Why can we spur unfused from rings?

[rewireIT]

Can someone explain electrically why we can spur unfused from a ring final on a 32a breaker when the cable is 2.5mm rated at max 27a?

 

[James]

Because even if it was a double socket, the maximum load would be 2 x 13A fused plugs.

 

[Dustydazzler]

As bove , the maximum current draw ( in theory ) will only reach 26 amps. but as we all know a 13a fuse will run way over 13amps so in reality its a folly rule but a rule none the less

 

[HappyHippyDad]

As bove , the maximum current draw ( in theory ) will only reach 26 amps. but as we all know a 13a fuse will run way over 13amps so in reality its a folly rule but a rule none the less

I'm not so sure it is a folly rule Dusty. I thought the same as you a few years back,.

An MCB (or even more so a fuse), will pretty much run for ever at it's rated current, or even a little above it. Even at say 20% more current running through it, it will still run for quite some time (20% is a guess, just to show the point although I haven't looked at the time/curve graph). I believe BS7671 takes this in to account, so in fact the insulation of the cable can take more than the CCC quoted in BS7671.

In fact, I'll give a proper example.

A 5A BS3036 fuse will take 0.75 secs to blow with 20A of current running through it. If it is wired in 1mm, then the 20A is too much, but I believe the cable would be designed to take this amount of current for (at least) the time given from the time/current graphs.

I would have thought this is the same for a 13A fuse (with slightly different actual figures).

This is a bit of guess work though, and I'm really not sure if it's right. Just seems logical that cable should be designed to take whatever current could be drawn until the fuse blows (as long as the electrician has designed the circuit correctly).

 

[Dustydazzler]

I'm not so sure it is a folly rule Dusty. I thought the same as you a few years back,.

An MCB (or even more so a fuse), will pretty much run for ever at it's rated current, or even a little above it. Even at say 20% more current running through it, it will still run for quite some time (20% is a guess, just to show the point although I haven't looked at the time/curve graph). I believe BS7671 takes this in to account, so in fact the insulation of the cable can take more than the CCC quoted in BS7671.

In fact, I'll give a proper example.

A 5A BS3036 fuse will take 0.75 secs to blow with 20A of current running through it. If it is wired in 1mm, then the 20A is too much, but I believe the cable would be designed to take this amount of current for (at least) the time given from the time/current graphs.

I would have thought this is the same for a 13A fuse (with slightly different actual figures).

This is a bit of guess work though, and I'm really not sure if it's right. Just seems logical that cable should be designed to take whatever current could be drawn until the fuse blows (as long as the electrician has designed the circuit correctly).

I agree with what you are saying and several youtube spraks have shown a short bit of 2.5 cable run 50-60 amps before its tarts to get hot,
my point is if you have a ring it should be kept a completed ring , allowing spurs , spider legs and lolly pops you might as well just fit radials in the first place and do away with ring mains