probably a stupid question - lighting circuits

[peterf83]

Hi,

I have 12 spot lights in my kitchen, having new kitchen fitted next month. Wife has just suggested she might want more spotlights. I have a 6amp mcb for downstairs lighting circuit. I have 17th edition CU, downstairs lighting with upstairs ring on RCD and visa versa.

I have changed all my lights to 5w LED bulbs from default 50w halogen. If I have 6a MCB my understanding is that I have a capacity of 1380w for my lighting circuit. Question is, seeing as I have changed my bulbs to LED does this mean my light fixing only draws 5w per spotlight seeing as that is all the bulb requires. Or due to light fixings being supplied with 50w halogen bulbs that each light fixing still has load of 50w per spotlight. They are mains voltage lights. I am sure common sense would suggest that the draw per light is 5w not 50 but wanted to confirm.

Also I currently have a light switch with two switches, one for one half of the lights, the other for the other. If I have converted to LED does this mean I could have all of my lights on one switch due to much lower load?

Thanks

 

[riveralt]

Hi,

I have 12 spot lights in my kitchen, having new kitchen fitted next month. Wife has just suggested she might want more spotlights. I have a 6amp mcb for downstairs lighting circuit. I have 17th edition CU, downstairs lighting with upstairs ring on RCD and visa versa.

More spotlights? might be better to have some undercupboard lights instead/as well.
No doubt someone will be along here shortly to give you some 'advice' on the lighting capabilities of downlights - ignore him.

I have changed all my lights to 5w LED bulbs from default 50w halogen. If I have 6a MCB my understanding is that I have a capacity of 1380w for my lighting circuit.

Correct, although with diversity you could have more.

Question is, seeing as I have changed my bulbs to LED does this mean my light fixing only draws 5w per spotlight seeing as that is all the bulb requires. Or due to light fixings being supplied with 50w halogen bulbs that each light fixing still has load of 50w per spotlight. They are mains voltage lights. I am sure common sense would suggest that the draw per light is 5w not 50 but wanted to confirm.

It is the demand that you base you calculations on - if someone else comes along later and changes to 50w then they should do the calculations themselves. - You could always change the MCB to 10A anyway.

Also I currently have a light switch with two switches, one for one half of the lights, the other for the other. If I have converted to LED does this mean I could have all of my lights on one switch due to much lower load?

Most light switches are rated for 10A anyway - so load is not such an issue and yes you could put both sets on lights on one switch.

 

[knowall]

You could consider using 8w wide beam lamps. I have installed Philips 8w GU10 lamps in my kitchen to replace 50w halogens, they are the biz.

Regards,

NA

 

[peterf83]

Perhaps I should have done more research into these led bulbs before I spent £400 on them, unfortunately the bulbs I have are here to stay.

 

[securespark]

Oops. Always best to buy a couple and see how they perform before busting your bank account!

Have you fitted them all? If not, you could send them back.

 

[winston1]

Hi,

You could always change the MCB to 10A anyway.

Surely that depends on whether it is wired in 1mm or 1.5mm cable.

 

[EFLImpudence]

No, 1mm² good for 16A (method C, of course)
1.5mm² only increases to 20A.

Bit pointless this fashion for 1.5mm² lighting circuits.

Immersion on 1mm² ok.

 

[winston1]

Immersion on 1mm² ok.

If that is the case why is it not done with the current ever increasing price of copper?

 

[OwainDIYer]

I have changed all my lights to 5w LED bulbs from default 50w halogen. If I have 6a MCB my understanding is that I have a capacity of 1380w for my lighting circuit.

Correct, although with diversity you could have more.

Since when was diversity permitted on lighting circuits? (if you say 17th Ed then I'll have to buy a new Big Coloured Book)

 

[riveralt]

Since when was diversity permitted on lighting circuits? (if you say 17th Ed then I'll have to buy a new Big Coloured Book)

BGB 311.1
On site guide (red book) Appendix 1 - Table 1b - Lighting circuit 66% of total current demand.

 

[securespark]

Immersion on 1mm² ok.

Used to be apparently.

But now the minimum CSA for power circuits is 1.5.

//www.diynot.com/forums/electr...tn-bs7671-imperfections.370254/page-2#2819760

 

[EFLImpudence]

Immersion on 1mm² ok.

If that is the case why is it not done with the current ever increasing price of copper?

Because tradition is a powerful thing.
(Although 1.5mm² is the minimum allowed on 'power' circuits (whatever they are).)

With 15A BS3036 fuses (rewirable) the cable had to be derated by 0.725.
So 15 / 0.725 = 20.69A therefore 2.5mm² had to be used - and still is.
The same for cookers and 6mm² with 30A fuse. 4mm² is fine with 32A MCB.

Far from saving copper, huge amounts must be being wasted because of such things as 1.5mm² on 6A lighting circuits and replacing meter tails with 25mm² 'because we do'.

 

[JohnW2]

Since when was diversity permitted on lighting circuits? (if you say 17th Ed then I'll have to buy a new Big Coloured Book)

On site guide (red book) Appendix 1 - Table 1b - Lighting circuit 66% of total current demand.

Indeed, but I wonder how long that figure for domestic dwellings will persist. It was probably very reasonable when most houses had a single lighting circuit, but I'm not so sure now that two circuits, with an upstairs/downstairs split, is common - I would think it quite common that all, or nearly all, of the downstairs lighting would be on simultaneously (hence more like the 'commercial' situation). Maybe, therefore, this domestic figure will be one day be revised to something closer to the 90% for commercial premises?

Having said that, the advent of low-energy/LED lamps obviously means that heavily-loaded domestic lighting circuits are beoming less and less prevalent.

Kind Regards, John

 

[Iggifer]

total load for my upstairs lighting circuit = 88w

 

[JohnW2]

(Although 1.5mm² is the minimum allowed on 'power' circuits (whatever they are).)

From the way the concept/Table is arranged, I presume that one has to assume that a 'power' circuit is any circuit which is not a lighting circuit.

With 15A BS3036 fuses (rewirable) the cable had to be derated by 0.725. So 15 / 0.725 = 20.69A therefore 2.5mm² had to be used - and still is.

I agree with your bottom line, but that's a rather odd way to approach the calculation. Surely the more relevent calculation is that a 1.5mm² cable only has a CCC (Method C) of 20A x 0.725 (i.e. 14.5A) when protected by a BS3036 fuse, hence fractionally too little to be protected by a 15A BS3036 fuse.

The same for cookers and 6mm² with 30A fuse. 4mm² is fine with 32A MCB.

Indeed so.

Kind Regards, John