explanation of using 2.5mm2 T+E for a spur from a ring 32A (ring)

[Rob678]

Why can we use 2.5mm2 for a spur from either the CB or a point on the ring ? Why does this not have to be 4mm2 when using a 32A breaker ?

 

[buzzlightyear]

Are we asking about 2.5 for a spur, in which it a 4mm.

 

[MJPD29]

Why would it have to be 4mm? If a spur is limited to one double socket without an fcu then ask yourself why would you need to use 4mm?

 

[littlespark]

For an UNFUSED spur there is a limit of only one point. Which, if a double socket, is maximum 26A if both outlets are used with a 13A plug.

Very unlikely that would happen, so we assume a max 20A load on that spur…. And 2.5mm cable can easily take 20A.

We use 32A on a 2.5 ring, because it IS a ring.
Any point is supplied by 2 x 2.5 cables…. Not just one.

 

[Rob678]

For an UNFUSED spur there is a limit of only one point. Which, if a double socket, is maximum 26A if both outlets are used with a 13A plug.

Very unlikely that would happen, so we assume a max 20A load on that spur…. And 2.5mm cable can easily take 20A.

We use 32A on a 2.5 ring, because it IS a ring.
Any point is supplied by 2 x 2.5 cables…. Not just one.

Thankyou littlespark, then my question is, if we can use 2x2.5mm T+E to create a ring on a 32A breaker. If I add a spur from a junction box or socket outlet, is there a calculation or any evidence in BS7671 to explain why this cable 2.5mm spur isn’t at risk of overheating ? I don’t understand the scientific/ numerical aspect behind this.

 

[cliffed]

2.5mm rated @ 27 amps I do believe… you are allowed 1 Twin socket on a spur .. thus max overload current would be 26 amps @ that point
32 amp RCBO/MCB protects the actual RM from short circuit, the fuse in the plug top provides overload protection

 

[littlespark]

It’s the fact that an unfused spur can ONLY take a maximum of two things plugged into a double socket limits how much current can actually run through the spur. 2 x 13A = 26.

That is why we don’t allow more than one point on an unfused spur. Then it could overheat with maybe 3 x 13 running through it.


The joys of fused plugs.

 

[Pretty Mouth]

Thankyou littlespark, then my question is, if we can use 2x2.5mm T+E to create a ring on a 32A breaker. If I add a spur from a junction box or socket outlet, is there a calculation or any evidence in BS7671 to explain why this cable 2.5mm spur isn’t at risk of overheating ? I don’t understand the scientific/ numerical aspect behind this.

433.3.1 (ii) applies in the case of a spur from a ring.

As @littlespark mentioned, a double socket is only expected to take a maximum continuous load of 20A (I believe that BS1363 only requires a twin socket to take a 20A load). This does not exceed the cable rating required from a ring final by 433.1.204

 

[timhoward]

I’m pleased 443.1.204 was mentioned!

It’s very common for the informative appendix to quoted as de facto regulations when in fact it’s simply an easy/common way to make sure 433.1.204 is complied with.

The key concept in the reg itself is expectation of likely use in terms of load and duration.
I’ve run more that one double socket on an unfused spur in a data cupboard where all six sockets had plug in transformers.
I don’t recommend doing this at the drop of a hat but it isn’t a banned practise as the cumulative expected load is (significantly) under 20amps.

I have no doubt that one day someone will insist a fused spur is inserted as it seems not many electricians read the actual reg about RFC’s, just the appendix.

 

[Julie.]

....443.1.204 ....

Go on then, what's regulation 443.1.204 ?

Or do you want to correct the typo?

 

[timhoward]

Go on then, what's regulation 443.1.204 ?

Or do you want to correct the typo?

443.1.204 : Early in the morning when spotting a mild hangover, phones shall not be used to type anything requiring any level of detail.
(Can’t correct now, too late!)

 

[Rob678]

2.5mm rated @ 27 amps I do believe… you are allowed 1 Twin socket on a spur .. thus max overload current would be 26 amps @ that point
32 amp RCBO/MCB protects the actual RM from short circuit, the fuse in the plug top provides overload protection

Thanks everyone, my question that that cliffed is: If in the event of a fault is there not a risk that the single leg 2.5mm spur to a double socket could become damaged ? If the MCB is rated to 32 Amps and the cable rated to 27 amps ? Thankyou

 

[pc1966]

Thanks everyone, my question that that cliffed is: If in the event of a fault is there not a risk that the single leg 2.5mm spur to a double socket could become damaged ?

Yes...but over a very long time.

If you go from 27A to 32A then the heating is increased by (32/27)^2 = 1.4 so your temperature rise goes up by roughly 40%, and from the usual assumption of 20C->70C defining the CCC then your 40% increase is taking the wire to 90C. Above its ratings for sure, but very far short of bursting in to flames.

The 20C excess temperature will typically increase the cable ageing by a factor of 2-3 depending on the type of reaction that degrades it, so the nominal 25+ year lifetime is now down to 10-ish years.

Assuming you run at full load all the time, and with 32A being about 7.3kW that would be around £1.5k per month. Which is a touch more than most folks can afford!

 

[ipf]

Thanks everyone, my question that that cliffed is: If in the event of a fault is there not a risk that the single leg 2.5mm spur to a double socket could become damaged ? If the MCB is rated to 32 Amps and the cable rated to 27 amps ? Thankyou

Take it up with the IEE.....but they won't explain it any better than you've seen already.