? I'm not seeing the need for pvc unless youre running it at a weird angle(even then I'm not sure why).

1mm is more than ample for lighting now days with LEDs and energy efficient lamps. 1mm would carry 10amps with ease. Any higher and I'd up cable size.

Mynight · 7 Dec 2015 at 18:03

I didnt even know there was 1mm

. (at least I forgot about it)

Soundood · 7 Dec 2015 at 18:06

as has been hinted there is a long answer and some calculations that can be done to adhere to regs,

but the short answer is technically the cable capacity is greater the cooler it is, inside conduit it could technically get hotter,

so the answer is run the cable without, taking care to make sure it is not straight across walkways/crawlspaces, maybe clipped to joists.

for the fun of it, here is the calculations from my spreadsheet I made in pages, I love this thing, I just change the values in the red boxes and I get the answers, saves faffing around in the on-site guide,

JohnnyG · 7 Dec 2015 at 18:20

Soundood said:
http://i.imgur.com/RGzE1gW.jpg[img][/QUOTE]

That's awesome :cool:
I don't suppose you'd like to share it? :)

Samueltastic · 7 Dec 2015 at 18:24

Soundood said:
as has been hinted there is a long answer and some calculations that can be done to adhere to regs,

but the short answer is technically the cable capacity is greater the cooler it is, inside conduit it could technically get hotter,

so the answer is run the cable without, taking care to make sure it is not straight across walkways/crawlspaces, maybe clipped to joists.

for the fun of it, here is the calculations from my spreadsheet I made in pages, I love this thing, I just change the values in the red boxes and I get the answers, saves faffing around in the on-site guide,

no cpc. naughty

JohnnyG said:
That's awesome
I don't suppose you'd like to share it?

Yes please if its going

Soundood · 7 Dec 2015 at 19:17

JohnnyG said:
That's awesome
I don't suppose you'd like to share it?

Samueltastic said:
no cpc. naughty

Yes please if its going

of course, here it is, you need Pages on ipad or iPhone, I use it on both, red areas can be changed, by either double tap and enter value, or tap and drop down menu,

https://www.dropbox.com/s/6bou4qybg7507jq/Circuit%20Planning%20Form%20Template.nmbtemplate?dl=0

Business Man · 7 Dec 2015 at 19:19

It's just to make it easy for any future work really, at the moment the ceiling is down but I'm sure electricians love conduit already in place to make it easier for them.

JohnnyG · 7 Dec 2015 at 19:30

Soundood said:
of course, here it is, you need Pages on ipad or iPhone, I use it on both, red areas can be changed, by either double tap and enter value, or tap and drop down menu,

https://www.dropbox.com/s/6bou4qybg7507jq/Circuit%20Planning%20Form%20Template.nmbtemplate?dl=0

Cheers

Business Man said:
It's just to make it easy for any future work really, at the moment the ceiling is down but I'm sure electricians love conduit already in place to make it easier for them.

Only when the conduit is plastered in a wall, it will make it more difficult to replace if it's just laying on the plasterboard.

Samueltastic · 7 Dec 2015 at 20:59

Business Man said:
It's just to make it easy for any future work really, at the moment the ceiling is down but I'm sure electricians love conduit already in place to make it easier for them.

If its only two meters and fairly open I wouldn't bother we have rods for these instances

Amp34 · 7 Dec 2015 at 21:22

Samueltastic said:
1mm is more than ample for lighting now days with LEDs and energy efficient lamps. 1mm would carry 10amps with ease. Any higher and I'd up cable size.

Problem is it's a light fitting that could have incandescents installed at a later date so you should really update it to the max, in case someone at a later date decides to install a 6 bulb light fitting full of 100W bulbs.

At least that's what I'd expect any inspection would come up with.

Samueltastic · 7 Dec 2015 at 21:34

Amp34 said:
Problem is it's a light fitting that could have incandescents installed at a later date so you should really update it to the max, in case someone at a later date decides to install a 6 bulb light fitting full of 100W bulbs.

At least that's what I'd expect any inspection would come up with.

Nope, as a lighting circuit should be on a 6amp breaker (norm in houses), the breaker is there to protect the cable so worse case would be that it would trip.

Even if they did do that then current drawn would be 2.6amp for that one fitting

Stanners · 7 Dec 2015 at 22:39

Amp34 said:
Problem is it's a light fitting that could have incandescents installed at a later date so you should really update it to the max, in case someone at a later date decides to install a 6 bulb light fitting full of 100W bulbs.

At least that's what I'd expect any inspection would come up with.

It all depends on the protective device doesn't it. Put a 6amp device on and you will never have a problem as you are limiting the let through energy.

The guy with the spreadsheet, have you allowed for Cmin that has now to be used since amendment 3 of BS7671.

Soundood · 7 Dec 2015 at 22:46

every light fitting must be calculated to be 100W, even 240v 50W halogens, and even if you plan on fitting 2.1W LED's,

I have a job coming up, with 8 x LED downlights being install in the living room, replacing the existing single rose, here is my calculations, and this is because I need to allow for the 100W per fitting, even though in reality it is much much lower, because all I know someone could pop in some Halogens in there, and also slightly odd as the EU is about to ban them next year, in favour of LED

Soundood · 7 Dec 2015 at 23:10

Stanners said:
The guy with the spreadsheet, have you allowed for Cmin that has now to be used since amendment 3 of BS7671.

edit: I was tired when I posted this, yes this takes into account the cmin reduction applying 95%, values in the sheet are taken from the 3rd amendment of the 17th

Amp34 · 9 Dec 2015 at 20:02

Samueltastic said:
Nope, as a lighting circuit should be on a 6amp breaker (norm in houses), the breaker is there to protect the cable so worse case would be that it would trip.

Even if they did do that then current drawn would be 2.6amp for that one fitting

The point was you can't install lighting wiring designed to take a load with only LEDs. As Soundood mentioned you have to allow for 100W per bulb. If the 1mm cable is fine for that then fine, but that wasn't what I was discussing in my post, I wasn't suggesting a larger cable than would normy be installed for lighting, just not a smaller one "because it will only have LEDs".