cable size

Greenie9657 · 18 May 2013

I know I'm changing the subject but I've just read somewhere a post about sockets on a radial for the attic. JohnW2 said that on a 4mm2 cable you can use a 32a MCB and on 2.5mm2 cable to use a 20a MCB. John (or anyone else really) is that laid down anywhere what MCB has to go on what? I was thinking of using a 16amp MCB for a 2.5mm2 radial in the attic. I'm not doing this work yet, just getting a massive plan together to sort this house out.

Greenie9657 · 18 May 2013

It was my own post, but thanks. noted

bernardgreen · 18 May 2013

The MCB rating must be lower than safe carrying current of the the "weakest" part of the cable. Weak in that current creates heating in the cable. More current more heating. Cable in thermal insulation will get hotter than cable in free air. The safe maximum current is that which does not overheat the cable at any point.

An MCB rated lower than the cable's rating is safe and acceptable.

Taylortwocities · 18 May 2013

Greenie9657 said:
JohnW2 said that on a 4mm2 cable you can use a 32a MCB and on 2.5mm2 cable to use a 20a MCB. John (or anyone else really) is that laid down anywhere what MCB has to go on what?

Yes, it is all a part of general electrical installation principles (cable size calculations). The data is included in numerous tables and charts that are all in BS7671, the wiring regulations.

Cable calculations take into account many environmental items that can influence the current carrying capacity of cables. eg installation method, temperature, etc etc.

The GENERAL radial circuit arrangements that John outlined are the GUIDE cable/CPD values shown in Appendix 15 of BS7671,
There is a noite to this that states the designer/installer must calculate the actual cable size by reference to Regulation 523.9.

PrenticeBoyofDerry · 18 May 2013

The protective device should be selected taking the load/demand the circuit will require in consideration.
Once the protective device has been seleceted, then the cable size can be calculated, this will take in a number of factors, such as ambient temperature, cable grouping, containment, thermal insulation, method of erection and routing.
Then the distance/length the cable is run, will apply a voltage drop, which again can reduce the safe current carry capacity of cables.
Most additional factors will reduce the cables maximum rated current carrying capacity by a percentage/factor. Known as de-rating.
Once the above calculation have been made you can select the cable size for the device, the cables calculated safe current must be equal to or greater than that of the device selected to protect it.
Links
http://www.tlc-direct.co.uk/Book/4.3.1.htm
http://www.csecables.com/technical/index.htm

ban-all-sheds · 18 May 2013

JohnW2 · 18 May 2013

Only just found this spin-off thread in which I'm being cited!

Taylortwocities said:
The GENERAL radial circuit arrangements that John outlined are the GUIDE cable/CPD values shown in Appendix 15 of BS7671, There is a noite to this that states the designer/installer must calculate the actual cable size by reference to Regulation 523.9.

Indeed. One obviously has to verify in each case but I think it's probably fair to say that the only situation in which those guide figures won't apply is if the cable is buried in insulation.

Kind Regards, John

ban-all-sheds · 18 May 2013

JohnW2 said:
I think it's probably fair to say that the only situation in which those guide figures won't apply is if the cable is buried in insulation.

Greenie9657 said:
JohnW2 said that on a 4mm2 cable you can use a 32a MCB.

JohnW2 · 18 May 2013

ban-all-sheds said:
JohnW2 said:

I think it's probably fair to say that the only situation in which those guide figures won't apply is if the cable is buried in insulation.

Click to expand...

Greenie9657 said:

JohnW2 said that on a 4mm2 cable you can use a 32a MCB.

Click to expand...

Fair enough. Maybe 'buried in insulation' was a bad choice of words - 'in contact with (or 'associated with') insulation' may have been better .. and, yes, I should have also mentioned cable in conduit, too.

However, I don't know if people always manage to avoid these installation methods or whether they simply ignore the effects of installation methods, because I've never seen a 4mm² radial with an OPD rated less than 30/32A, have you?

Kind Regards, John

RF Lighting · 18 May 2013

Not domestiaclly, but it's not unusual to have 32A ring finals and 16A / 20A radials wired in 4.0mm² on commercial jobs, but it tends to be more for prevention of excessive VD, rather than CCC of the cable.

Iggifer · 18 May 2013

JohnW2 said:
However, I don't know if people always manage to avoid these installation methods or whether they simply ignore the effects of installation methods, because I've never seen a 4mm² radial with an OPD rated less than 30/32A, have you?

As RF says, I have seen plenty wired in 4.0mm, and usually to negate the effects of VD, I've seen a few radials on a 32a OPD, wired in 4.0mm supplying one point. Now that's some poor design

bernardgreen · 18 May 2013

JohnW2 said:
I've never seen a 4mm² radial with an OPD rated less than 30/32A, have you?

Yes, in my cottage. I wired them in 4 mm for volt drop reasons..

JohnW2 · 18 May 2013

bernardgreen said:
JohnW2 said:

I've never seen a 4mm² radial with an OPD rated less than 30/32A, have you?

Click to expand...

Yes, in my cottage. I wired them in 4 mm for volt drop reasons..

As you quote, I was talking about radials wired in 4mm²

Are you saying that the OPD was less than 32A and, if so, was that because of cable installation methods, or what?

As for doing it for VD, unless you have a mega-cottage, I presume you must have much more exactly targets that the regs require. Is this to save 'wasted electricity' (in summer)?

Kind Regards, John

JohnW2 · 18 May 2013

RF Lighting said:
Not domestiaclly, but it's not unusual to have 32A ring finals and 16A / 20A radials wired in 4.0mm² on commercial jobs, but it tends to be more for prevention of excessive VD, rather than CCC of the cable.

Fair enough. Needless to say, my (limited) experience is almost exclusively of domestic installations.

Kind Regards, John

333rocky333 · 18 May 2013

Iggifer said:
I've seen a few radials on a 32a OPD, wired in 4.0mm supplying one point. Now that's some poor design

Why is it a poor design

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cable size

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