2 x 2.5 T&E

[Ukwoody]

Morning all.

Aside from regs, can someone explain to me why you cannot use 2 x 2.5 t&e instead of 1 x 4mm cable? What is the actual problem?

For an example. I want to run electrics to a workshop 40 mtrs away. max draw about 4KW. Cables clipped to outside (covered) wall. So since there would be easy heat dissipation why do I have to use 4mm cable instead of 2 x 2.5mm which would actually amount to a larger size (?).
Just curious

many thanks
Woody

 

[ericmark]

You can use two cables in parallel there are rules:-

433.4 Overload protection of conductors in parallel
Where a single protective device protects two or more conductors in parallel there shall be no branch circuits or
devices for isolation or switching in the parallel conductors.

This regulation does not preclude the use of ring final circuits with or without spur connections.
433.4.1 Equal current sharing between parallel conductors
Except for a ring final circuit, where spurs are permitted, where a single device protects conductors in parallel and
the conductors are sharing currents equally, the value of Iz to be used in Regulation 433.1.1 is the sum of the
current-carrying capacities of the parallel conductors.

It is deemed that current sharing is equal if the requirements of the first indent of Regulation 523.8(i) are satisfied.

But using two x 2.5mm² cables to feed a grid switch is common. Simply hard to get 4mm² into the terminals so 2.5 is used. If equal current sharing i.e. not part of a ring final then the overload used can reflect this so two x 2.5mm² cables could have a 40 or 45 amp (according to installation method) MCB to protect them rather than the 32 amp max for a ring final.

 

[DaveHerns]

If one cable fails without you knowing then the other one would be overloaded . What's the point in running two cables when one is better?

 

[Lectrician]

Because he's probably trying to use up what he has. It's permissible, but bodgy IMO.

 

[Taylortwocities]

Most houses use 2x2.5mm² cable for socket circuits (a ring final circuit). So there isn't a technical reason why this cannot be done.

There is a practical problem with your proposal however:

Twin and earth cable is not suitable for use outside, as frost and infra red (sunlight) will degrade the plastic in a relatively short time and the cable will fail.

SWA would be my choice. 4mm² is on the edge for vd - I would use 6mm² would give a margin for future additions.

 

[Ukwoody]

Lectrician, yes you are correct I have 100mtrs of 2.5 t&e and a limited budget,lol. I also thought that 2x 2.5 would actually give a greater tolerence to drawing a bit more power on brief occasions. I am trying to save money, but certaintly do not want to create a dangerous situation (it is my own not for a customer). I just couldn't quite see what the actual harm is. I do see the point about 1 circuit going down, but I would have thought that was fairly unlikely in the given circumstance. Just trying to get my head around why, lol

 

[JohnW2]

But using two x 2.5mm² cables to feed a grid switch is common.

... which, if both cables originate from the CU, makes it more-or-less qualify as a ring final circuit. Interestingly, if you regarded it as a ring final, the maximum permissible MCB would be 32A. If you regarded it as two cables in parallel, then, as far as cable protection was concerned, you would be allowed a 40A MCB, maybe a 50A one (if Method C).

Kind Regards, John.

 

[JohnW2]

...I just couldn't quite see what the actual harm is. I do see the point about 1 circuit going down, but I would have thought that was fairly unlikely in the given circumstance. Just trying to get my head around why, lol

That's probably the only 'problem' with cables in parallel, but is why, as has been said, many would regard it as somewhat 'bodgy', even though compliant with regulations.

In your case, of course, if the maximum demand really is only 4kW, then just one 2.5mm² cable should be adequate from the viewpoint of current-carrying capacity (might be slightly 'under-sized' as regards guideline voltage-drop considerations), so a second in parallel with it would essentially just be adding 'an additional element of safety'

Kind Regards, John

 

[OwainDIYer]

If you can split the load across circuits you could run 2 x 20A radial circuits from the supply end (all other design criteria being met).

 

[JohnW2]

If you can split the load across circuits you could run 2 x 20A radial circuits from the supply end (all other design criteria being met).

One could. However, as I've just written, if the max load really is 4kW then a single 2.5mm²/20A circuit ought to be adequate, CCC wise. Even in terms of VD, if (as usual) the 4kW relates to 240V, then at 230V, the VD with a single 2.5mm² cable would only be about 5.009% - which, by my reckoning is as close to 5% as makes no difference

I'm not really suggesting that the OP goes with a single 2.5mm² cable but, since that would probably, strictly speaking, be adequate, adding a second cable in parallel would merely be 'icing on the cake'. I therefore don't think one should be too concerned about one of the parallel cables becoming disconnected (which, as I've said, is about the only downside of parallel cables).

Kindest Regards, John

 

[Risteard]

But using two x 2.5mm² cables to feed a grid switch is common.

... which, if both cables originate from the CU, makes it more-or-less qualify as a ring final circuit. Interestingly, if you regarded it as a ring final, the maximum permissible MCB would be 32A. If you regarded it as two cables in parallel, then, as far as cable protection was concerned, you would be allowed a 40A MCB, maybe a 50A one (if Method C).

Kind Regards, John.

I wouldn't consider two cables following the same route to be a ring final circuit, given that they do not form a ring. It would clearly be conductors in parallel.

Really parallel conductors might be appropriate for a distribution circuit, but not really for a final circuit. In fact other national standards prohibit their use for a final circuit. (And before anyone mentions it I am well aware that BS 7671 does not, but there really is no advantage in this scenario.)

 

[JohnW2]

I wouldn't consider two cables following the same route to be a ring final circuit, given that they do not form a ring. It would clearly be conductors in parallel.

You're obviously free to consider what you like, but a pair of cables originating from the same OPD in a CU and connected to just one accessory as much "form a ring" (electrically) as they constitute two conductors in parallel. If you really believe what you've written, are you implying that the two parallel conductors would 'form a ring' if they took slightly different routes (but were identical in length)?

Kind Regards, John

 

[EFLImpudence]

That is not allowed for conductors in parallel.

 

[JohnW2]

That is not allowed for conductors in parallel.

Reg number? In fact, 523.7 only requires that parallel conductors have "approximately" the same length - and if their lengths are not identical, it would clearly be impossible for them to have absolutely identical routes.

Kind Regards, John

 

[EFLImpudence]

That is not allowed for conductors in parallel.

Reg number? In fact, 523.7 only requires that parallel conductors have "approximately" the same length - and if their lengths are not identical, it would clearly be impossible for them to have absolutely identical routes.

Oh. I always thought that was the case.