Wiring multiple sockets on fused spur - what is the electrician calculating?

[Justin0]

The electrician has returned to his point that the fuse won't protect against long-term overload.

As he said before: "that cable will be unprotected to sustained overload & long term damage despite being protected by a limiting fuse. Fuses work fine in a short circuit but in an over load can take as much as 1.8 to 2 times there rated current & there have been cases where this has been even higher."

He now continues:

"The fuse protecting the cable between the socket & FSU only has potential to blow quickly under short circuit conditions, under over load it will carry twice its value 26a+. The way you have it wired the FSU is the one & only accessory allowed from the ring radially regardless of what you wire downstream of the load side of the FSU. The potential for a dangerous situation to occur is evident to see, the regs are there to follow & prevent this."

Leaving aside the question of why he thinks it makes a difference whether the cable is before or after the FCU, we've dealt with that.

The cable is 2.5mm T&E clipped direct, so I believe that is rated to 27A under BS7671, so actually covers the double-rating of 26A that he is talking about. But that makes me wonder about the double socket that was there before, installed by the housebuilder (which I have replaced with an FCU).

If a 13A fuse can allow greater that 13A under a slow-overload condition (which I think is right) then the double socket could theoretically have been drawing more than 26A (via two plugs each with 13A fuses and each running overloaded above 13A, but not enough to blow the fuse).

Or to put it another way, if I now wanted (which I don't!) to install the same T&E with installation method 103 "In a stud wall with thermal insulation with the cable NOT touching the inner wall surface" which is then downrated to 13.5A, would the 13A fuse be too high to protect that cable, given a slow overload could occur?

 

[EFLImpudence]

The electrician has returned to his point that the fuse won't protect against long-term overload.

Yes, it will. That is what they do.

As he said before: "that cable will be unprotected to sustained overload & long term damage despite being protected by a limiting fuse.

That doesn't make sense. It is protected by the limiting fuse; that is what it does.

Fuses work fine in a short circuit but in an over load can take as much as 1.8 to 2 times there rated current & there have been cases where this has been even higher."

They can and that is allowed for in the ratings.
Anyway the 2.5mm² cable can cope with twice the 13A. It is good for 27A even though a smaller cable would still be alright..

"The fuse protecting the cable between the socket & FSU only has potential to blow quickly under short circuit conditions, under over load it will carry twice its value 26a+.

...but that is alright.

The way you have it wired the FSU is the one & only accessory allowed from the ring radially regardless of what you wire downstream of the load side of the FSU. The potential for a dangerous situation to occur is evident to see, the regs are there to follow & prevent this."

I don't understand that.

Leaving aside the question of why he thinks it makes a difference whether the cable is before or after the FCU, we've dealt with that.

QED

The cable is 2.5mm T&E clipped direct, so I believe that is rated to 27A under BS7671, so actually covers the double-rating of 26A that he is talking about. But that makes me wonder about the double socket that was there before, installed by the housebuilder (which I have replaced with an FCU).

You are correct.
A double socket is unlikely to have 2 x 13A loads but even if it did the circuit is still alright.

If a 13A fuse can allow greater that 13A under a slow-overload condition (which I think is right) then the double socket could theoretically have been drawing more than 26A (via two plugs each with 13A fuses and each running overloaded above 13A, but not enough to blow the fuse).

Yes, but that is how it works. Fuses don't blow at 13.1A and the cable doesn't melt at 27.1A.
The figures for current rating allow for this and are very conservative.

Or to put it another way, if I now wanted (which I don't!) to install the same T&E with installation method 103 "In a stud wall with thermal insulation with the cable NOT touching the inner wall surface" which is then downrated to 13.5A, would the 13A fuse be too high to protect that cable, given a slow overload could occur?

One fuse wouldn't be too high. Obviously more than one would.

 

[EFLImpudence]

Incidentally, 30A fuse wire has a size of 0.6mm² so ...

 

[JohnW2]

The electrician has returned to his point that the fuse won't protect against long-term overload....

EFLI beat me to it, but I totally agree with everything he's recently written.

As per my previous comments, I really think that you would do well to find a different 'electrician'!

Kind Regards, John

 

[mungo scott]

Dear Justinho
Even though you intend using only 13 amps in your garage, subsequent owners/users of the property will be unaware of your bespoke electrical setup, and may therefore overload your spur. This is why you need to convert your garage (very simply) to a ring circuit. All your new outlets can be linked together in one continuous line (straight or curvy, long or short) . One end of the line connects to your original socket and the other connects to the next socket in your domestic circuit. You then disconnect the cable linking these two original sockets and hey presto ....you have diverted and extended your original ring circuit safely. Electrics are a science governed by logic. If you make the correct connections, it cannot fail to work.

 

[Justin0]

Yes, it will. That is what they do.
They can and that is allowed for in the ratings.
...
Yes, but that is how it works. Fuses don't blow at 13.1A and the cable doesn't melt at 27.1A.
The figures for current rating allow for this and are very conservative.

That makes a lot of sense, and is what I had always assumed, thanks.

Does anyone have the ability to post a picture of somewhere in the regulations where it might indicate that cable ratings allow for this overload? And furthermore, JohnW2 indicated there was an example of an FCU "inline" on a spur, that would be incredibly helpful. If copyright is a concern, maybe just the section numbers in the current edition if someone has that? Then I could point him in that direction. I don't have access to any of the literature myself. Apologies if that is asking too much!

 

[Justin0]

As per my previous comments, I really think that you would do well to find a different 'electrician'!

You're right about that but we've been liaising for quite a while and I don't want to ghost him, I'd like to let him know my reasons as authoritatively as possible.

In his last message he said "the interpretations of an untrained member of the public that reads regulations from a book or off the net are easily misunderstood or misinterpreted when not backed up by experience & training".

 

[JohnW2]

In his last message he said "the interpretations of an untrained member of the public that reads regulations from a book or off the net are easily misunderstood or misinterpreted when not backed up by experience & training".

Hmmm! As as has been said before (and not only by me), I think it's really the converse of that - i.e. that the situation seems to be that a person (I hesitate to say 'electrician') who does not have enough understanding, training and experience to understand or interpret the regulations correctly!

That's obviously only my opinion, but I don't seem to be alone.

Kind Regards, John

 

[SFK]

Justin,
With regards to wanting written evidence - Are you overthinking this?
You have a 13A fuse, 2.5mm2 cable, and 8 sockets.
Your setup is the same setup as this (in fact yours is better as you have better cables, and sockets). :

The 13A fuse in the plug/FCU will protect the 2.5mm2 cable, whatever order or number or connection of the subsequent sockets.
Your circuit is correct and the 13A fuse will protect the cable.

SFK

 

[JohnW2]

Even though you intend using only 13 amps in your garage, subsequent owners/users of the property will be unaware of your bespoke electrical setup, and may therefore overload your spur.

That could be said of any multi-socket spur from a ring final - or, indeed, any plugged-in multi-output 'extrension lead'. Subsequent users of the installation (or subsequent users of any sockets on a fused spur, or an extension lead) might, as you say, attempt to overload the circuit, but the 13A fuse will ensure that the cables come to no harm as a result.

Kind Regards, John

 

[Justin0]

With regards to wanting written evidence - Are you overthinking this?

I actually used that exact same comparison with him right at the beginning, before he convinced me to rewire everything in a line, but he didn't accept it was the same thing or met regulations.

 

[SFK]

Justin,
Sorry to say but he is the problem, putting doubt into your mind.

And if you want to be shown how secure the system is, when you say you have thoughts about..

If a 13A fuse can allow greater that 13A under a slow-overload condition (which I think is right) then the double socket could theoretically have been drawing more than 26A (via two plugs each with 13A fuses and each running overloaded above 13A, but not enough to blow the fuse).

...think about your kitchen circuit that has Kettle, Toaster, WMachine, TDryer, Phone chargers, Deep fat fryer, Undercounter lights, Extractor fan, Popcornmaker, etc etc all plugged in and ready to go on one (admittedly ring) cable that is inside the wall. It works perfectly okay without a 13A fuse.

SFK

 

[JohnW2]

I actually used that exact same comparison with him right at the beginning, before he convinced me to rewire everything in a line, but he didn't accept it was the same thing or met regulations.

At risk of sounding like a broken record - I really do think that you need a different ('proper') electrician!

Kind Regards, John

 

[JohnW2]

... furthermore, JohnW2 indicated there was an example of an FCU "inline" on a spur, that would be incredibly helpful. If copyright is a concern, maybe just the section numbers in the current edition if someone has that?

From Appendix 15 (which is 'informative'/guidance, not actually 'regulations') of the Wiring Regulations (BS7671:2018) ...

I'm not totally sure what the comment about 'number of socket outlets' is actually meant to mean, but I would not worry too much about it! On any standard ring or radial sockets circuit, the number of permissible sockets is 'unlimited', regardless of the fact that the circuit is protected by a device (commonly a 32A MCB) with a certain rating (far less than the total potential load that the sockets could result in).

Kind Regards, John

 

[Justin0]

Thanks JohnW2 for digging that out.

But actually looking at it I feel like it might back up the electrician's case, because:

1) The cable going into the FCU is larger than that coming out (ignoring the fact that it's 2.5 going in, as in my setup)

2) That comment about the number of sockets being dependent on the load characteristics - I mean, it shouldn't make any difference, should it?

Could he be onto something?