Cable size

[Davey]

I've had a gas fire removed and we want to replace it with a modern electric one.

The gas fire had a FCU for the electronic ignition and I want to change this to a single socket to plug the electric fire in.

But I've since found the FCU has been spurred off a nearby double socket in 1mm T&E.

The length of 1mm is less than 1M and it's buried in plaster - no conduit. I believe this is method C and so the 1mm cable can carry 16A?

Can somebody please confirm this is correct and so the cable will be ok if I replace the FCU with a single socket?

Thank you

 

[Taylortwocities]

No. You must replace the cable with 2.5mm.
The cable must be capable of carrying the maximum fault current, and that is probably 32A

 

[Risteard]

The fault current will be very much higher than 32A, but this only needs to be sustained until disconnection occurs. This could be calculated adiabatically, so there is every chance that it is not unsafe (aside from 1mm^2 cable being used other than for a lighting circuit).

 

[Davey]

I'm sorry but I don't understand what appears to be conflicting advice.

If there was a fault then wouldn't the fire be protected by the 13A fuse in the plug?

 

[EFLImpudence]

Can somebody please confirm this is correct

What you say IS correct.

However, in the regulations there is a very obscure table, that some of us think is ridiculous, which states the minimum size of cable for a 'power' circuit is 1.5mm².
It does not define 'power circuit' nor which circuits do not supply power.

Your cable is adequate for 13A and the fire cannot draw more than that.
The 'fault current' referred to above is the short-circuit (L-N) or earth-fault (L-E) current until the fuse blows which the cable is very likely able to handle but measuring is the only way to tell for certain.

 

[Davey]

Thank you for the advice and apologies for the delay in responding.

I'm not questioning the advice I've received but, to satisfy my curiosity, can somebody please explain about the fault current?

The fire came with a fitted 13A plug on 1m of 3core 1.0mm cable.

Why is this acceptable if the fault current can be much greater?

I understand this cable only has to carry the fault current until the fuse blows but - if this is acceptable - why do the Regs require the cable feeding the single socket to be 2.5mm?

Surely it's the same fault?

Apologies if this sounds argumentative (or if it's a stupid question!). SWMBO won't be impressed if we need to redecorate to replace the cable with 2.5mm so I'm just trying to understand the reasoning.

Thank you

 

[sparkwright]

Is this wiring on an outside wall?

If so, can you run a 2.5 mm2 cable along the outside, properly protected of course.

In the old days a cable would be fished in the brick cavity...

 

[flameport]

why do the Regs require the cable feeding the single socket to be 2.5mm?

A ring circuit is a very specific design, and unlike all other circuits there is a specific regulation which requires that the cables must be of a certain size (2.5mm)
Therefore the 1.0mm does not comply.
There is also the unhelpful table about 'power' circuits having a minimum of 1.5mm and 'lighting' being the only valid use for 1.0mm

As previously stated it is likely that the existing cable is safe, which can be proven by a few measurements and calculation.
It still won't comply with BS7671 (wiring regulations) for the reasons above.

 

[JohnW2]

I understand this cable only has to carry the fault current until the fuse blows but - if this is acceptable - why do the Regs require the cable feeding the single socket to be 2.5mm?

They don't necessarily. The cable feeding the socket is primarily protected by whatever fuse/breaker is in your CU. Commonly, that might be 32A for a sockets circuit. However, if the device in the CU was 16A, then you could use 1.5mm² cable. Indeed, depending on how it was insured installed, even 1.0mm² cable might well be acceptable, were it not for the fact that the regs don't allow anything smaller than 1.5mm² other than for 'lighting' circuits.

Kind Regards, John
Edit: crazy typo corrected!

 

[EFLImpudence]

I'm not questioning the advice I've received but, to satisfy my curiosity, can somebody please explain about the fault current?

It is the result of the live directly touching the neutral or earth.
It may be hundreds of amps but the fuse/MCB will disconnect the supply in a split second (0.1s or quicker) before the cable has time to heat and/or melt.

The fire came with a fitted 13A plug on 1m of 3core 1.0mm cable.

There are different rules for appliance flexes.
As I mentioned above 1.5mm² is given in the regulations as the minimum for a power circuit but there is no electrical reason for this.
You may well think that lights use power as well.
Also in the table mentioned by Flameport is a smaller size allowed for flexible cable, which generally have a lower rating than twin&earth. Go figure

Why is this acceptable if the fault current can be much greater?

As above.

I understand this cable only has to carry the fault current until the fuse blows but - if this is acceptable - why do the Regs require the cable feeding the single socket to be 2.5mm?

They don't. As above.

Apologies if this sounds argumentative (or if it's a stupid question!).

Not at all, but I may have to issue an "Independent Thought Alert".

 

[EFLImpudence]

Is the new fire still going to be covered by the FCU or plug fuse?

 

[333rocky333]

Indeed, depending on how it was insured

 

[JohnW2]

Whoops. That was my brain/typing fingers in 'semi-phonetic typing' mode. It should have been "installed, not "insured"! Now corrected. Thanks for noticing!

Kind Regards, John

 

[Davey]

Thanks for your replies gents.

Is the new fire still going to be covered by the FCU or plug fuse?

Yes the arrangement would result in a single socket being fed via a 1.0mm spur from an existing twin socket on a 32amp ring. The fire would be plugged into this single socket and still protected by its 13amp plug fuse.

I understand this is not permitted by the Regs but, if I've understood you correctly, the only reason is because this is fixed wiring in a power circuit?

So presumably if I used a 1.0mm extension lead to plug directly into the double socket then it would comply with the Regs?

If so, then I suppose the question I'm asking is why as I can't see any difference electrically?

Thank you

 

[Taylortwocities]

The difference is that the plug has a 13A fuse. That limits the maximum current and makes the use of 1mm cable acceptable.