Thread: 3 phase power to my Shed!

Hi simonl,

Starting with an upgrade to your meter box is a good idea. From reading your thread it seems like the reticulation in the street is underground and your meter box is not far from the pit. On this basis your mains to the meter box don't have to be huge. 4 x 16mm Copper may do the job. It will depend on the electrical load in house. One phase will end up with shed load plus the house and it has to be big enough for the total. The other two phases will be the same size. Given the short length of your mains 4 x 25mm copper would probably not cost much more in the big scheme of things. The thing to watch will be how much space you have in the water pipe under the driveway as 25mm mains may require a bigger conduit than will fit with the water mains in place. BTW underground electrical conduit comes in 32, 40 and 50mm dia, so you may get an idea of how much space you have from that. 16mm copper is usually, but not always, rated at around 80 amps per phase. Aluminium conductors somewhat less.

With regards to the sub mains from the meter box to the shed, ask your sparky if he is going to run an earth wire to the shed or will he drive an earth stake at the shed and create a MEN point there. The later is probably the cheaper option. Disclaimer. I don't know if you can do that in your state or not.

The size cable you run will be determined by voltage drop and that depends on your proposed load. How the cables are run will affect the amount of load they can carry without overheating. The more accurately you calculate the load you are going to connect, the easier it will be for the sparky to calculate the cable size you will need. Given your shed is 110m from the meter box (plus 5 or 6 meters to get down to the underground conduit and back up again) voltage drop will be your enemy. Make sure your sparky knows the ins and outs of AS3008 which is the gospel on cable ratings. In my experience, a lot of sparkys are lacking in this area.

Cheers

The Beryl Bloke

Hi Beryl,

Yes the sparky told me that he intends to run 4 core + earth. I asked if that was necessary and he said yes. So that's that sorted. He also said 16mm cable will be ample for my intended use, allowing 120M in total length. Going to use 50mm conduit, although when I did the slab in my shed I cast 32mm conduit up through the slab. I have test fitted 16mm 4 core + earth and it just fits, hoping that I will be able to use the existing 32mm conduit and somehow find a 32mm to 50mm adapter. The pipe under the driveway is 100mm sewer and it has 32mm blue line class 12 poly pipe running through so there should be ample room.

When I started this thread, I was only looking into the possibility of 3 phase, now after doing the research it's looking more and more likely that I will pursue it.

One thing I forgot to ask him, how does earth leakage protection work with 3 phase machinery? Or do you only get that luxury with single phase stuff that uses the neutral return? Given that the concept works by comparing the current flow in the active to the current flow back through the neutral, and any discrepancy is attributed to an earth leakage somewhere. How would a system determine an earth leakage in a 3 phase system (for example an induction motor), where if all phases are equal, no current flows through the neutral?

Simon

Hi simonl,

Three phase earth leakage works the same as single phase. It just looks for current balance in the circuit, if it is not balanced the circuit breaker assumes the imbalance is current flow to earth and will trip. It doesn't matter if the neutral is in use or not. Quite a few three phase machine tools have some single phase load for the lights, DRO, coolant pumps or control circuitry although the later two could also be 415 volt items. Providing the current going into the machine on the active conductors equals the current coming back out on the active conductors plus any neutral current, all will be well with your RCD.

When earth leakage started to get popular (legislated for power circuits perhaps?) the circuit breakers were not cheap and one often used trick was to use a single three phase breaker with 2 single phase power circuits through it. I don't remember how that was wired up.

There are adapters available to go up or down one conduit size and you can use a couple of these together to go as many sizes as you like.

Cheers

The Beryl Bloke

Originally Posted by Theberylbloke

Hi simonl,

Three phase earth leakage works the same as single phase. It just looks for current balance in the circuit, if it is not balanced the circuit breaker assumes the imbalance is current flow to earth and will trip. It doesn't matter if the neutral is in use or not. Quite a few three phase machine tools have some single phase load for the lights, DRO, coolant pumps or control circuitry although the later two could also be 415 volt items. Providing the current going into the machine on the active conductors equals the current coming back out on the active conductors plus any neutral current, all will be well with your RCD.

When earth leakage started to get popular (legislated for power circuits perhaps?) the circuit breakers were not cheap and one often used trick was to use a single three phase breaker with 2 single phase power circuits through it. I don't remember how that was wired up.

There are adapters available to go up or down one conduit size and you can use a couple of these together to go as many sizes as you like.

Cheers

The Beryl Bloke

Once again Beryl, Thanks!

Simon

Simon,

Just a heads up - 16mm 4 core + Earth requires a minimum of 50mm conduit. 25mm 4 core + Earth requires 63mm.

Whilst you can fit either sized cables in smaller conduit, it wouldn't pass code.

Hope this helps.

Cheers
Jon

Originally Posted by Jon_77

Simon,

Just a heads up - 16mm 4 core + Earth requires a minimum of 50mm conduit. 25mm 4 core + Earth requires 63mm.

Whilst you can fit either sized cables in smaller conduit, it wouldn't pass code.

Hope this helps.

Cheers
Jon

Thanks Jon, my sparky did not mention this. Looks like I may have 32mm conduit coming up through my slab for zero use! I haven't had time to scratch myself last couple of weeks. Now I'm away for 4 days on a course. Hopefully September will quieten down a bit work and home wise and I can follow this up with some actual work.

Simon