120 V 3 phase delta

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
So I have an application for 120 V 3 phase delta power. I know it is a little unusual and I figure I will have to use three single phase transformers banked together because probably no one makes a 3 phase unit like this, at least off the shelf.

So I was thinking about this. A nine kVA 3 phase transformer would be 10.8 Amps primary current, and the primary OCPD could be 250% of that number.

My first thought was the three transformer bank could use the same arrangement for OCPD.

However, I thought about it some more, and came to the conclusion that one single phase transformer is rated at 6.25 A primary current, so even though it is wired as a three phase bank, it would need to have OCPD as if it is a single phase transformer.

Any comments.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Does article 450 still contain:

450.3 Overcurrent Protection.
Overcurrent protection of transformers shall comply with 450.3(A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers operating as a unit.
(sorry, don't have a current code book here)

-Jon
 
120-volt three phase power used to be popular in aircraft. (maybe it still is?) But it was also 400 Hz.
Delta transformer secondaries have fallen out of favor because a little bit of harmonic current or phase imbalance can result in circulating currents. But no configuration that creates 120Y69 springs immediately to mind.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Does article 450 still contain:

450.3 Overcurrent Protection.
Overcurrent protection of transformers shall comply with 450.3(A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers operating as a unit.

-Jon
Yes, the 2020 code has the same statement.
 
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