Cable Calcs - can anyone check my logic

[Noddy67]

Hi,

Calculating - for the first time - SWA buried cable.

The design current (Ib) is 12 amps
The cable run will be 20 metres, buried in the ground.
I'm looking at 2 core SWA, using the armour as earth
(no grouping, ambient temp changes (other than buried) or rewireable fuse factor to consider)

I'm going to join the home (Zs's permitting) off the downstairs ring main (RCD'd) with a external fused spur at 13 amps (so In =13a).

Now this is the fuzzy bit... do I

1. look at tabulated current (It) based on the 2 core Multicore Armoured - Ref Method D (P147 of unite book) to see that 2.5mm works (up to 29 amps?) and doesn't suffer from excessive volt drop (18x20x12/1000=4.32v, well within 5%)

or

2. look at 4B3 on page 128 to calculate factor in which case I have no idea of k.m/w!

I've assumed it's the former, but given the higher ampage than 2.5mm T&E I'm suddenly wary.

(Yes, this cable calc business is so much easier with a tutor standing close by!)

Any help/thougths on these cable calcs would be appreciated.

 

[Guest123]

For 12A over 20M, a 2.5 SWA is more than adequate.

The first formula is correct for this scenario.

 

[Noddy67]

Thanks Lenny. Much appreciated.

Out of interest (and feel free to ignore this) does this mean that the increased heat of buried cable actually have very little impact on it's current carrying capacity. Lookng at SWA Ref D vs Clipped Direct it's virtually the same.

whatever, thanks again.

 

[jeremy]

I would have thought the earth that the cable is buried in would keep it cool, especially if there's any moisture there. In physics class (long time ago!) we were taught that if you touch something at ambient temperature and it feels cool it's because it's transferring heat away from your hand . Sorry, wandering thoughts again!

 

[marksparky]

could be wrong but is it not that swa xple insulated is rated higher because the insulation is xple instead of pvc and can withstand higher temperature.

 

[Noddy67]

I could believe that Jeremy. But equally I'd believe it was hotter since if you go a few metres down you use those Ground Source Heat Pumps to partially heat water.

Seems I believe in a lot of things given i'm an athiest.

 

[Noddy67]

Hi Marksparkys,

So it can get hotter without damaging the basic insulation? That seems right.

R

 

[jeremy]

ground temperature stays at a fairly constant 10 - 16 Celcius at c.500mm depth so very able to cool cables . GSHP works by removing the thermal energy putting it into your heating system, then going back for more and so on. this eventually heats up your water. Its not instant but a fridge in reverse.

 

[Noddy67]

all interesting stuff - I'm on a steep learning curve still.

Thanks all.

 

[marksparky]

as far as im aware noddy, xple can carry higher currents than pvc insulation for eg a test to prove the therory if u try and burn xple insulation wit a lighter it doesnt burn it sort of extinguishes itself, although if u do the same to pvc it will burn very easily.