Lights and sockets are usually on different circuits but must they be?
Can I have a 35ish meter 2.5mm radial from 16mm RCD running a few sockets and lights in what was once Ladies and Gents toilets, sockets to be used for occasional garden extension leads and a 480 watt frost heater?
If so must the wires from the switches to the lights be in 2.5mm too?
Or could i use 1mm wire from switch to lights as it would only run one bulb?
But the whole circuit will be on a 30ma RCD at the consumer unit.
Does it need another one at the garden socket?
In my experience both would probably trip together if they have the same characteristics
Our conservatory lights are run off the ring main through a 5A fused FSU. Should imagine you could do the same off a radial circuit depending on wire cross-section and the number of sockets already on that radial.
No, they don't have to be. It would be up to the installer to decide how
acceptable that would be, and what (if any) mitigation would be required
in the circumstances.
Probably prudent to check the voltage drop though since we are deviating
from one of the tabulated circuits:
If we assume 35m of 2.5mm^2 and say 4m of 1.0mm^2. If we put 2A of load
on the end of the thinner cable, and 14A of load on the thicker one
(over and above the 2A - making 16A total load), we get a voltage drop
on the sockets of:
25 x 14 x 0.018 = 6.3V on the sockets, which is acceptable.
For the lighting circuit bit we get an additional 4 x 2 x 0.044 = 0.35V
giving a total of 6.65V of drop. That's just inside the 6.9V allowed
(i.e. If so must the wires from the switches to the lights be in 2.5mm
The elegant and sensible solution would be to take a 5A fused spur and
feed the lights from that. Possibly also include at least one non
maintained emergency light if loss of lighting would be a safety issue.
However, as an academic exercise, we can probably demonstrate that the
fuse could safely be omitted.
Depending on the installation method the 1.0 mm^2 T&E can have a
capacity of as much as 16A (method C), the lighting load will be
substantially less than that in all likelihood, and the possibility for
the user to create an overload by changing lamps etc seems very slim. So
overload protection for the 1.00 mm^2 cable does not need to be provided
by the MCB at the origin of the circuit. It only needs to provide fault
So we can do an adiabatic calculation check on the cable to ensure it
could withstand a fault at the far end.
So if we say 35m of 2.5mm^2, and lets allow 4m of 1mm^2, that gives a
loop impedance contribution from the cable of:
35 x 17.78 + 4 x 43.4 = 796 mOhms.
(that's for a L to N short on the thinner cable - we will assume the RCD
will protect for the L to E short, and its slightly higher loop
impedance. We are also using the "hot" resistance values for the
conductors - i.e. assuming they are already running at full load. That's
being very pessimistic for the 1.00mm^2 cable)
You have not said what the supply earthing system is, and we don't have
a measurement of the external earth loop impedance, so lets go with the
nominal TN-C-S and 0.35 Ohms.
That way we get a total loop impedance at the far end of the circuit of
So that means the prospective fault current is going to be:
230 / 1.16 = 198A
That is adequate to trip a B16 MCB on the "instant" (i.e. magnetic) trip
mechanism (it takes 5x In or >= 80A for that), so we will assume a
disconnect time of 0.1 secs. So an adiabatic check:
s = sqrt( I^2 x t ) / k
PVC cable, so take k as 115
s = sqrt( 198^2 x 0.1 ) / 115 = 0.54mm^2
So there is plenty of spare CSA in 1mm^2 cable to withstand that fault
(note that BS7671 normally requires a protective device to be present at
each reduction in conductor size in a circuit - so we are deviating
from that based on the demonstration that the head end circuit
protection is actually sized to protect the smaller conductor sized
Gosh that's some calculations from John Rumm:
his use of the word 'elegant' reminds me of maths at Uni...
I'm going to keep his calculations to justify using 3 core 1mm for light switches from 1.5mm circuits without additional fuses.
But for this job I'm going to use 2 separate wires,
one for sockets and one for lights,
its far simpler to justify to Building Control when I get to that stage.
I want as few fuses as possible around the house,
(I found it with 3 different locations of fuse boxes);
all circuits to be from RCDs at the consumer units by the back door
will make it easier for whoever has to deal with trips.
And no ring mains will make it easier for testing
Well, part of the elegance of the proposed fused solution, was it avoids
any doubt and any need for calculations. Its also what would be
expected, which is usually the best way to go unless there is a
particular reason not to.
Assuming the circuits are protected by a B6 MCB or similar, then they
would be fine wired all in 1.00mm^2 in most cases... (the larger cable
gives you longer maximum cable runs, and a bit more mechanical strength
- you don't typically use it for additional current carrying capacity on
a lighting circuit).