2off 2.5mm T&E cables in 20mm Plastic Conduit....

[BS3036]

I did say DIVIDE by the sq root of 2, which is multiplying by 0.7 ish. The regs state that each leg of the ring must carry 20A minimum. Usually where there are two cables run together they are carrying the same current (eg drop to a socket). So a reference methos which gives around 20A for one cable, is going to allow only 14A for 2.

I know most people ignore the problem, but if you read the regs and the OSG it's hard to come to any other conclusion.

 

[boyyo]

Hello again,

Thanks for all the info'...some of which has gone over my head.

So to check, I should be OK derating the MCB to 20A for the Ring 0f 4 double skts?......although I thought someone suggested that 32A should still be OK?

 

[ban-all-sheds]

This'll make you laugh:

If a 20A circuit is OK for your needs you could have just installed a 20A radial using 2.5mm² and not had to struggle to get 2 runs of cable into the conduit.

The only ring final which complies with the regulations is a 30/32A one wired in 2.5mm² with a current carrying capacity of at least 20A.....


On an overall note - you don't come across as being fully up to the job of designing circuits - when you applied for Building Regulations approval what did you say you'd be doing to ensure that your work complies with P1?

 

[BS3036]

To be perfectly honest, I've just been looking at the books and I can't find the part that gave me the concerns before, and I don't have time now to find the part that I thought I remembered. In fact Eric seems to have been pretty close. So I withdraw my statement about the 20A MCB with apologies.

At the end of the day though, the designer of the circuit is responsible for its characteristics and installation method. You need to be able to show Building Control how you arrived at the design, and I'm not sure that they will buy 'some internet forum'.

 

[ban-all-sheds]

No, but an Internet forum can point him at information from which he can learn about voltage drop, cable de-rating factors, installation methods, the importance of R1+R2 etc etc....

 

[BS3036]

No, but an Internet forum can point him at information from which he can learn about voltage drop, cable de-rating factors, installation methods, the importance of R1+R2 etc etc....

I quite agree. Though that's a bit different from 'Can I or can't I?'.

BTW, for anyone interested, my reasoning on derating the MCBs went as follows.

433.1.5 requires the cable used in ring finals to be able to carry not less than 20A.

The closest reference method to the conduit would be Reference Method B.

The Current Carrying Capacity table which applies to flat cable is 4D5. This table does not specify Reference Method B values but does give figures for A and C. By examination, B is clearly a case somewhere between A and C, and therefore a single flat cable will have a capacity of between 20 and 27A.

At this point, for two cables, I should have applied the grouping factor in Table 4C1. For 2 cables this gives a factor of 0.8. Using this factor, the current carrying capacity of between 0.8 * 20A and 0.8 * 27A. i.e. between 16A and 21.6A. Most of that range lies below 20A, so it is extremely questionable whether 2 x 2.5mm cables in conduit comply.

I would be interested in any feedback on this, as I have a current interest in the problem. My instinct is probably not to worry about it in practice.

(In my original recollection I was using the grouping factor specified in 2.3.3.1, which gives the grouping factor as 1 over the square root of the number of cables. For 2 cables this gives 0.71.

That would give a capacity of between 14A and 19A, which clearly doesn't comply. However 2.3.3.1 is a 'play-it-safe' approach specifically for different size cables, so it is not necessary to use it here.)

 

[ban-all-sheds]

I quite agree. Though that's a bit different from 'Can I or can't I?'.

Yup - and one of the things I most regularly get stick for is telling people that they should go off and find out enough so that they can answer the "can I or can't I?" question themselves before starting the work.

BTW, for anyone interested, my reasoning on derating the MCBs went as follows.

433.1.5 requires the cable used in ring finals to be able to carry not less than 20A.

Yup - and if it doesn't then you may not de-rate the MCBs and create a 20A ring, or a 16A ring etc - the only one allowed is a 30/32A one, so if the cables can't carry 20A then you may not have a ring final at all.

 

[BS3036]

the only one allowed is a 30/32A one, so if the cables can't carry 20A then you may not have a ring final at all.

That's where I'd take issue with you. The standard circuits are surely defined to save electricians from having to design each circuit themselves. If you can provide a comprehensive design for a circuit which does not fit the standard circuits, fully justifying its safety, then such a circuit is surely allowed.

Isn't that what, for example, the OSG means when it says: 'Table 7.1 has been designed to enable a radial or ring final to be installed without calculation...'?

 

[ban-all-sheds]

523.8

Yes - you could do what you propose, in the same way that in theory you may do anything you like which contravenes the regulations as long as you are confident that you then comply with 120.3 and that your declaration on the EIC is sound.

But it's more aggro, requires more onerous creation and retention of installation documentation, and carries the risk that any future electrician who happens not to agree with you will code it accordingly on a PIR or refuse to do any alterations to the circuit, e.g. add another socket.

 

[ericmark]

I do see some of the points made.

It is considered that a 32A ring main has 20 amp draw at centre of ring and the remaining 12 amp spread even throughout the ring for calculation purposes.

This as you say does mean that to reduce the feed MCB the first 12 amps would be considered as the twelve spread even throughout the ring and one would still consider 20 amp at centre until the MCB was less than 20 amp.

If the two cables were put in oval conduit and shoved into a stud wall then I would think in still air there could be a problem.

However if plastered into a wall the plaster is quite a good conductor of heat and I would not expect any problems and it is really immaterial as to if the cables are touching inside the conduit or under capping the heat escape is the same.

So in a plastered wall I would not be worried about two cables in conduit however I would in a stud wall.

However twin and earth is not the recommended cable for a stud wall the manufactures now make Ali-tube cable which is recommended for use in stud walls. Because this cable is rated at 90 deg not 70 deg and has a very good conductor near to surface it is able to run with less problems of internal cable damage. However there is more of a chance of external damage from the heat.

However I would think the Ali-tube cable is still not used in stud walls even by the electricians who love to point out how we need to follow manufactures recommendations to comply with the BS7671:2008.

And considering the extra cost I can see why it is not used.

However to tell this poor chap he can't use two cables in oval conduit buried in plaster when we all know that is the normal way as one needs a smaller channel cutting is not really being fair on him.

However I will repeat in still air inside stud walls this is not a good idea at least in plastic. However I would not worry in steel conduit as it does conduct the heat away better.

Lets face it 4mm is only rated at 30 amp in conduit and yet is still permitted on a 32 amp MCB as a radial circuit if we exceed the rating by 2 amp it will not burst into flames. Forget the regulations and use some common sense.

 

[ban-all-sheds]

By all means use common sense.

By all means use sound engineering principles - earlier versions of the regs specified a cable rated at ⅔ of the breaker rating for ring finals.

But don't lose sight of the fact that in the current version of the regs no ring final other than a 30/32A one wired with cable with a CCC of at least 20A is permitted.

 

[securespark]

Hmm. Not sure your final comment is best advice.

I personally feel (providing there's no insulation involved in the equation) that regardless of the wall, the cable(s) will dissipate heat at the same rate if in conduit.