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.........

 

[DNS1]

I'm not expert on three phase, but I don't see anything wrong with what you're suggesting

 

[Darkwood]

Would be nice if you expressed voltages here input and output....

What protection is there, what load are we talking is this a building site ???????

 

[Crosswire]

Would be nice if you expressed voltages here input and output....

What protection is there, what load are we talking is this a building site ???????

1) Input voltage is standard 415v (400v) nominal , output voltage is 110v

2) Circuit protected by 3phase 32A MCB, whole site protected by 100mA RCD on the generator

3) Trannies are 10KvA, and will be used to supply temporary lighting, and outlets for 110v volt equipment

and yes, this is a building site

 

[Darkwood]

Still Ambiguous - .... if you were to piggy back second transformer of the supply to the first as i see in your OP then the inrush on start-up would be a big issue if you thinking of taking it off the out-going side ??? then i assume youll have 110v between phases 55v to star point.... you kinda of lost me here unless you have a multi-tapped unit.

The primary current of a 10Kva 3ph transformer is approx 14amp per phase (requiring 25amp-D)
The primary current of a 10Kva 1ph transformer is approx 42amps (requiring 63amp-D mcb)

Either way you would have problems turning the units on as inrush would be cause for concern as well as the demand on the primary side of the 1ph unit.

A separate supply and est using the 3ph transformer is your best option IMHO unless ive mistook your info or you have missed anything out that is relevant.

 

[Crosswire]

Hmmm some good points about start up current there Darkwood, .......but would it trip the RCD?

 

[Darkwood]

Rcd's monitor Earth Leakage which shouldn't be a concern for inductive inrush currents as the PH/N are still balanced throughout the inrush, although its not unheard off with some makes of rcd that the magnetic spikes within the rcd on inrush can create nuisance tripping. Most quality brands out there are not really prone to this as the inrush itself is usually over with within a few miliseconds.

 

[cossie400]

I'd piggy back off of one of the incoming phases to the 7th floor tranny in singles (something like coil end cable) to an enclosure with a 2pole breaker in, bolted to the side of the transformer, then run an appropriately sized 3 core SWA up to your new single phase tranny on the 8th. Done.

 

[Engineer54]

Hmmm some good points about start up current there Darkwood, .......but would it trip the RCD?

I don't think you have caught on to what Darkwood is saying here. How are you going to piggy back off a 415V/110V 3 Phase TX with a single Phase TX 230V/110V??

 

[Darkwood]

I'd piggy back off of one of the incoming phases to the 7th floor tranny in singles (something like coil end cable) to an enclosure with a 2pole breaker in, bolted to the side of the transformer, then run an appropriately sized 3 core SWA up to your new single phase tranny on the 8th. Done.

Did you not read my post on primary side current demands, if you piggy back of the supply side of the 3ph transformer then the supply to that transformer would need to handle 56amp + cable calcs as oppose to 14amps FLC also with a 32amp protective device there would be tripping issues, also will the 3ph supply to the 3ph transformer have a N?

 

[Knobhead]

For gods sake, get someone that knows what they are doing or at least what they are talking about!

 

[cossie400]

Isn't he just on about a temporary site transformer here for **** sake?
Wouldnt it just be easier to take a new single phase supply from the same board thats feeding the 7th floor tranny, up one floor to feed the new one?

 

[Darkwood]

No mention of a DB here at all its a building site with a 3ph Tx rigged up on 7TH floor, if OP had a DB to connect to I assume this thread wouldnt have been created and temp or not the install still has to comply

 

[Tanker222]

Isn't he just on about a temporary site transformer here for **** sake?
Wouldnt it just be easier to take a new single phase supply from the same board thats feeding the 7th floor tranny, up one floor to feed the new one?

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.

 

[Knobhead]

The 3Ph transformer would be ∆Y therefore no neural on the primary input. He referred to using the star point, the only star point is on the secondary side. This star point should be earthed.

To get 230V from the ∆Y transformer the primary neutral would effectively be connected to earth and using the earth as its neutral conductor.

I hope I’m wrong but this has to be one of the most stupid and dangerous ideas suggested on here yet.