Security lights from fused spur in loft

[Minime46]

Hi all.

I'm looking at doing a mains compliance course in feb to learn more about testing circuits and filling in minor works certs etc.

My question is:

I have spot lights in my upstairs, electrics are all new within last 4 years. They are halogen fittings but with LED bulbs. It's a 3 bedroom house. Could I take a spur off the upstairs lighting circuit (in the loft) and run 4 security lights outside in series off that 1 spur if they are LED lights?

I am unsure of how to check what the current total load is on the lighting circuit unless I am meant to just walk round and tot up the upstairs lighting watts and then do my ohms law equation? So long as it all comes under 5amps including fused spur security lights then I would be ok and the lighting circuit RCD Shouldnt trip?

Any advice would be helpful. Trying to learn all I can.

Thanks in advance

 

[JohnD]

you should add them all together and calculate the max load.

As you have LED lamps you will find, when you finish your calculations, that you are way under the 6A.

In the old days we would have taken each lampholder as potentially a 100W load, and you could have more than a dozen. Most houses had less than that per floor. Long ago, people used 60W and even 40W bulbs in small rooms and landings, so they could run a whole house on a single 5A lighting circuit.

 

[Minime46]

Thank you. I will have a tot up and do the maths but that's very helpful. Thanks

 

[EFLImpudence]

I'm looking at doing a mains compliance course in feb to learn more about testing circuits and filling in minor works certs etc.

Ok. I think; not sure what that is.

It's a 3 bedroom house. Could I take a spur off the upstairs lighting circuit (in the loft) and run 4 security lights outside in series off that 1 spur if they are LED lights?

Yes, but they will be in parallel; not series.

I am unsure of how to check what the current total load is on the lighting circuit unless I am meant to just walk round and tot up the upstairs lighting watts and then do my ohms law equation? So long as it all comes under 5amps including fused spur security lights then I would be ok

Well, sort of. You could switch them all on and measure the current with a clamp meter, or
just count the number of lights and give a Wattage to each lamp.
As said - used to be 100W; now more likely 5W (possibly 10W) for LEDs.

and the lighting circuit RCD Shouldnt trip?

RCDs (Residual Current Devices) only trip with faults to earth.
MCBs (Miniature Circuit Breakers) and Fuses trip on overload - but likely more overload than you think.
So, 6A @ 230V equals 1380W - 138 x 10W lamps. You probablyly don't have that many.
As not all lights will normally be used at once, 50% more than the 6A is acceptable.

 

[Minime46]

As not all lights will normally be used at once, 50% more than the 6A is acceptable.

Thanks for that, though could you please explain what you mean by this part?

1380 watts is the total for 6amp - if bulbs are 10w.... but for lighting you could actually go over this by an extra 50% watts on top?? So 2070 total watts?

1380 + 50% = 2070watts

Hope I'm not confusing you.

Cheers

 

[SFK]

230V (supply) x 6 A (MCB) means that you have 1,380W available on that circuit.

So you can have 138 lamps on your 6A circuit if 'every lamp is 10W and they are ALL on at same time' (1380W / 10W per lamp = 138 lamps).

But as you are unlikely to have them ALL on at the same time,
and many of they lamps might be 5W instead of 10W you could:
+ assume that you can have 50% extra lamps (=207 lamps), or
+ assume you have 50% extra Watts available for lighting (=2,070W).

But (this is where percentages get weird) you should remember to only illuminate two thirds of them and leave a third of them off (138 on lamps / 207 total lamps = 66% can be on at same time).

In summary, you should be more than okay adding 4 security lights.
SFK

 

[EFLImpudence]

1380 watts is the total for 6amp - if bulbs are 10w....

Well, 1380W whatever the bulbs are.

but for lighting you could actually go over this by an extra 50% watts on top?? So 2070 total watts?
1380 + 50% = 2070watts

Yes, it's what we call diversity.
Because not all of the lamps will be on at once, determine the total of all the lamps - say your 2070W or 9A, and then 66% of that is deemed to be the design current of the circuit, i.e. 6A.
Of course, with LEDs nowadays it will be nowhere near that 2070W.

This diversity also works for other appliances.
For example, cooking appliances, for the same reason that not all elements are on at once even when switched on because of thermostats cycling on and off, circuits are designed by the full load current (total of all elements) using 10A + 30% of the remainder.
That is for a full load of, say 70A, 10A + (60 x 0.3) = 28A. That's why most cookers are on a 32A circuit.
It might seem wrong but it works.

Hope I'm not confusing you.

No, ask anything you want.

 

[Minime46]

Excellent info guys. Really appreciate that!

 

[bernardgreen]

Provide a means of double pole ( Live and Neutral ) isolation to the outside lamps.

Then if ( when ) an outside lamp is water logged and/or damaged then you can still have lights inside the house. Without double pole isolation a fault in the outside lamp will trip the RCD even if the lights are switched off.

 

[Minime46]

Provide a means of double pole ( Live and Neutral ) isolation to the outside lamps.

Then if ( when ) an outside lamp is water logged and/or damaged then you can still have lights inside the house. Without double pole isolation a fault in the outside lamp will trip the RCD even if the lights are switched off.

How would I achieve this please?

Do I install a double pole isolator switch after the fused spur and then run to the lights after that?

 

[SFK]

Most of these are Double Pole Switched and Fused.
But you have to check the description to make sure that they are double pole:
https://www.screwfix.com/c/electric...gory=cat830530&switchessocketsproducttype=fcu

 

[Minime46]

Most of these are Double Pole Switched and Fused.
But you have to check the description to make sure that they are double pole:
https://www.screwfix.com/c/electric...gory=cat830530&switchessocketsproducttype=fcu

Cheers.

Newbie question,
Does my last security light then have to run cable back to the fused spur or can it terminate at the last light?

 

[JohnW2]

Most of these are Double Pole Switched and Fused. But you have to check the description to make sure that they are double pole:

Whilst I don't disagree with the need to check ('to be sure'), I don't think one would be able to find a new switched FCU for sale which did not have a double-pole switch.

It's worth noting that, even when a fuse is not really required, and for reasons which I suspect relate to relative market size, switched FCUs are generally appreciably cheaper than standalone double-pole switches (which are invariably rated at 20A or more).

Kind Regards, John

 

[SFK]

John , Agreed, not sure why/how but I once manged to get a SP version (possibly becuase I used to buy the cheapest version/brand), and have ever since always double checked to make sure DP.

 

[SFK]

Single cable that ends at last Lamp (does not return as a ring to Switch).
SFK