Why are you only allowed one spur per socket on a RFC?

[HappyHippyDad]

This is something I have been racking my brain over for ages and its doing my head in!! :crazy:

Please could someone explain why you are only allowed to have one unfused spur per socket on a RFC?

I'll explain a little why I'm confused.

A ring main is allowed as many sockets as you like on the ring (over given area). So as an example, say I have 8 sockets on my RFC, what would be the difference in having a ring with 5 sockets and then a spur coming off one of them which led to another and then another spur (i.e 3 spurs, so 8 sockets in total)

The 3 sockets coming off the one socket would be like a radial, the 2.5mm[SUP]2 [/SUP]cable supplying these 3 sockets would only be carrying current for these 3 sockets (starting from the socket supplying the spurs) which is fine. The rest of the 2.5mm[SUP]2 [/SUP]cable on the ring would be equally (roughly) suppling the extra current for the 3 sockets which it would be doing anyway if they were part of the ring??

When I qualify I will never supply more than one spur per socket as it is against the regs but I'd really like to understand why? :confused5:

Thanks.

 

[Richard]

I think that it's because you are only limited to a 13A fuse in the plugtop of any appliance that could plug into it. Even with a double socket, you could only pull 26A max, which would rearly ever happen.

Edit: however if you had a situation where you spured additional sockets off an existing, you could realisticly overload the cable.

 

[timo1]

If you have a radial on 2.5mm then the circuit should be limited by a 20amp MCB. 2.5mm can handle about this and the mcb will trip if you pull more than 20A through the circuit, for example if you plug in two welders pulling 20A each then the MCB will trip and protect the cable.

On a RFC on say 30A or 32A if you have a spur on a spur without an FCU then the circuit will allow you to pull over 32A through the spurs, the MCB set at 30A wont trip and the cable could melt. With an FCU then the fuse will blow protecting the cable. Regs allow you to have as many spurs as you like off an FCU as that leg will never pull more than the fuse would allow (think of how most conservatory's are wired)

Hope that helps a bit

 

[Tonkatoy27]

Putting it simply.
A line of spurs from spurs is dodgy because of the potential load that could be drawn through the 1st piece of 2.5.
Find out what load a piece of 2.5 can handle and compare it to the potential load that could be applied to the 3 twin sockets if say, 1x fan heater, 1x kettle and a washing machine were to be connected. You'll soon draw your own conclusion.

 

[Philpot]

Good point why?......well you have answered part of your question....it says so in the regs. But that isn't the answer you want.

Kirchoffs Law is the most likely reason. the currents running to a point equal those running away. ( I wish I had thought of the and it would be Philpots Law)

The current distribution in a ring CCt is not equal so the currents running to a socket on the ring will not be equal. If you add to that the possible currents from multiple sockets on a single spurs then one or other of the two legs of the ring could "theoretically" be overloaded.

 

[Philpot]

That doesn't explain why they allow 3 sockets on a 2.5mm. But then regs were never meant to be logical.

Also the spur has a 32a cb and the 2.5mm with 3 sockets has a 22A. And what Timo said.

 

[Philpot]

sorry Tonkatoy.

 

[Adam]

2.5mm2 can safely carry 27 amps.

One twin socket has the potential of carrying 26 amps (2 x 13A Plugs). That is why we are not able to add more than one socket per spur (unless we protect the spur with a fuse).

 

[HappyHippyDad]

Thankyou verymuch..

I was getting carried away with the cable and not thinking about the MCB. It makes perfect sense! I could have 3 (or more) sockets on a radial but would have to have a 20A MCB, which I wouldn't have!

Of course I'm now looking through the on-site guide and I see that 2.5mm[SUP]2 [/SUP]cable (twin and earth) (enclosed in in conduit in a thermally insulated wall) has a current carrying capacity of 18.5A. Would you then simply use a 16A MCB? I'm guessing with most radials the cable is usually tucked away in the wall so is it common practice to use a 20A and would that not be a unsafe?

 

[sedgy34]

simply because the book says so!

 

[Adam W]

Of course I'm now looking through the on-site guide and I see that 2.5mm[SUP]2 [/SUP]cable (twin and earth) (enclosed in in conduit in a thermally insulated wall) has a current carrying capacity of 18.5A.

Which table are you looking at? In table 6F column 7 (p134 in the red book) it states maximum current of 20A for flat cable with protective conductor if enclosed in conduit in an insulated wall.
Remember if the cable is in conduit there will be a certain amount of free air in the conduit around the cables to allow heat to dissipate.

 

[nickblake]

all about over load and have see a few melted 2.5mm's due to it

 

[HappyHippyDad]

Which table are you looking at? In table 6F column 7 (p134 in the red book) it states maximum current of 20A for flat cable with protective conductor if enclosed in conduit in an insulated wall.
Remember if the cable is in conduit there will be a certain amount of free air in the conduit around the cables to allow heat to dissipate.

I'm looking in the green on-site guide page 147 Appendix F, Table F5(i)

 

[Guitarist]

Just remember, the circuit protective device (MCB) is there to protect the cable.