16A MCB on lighting circuit

ajrobb · 7 Sep 2010

While trying to understand the 2-way wiring for a landing light, I went through the MCBs on the CU to identify which lights were on which circuit.

Apart from obvious howlers, like metal switch plates not earthed on pattress boxes with 2 plastic sliding lugs, I am somewhat concerned that the wiring in the switches is quite thin (either 1.5mm² or even 1mm²) but the MCB is 16A.

What is the smallest wiring that can be used with a 16A MCB?

I tried to see the circuit wiring but the ceiling roses only have switched live, neutral and earth. The 2-way switches only have a single 3-way cable at both ends and nothing else. I cannot see any trace of the main circuit!

This is a 1930s chalet bungalow. Where are the junctions likely to be hidden?

sparkwright · 7 Sep 2010

1.0 or 1.5 mm cable is the norm for lighting circuits.

How many lights on the circuit with the 16 amp mcb?

6 amp would be more usual, ocassionally 10 amp.

PrenticeBoyofDerry · 7 Sep 2010

Depends on the method of installation, both 1.00mm and 1.5mm T&E can carry 16Amps if clipped direct.
Junction Boxes in the loft space.

ajrobb · 7 Sep 2010

sparkwright said:
How many lights on the circuit with the 16 amp mcb?

hall - 2 switches controlling 3 lights
bathroom - 1 light
downstairs toilet - 1 light
reception - 1 light (3x100W spot)
office - 1 light (2x60W)
reception - 2 switches controlling 4 wall lights
landing - 1 light
upstairs toilet - 1 light
bedroom - 1 light
bedroom - 1 light

total - 12 lights (switched circuits)

sparkwright · 7 Sep 2010

The junctions may be in the loft spaces.

If not they will be under the floor.
Look for traps in the floorboards, usually above each light, each switch, or somewhere on the landing.

Just looking in the roses and switches should tell you enough about the cable size though.

ColJack · 7 Sep 2010

1mm T+E is 16A clipped direct ( that includes burried in plaster ), so 1.5 is more than man enough ( providing no de-rating factors etc.. ).

PrenticeBoyofDerry · 7 Sep 2010

So you have 13 lamps on your lighting circuit, worst case is that each lamp is valued at 100w.
so you have 1300 Watts of power, A 6A MCB will be okay to protect this circuit, but 16A is fine on 1.00 and 1.5mm T&E if the method is clipped direct.
Looking back at you set up you have 18 lamps.
So worst case is a 10Amp breaker

ajrobb · 7 Sep 2010

ColJack said:
providing no de-rating factors etc..

We had the cowboys in to do loft insulation last year. They only managed to insulate ceilings and lighting wires above rooms on 6A circuits. Unfortunately, I don't know where the 16A circuit gets upstairs but there is a good chance it is not buried in insulation.

Actually, there might be a problem over the garage above the CU. Its not heated but they could insulate it, so they did.

ricicle · 7 Sep 2010

What is the likelihood of all lights on the circuit ever being on at once ?

Lyngarth · 8 Sep 2010

ricicle said:
What is the likelihood of all lights on the circuit ever being on at once ?

If you have a wife and daughter like mine, then its very possible every dam light will be on.

OwainDIYer · 8 Sep 2010

ricicle said:
What is the likelihood of all lights on the circuit ever being on at once ?

Quite high actually; no diversity is allowed on lighting circuits.

ricicle · 8 Sep 2010

OwainDIYer said:
ricicle said:

What is the likelihood of all lights on the circuit ever being on at once ?

Click to expand...

Quite high actually; no diversity is allowed on lighting circuits.

I was refering to the concern about the loft insulation over the lighting cables. The fact there will only be one or two cables in the loft carrying full current for the circuits up there if all the lights are on. Also some of those circuit lights are downstairs !

Spark123 · 8 Sep 2010

OwainDIYer said:
ricicle said:

What is the likelihood of all lights on the circuit ever being on at once ?

Click to expand...

Quite high actually; no diversity is allowed on lighting circuits.

The IET on site guide does allow diversity for lighting circuits.

JohnD · 8 Sep 2010

quite apart from the capacity of the cable, UK light switches and fittings are usually rated at 10A max. Some, especially smaller ones that take a candle bulb, and some spotlights, are rated at 5A or 6A max. Some people use those horrible little 6A junction boxes on lighting circuits. So in the event of a fault, the switch or fitting might fail before the cable.

Fit a few CFLs and get the MCB changed to 6A.

ajrobb · 8 Sep 2010

JohnD said:
Fit a few CFLs and get the MCB changed to 6A.

I already have CFL lamps on 8 switched circuits, but there are still 8 tungsten spots on 4 switched circuits.

Given that the cowboys might have insulated over 32A rings as well, I'll get a sparks in to give things a once over and update the CU (there is no RCD protection). Even I think it's too complicated to attempt to DIY under Part P.

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16A MCB on lighting circuit

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