Cable sizing

[bagendben]

I'm trying to run a feasability study into replacing a boiler and would like to calculate a new supply cable size. I've tried several online calculators and have varied results. The supply will be fused at 200A to supply a 115kw boiler on a 3 phase supply at 400v over a run of 27m. The isolator will be at 23m from the fused supply with a 4m feed direct to the boiler. The cable will have to run for 5 metres underground but the rest will be through conduit or clipped to walls etc. I've been told that every meter counts on this kind of run, but the calculators (usually with a 70mm result) do not change their suggested size from the 30m mark up until over 100m. Any suggestions, formulae or links would be appreciated.


Thanks.

 

[ericmark]

There are three things to consider.
1) Earth loop impedance
2) Volt drops
3) Current handling ability of cable with installation method.
Where an RCD is used then the ELI will not really be an issue.

At a 166A and 27m to keep within the 5% volt drop it would need a mV/A/m of around 2.6 so that would leave 16mm too small and 25mm would be required.

However 25mm only rated at around the 102A so closest to 166A is 70mm at 192 amp OK looking at SWA but basic principle is what I am trying to show.

So yes the calculators have selected the worse of three considerations and correctly selected 70mm cable.

 

[BSBS]

However 25mm only rated at around the 102A so closest to 166A is 70mm at 192 amp OK looking at SWA but basic principle is what I am trying to show.

So yes the calculators have selected the worse of three considerations and correctly selected 70mm cable.

Doesnt the OP say that it will be fused at 200A, if 70mm is rated at 192A then it would not comply, also being a boiler room im gussing ambient temp is going to play its part

 

[ericmark]

The 8A is not worth worrying about he will only be drawing 166A and 200+ will only be case under fault conditions.

I have just wired up 2 x 35mm cables with a peak amps of 500A and not at all worried about overload as I know there is a limited time the batteries can produce that amount of power for.