small tails from service fuse

[Deleted member 26818]

Says 25mm² in the OSG, so those 16mm² tails must be wrong.

 

[adammid]

Please tell me you are fitting a switch fuse between the 100amp intake and the shower. If the switch fuse is within close proximity to the 100amp service fuse then I would say its fine but its not if you are going to feed the 6mm straight off the service fuse and not protect it.

 

[Engineer54]

I see someone laying down a trap, and waiting for the unwary to fall into above!! lol!! :ihih:

 

[jonnyboy1]

Agreed, told the firm I do some work for to get 16mm2 tails for our solar Pv boards. Answer was no it needs to be 25mm!!

 

[adammid]

usually in those cases, the undersized tails feed a switch/fuse or small CU which is fused down within a few inches of the henleys, so theoretically those undersized tails can't be overloaded.

AS the above, as telectrix states if there is a switched fuse or small CU installed within a few inches of a cut out then the tails cannot be overloaded, however the tails are still only protected by the main cut out, which is not correct.

All fuses should have a smaller rating than the cable that they are protecting. This is BASIC stuff, yet probably one of the most important.

 

[Rockingit]

Admittedly I've only got the previous red OSG, but it sites 514.3 against 2.2.3 - Meter Tails. 514.3 is colour identification and has no comment regarding minimum cc's for tails, just like the rest of BS7671 as far as I'm aware.

What I am aware of is BS7671 giving us lots of guidance on how to design and calculate circuits properly as best befits the task in hand. But then, that takes more than a few days in a cram-class to understand.

 

[Geoffsd]

Says 25mm² in the OSG, so those 16mm² tails must be wrong.

The On Site GUIDE is one size fits all without having to think.

It also refers to section 1.3.

Why would you need 25mm² with a 60A (or 80A) service fuse?

 

[Deleted member 26818]

I see someone laying down a trap, and waiting for the unwary to fall into above!! lol!! :ihih:

Seems like the trap has sprung.

- - - Updated - - -

The On Site GUIDE is one size fits all without having to think.

It also refers to section 1.3.

Why would you need 25mm² with a 60A (or 80A) service fuse?

Refer to above.

 

[Geoffsd]

Ah. I see.

So how do I tell when you are genuinely wrong?

Was andytucker you in disguise?

 

[keniff]

I'm a bit lost here, if your feeding a small 2 way board for example and the loads are say 6 amp and 6 amp mcb. What's the point of using 25mm? If the loads never going to be over 12amps you can use a smaller csa tail to feed it

 

[Geoffsd]

I'm a bit lost here

[BEING ACTUALLY HELPFUL]It would seem to be a plot to wind us up.[/BEING ACTUALLY HELPFUL]

 

[adammid]

I'm a bit lost here, if your feeding a small 2 way board for example and the loads are say 6 amp and 6 amp mcb. What's the point of using 25mm? If the loads never going to be over 12amps you can use a smaller csa tail to feed it

because what if the small 2 way board was replaced with a larger load, eg a six way board, feeding a ring main, a couple of heaters etc. yes the responsible spark should check the tails feeding it but with the Electrical Trainee out there, would this actually happen. Like it quite clearly states in the regs all cables should be suitably protected.

 

[Rockingit]

because what if the small 2 way board was replaced with a larger load, eg a six way board, feeding a ring main, a couple of heaters etc. yes the responsible spark should check the tails feeding it but with the Electrical Trainee out there, would this actually happen. Like it quite clearly states in the regs all cables should be suitably protected.

Adam - we simply cannot future proof for every idiot out there in the world who is not as good at his job as we might be. If we did, imagine where we'd get to. AND - to what edition of reg's would we proof to - 18th? 19th? 20th?

 

[adammid]

I do agree somewhat but when you are talking 6mm tails fed from a 100 amp fuse, I think that's a bit extreme?

 

[Octopus]

I do agree somewhat but when you are talking 6mm tails fed from a 100 amp fuse, I think that's a bit extreme?

But nobody would install 6mm tails these days would they?