SWA Cable sizing

[m109254]

Why is it that a supplier on this site states that 4mm SWA cable has a current carrying capacity of 45 amps whereas my local electrician now tells me it needs to be at least 10mm if not 16mm?

The run is 100m and we need a 32 amp supply to run a small welder - otherwise it's only for lights and power tools. I reckoned the voltage drop would be ok according to the specifications for the cable as did the supplier.

This is going to cost me a great deal of effort to re lay the cable although luckily it's in a duct - let alone the cost of buying the new stuff and dumping the original.

Have i just asked 1 too many people for advice - i.e. 2?

Signed - a not very happy person who tried to do the right thing by talking to the experts and reading the data sheets.

 

[Driller]

The tabulated current rating is the maximum the cable can carry. Once you've factored in correction factors and volts drop this can be significantly reduced.

I've just put 32A @ 100m into Cablecalc and it's coming up with 16.00mm 3 core. anything less won't meet the voltsdrop per metre limit.

 

[Pensdown]

Your electrician is right.

The calculation above is based on a 3% volt drop end to end which may apply if the new cable was fed directly from your consumer unit and terminated within a few metres of the furthest outlet in your workshop.

Personally I calculate sub-mains on 2% volt drop as above which would mean a 16mm cable

PS..Ignore all calcs in the right hand column as they are site specific

 

[Lectrician]

Pensdown, change it to a single phase circuit for better results

Still agree though - 4mm aint gonna do it for you!!!

 

[Pensdown]

You go straight to the top of the class

 

[m109254]

Thanks chaps - appreciate the detailed replies. We've put 16mm in and it's working fine.

One final question (i'm not an electrician but I thought I understood the basic physics...) - why is the cable size recommended as 10mm with a 3% drop and a bigger cable size with a smaller drop - or am I misunderstanding how cablecalc works? In my innocence I would have thought that if you expected a bigger voltage drop you'd spec a bigger cable?

Looks like I'd better stick to farming....

 

[Adam_151]

Bigger cable drops less volts per m, per A

The low the volt drop you intend to achieve, the bigger the cable is going to have to be!

 

[RF Lighting]

Very laymans terms here to hopefully make things a little clearer:


If you have a thin cable it is harder for the electric to get down it

If you have a thick cable it is easier for the electric to get down it.

This is due to resistance. The larger the Cross Sectional Area of the cable, the lower its resistance per meter, and therefor the less millivolts per amp per meter it will lose.