Taking a single phase suply from a 3phase transformer.....

[Crosswire]

Have got a Three phase transformer wired in star on the 7th floor of a part built tower block and the builder wants another tranny on the 8th floor to supply the works on that floor.

There is already a single phase tranny on the 8th.. (not yet wired up)

Is it possible to take a single phase supply for the 8th floor single phase tranny from the three phase tranny on the 7th, using one of the phases and the star point as a neutral, or would this trip the RCD?

I really don't want to carry a 3 phase tranny up 16 flights of stairs.........

 

[cossie400]

Just thinking the same . . use 230 v single-phase taken from 7th floor (one line plus neutral) to feed the 8th floor 230/110 v single-phase transformer.

it seems to me (possibly due to limited info from the OP) that some are over complicating this.
The way I read the OP was that there is some sort of office refurb going on and there is an existing site transformer being used by trades and basically all he wants is an additional one on the floor above.
It does seem odd that it has an RCD protecting it but I have been on sites where trannys have been daisy chained, failing that just run another supply up one floor like we've said!

 

[Darkwood]

I read it as tony does too, its a building site... as we have been informed , if their was available DB's then the OP wouldn't be asking the initial question
The OP has explained the secondary is 110v on both TX's and in the case of the 3ph TX on Floor 7 it its primary will be 400v (No Neutral!) please explain where your going to get this N from Cossie.... i originally put this to the OP who clearly states i make good points ... if i was wrong in my thinking of the OP set-up he would have corrected me back then.

Far from over complicating things here we are trying to help the OP avoid a potentially dangerous set-up . ....but happy to see your oblivious to this if not a little scary, yes agree the info is limited but i wouldn't just say yes sounds ok without high-lighting the dangers and possible issues. This way the OP has all the info he needs to which he can make a decision .

If their was an option at to plug into a 230v secondary output on the 3ph TX (Noting the OP hasn't said this is the case) i would still way up the whether the TX was up to it long before i plugged it in this way you dont have to fob you way out of situations when the foreman complains the power keeps tripping. If i was to set up a temp' system on any site i was on it wouldn't bode well if the mcb's kept nuisance tripping or other related power problems nor would it give good impression.

I make my original assumptions to the fact that the OP states the TX's are 10Kva rated so i would suggest this is a large building site if they have a 3ph one fo the 7th floor hence caution to loading and were it doubtfully be at all possible to daisy chain as you suggest and given the 1ph TX is also 10Kw are you aware of the primary inrush current could be in excess of 600amps a mean feat of any mcb rated under 63(d) not to trip on power up..... so makes me wonder how the 32amp protecting the 3ph Tx will react?

 

[Crosswire]

OK everyone calm down.

It is a bog standard tranny on a bog standard site. There is obviously a DB feeding it but it is on the ground floor and would involve a 50m run of armoured to feed one tranny, which would obviously cost money.

There are dozens of 3 phase trannies on this site and all are fed off type C 32amp mcb's, and have been working happily for many months with no nuisance tripping

I was obviously thinking about daisy chaining the trannies on the primary side, but have decided against it , and found a single phase tranny on the 7th to come off. This was done today and everything is fine.

I too have worked on sites where there are no RCD's but this one does have one, as I believe temporary installations are required to, when TT'd.

Anyway thanks for contributing in a positive way guys, as I am always keen to listen to people with constructive criticism .

 

[Darkwood]

At least your sorted and did say 25amp (d) for 10Kva so a 32(c) is understandable in that it doesn't or rarely trips.... when we have limited info we have to assume alot hence the threads can get deeper lol.... its the norm on here

 

[Pete999]

Yep as long as the new SP is protected properly