Help with Calculations fo cable size

[renobloodyvation]

Hi - designing specification for partial rewire/ addition of circuits

Would appreciate some help with first attempt @ understanding cable size calculations
Useful to know if i've the complete wrong end of the stick, other information i need to collect/consider

An example below for shower circuit - Ideally would like to achieve 9.6kw shower run on 10mm cable, 45Amp MCB

Factors considered so far
Length of run maximum 22m
route: CU - through boards/ ceiling space 12-14m - clipped to to wall open 6m, In conduit plastered 1-2m

maxiumum ambient temp 30C so Ca =1

grouped cables under roof space & along wall variable but maximum 10 other cables. will be spaced so that at least width of cable space between. So Cg = 0.9

However for width of 1 beam 3.5''only, would be easier if 4 of 8 cables were to be grouped touching through beam - How critical is this? Would You use Cg =0.7 or 0.48?

No thermal insulation - so Ci=1

Calc.
Lb =9600/230 = 41.74A
lz= 45A/0.9 =50A - with 0.7 = 64A with 0.48 =93A!

So 10mm cable rateage - 52- 63 inconduit/clipped direct acceptable @0.9

Not with Cg@ 0.7 0r less.

Hence the critical question of those few inces through beam?

voltage drop calc - - 10mm - 4.4mV/A/m
so 4.4X 41.74x22/1000 =4.0404 so i.e approx 1.76% - under 45 tolerance - acceptable?

Ok need to go & get a complete set of lessons - or on the right track?

Thanks

Andy

 

[GaryMo]

Be careful with grouping factors otherwise you'll end up with silly cable sizes, as you're well aware!
Remember that any cable that is expected to carry not more than 30% of its grouped rated can be omitted.
Also, the grouping factor table (4C1) is for groups of uniform cables, equally loaded.

Domestically I think reference to the factors in 4C1 will be overkill.

 

[renobloodyvation]

Thanks for that
I guess I'm broadly on the right track

If I'm new to something I tend to sart from the safe side in theory - then look @ the practicalities & then look for compromise .

As you say in this & the same will apply on a number of the other circuits If I'm not careful I'll end up with overkill. The house is a fairly large rambling house over 4 stories most of which is in a state of renovation - but a lot of circuits will need to run over fairly long distances - but many of which will never run @anything like capacity.

But as a newbie want to make sure I'm not missing any biggies. And useful to get other opinions.

The '30% stuff' is useful - not something i was aware of.