3phase inverter on domestic supplly

ga8ry · Sep 15, 2015

A friend of mine is building a workshop out his back. One of the machines is 3-phase with a 7.5ka load. It's a 40m run from his fuse board in the house to the one I want to put in workshop. There is only going to be a 2 twin 5ft flossys, 6 double sockets(no big load just hand tools, radio etc.....) and the invertor that he ordered with the machine.

I I was going to put a 80a 30ma rcd at the house escos run 16mm radial out to the board in the workshop with another rcd and a 6, 32, and 40a(type c) breakers to run the 3 circuits.

Does this sound alright to you's? Ive never sized cable runs etc... Is the 16mm mains sound ok and the rcd and mcb types and rating seem ok??

really appreciate any help

cprfenom · Sep 15, 2015

I guess you are using a time delay rcd at the main board? And if using SWA running surfaced, would you really need an RCD at the main board?

In terms of length, you will need to confirm VD over length to see if 16mm will take the load over the length. You need to make sure that you look at the total load too, and not just for the 3 phase inverter.

ga8ry · Sep 15, 2015

Was going to use swa, would i be ok putting the swa tails straight into the main escos and just having any rcd at the workshop board. Also would it better to have say a 63a dp switch with a time delay rcbo for the the inverter and 2 other rcbos for the other circuits

Spoon · Sep 15, 2015

As above. Why the RCD at the main board. Use armoured cable and just put a RCD in the workshop. Volt drop is ok over 40 meters. 8.96V 2.24%. 16mm? What is the installation method?

davesparks · Sep 15, 2015

ga8ry said:
Was going to use swa, would i be ok putting the swa tails straight into the main escos and just having any rcd at the workshop board. Also would it better to have say a 63a dp switch with a time delay rcbo for the the inverter and 2 other rcbos for the other circuits

No you can't just connect the SWA to a set of isco blocks!
That you you have even suggested this makes me think the best advice is that you leave well alone and hire an electrician to carry out the work safely.

Richard Burns · Sep 15, 2015

You need to protect the SWA from overcurrent so you need a circuit breaker or preferably a fuse at the start of the run.
In the far end you will need 30mA RCD protection, at least for the sockets, as well as the local overcurrent protection for each circuit.
Separating the RCD protection would be a good idea.

Darkwood · Sep 15, 2015

Hi ga8ry..

Firstly I would be looking at the Electrical requirements of an inverter for a 7.5Kw motor 1ph to 3ph as you may be surprised about the required fusing.

As above - you shouldn't have rcd's in series in this case and you should have front end overload protection of your garage supply.

You say you have never really done cable calcs - now don't get me wrong here when I ask because you have a profile expressing a time served Electrician, I'm going to refrain from making assumptions here so how is it a time served Electrician hasn't used cable calcs before and noting the points made in reply doesn't seem to be aware of some of the more basic of regulations?

Lastly to add to the above regarding the lathe.. a 7.5Kw motor has a full load of approx 15amps per phase, if you derive this through a inverter 1ph to 3ph are you getting 3ph 200/230v output and is your motor able to be reconfigured for 230v 3ph?.. if so can the supply handle the 40-50amp demand that the lathe may pull if loading motor up full.