Single Phase transformer

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john37

Senior Member
I have two mammogram machines that are rated at 35 amps, 208V, 1 phase each. I need to pull power from an existing 480/277V 3 phase panel. (The existing 208/120V 3 phase panel is full) I was thinking on providing a 30KVA 480-208/120V single phase transformer to power the two 35 amp, 208V 1 phase mammogram machines. Sorry if this is a dumb question but would I provide a 3P50A breaker on the primary side if it's a single phase transformer?
I need to split the secondary side to feed each mammogram machine that is located in two seperate rooms. I was planning on providing a gutter in the electrical room where I am putting the single phase transformer and then providing two 2P35A enclosed breakers and then run the feeder to the 2P35A shunt breaker (required by the manufacturer) which will be located in each mammo room. Does this sound right?
 

Smart $

Esteemed Member
Location
Ohio
... Does this sound right?
Gonna need a little tweaking ;)

Problem areas:
1) To feed a line-to-line primary of a single phase transformer requires only a 2-pole breaker... but,
2) There is no such thing as a 208/120 single phase secondary...
3) The gutter sounds ok, but you will have to run separate secondary conductors to each 2P35 enclosed breakers while meeting the other requirements of secondary tap rules for distance, conductor size...
 

BJ Conner

Senior Member
Location
97006
208 power

208 power

John37
Interesting job.
The smallest 3 phase transformer you can get to do the job is a 15KVA. The two 35 amp breakers are the secondarys for the transformer.

If I were the engineer I would specify a 30 KVA transformer and a 100 amp panel with a 100 amp MCB. Specify 2 -2 pole 35 amp breakers for your machines 38 single pole 20 amp breakers.

Reasons-1. Your going to hang a transformer in any case. Teh 15 KVA is marginal for the job. 2.) A panel is faster to hang than 2 breakers and a gutter. 3.) Check your prices. IME a populated panel is cheaper to buy and install Than two 35 amp breakers in a nema 1 enclosures.

Lastly there may be something they didn't tell you about (usually because they don't know). The machine may have periffian devices (like printers) etc.
 

john37

Senior Member
Thank you all for your replies. Sorry for the "208/120V single phase" I had a brain fart.

BJ Connor. The reason why I thought of going the gutter and enclosed breaker route is because this is only going to be a temporary installation until they build out the permanent space. I thought it may be easier this way but you have a great point.
 

cadpoint

Senior Member
Location
Durham, NC
John37
Interesting job....

The smallest 3 phase transformer you can get to do the job is a 15KVA. The two 35 amp breakers are the secondarys for the transformer.
....
If I were the engineer I would specify a 30 KVA transformer and a 100 amp panel with a 100 amp MCB. Specify 2 -2 pole 35 amp breakers for your machines 38 single pole 20 amp breakers..



Do they make a 42 space - 100 amp panel?
I'm lost with the gutter? Better yet, I'm a box of rocks trying to play along at home! I guess as usuall I'm missing something... :grin:

Is this a space limited application, or your going to tap off in this gutter; frankly any gutter means more work to me and that includes all sizes. How can this be done with a with 25' rule, and under very limited conditions, nothing ever seems that "close".

Can't you use one conduit all the wire(upsized) in line maybe use a junction box, split the services once out side elect. room at the closet point?

Or better yet, existing panel, a breaker for the new transformer, a conduit out of room, to new panel, split circuits, disconnects at machines?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I would suggest double checking the detailed electrical requirements of the machines. If they have X-ray tubes in them, then they _may_ need momentary pulses of much higher than rated current, and might have requirements for minimum supply impedance (maximum voltage drop, etc.)

-Jon
 
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