Justification for using 2.5mm2 to feed a spur on a ring?

[brman]

I feel a little embarrassed asking a this as it is pretty fundamental and I thought I knew the answer but someone asked me this today and I couldn't justify it according to the regs. So.....

How come we can use 2.5mm2 to feed an unfused spur from a ring? (eg a double socket or even a FCU not actually on the ring)

2.5mm2 won't take 32A so isn't protected upstream by the 32A MCB. I thought it relied on downstream fusing but.........
Reg 434.2.1 says you can downstream protect a cable if there are not branches and it is less than 3m long BUT (ii) states that the cable must be "installed in a manner to reduce the risk of a fault to a minimum" and notes that, for example this could be "reinforcing the protection of the wiring" which I take to mean mechanical protection.

So, the way I read it, a standard 2.5mm2 cable to an unfused spur isn't allowed because it doesn't meet 434.2.1 part (ii). However it is of course standard practice and shown in Appendix 15, standard circuits.

What have I missed?

 

[telectrix]

basically the fact that a single socket can't draw more than 13A. a double 20A. these are below the ccc of 2.5mm cable. as long as only 1 outlet is fitted on an unfused spur.

 

[jumpin jax]

I think if you think what the max load that could be plugged into the socket might be a clue and the ccc of the 2.5


tel beat me to it:shame:

 

[brman]

yes, I know that guys but I am trying to match it to the regs.

This appears to be downstream fusing (for the cable, not the equipment), and, as far as I can tell covered by 434.2.1. Trouble is, 434.2.1 has that little clause about "reinforcing the protection of the wiring" which we do not do.
So where in the regs does it allow fusing at the end of the cable, not the start and without "reinforcing the protection of the wiring"?

 

[brman]

btw. 434.2.1 also puts a 3m limit on the length before the fuse, I've see plenty of unfused spurs in 2.5mm2 longer than 3m so I am obviously missing something.....

 

[Geoffsd]

434.2.1 deals with fault current conditions with which the cable must still comply.

You want 433.3.1

The downstream fuse will be in the plug.

Reinforcing is one method of protection against external influences - there may be none to consider.

 

[brman]

434.2.1 deals with fault current conditions with which the cable must still comply.

You want 433.3.1

The downstream fuse will be in the plug.

Reinforcing is one method of protection against external influences - there may be none to consider.

Ah, ok, starting to make sense now. So 433.3.1 (part ii specifically) allows this as the plug fuses limit the potential overload. However 433.3.1 still wants 434 to apply (fault protection). However what I didn't check is whether 32A MCB will protect a 2.5mm2 cable against fault. I guess if I used the adiabatic I might find that it does?

 

[Geoffsd]

Yup.

Also, forgot - a lot of people ignore the 3m bit.

 

[brman]

thanks guys. Makes sense now, I can now argue the point next time someone asks me to justify it :lol:

 

[SparkyJoe]

Its a crap way of working anyhow, Just take 2 legs and crimp one in the back of the socket your spurring off, Its not normally a mega run anyway.

 

[Deleted member 26818]

The Regulation that you are missing is 434.2.2, which then refers you to Regulation 434.5.2 and Table 43.1.
Using the formula t = (k² x S²) / I².
As long as 't' exceeds the 0.1s operating time of the protective device at the current denoted by 'I', and fault protection is provided by the CPD, then there is no requirement to provide overload protection as per Regulation 433.1.1.
Ib < In < Iz does not have to be satisfied.

 

[brman]

Its a crap way of working anyhow, Just take 2 legs and crimp one in the back of the socket your spurring off, Its not normally a mega run anyway.

Often the case and I tried to do this today on a job as I had two sockets to run off the existing ring. 2 hours of swearing later I gave up and just ran a single to the last socket! Not enough space, oval conduit that had kinked and I hadn't noticed, pulled too hard and the conduit came adrift in the wall - you name it, it all went wrong!
One of those days when I wished I had stayed in bed.... :mad2:

 

[brman]

The Regulation that you are missing is 434.2.2, which then refers you to Regulation 434.5.2 and Table 43.1.
Using the formula t = (k² x S²) / I².
As long as 't' exceeds the 0.1s operating time of the protective device at the current denoted by 'I', and fault protection is provided by the CPD, then there is no requirement to provide overload protection as per Regulation 433.1.1.
Ib < In < Iz does not have to be satisfied.

Got it, thanks. I hadn't previously read 434.5.2 properly but I guessed it was something to do with the adiabatic.