Cable sizes & fuse ratings . . . .

[chocolate]

Hi.

Hoping that many of you experienced girls and guys out there will be able to help me out.

A disagreement regarding cable sizes and fuse ratings.

For example : I believe a 2.5mm2 radial ( assuming it is more than 5 metres in length ) should be rated at 16A unless there is convincing evidence that a 20A rating will not induce heating of the cable to an extent that could cause future damage.

As the lesser experienced sparks, my opinions are questioned.

More so with 4mm2 T&E. I believe a 20A rating ( again assuming a measureable run of cable ) rather than a 32A.

My query relates to cables installed as in a standard domestic environment.

Thanks for any thoughts on this matter.

 

[scotsparky]

have a look at the back couple of pages of the regs abbout appendix 15.

2.5mm will happlity take 20A so a 20A mcb is fine
4.mm will take 32A so 32Amcb is fine.

 

[Mac]

Cable Size Selector Calculator and online cable selector for electricians

 

[ajelec10]

Also i would think the the maxium currents rated in bs 7671 are probally very reduced from what would actually make the cable heat up or melt, take for instance the safe working load of lifting equpiment the item will have to take 7 times its rated swl ie a 1 ton strop should be on its limits at 7 ton but shouldn't break
Hope that put your mind at rest
Regards
AJ

 

[scotsparky]

ajelec the current rating in the BRB is based at an operatin temperature of 70c as the normal ambient temperature is 25 which means there is a larger C.C.C of a factor of about 1.02 (not 100% sure OSG in van)

 

[rme]

Hi
Sorry if I’ve posted this in the wrong area.

Do the tabulated values for the current carrying capacity of conductors in bs7671 include the 1.45 rule of thumb?

I.e. a 32amp type B should trip within an 1hr at 1.45x it’s In (46.4amps)

So do I look for a cable that can take 46.4amps, even if the Ib is only 32amps?

I’m sure I’ve seen a reg that states the 1.45 is satisfied when using the tabulated values for twin & earth.

This is all hypothetical and I’m trying to keep my question simple (leaving out the other correction factors and install method)

Cheers