cable calcs

[still learning.]

hi, could someone please explain how to do cable calcs and de-rating.

cheers

 

[Guest123]

Well in order to derate a cable you would first need to work out which factors applied in a given scenario, i.e ambient temperature, grouping, thermal insulation, protection by a BS 3036 fuse (Ca, Cg, Ci, and Cc respectively)

Obviously calculate design current (Ib) select protective device rating (In) then apply factors to tabulated CCC (It) to obtain CCC of cable in particular service conditions (Iz).

Another way is to apply any factors to (In), then select a cable with an (It) greater than (In).

 

[still learning.]

for example i am fitting an outside light under a plastic lean-to, I am using a 6 A fuse in the CU, the cable run is 6 metres, the ambiant temperature during the day will be high but at night when the lights are to be used will be cooler say 15 - 20 degree's from this info what would be the calculation to determine cable size?

cheers

 

[Guest123]

Well seeing as you already have (In) then any factors you would apply to this, ie - (In)/CaxCgxCixCc.

= 6/.........

All the factors you will find in Bs 7671.

 

[still learning.]

hi, sorry i'm even more confused now. first what does CaxCgxCixCc stand for and to de-rate a cable i was told to / the conductor cross sectional area by the de-rating factor so what do i devide 6 by? and how do i factor the length of cable into it, can you give me an example and then the calculation so i can then apply that to other situations so i can understand and remember how to do it.
cheers

 

[ezzzekiel]

suggest you read up its quite long winded
Apply any corrections (for temp, grouping, thermal, fusing ect)
these values are all in brb or osg and will vary depending on installation

above is only one part of the whole design guide.

as im too lazy to type it up:
http://www.tlc-direct.co.uk/Book/4.3.9.htm

 

[still learning.]

hi, thanks i think i'm getting the hang of it i found another thread that explains it a bit i'm just trying to find where i can get the tabulated rated current from to do the calc.

 

[Guest123]

You'll find the values in appendix 4 of BS 7671.

 

[still learning.]

so is the tabulated current the size of the fuse or in my case for 1.5 t&e clipped direct the CCC which is 16A and that is timesed by the ambiant air temp derating factor which if i am correct will be 0 because in table 4D5 that is the CCC at 30 degrees. or because it's outside do i have to take anyother factors into consideration.
also i am putting a switch/socket on a garden wall 6 meter run is SWA overkill or a must?

 

[J@mes]

While it is great to be able to do the calcs by hand (I have been doing it for about 6 months now) I have just bought off ebay a cable calculation program which claims to be to the 17th and designed by an electrician.

Gonna compare it to what I get and see how it measures up - at £12 it's not a lot to lose if its junk!

 

[pennywise]

Get a Brian Scadden text book, very informative, clear formulas and examples etc, really good books