3 x Twin & Earth In 20mm Conduit

[ScubaCollie]

Hi All,

I've spurred off a garage socket and run another T&E out of the socket via the conduit connector, up approx 6ft of 20mm conduit, and then cut into the conduit and fixed a T Piece to run along the wall, then around an elbow and back down the wall to a new 2 gang socket.

To summarise i've got 3 x lengths of T&E 2.5mm cables running up approx 6ft of conduit (the original 2 plus the new length I've added) from the spurred socket and then one of those T&E's T's off whilst the other two continue up the wall on their own as they were originally.

I admit it was a tight squeeze getting the new run inside, but i got them through in the end, but my worry now is whether it's dangerous and if they will overheat due to being in the conduit too tight?

I did a really neat job of it all, and will be a shame to rip it all out and start again, but im wondering if I should remove it all and put in 25mm instead?

Is it dangerous the way it is?

 

[ericmark]

Unless all cables taking max likely safer, I found when cables burnt in vehicles where in a harness they were OK as the other wires removed the heat.

 

[ScubaCollie]

Unless all cables taking max likely safer, I found when cables burnt in vehicles where in a harness they were OK as the other wires removed the heat.

Sorry mate could you clarify? Are you saying what I've done should be OK?

 

[Taylortwocities]

There’s a very helpful reply to this question on another forum.

 

[ericmark]

Yes

 

[ScubaCollie]

Yes

Many Thanks Eric, the cables won't overheat where I've squeezed 3 x T&E in the 20mm conduit? I'm worried about it burning and catching fire?

 

[Taylortwocities]

Cables heat up in use. When calculating cables sizes, a number of factors are considered. One of these is grouping of cables. Several cables, together, in an enclosed space means that the designer may need to use thicker cables than "standard" to compensate. see https://www.tlc-direct.co.uk/Book/4.3.5.htm
Otherwise the cables heat up, melt with obvious consequences.
How much bigger the cable needs to be will depend on the cable calculation, which you haven't done.
The amount of current in the cables is a major factor. Your cables won't over heat if there's no current! Plug a couple of 2KW heaters in and things can get interesting.

Then we come to the cable capacity of the conduit. There are lots of tables on conduit capacities but all of these cater for single conductors in conduit, because nobody in their right mind would try and cram T&E down a length of conduit. Here's a guide
https://www.tlc-direct.co.uk/Book/4.5.3.htm
The capacity includes a space factor, which you don't have. The conduit is rammed. My gut feel is that you need 25mm conduit, or strip out the inner cores and run the conductors in the 20mm conduit as singles. At least they then can breathe.

Then we can turn to if you can just tag this on to your existing socket. Is this on a ring final, or a radial. What is the maximum current for this circuit? Is it RCD protected?

 

[davelx]

Many Thanks Eric, the cables won't overheat where I've squeezed 3 x T&E in the 20mm conduit? I'm worried about it burning and catching fire?

2.5mm² T&E cable in conduit (Ref method B) is rated 23A. With three cables grouped together, apply a grouping factor Cg=1/√n (where n is the number of cables) of 0.58. (Appendix 4, para 2.3.3.1)

0.58 x 23A = 13.34A per cable.

Therefore the total load should be <=13A per cable, so it depends what is plugged in where and when. In practice, unless you are running heaters etc. for long periods of time, you should be fine.

 

[ScubaCollie]

Hi Guys I'm permanently running a 4ft tubular heater off the Spurred socket I've installed which obviously comes off the original socket that the 3 x cables are running to/from?

 

[ScubaCollie]

The house was built 10 years ago and has a modern consumer unit with a Wylex NSB32 installed in the Downstairs Socket Circuit. The circuits are also protected by 2 x Wylex WRS 80/2 80A 30mA RCD's.

The Original socket I spurred from isn't really used as its in the middle of the garage and I have another near the front of the garage, but if the original one was used it would be for power tools like drills and cutting tools etc and a power washer, but the new spur i put in currently has a plugged in wall mounted 4ft Tubular Heater for the rabbits plugged into one of the 2 gang sockets and the other may be used for a lawn mower at the same time as the tubular heater.

The garage is running off the downstairs socket circuit and has the NSB32 installed, Amazon have them and state they are 32amp, I assume that's what the 32 stands for.

 

[davelx]

Hi Guys I'm permanently running a 4ft tubular heater off the Spurred socket I've installed which obviously comes off the original socket that the 3 x cables are running to/from?

What is the power rating in Watts, of the heater? - the fact that it is 4ft doesn't tell us anything, I'm afraid

 

[ericmark]

If the garage supply is 20 amp, then clearly the total power used is 20 amp, so it one cable can carry 20 amp then the other two makes no difference, as combined they still can't carry more than 20 amp as the garage supply would trip.

If you have 3 x 20 amp trips each supplying a cables then total could be 60 amp and de-rating would be required, but having three cables shoved together sharing 20 amp means each cable acts as a heat sink for the other cable it does not add to total.

I see nothing wrong with what you have following what you have said each cable can carry.

 

[Taylortwocities]

4ft tube heaters are only about 160watt.
Should be ok, providing the original socket can be connected to as a spur.

 

[davelx]

If the garage supply is 20 amp, then clearly the total power used is 20 amp, so it one cable can carry 20 amp then the other two makes no difference, as combined they still can't carry more than 20 amp as the garage supply would trip.

The garage is running off the downstairs socket circuit and has the NSB32 installed

Eric, there is no mention 20amps anwhere. It's fed from a 32A MCB. I think you know that a 32A type B MCB will pass 46A continuously without tripping.

heat sink for the other cable it does not add to total.

each cable is still passing that current though and due to its resistance will dissipate heat. So the currents DO add. If they didn't, then a coiled extension lead with a 3kW fan heater on the end would never burst into flames.

That said, I see nothing wrong with what he has either.

 

[ericmark]

The house was built 10 years ago and has a modern consumer unit with a Wylex NSB32 installed in the Downstairs Socket Circuit. The circuits are also protected by 2 x Wylex WRS 80/2 80A 30mA RCD's.

To supply from the ring circuit it would either need a FCU (fused connection unit) or be connected with 4 mm² or larger directly to the 32 amp MCB, I would guess a FCU so all goes through a 13 amp fuse?

The Original socket I spurred from isn't really used as its in the middle of the garage and I have another near the front of the garage, but if the original one was used it would be for power tools like drills and cutting tools etc and a power washer, but the new spur i put in currently has a plugged in wall mounted 4ft Tubular Heater for the rabbits plugged into one of the 2 gang sockets and the other may be used for a lawn mower at the same time as the tubular heater.

A 4 foot tubular heater is likely to be no more than 250 watt, so really not worried about that.

The garage is running off the downstairs socket circuit and has the NSB32 installed, Amazon have them and state they are 32amp, I assume that's what the 32 stands for.

Yes a NSB32 is a 32 amp B type MCB and 2.5 mm² is rated around 20 to 25 amp depending on how installed, so some thing would need to reduce that to around 26 amp max, having one only double socket for example 2 x 13 = 26 amp in real terms unlikely to exceed 20 amp so permitted, other wise you need to fuse down, a FCU is most likely and since I would assume garage has lights and socket in real terms that is most likely.

So likely the max that can be drawn for any length of time is 13 amp. In which case your fine, if not then your looking at more than changing the wires in the conduit, you will need to fit either 4 mm² cable or larger house to garage, and a consumer unit in garage or at least a switched FCU in garage for lights, as 16 amp is max for lights, or fit FCU in house.