What wire to power a little workshop,

Handyman1

Hi Folks

I just build a little workshop at the end of my yard and need to run some power into it from my house,

Would like to be able to use a little Mig welder on small jobs, also have a few wood working machines such as Chop saw / table saw and belt saw to fit in,

I will be the only one using the tools So only one of the above tools + Small shop Vacuum and lighting will be drawing power at one time, I need to run the wire 40 meters back to the fuse board in the house,

Been told no matter what wire I use A 40amp fuse will need to be fitted.. Does this sound about right ?

I want to run the Cable / wire in before get a sparky to wire it up for me.

In anticipation of your kind help I thank you's :cool:

What size wire should I use ?

Steve

With my enginineer hat on I would say 3x10sq SWA... 6sq would probably do with a 32A MCB at the fuse board.

Ask your spark, he'll be the one writing the cert.

2011

Steve wrote: »

With my enginineer hat on I would say 3x10sq SWA... 6sq would probably do with a 32A MCB at the fuse board.

My engineer's hat is bigger than yours :D:P

The resistance per meter per core of a 10 mm sq. SWA is about 1.83 mΩ (with a relatively high ambient temperature of 20 DegC).

See data sheet

The conductor length (phase & neutral) = 40 x 2 = 80m
Therefore the resistance = 80 x 0.00183 =0.1464 Ω

So if the current was at it's theoretical maximum of 40 A the volt drop on the cable = 40 x 0.1464 = 5.856 V

This is about 2.5% of the nominal voltage (230 V), so from a volt drop and current carrying capacity perspective this looks like a good choice.
Max allowable volt drop from cut out to furthest point in a circuit = 5% of nominal (which is 11.5 V).

A few points to note:

● The OP is taking this supply from the main distribution board. There will be a volt drop to this point (in the cable supplying the main board) that needs to be taken into account.

● The volts drop must be calculated to the furthest point on the worst case final circuit (fed from the sub distribution board). In other words the volt drop is only partly made up by the 40m SWA cable supplying the sub distribution board in the shed.

● It must be determined that the final installation (including final circuits) has sufficiently low earth fault loop impedance to ensure that the required disconnection times are met.

Handyman1

Thanks for the replies guy's.

So am I safe to go with 2.5mm swa 3 core or should I go with the 4.0mm,

on very rare occasions I will be running a wire welder that has a 16amp plug,

Steve

Handyman1 wrote: »

Thanks for the replies guy's.

So am I safe to go with 2.5mm swa 3 core or should I go with the 4.0mm,

on very rare occasions I will be running a wire welder that has a 16amp plug,

Go with 6sq minimum. The price difference is minimal, and as I said, ask the sparks who will have to cert it to be sure.

2.5 / 4.0 is too small.

What would a fire cost you? More than the cost of the cable I'll bet.

2011

Handyman1 wrote: »

Thanks for the replies guy's.

So am I safe to go with 2.5mm swa 3 core or should I go with the 4.0mm,

on very rare occasions I will be running a wire welder that has a 16amp plug,

+1 to Steve's points above.

In addition neither a 2.5 nor a 4 sq. cable can be protected / supplied from a 40A MCB. Most likely 20 and 25A MCBs respectively could be used. So what does this mean? Essentially less power can be drawn from the shed.

Handyman1

Steve wrote: »

Go with 6sq minimum. The price difference is minimal, and as I said, ask the sparks who will have to cert it to be sure.

2.5 / 4.0 is too small.

What would a fire cost you? More than the cost of the cable I'll bet.

Thank you Steve,

I will get it sorted. Thank you both again for your help :cool: