lighting solution

Postby tt » Tue Jan 22, 2008 11:11 pm


this is a made up problem,

there is no work involved.

A house has 1 x 6a light circuit but this has running off of it

front room
has 5 up lighters with 40w bulbs in each, a double 250 watt dimmer also has two 40w ceiling fans with 3 lamps on each all 40w bulbs

has two 40w ceiling fans with 3 lamps on each at 40w bulbs

the other 5 rooms have 40w single bulbs.

My question is,

If asked to sort this out, what is the proccess I would have to follow

Would I be right to think along the lines of adding another 6a mcb in the cu and split the light circuit into two seperate ringed circiuts, as for the front room having it's own because there is more than 10 lights,

Anyone explain the calculations involved to get it right.

I'v only done a basic electrical course so far, so more towards understanding rather than doing

thank you for any guidence

Posts: 18
Joined: Sat Aug 11, 2007 4:10 pm


Simply Build It

Postby ericmark » Wed Jan 23, 2008 2:16 am

With the new 17th Edition the fuse sizes may change and at this point it is pointless working out what can be done as by June it will all change. There are two major problems with lighting design.
1. The amount of lights switched on together. If as seen in posts on this forum you get 5 or 6 GU10 bulbs at 50 watt switched on together they can trip the MCB Osram web site stated over 10 amp on 150 watt bulb when first switched on. So the more control i.e. switches the better.
2. Although the old 16th Edition shows which bulbs are limited to 6 amp overcurrent protection and those where 16 amp is allowed there are many types not listed for example GU10 and I have failed to find any recommended size for these.
The more that can be done to prevent lighting loss the better. With the new requirement on cables buried in walls I can see a big increase in the use of RCBOs
This will increase to cost per circuit as they are 6 times more expensive. Splitting a lighting circuit in two at £6 cost of cable and MCB may be acceptable but at £35 one has to be more careful. Also Part L2 regulations limit lighting power and in the future lighting bars with rows of spot lights are unlikely to be allowed unless using discharge or LED lighting. And with 10 lights if to power them you need an extra RCBO then using more expensive low energy lamps becomes more of an option. Re-ask you question in 3 months once we all have read the new 17th Edition and worked out our selves what the changes really mean.

Postby kbrownie » Wed Jan 23, 2008 8:27 am

Hi tt.
The calculation without getting over complicated is done by using ohms law and diversity.


For example
You add up all the power the lighting circuit could run on maximum demand, everthing on.
Say 10 lights at 40w, 6 lights at 50w and 2 at 100w that would equal 900w
that would be devided by your voltage domestic being 230v
That will give you your design current of 3.91. MCB must be equal or above design current, so that would be a 6amps.

On larger installation diversity could be used:
which allows household installation to take in to account only 66% of total current demand, this is on the presumption that every light will not be switched on at the same time.
So if your total power was 2kw.
2000w/230v=8.696amp(total current demand)
Then accounting for diversity of 66%(Household lighting installation only)
8.696amp would become 5.74 amps allowing use of 6amp MCB.
Hope that is clear, these are only the basic calculations. Some more complicated installation need more thought.
Posts: 1759
Joined: Mon Mar 05, 2007 9:36 pm

Postby tt » Wed Jan 23, 2008 11:36 pm

many thanks to KB & ericmark for replying

the way kb has broken it down, the easier it is to understand, the diversity is the word I've not got to gripes with

Am I correct in thinking/saying that for example 20 bulbs at 40 w = 800w

the 800w / supply current of 230v will = 3.478amps x 66% = 2.295amps

is the 3.478 the total demand in amps ?

the 66 % is to allow for diversity ?

and the 2.295 amps is whats most likley to be used at anyone time ?

even doubling the wattage of the bulbs would only just go over the 6 amps 6.956 then x by the 66% takes it back down to 4.591 but I couldn't ever do that because of the possibility of 100 w builbs being used

KB am I anywhere near getting it right.
I'v got exel so If you do give me an examples like you have I can build up the calculations to help me remember how to do them.

thank you for your help
Posts: 18
Joined: Sat Aug 11, 2007 4:10 pm

Postby kbrownie » Thu Jan 24, 2008 9:28 am

you've got it bang on, if you give me sometime i'll knock up some calc for you later on.
Posts: 1759
Joined: Mon Mar 05, 2007 9:36 pm

Postby kbrownie » Fri Jan 25, 2008 4:15 pm

Hi try this little puzzle, should be easy.
Bungalow with 3 bedrooms.
bed 1; has 2 wall lights with 2 x 40 watts bulbs and a ceiling mounted light with 5 x 50 watts bulbs.
bed 2; has 2 wall lights with 2 x 40 watts bulbs and ceiling mounted light with 3 x 60wattsbulbs.
bed 3; 5 halogen downlights each rated at 50 watts
Study has also has 5 halogen downlights rated at 50 watts
Kitchen has a double strip light both tubes rating 70 watts
Hall way has 2 wall lights with 2 x 60watts bulbs in each and pendant light with 3 x 40 watts bulbs.
Bathroom contains 4 x 35 watts downlights
Dining has 2 wall lights both rated at 60 watts and 3 pendant lights each with bulbs rated at 40 watts.
Work out
Design Current (maximum demand)
The Design current rated fuse or mcb
The current after diversity
and recomended rating of fuse or mcb
Posts: 1759
Joined: Mon Mar 05, 2007 9:36 pm

Postby ericmark » Fri Jan 25, 2008 7:16 pm

That's sneaky you haven't told him he has to allow 100 watts for each lamp holder! or that discharge lights are times 1.8! or what the diversity factor for lighting is? come on give him a chance.

Postby kbrownie » Sat Jan 26, 2008 6:30 am

HI, ericmark point taken regarding discharge lamps not a trick but good point.
Just a basic add up calcs and ohms law, already explained basic domestic lighting diversity and tt should now have on-site guide for help understand it a little better.
Not trying to be over complicated and no trick questions.
Posts: 1759
Joined: Mon Mar 05, 2007 9:36 pm

Postby tt » Sat Jan 26, 2008 6:05 pm

hello KB

thank you for the challange,also ericmark I've seen that about the 100w per lamp holder

here goes with or without mistakes

with the information you've given,

at present the disign current is 1970w 8.56 amps
with diversity x 66% = 1300.2 w 5.65 amps

with 100w bulbs this would change to 39 x 100w + 2 x 70w strip lights = 4040w 17.56 amps

diversity x 66% = 2666.4 w 11.59 amps

1 x 6a mcb = 1380w

if the diversity figure was divided by 2 x 6a mcb which will give upto 2760w 12 amps

Based on the present demand of 1970w 8.56amps this will give a demand of 4.82 amps 984.4 watts on each curcuit leaving 1.72a 395.6w remaining which i think is better than maximum demand all the time at least there is a little left over for expantion such as loft conversion or extention etc


Design current would be 17.56 amp

current after diversity 11.59 amps

I would say the reccomended mcbs would be 2 x 6a

If evenly split 5.79a

with the present demand this would be 4.28a on each 6a mcb

I have the on site guide, electricians guide to part p and the 17th edition.

I'm hoping i've got the right idea

thanks again

Q does the strip lights change anything having starters
what are discharge lights
how about low voltage lighting any allowances to be made

Q We have a 7000w 30.43 cooker
is it 10a + 30% of the 30.43 which = 9.129 = 10a + 9.129 + 5 for outlet socket = 24.129 amps
Posts: 18
Joined: Sat Aug 11, 2007 4:10 pm

Postby ericmark » Sat Jan 26, 2008 11:33 pm

I took the spot lights as 75 watt max as to the best of my knowledge that's the largest size available as a result slightly lower result to you at 15.22 amp and upstairs at 1.875Kw gives 8.15 amp with diversity 5.3 and downstairs 1.627Kw gives 7.07 amp with diversity 4.66 amp so as you say two circuits both using 6 amp fuse or MCB. Discharge lighting covers florescent, energy saving, Cold cathode, High pressure mercury vapor, and sodium lighting etc.
I think your correct with cooker but lost my guide so not sure.
Remember the word "Guide" you must use some common sense, For example a Baby Belling cooker has two rings an oven plus grill but because it has a selector switch you would not add them together it's just 13 amp.
As to discharge lighting you will find some give information from the manufacturers as to diversity this takes precedence over figures in "Guide". If you have a house right next to a power station as I did in Sizewell you may need to allow more that the 1.8 as you can get over voltage greater than normal. Or use switch mode.
Switch mode converts the AC to DC then back to AC at very high frequency and also corrects any small under or over voltage at the same time so uses less power than old system. Also the tubes last longer. They can be a complete pain for RF interference but are OK with rotating machinery and more like X 1.2 on diversity. I think they will soon start to be fitted at home and are very common in industry. Some tubes only have single contact either side with no heaters. They also have some very high voltages don't play.

Postby tt » Sun Jan 27, 2008 7:27 pm

hello ericmark,

thank you for your reply

In a standard domestic install is there a limit on how many lighting curcuits you can have say 3 Living room,kitchen and bathroom,then upstairs and loft for example, Is this a common sense thing aslong as the cu has the room.

Can you explain in simple terms the difference between the types of earthing

I think my house is a tn-c-s the two phases come into the house there is a large fuse housing,then the meter and tails to the cu, for the phase conductor.
there's a black box on the neutral conductor and I'd say about 16mm aswell which is going to the cu, and the neutral goes meter,cu

the other earthing is going to gas meter, aswell as the hot & cold water pipes plus heating pipes it terminates at the bath, there was nothing on the shower or the pipes for toilet or the kitchen sink pipes so I've done it already should this be tested, I've used 10mm the same as what is already been used also I used copper connectors to the straps it looks
neater rather than rapping it around the screw.

thanks tt
Posts: 18
Joined: Sat Aug 11, 2007 4:10 pm

Postby ericmark » Sun Jan 27, 2008 10:04 pm

The supply authority does not need to supply an earth and when it does not supply an earth it is normally referred to as a TT supply. If they do supply an earth this may be a separate wire and is called TN-S but this is rare and in most cases it is combined with the neutral and is called TN-C-S this was called PME.
Years ago we were allowed to use any handy metal as an earth but then problems modified the ideas and any metal used as an earth has to be selected so it will never be removed without the user being aware of its removal. The use of gas and water as earths was stopped. But we still need to ensure these services do remain at the same voltage as the rest of the house so still have earth wires connected but now they connect to pipe to house earth rather than earth the house via the service pipe.
We have therefore four types of earth in the house.
1. The ones inside the wires are called “Circuit Protective Conductorsâ€

Postby kbrownie » Mon Jan 28, 2008 9:02 am

Sorry been missing!
Her indoors has had me out looking at new kitchens and bathrooms, which mean't we ended up with a corner suite for the front room and some wooden flooring. So total baffled and head full of BLAH!.
Seems like with moved on a little!
Firstly loose ends.
Little puzzle that seemed ana ge ago and a few quid spent since.
Basic Calcs
After Diversity=6.26A

If Dischage lighting is calculated at 1.8
After Diversity=6.43A

100 watts per Lampholder including Discharge lights and calcs
After Diversity=13.09

Everthing at 75watts plus accounting for discahare lights;
After Diversity=9.81A

By the way I worked out that 44 lamps on circuit

Q: We have a 7000w 30.43 cooker
is it 10a + 30% of the 30.43 which = 9.129 = 10a + 9.129 + 5 for outlet socket = 24.129 amps

Ans: Ib=30.43
minus first 10amps
Plus 30% of remainder, so 30% of (30.43A - 10A)
20.43A X 30%=6.13A
Plus 5A for socket

So 10+6.13+5 = 21.13A

Sorry again for being late back, hope you all understand.
Posts: 1759
Joined: Mon Mar 05, 2007 9:36 pm

Postby tt » Mon Jan 28, 2008 7:39 pm

thanks to KB & ericmarks replies

It's good to see you back KB

that challange was good my answer wasn't 100% but I've printed it off and I'll do it again the answer was best I could do without knowing little things like the spots having a max that ericmark pointed out also i missed the 3 pendants with 40w bulbs,

So as a first attempt any good, this time last week i had no idea how to break it down and even less how to use the 66% diversity.

Posted new topic to learn
Posts: 18
Joined: Sat Aug 11, 2007 4:10 pm

Postby tt » Mon Jan 28, 2008 10:23 pm


this is the place I'm hoping to do my 2381 17th edition It's called

Have a look if you don't mind.

Posts: 18
Joined: Sat Aug 11, 2007 4:10 pm

Display posts from previous
Sort by
Order by


  • Related Topics