Radial circuit and the use of spurs

Tradewind25 · Sep 11, 2015

I currently have an integral garage fed by a radial circuit protected by a 32A MCB and wired with 4mm T&E cable. This feeds just two power points in the garage (and nothing else).
i wish to redesign this circuit to provide several more power points and would like to check whether my proposed design is within regulations. What I foresee is a number of junction boxes wired in series in 4mm cable. From each box would be a single power point wired in 2.5mm T&E cable. In effect, all outlets on the circuit would be spurs.

i would be grateful of your professional comments on this design. (I am aware of tge 33m total cable length restriction.)

Regards
Geoff

Darkwood · Sep 11, 2015

Hi Tradewind25 welcome to the forum ... you are down as retired but don't specify, so with can put your query in the correct section can you tell us if you have any electrical background thanks.

Fox · Sep 11, 2015

OP - Be very careful, your design describes dropping cable size from the junction boxes which is a no go.

Please make sure you complete your circuit design calculations before undertaking any work - what is your proposed maximum demand? Also, what is the supply type?

telectrix · Sep 11, 2015

just carry on with 4mm, socket to socket. any branches in 4mm. and they're not spurs unless off a ring, on a radial they're branches. just be careful that your calculated load is < 32A. if you want lights, fit a FCU fused @ 5A.

Tradewind25 · Sep 11, 2015

The total load at any one time is unlikely to exceed 13A. I simply wish to have a greater choice of sockets to use around the garage wall. The space in question is approx 6x7 metres. There will be no lighting off this circuit.

I read in several threads on the topic of radial circuits (not necessarily on this forum) that, although a 32A protected circuit has to be wired in 4mm cable, "spurs" may be taken from a socket on the circuit and wired in 2.5mm cable. As I see it, under my proposed design I am simply replacing the radial sockets with junction boxes.

The reason I do not wish to create a simple radial circuit is that I do not want horizontal runs of cable at socket height; I prefer the main cable run to be at ceiling height. To then drop two cables to each socket would inevitably mean my contravening the 33m limit.

murray2707 · Sep 11, 2015

Where do you get the 33m limit from?

7029 dave · Sep 11, 2015

Go with Tel's post, keep with the 4mm, if you insist on spuring with 2.5 with your drops, change the mcb to 20amp.

Pete999 · Sep 11, 2015

7029 dave said:
Go with Tel's post, keep with the 4mm, if you insist on spuring with 2.5 with your drops, change the mcb to 20amp.

Yeah right on.

Octopus · Sep 11, 2015

Not sure I'd be changing the cable size for the spurs

Tradewind25 · Sep 11, 2015

OK guys, I give in - I'll use 4mm cable throughout.
Just to be clear, I am correct in thinking that the rest of mynproposed design is OK? I.e. All sockets are wired as spurs from junction boxes.

Marvo · Sep 11, 2015

Why would you want to do it this way rather than cable in and back out of each socket? The more joints the more problems and the less reliable.

Deleted member 9648 · Sep 11, 2015

Marvo said:
Why would you want to do it this way rather than cable in and back out of each socket? The more joints the more problems and the less reliable.

DIYer's luv a JB or three!

timo1 · Sep 11, 2015

I may be misunderstanding, but they wouldn't be Spurs as its a radial circuit. the junction boxes would be joints in the circuit.

Tradewind25 · Sep 11, 2015

As mentioned earlier, I was under the impression that there was a 33 metre limit on total cable length with this type of circuit. This I have seen referenced in various places e.g. Domestic Circuits - SimplifyDIY - DIY and Home Improvement Solutions.

Tradewind25 · Sep 11, 2015

wirepuller said:
DIYer's luv a JB or three!

FYI: This DIYer has a first class Electrical Engineering degree. However that was many years ago and I am far from up to speed with currect Regs. I had hoped that this forum would provide reliable advice, rather than facetious comments.

davesparks · Sep 11, 2015

Tradewind25 said:
FYI: This DIYer has a first class Electrical Engineering degree. However that was many years ago and I am far from up to speed with currect Regs. I had hoped that this forum would provide reliable advice, rather than facetious comments.

Forget the degree, just use a little common sense!
Every joint is a potential point of failure, common sense says you don't add a load of unnecessary joints.
If you are insistent on adding unnecessary joints then why not use 4mm for the branches?

Octopus · Sep 11, 2015

Tradewind25 said:
As mentioned earlier, I was under the impression that there was a 33 metre limit on total cable length with this type of circuit. This I have seen referenced in various places e.g. Domestic Circuits - SimplifyDIY - DIY and Home Improvement Solutions.

Hum..........

davesparks · Sep 11, 2015

Tradewind25 said:
As mentioned earlier, I was under the impression that there was a 33 metre limit on total cable length with this type of circuit. This I have seen referenced in various places e.g. Domestic Circuits - SimplifyDIY - DIY and Home Improvement Solutions.

What a load of utter nonsense, how can people get away with publishing such Ill informed and incorrect rubbish?

Ignore the ramblings of strangers on the internet and follow the wiring regulations!

Tradewind25 · Sep 11, 2015

davesparks said:
If you are insistent on adding unnecessary joints then why not use 4mm for the branches?

1. I have already agreed that 4mm throughout would be better.
2. The "unnecessary" joints are due to my understanding that there is a max cable length.

Octopus · Sep 11, 2015

You can lead a horse to water......."

Robson689 · Sep 11, 2015

davesparks said:
What a load of utter nonsense, how can people get away with publishing such Ill informed and incorrect rubbish?

Ignore the ramblings of strangers on the internet and follow the wiring regulations!

I think it's on about the volt drop for clipped direct t&e it doesnt go into enough detail though

Richard Burns · Sep 11, 2015

After 33m the volt drop on a 4mm² cable will exceed the 5% limit indicated by the wiring regulations.
A run of cable in smaller size than the supply cable would be considered a spur, rather than a branch.
It is recommended that the number of spurs is fewer than the number of sockets directly on the circuit.

Whilst this is a potentially possible solution it is by no means ideal, also you can have exactly the same arrangement in 4mm² cable as you have in 2.5mm² cable you will have the same cable length, which is presumably less than 33m.

If you were to run the spurs as you have suggested then you are even more likely to have problems with voltage drop as the reduced conductor size will increase the volt drop and so reduce the allowable circuit length.

Darkwood · Sep 11, 2015

Tradewind25 said:
FYI: This DIYer has a first class Electrical Engineering degree. However that was many years ago and I am far from up to speed with currect Regs. I had hoped that this forum would provide reliable advice, rather than facetious comments.

It is clear your not up to date with regulations and tbh Electrical Engineering does not in itself give you the knowledge for domestic work so with all respect I would ignore the invited comments and let's move on with the thread ...

We all know the OP's background now ... had he answered my post 2 we could have avoided all this, so can we either offer good advice or refrain from comment