lighting load and a 6 amp MCB

misterlister · 22 Jan 2011

Hi

I have just moved in to a new house that has been extended subtanatiialy over many years.

The existing consumer unit only has a single 6 amp type B MCB for the lighting circuit. This has tripped out a few times since moving in and whilst it is not a nuisance it did make me work out what load is currently on the circuit. There are masses of halogen spotlights in the house which with everyhting else adds up to 2.9 kW on the one circuit which I think is 12 amps on full load but probably around 9 in winter when my wife leaves most of the lights on...

Question is - I have heard of some people upgrading lighting circuit MCBs to 10 amp, but it is obviously not as simple as this - what do I need to do to ensure that the circuit is safe - splitting to two loops would be a major hassle in the house but I want to make sure either the existing 6 amp MCB is suitable (poss by downgrading the wattage of the bulbs, although this would obviously require putting a fair amount of lower wattage bulbs in ) or getting a 10 amp MCB fitted (which would obviosuly allow a greater current on the lighting circuit but which may not be suitable for the current rating of some of the light fittings).

Thankful for any advice on next steps. In particular, is there an issue wih leaving the 6 amp MCB in an putting up with a trip once a month or so? Happy with this if it is safer than dumping more current on the circuit.

Cheers

ricicle · 22 Jan 2011

Simpliest solution would be to reduce the load ie lower wattage lamps. Also constant nagging about leaving lights on might be the way forward.
A 10A swap might be possible as well but this will depend on the earth fault loop impedance of the circuit at it's extremities. This will require testing with a proper instrument.

pjcomp · 22 Jan 2011

Just a thought - only one MCB but there might be two or more radials from it. Have you checked inside the CU? Just possible the extra lights were run from the CU and someone didn't bother to add a circuit breaker for them (maybe no spare slots ..). Anyway, it might make it easier to split the load.

PJ

martinxxxxxx · 22 Jan 2011

In all hallways and kitchen, toilets and bathrooms have occupancy switches switches fitted instead of the normal one or if you don't fancy the upheaval of changing the wiring, you could try these Click

I would look at the ideas above as well, you can tell we are all more concerned about the environment here than just getting work done unless it is absolutely necessary.

Hope this helps

Martin

Just rerread and maybe you should get those halogens changed for LEDs too

misterlister · 22 Jan 2011

Thanks for the comments

I have tried nagging - it doesn't work

Good point about the CU - haven't checked inside so I don't know how the additional loops have been added (there are a hell of a lot of junction boxes in the loft though....) - I'll have a look - not sure there is any room left for further MCBs in the CU even if there are separate lighting loops. S'pose nothing is ever simple!!!

riveralt · 22 Jan 2011

The methodology for determining the load on a circuit according to the On Site Guide is 100w per lamp.

How old is the house and does the lighting circuit have a CPC (earth)?
What is the lighting circuit cable size and how is it run through the house?

The current carrying capacity of 1mm cable ranges from 16Amps when clipped direct to the wall or buried in plaster down to 8Amps when buried in thermal insulation. So it is possible that you could fit a 10Amp MCB in certain circumstances.

However, i would also be concerned as to why the MCB is tripping?

misterlister · 22 Jan 2011

House built in 1935

What I can see of the wiring is 1.5 mm T&E some is run in plaster down to switches none in thermal insluation to my knowledge.

Looking at my lights again, the kitchen and stairs account for over 1200 watts of the total with all being 50 w halogens spots

Taylortwocities · 22 Jan 2011

riveralt said:
The methodology for determining the load on a circuit according to the On Site Guide is 100w per lamp.

That is based on tungsten lamps.

You can ease the situation simply by changing some of the lamps to low energy.

misterlister · 22 Jan 2011

Just had a look in the CU - there are two feeds from the 6 amp MCB so it does look like further lighting has been added to the circuit at some point - there is deffo no room for any further MCBs in the current CU so that is probably why they stuck another radial on the existing MCB?

I also want to do this as cost effectively as possible, having spent out a lot of cash on moving and note that LED spot are 25 quid a pop - that would cost about 550 quid to replace the offenders

Are 7 watt GU10 LED lights as good as the halogens?

riveralt · 22 Jan 2011

misterlister said:
House built in 1935

What I can see of the wiring is 1.5 mm T&E some is run in plaster down to switches none in thermal insluation to my knowledge.

Looking at my lights again, the kitchen and stairs account for over 1200 watts of the total with all being 50 w halogens spots

If the wiring is 1.5mm then there should no problem in swopping to a 10Amp MCB.

To calculate you lamp demand add up all the lamps in the house and multiply by 100w. Then apply a diversity factor by multiplying the result by .66. If that figure is below 2300w then you can use a 10Amp MCB. Remember that the current carrying capacity of the cable allways has be larger than the MCB - so the MCB trips before the cable burns!!

Most houses built up to the mid to late sixties did not have an earth in the lighting circuit. You need to check your lighting has an earth otherwise you cannot have any Class 1 metal fittings in your lamps or switches - you can only have plastic fittings.

riveralt · 22 Jan 2011

Taylortwocities said:
riveralt said:

The methodology for determining the load on a circuit according to the On Site Guide is 100w per lamp.

Click to expand...

That is based on tungsten lamps.

Yes I was told that as well but I have been unable to track down its source.

If the OP is basing his 12Amps on the actual wattage of each lamp then he has an awful lot of lamps and probably a high electricity bill.

misterlister · 22 Jan 2011

What is currently on the system (actual) is 2.9 kW - if I multiply each fitting by 100 watts it is truly massive!! - we have several chandaliers with low wattage bulbs in eight fittings so these would add up to a lot!

At the current 2.9 kW multiplied by 0.66 for diversity the current would be over 8 amps so that is why I am concerned about the 6 amp MCB

The lighting circuit deffo has an earth in the CU

Thanks again for all the help guys

riveralt · 22 Jan 2011

misterlister said:
Just had a look in the CU - there are two feeds from the 6 amp MCB so it does look like further lighting has been added to the circuit at some point - there is deffo no room for any further MCBs in the current CU so that is probably why they stuck another radial on the existing MCB?

Is this a Wylex board with plug in MCB's?

I also want to do this as cost effectively as possible, having spent out a lot of cash on moving and note that LED spot are 25 quid a pop - that would cost about 550 quid to replace the offenders

Are 7 watt GU10 LED lights as good as the halogens?

downlight spotlamps and LED spotlamps are just that - pools of little light spots on the floor - hence the reason you have so many.

LED claim to be equivalent to spotlights and I have no reason to doubt them - its not a route I would ever go down though.

If you do go for GU10 LEDs then beware that some LEDs do not have the constant current driver built into them and you will need to buy one of these - possibly for each lamp - as well.

riveralt · 22 Jan 2011

Now I have re-read the figures you quoted - you probably will need to downsize the actual load. One other option would be use 25w bulbs instead of the 50w - much less costly than LED - might be a bit darker though.

I would still change the MCB to 10Amps.

misterlister · 22 Jan 2011

I am unfamiliar with what a Wylex board is - the CU is made by Hager and looks like the MCBs clip on to a rail and the screw on to the live bar.

Both cable coming out of the MCB are deffo 1.5mm - can't see where it goes after that under the bathroom floor... all cable I can see in the loft are also 1.5 mm

Bottom line I suppose is that I want it to be safe - I won't fit a 10 amp MCB if it brings in a greater risk of cable overheating - rather stick with a 6 amp MCB if that is safer - my worry with the 6 amp MCB was that it, in itself, may be carrying higher currents than it should be handling - I know it should trip if ther is a significant over-current, but wasn't sure if sustained periods at around 8 amps would create a risk in the CU?

I may be taking rubbish here, so please humour me - I've got a reasonable electrical knowledge but not in terms of wiring regs and consumer electrics

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lighting load and a 6 amp MCB

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