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Posted By Topic: Single phase off a 3 phase transformer, problem?

Sep 30 2018 21:24

Hi, can somebody please help?
The electrician run 2 single core 185mm aluminium cables from pole to house. 170m long, 40 amp load.
Now he says he can\'t connect it because the transformer is 3 phase.
I don\'t understand why single phase cannot be connected to a 3 phase transformer.
Is it because I\'ll be the only user connected and transformer will be \"unbalanced\"?
All appliances in the house will be single phase.

Sep 30 2018 22:39

Find another electrician. This one appears to be an idiot.

Oct 01 2018 09:01

It may be a network issue, still the electricians fault. If you are rural some networks insist that you run your mains 3 phase to minimise balancing on their side.

I have run into this before with a electrical engineer who ran his own 2x ali mains in called us in to connect and Powerco would not accept it. The mains had to be re-run, I think we ended up running in 2x more ali conductors.


Oct 01 2018 09:44

Sparky\'s cock-up; so sparky\'s job to fix it. At no cost to you (unless you were the one who specified the mains be single-phase).

Oct 01 2018 13:37

Thank you all for your replies.
I didn\'t ask the electrician to run single phase. He asked me what I needed and I told him \"power for a normal residential house\", as I\'m not an electrician and there was no way for me to know what\'s possible and what\'s not.
I talked to UNISON, who owns the network and transformer. They said the transformer is small and I have to run 3 phase to the house and split the load between phases at the switchboard.
Thanks again for your help. At least I know that it wasn\'t my fault.

Oct 01 2018 13:50

Something is not right here , you say your load is 40Amps , Most single phase residential connection are going to have a 60 amp fuse anyway .
I would be going back to the network to find out exactly whats going on ,Maybe they need to look at upgrading the Transformer !
How many other connections are coming off it already ?


Oct 01 2018 14:34

I asked UNISON if they would change the transformer. They say the transformer will allow for 2 houses to be connected to it and that they will eventually upgrade it if there are more than two connections, which is unlikely at this stage.
I\'ll be the first to be connect to it.

It\'s a rural area and, as mrsparky said, some networks require rural properties to have 3 phase. I think this is the case here. It may just be UNISON policy, but I\'ve came across something similar when reading other posts on internet

Something has not been right from the beginning. It\'s hard when you are not an expert and the people you are dealing with try to cover their backs.

It\'s good that there is a forum like this where you can get independent advice.



Oct 02 2018 07:38

My local network frowned upon three phase being put into my house - fully balanced as I spent a heap of time with a cable clamp getting it so. Their \'concern\' was that of safety in that two adjacent appliances of two separate phases would effectively be 400v apart. But realistically for that to be a risk, there\'d need to be a fault condition on both appliances simultaneously (in which case at least one of the RCDs should have let go).

Oct 02 2018 08:05

medistat notes:

> Their \'concern\' was that of safety in that two adjacent appliances of two separate
> phases would effectively be 400v apart. But realistically for that to be a risk,
> there\'d need to be a fault condition on both appliances simultaneously...

And an undetected or unprotected double fault at that.

In the UK there used to be the \"six foot rule\", which was that sockets on different phases needed to be a minimum of six foot apart, and there be a 415V risk warning label affixed adjacent.

This rule was scrapped decades ago, as somehow the light eventually dawned on the regulatory bodies that the chances of someone actually getting a 415V shock was vanishingly small.

Oct 02 2018 08:10

No-one ever accused a network of being entirely logical.


Oct 02 2018 11:04

There seems to be a fundamental problem with the order of events here. Did he need to use that particular cable because he had it to get \'rid\' of? I\'d be keen to see the basis used in selecting that cable and the best value proposition for the customer.

Oct 06 2018 00:00

For what it\'s worth, GenCalc says 185 Al, single phase, at 170m is good for about 90A.

Definitely sounds like trying to get rid of unwanted stock.

Nov 27 2018 22:35

...Thread dredge.

Tried to do this in reverse for a rural customer that was paying for 3ph when they only needed 1ph.

Inspector said that because of the load balancing it HAD to be 3ph supply.


Nov 27 2018 23:08

Must be different in different parts of the country. I live in the Waikato and have a 3 phase supply. I looked at going to single and the supply charge was the same.

Dec 28 2018 11:26

The powerco mandate the phasing. Most properties in my area are 3-phase, whether you like it or not. Important to get a permit to connect before any works start with the powerco, which will outline phasing and maximum supply current. I just ran in 175m of 185mm 4-core AL sector cable for a cabin which has predominantly gas appliances. I derated the supply to 50A per phase with a main meter station at the boundary... always handy to have power at the gate IMO. Electricians need to get the facts from the powerco before initiating works. I once had a woolshed down the end of a long shingle road which the new owners were going to convert to a house. The only existing supply fed a farm water pump and the woolshed had been long disconnected. There was a 32A pole fuse insitu. Upon submitting a permit, the powerco said that the pole fuse would need to be derated to 10A. If the owner wanted more, they'd need to pay to upgrade around 7kms of lines to increase capacity. Long story short, they sold up and moved on. Pays to do your homework first.