Applying It>= In/CaCiCgCr: "Cable is king"

[rlc]

Scenario : 8kW shower, Ambient temp 30degC, clipping direct ref method 1, thermal insulation 400mm, touching / grouped with 3 other circuits

8kw/230 Ib = 34.8A,
In >= Ib, 40A MCB
Cg = 0.75
Ca = 1 @ 30deg C
Cr = 1 as MCB

It = 40 / (0.75 * 0.55 * 1 * 1)
= 97A

OSG Cable selection from Table 6E1, page 123 = 25mm^2!!!

So practically, how is this value ever used?

 

[markie]

Thats the worsed confusing post i've ever read, but i'm no sparky so what do i know

ps the op's subject typing done it i think, cable is king

 

[Spark123]

If using flat twin and earth then use table 4D5A, but will still come out pretty large as you have almost halved the CCC of the cable by running it through thermal insulastion.

 

[rlc]

Yeah sorry having re-read I'd have to agree at this _late_ time.....I'm clearly bored and was just trying to establish the application of the calculation for current carrying capacity of a cable based on the design equation as stated in the On-site-guide "It>=In/CaCiCgCr" where:

Ib = design current
In = overcurrent device rating
It: is the tabulated current carrying capacity
Ca is the correction factor for thermal insulation
Cg is correction factor for grouping
Cr is correction factor for semi enclosed fuses to BS3036

Anyway, I think the answer is "calcuation equals 97A by the book if it was a constant load" - practically you are governed by what the fitting ultimately will accommodate i.e 10mm^2, as the usage is typically bursty in nature thats another factor for derating possibly?

Thanks Spark123 just read you reply after posting : basically killer is the thermal insulation and best case would be to remove any need for derating completely to have the design equation stand up i.e Cg=1, Ca=1 & Ci = 1, leaving only derating factor the method of clipping which equates to 40/0.75 = 53.3A, table 4D5A = 64A = 10mm^2

 

[Spark123]

I think that further care should be taken to avoid running the cable through thermal insulation!!
You cannot just run a 10mm² all the way from CU to appliance including a run through through thermal insulation just because it is the only one that fits in the terminals! A shower is a constant load btw .
P.S. there is nothing stopping the cable size being increased for a run through thermal insulation then decreased after the insulation, as long as the cable is correctly sized for its environments etc for its entire length.

 

[markie]

rlc sorry for the post like i said im no sparky hence my post

 

[rlc]

thanks for the clarification on this - I was just working some basic numbers on a simple test scenario passing some time + like the odd small calculation now and again...

You cannot just run a 10mm² all the way from CU to appliance including a run through through thermal insulation just because it is the only one that fits in the terminals!

Totally agree...

rlc sorry for the post like i said im no sparky hence my post

no problem mate - i did laugh when I looked at the title again ...confused myself the second time

 

[dingbat]

Just because a cable calculation involves several de-rating factors, you don't need to apply them all together unless they act simultaneously on the cable. For instance, grouping is only an issue here if it coincides with the run through insulation (read Appendix 4 of BS 7671).

There is no doubt, however, that insulation is the enemy, if you can avoid it, do so. But there is other guidance at hand. The Summer 2005 edition of the IEE's Wiring Matters carried an article about insulation, cable installtion methods and current carrying capacities:

IT's a good read: http://www.iee.org/Publish/WireRegs/WiringMatters/index.cfm