A spur off a ring with FCU (UK)

[DarrenA]

Hi guys, newbie posting!

Does the following sound ok...

Existing circuit:
SOCKET on a ring -> spur to SWITCHED 3A FCU -> SWITCH -> hardwired FAN

Proposed:
A new UNSWITCHED 13A FCU will be inserted between the SOCKET on a ring and the SWITCHED 3A FCU (which protects the FAN to 3A). Then that same new UNSWITCHED 13A FCU (from load side of course) will separately feed a new DBL SOCKET. This means that the spur off the ring could not exceed 13A in total, noting also that at least 1.5mm cable is/will be used.

Circuit becomes:
SOCKET on a ring -> spur to new UNSWITCHED 13A FCU -> original SWITCHED 3A FCU -> SWITCH -> hardwired FAN
and...
same new UNSWITCHED 13A FCU -> DBL SOCKET

I have coloured the original existing parts green to help them stand out.

It's quite simple but it looks complicated written down, and the above will minimise the amount of work - just need to add a in-line FCU and run it to a dbl socket.

 

[littlespark]

Yes… as long as everything is on load side of 13A fuse.

 

[Avo Mk8]

noting also that at least 1.5mm cable is/will be used.

clarifying that the cable between SOCKET on a ring -> spur to new UNSW FCU is in 2.5mm2, which it presumably is already?

 

[timhoward]

I agree that's all fine.
Two developmental thoughts as I love complicating things:
-Does it need to be a double socket. As a single socket could be fed from the supply side of the existing 3A spur.
-You could fit one of those 3 gang sockets incorporating a 13 amp fuse, again from the supply side of the existing 3 amp spur. Max load on spur off ring is therefore 16 amps which meets 433.1.204

 

[DarrenA]

clarifying that the cable between SOCKET on a ring -> spur to new UNSW FCU is in 2.5mm2, which it presumably is already?

That's a good question. I have owned the house since new in the late 90s so I know that no dodgy work has been done. Assuming the house builders electrical subcontractors were professional and followed regs, do regs not say that a spur must be 2.5mm? Is the only way to tell to do a visual? Is the difference between 1.5 (or less) and 2.5 very obvious?

I am also assuming that the original circuit for an extraction fan is spurred off a ring - isn't that the norm for a new build in the 90s? I will verify this by switching off the ring MCB and then a visual on the sockets I am guessing its connected to (it the nearest other than another spur to an immersion heater). If the socket has 3 cables (ring in, ring out, spur), I can then do a continuity test from them to the spurred FCU to verify 100% that its the feed?

 

[DarrenA]

I agree that's all fine.
Two developmental thoughts as I love complicating things:
-Does it need to be a double socket. As a single socket could be fed from the supply side of the existing 3A spur.
-You could fit one of those 3 gang sockets incorporating a 13 amp fuse, again from the supply side of the existing 3 amp spur. Max load on spur off ring is therefore 16 amps which meets 433.1.204

If I have understood what you are suggesting, would that not effectively mean 2 x FCUs from a spur when only 1 FCU is allowed or one socket (which can be a dbl)?

The 3 gang fused socket would be no good as (1) the fan is a hardwired extractor fan and (2) the dbl sockets need to be several meters away in the loft.

 

[nicebutdim]

A lot of kitchen extractors are fed from lighting circuits. I wouldn't make any assumption to the contrary and would verify where it is supplied from.

 

[DarrenA]

A lot of kitchen extractors are fed from lighting circuits. I wouldn't make any assumption to the contrary and would verify where it is supplied from.

Fair enough, I will check. The fan is upstairs and ducted for the bathroom and ensuite.

I have learnt not to 100% trust professionals as they labelled the consumer unit as sockets upstairs and sockets downstairs only for me to find they are really sockets left side and sockets right side of the house.

 

[timhoward]

If I have understood what you are suggesting, would that not effectively mean 2 x FCUs from a spur when only 1 FCU is allowed or one socket (which can be a dbl)?

It's true that Appendix 15 in the regs gives example circuits and the examples all have one spur off a ring.
It's also true that more people are familiar with the appendix than the actual regulation, so there is some merit is sticking to the examples.

For the record, the actual regulation says:
Accessories to BS 1363 may be supplied through a ring final circuit, with or without unfused spurs, [ .... ]
Such circuits are deemed to meet the requirements of Regulation 433.1.1 if the current-carrying capacity (I,) of the cable is not less than 20 A and if, under the intended conditions of use, the load current in any part of the circuit is unlikely to exceed for long periods the current-carrying capacity (I,) of the cable.

Both my suggestions meet that as the max load current will be less than the CCC of the cable in both cases.
However it they are impractical for other reasons, fair enough!

 

[nicebutdim]

Fair enough, I will check. The fan is upstairs and ducted for the bathroom and ensuite.

I have learnt not to 100% trust professionals as they labelled the consumer unit as sockets upstairs and sockets downstairs only for me to find they are really sockets left side and sockets right side of the house.

Bathroom extractors would rarely be supplied from anywhere other than lighting.

Trust no one where electricity is concerned and prove everything.

 

[DarrenA]

It's true that Appendix 15 in the regs gives example circuits and the examples all have one spur off a ring.
It's also true that more people are familiar with the appendix than the actual regulation, so there is some merit is sticking to the examples.

For the record, the actual regulation says:
Accessories to BS 1363 may be supplied through a ring final circuit, with or without unfused spurs, [ .... ]
Such circuits are deemed to meet the requirements of Regulation 433.1.1 if the current-carrying capacity (I,) of the cable is not less than 20 A and if, under the intended conditions of use, the load current in any part of the circuit is unlikely to exceed for long periods the current-carrying capacity (I,) of the cable.

Both my suggestions meet that as the max load current will be less than the CCC of the cable in both cases.
However it they are impractical for other reasons, fair enough!

Maybe elsewhere in the regs it states that a spur can only have one sngl/dbl socket or one FCU? Its just that while reading up about all this I have come across several posts that say according to regs you can't have more than one FCU off a single spur.

It does make sense to be able to have 2 though as 2 x 13A FCU = 26A which is within the 2.5mm cable limits and an unfused dbl socket would allow the same. That is unless you also add those unfused "cube" socket doublers, though that's a different conversation!

 

[DarrenA]

Bathroom extractors would rarely be supplied from anywhere other than lighting.

Trust no one where electricity is concerned and prove everything.

Thanks for the advice.