Objectionable Neutral Current on Wye Secondary

[Mike Scarry]

I have a 208:480 delta:delta transformer, corner grounded, so this question doesn't really pertain to my current installation, but in going through Art. 250 I wondered: If I had a wye secondary with unbalanced phase loads, wouldn't objectionable neutral current flow through the XF case and on the EGC from the supply? Or is the idea that the neutral current is supposed to flow on the GEC for the separately derived system (the secondary) to the Grounding electrode? This doesn't seem right either, because then you would possibly have objectionable current on exposed structural steel, for example? Any insight as to what I am missing here would be greatly appreciated.

 

[charlie b]

What you are missing is simply that current returns to its source. Current leaves the secondary of a transformer on one phase, passes through a load, and returns to the transformer secondary on the other phases or on the neutral. At that point it has already returned to its source, and thus it has nowhere else to go. The driving force that set the current into motion in the first place was a voltage generated in the secondary windings. There is no driving force (i.e., no source of voltage) capable of sending current from the secondary WYE center point, along the GEC, to a grounding electrode, or along the primary feeder?s EGC back to the panel feeding the transformer.

Welcome to the forum.

 

[Mike Scarry]

Or is all current flow in the windings only, even when loads are unbalanced?

 

[Mike Scarry]

Charlie,
That makes perfect sense. Thank you for your prompt reply and kind welcome.

-Mike

 

[charlie b]

If the loads are unbalanced, then some of the current passing through the load will return to the secondary windings via the neutral wire. At any point in time, there will be current in all three phases and in the neutral, some of which is heading towards the load and some of which is heading back to the transformer. The sum of the four currents (A, B, C, N) is zero at all times.