Lighting Circuit.. Surely This Isn’t Right?

[Stuart King]

Its the same question as a year ago but there was no answer
50w at 230v i=p/v i=50/230 i=0.28 amps
50w at 12v i=50/12 i=4.2 amps
Is this correct?

 

[Tag]

50w at 12v i=50/12 i=4.16

 

[Stuart King]

Always round up.
Is the 12v sum correct?

 

[Tag]

I was told to never round up... I think that WAS how it was done but I was recently told different

 

[Stuart King]

Anyway,is the 12v sum correct?

 

[Tag]

yes it is

 

[Doomed]

yes (for the secondary side of the transformer)

Now calculate the primary current

 

[Modernelectric]

yes it is correct.

 

[Stuart King]

The primary side is fine, just can't get my head around the secondary

 

[sparks1234]

You are forgetting that you are working it out at 12 volts, if you transpose it to 240 volts (for ease of calculation then divide your 4.2 amps by 20 which comes to 0.21 amp, so 5 LV lights to 1 ampish, so why not put 20 or so lights on a circuit, calcs work out alright.

 

[Stroppy]

Don't forget to add in the power factor load of the transformers

On a serious note, all these quotations of loads on the 1.0 don't mention that the cable should be de-rated as, being a lighting cable, it will undoubtedly be going through loft insulation, bunched with other cables, and goodness knows what else on its route, knocking the CCC down considerably. I know 'official' cable calcs often give ridiculously pessimistic results, like demanding a 10mm cable for a lighting circuit because of all the factors, but in this case 1.0 really doesn't seem enough.

It would be be interesting to do a proper calc with the given load, and all the insulation, grouping, and other factors, and real length and see what it comes up with.

 

[i=p/u]

and did any one mention diversity..... ive pushed circuits to limits .... forget future proofing...... will keep us in work and are quotes will be slightly cheaper

 

[Engineer54]

You can probably count the amount of times ALL of a circuits lights are on, within a domestic setting on one hand over the lifetime of the installation!! As far as i'm aware, 100W incandescent lamps are no longer available, and 60W lamps are soon to follow.

I'm not saying that the circuit as described by the OP is a particularly good one, but most are over assessing the actual demand load that this circuit will ever see!! Diversity is a wonderful thing, to those that know how to use it!!! lol!!

 

[jumpin jax]

This is 2009, time warp

 

[Engineer54]

Just Noticed that!!! lol!! All Stroppy's fault digging up an ancient post!!... :56:

 

[Stroppy]

Oops didn't read the date! It was on the front page so I thought it was quite recent. Never mind, that house has probably burnt down by now, from a 1.0 cable melting onto some newspaper in the loft!

 

[Engineer54]

Nah, ...It'll probably be good till the day the house get's an overall re-wire!!

 

[keniff]

Isn't it 66% diversity on lights on domestic? I know old post lol

 

[Stroppy]

Yes it is. I've never liked the idea of diversity on lights. In houses inhabited by rich and/or lazy people it is common to have all lights blazing all day and most of the night whether anyone's in the house or not. In my two-room hovel it is very common to have both candles alight at once, so no diversity here...

 

[Engineer54]

haha!! Are you saying that it's common for householders to have every lamp switched on a circuit, including exterior lighting?? I don't know what world your living in but it's not mine. Diversity ''always'' needs to be applied, especially in domestic installations, or you'll be asking the DNO for a cut out fuse of around 160+A.

 

[dagrat]

haha!! Are you saying that it's common for householders to have every lamp switched on a circuit, including exterior lighting?? I don't know what world your living in but it's not mine. Diversity ''always'' needs to be applied, especially in domestic installations, or you'll be asking the DNO for a cut out fuse of around 160+A.

not in my house, kids leave bathroom lights on , bedroom lights on, living room lights on.
my kids don't do diversity...........:coolgleamA:

 

[shocking_eg]

diversity ?!?!?! come on !

no probs mixing 1.0 and 1.5mm together.. just select overcurrent device suitable for the smaller CSA cable.

 

[shocking_eg]

plus the amount of LED's being installed now means you can have more lights per circuit.

dont forget the startup current on LED's though !

 

[Engineer54]

not in my house, kids leave bathroom lights on , bedroom lights on, living room lights on.
my kids don't do diversity...........:coolgleamA:

Well that would be across 2 circuits from your description here, up and down stairs circuits. .lol.!! If you let your kids go round switching every light switch on in the house, and leaving them on, then more fool you.

In domestic surroundings, it's unlikely that the lighting circuits will be drawing a fraction of the cables or protection devices rating!!