Cable calculations ?

[Morley2012]

Just seeing if im doing this correctly so im using a post from the main forum that someone has asked for help with.

cable calc for 9kw electric boiler.

This is my first post and not sure if in right place! My company have asked me to quote for a supply to be run for a 9kw electric boiler, the run will be through a ceiling void with under 100mm of insulation for about 20m, I used an online cable calc as I dont have my regs book to hand, which tells me a 16mm t +e is the minimum acceptable cable, I thought I would get a second opinion as I can imagine a 16mm is going to be extremely difficult to run through a ceiling void. Any help will be much appreciated.

Ib = 9000/230 = 39.13A
In = 40A

Iz = In/(1.00*1.0*1*0.78) = 51.28A or In/0.78 = 51.28A

So looking at Appendix F table 4D5 he would need a minimum CSA of 16mm (57A)

and the voltage drop would then be 2.8x10-3*39.13*20 = 2.19V

Only had one lesson on this so hopefully im nearly there.​

 

[SirKit Breaker]

What about diversity? Need more info about the boiler, as most boilers/water heaters don't run at full load for most of the time, unlike showers. Take diversity into account and your cable CSA may well come down considerably. Without having the full design spec, an off the top of my head cable size for this would be 10mm, but it isn't clear as to whether the cable will run under or above the insulation, and even if it does, the diversity calc may still bring down your cable CSA.

Cheers............Howard

I havent checked your calcs, but they look good.

 

[Morley2012]

Thanks bud, would I use the same diversity figures as a cooker e.g first 10a x30% re add the 10a and plus 5a for a socket ?

 

[telectrix]

no. not a fixed figure like a cooker as that has several elements. your diversity estimate can only come from experience. 5 different sparks will come up with 5 different values. agree with howard. 10mm should be adequate under 4" insulation. a far better method is to route the cable above the insulation.

 

[Morley2012]

Thanks for the replies guys appreciate the input and hopefully one day I will be able to make a good judgment myself.