Measuring MWBC neutral current 240/120 Delta

tortuga

Senior Member
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field supervisor
If I have 2 equal banks of 120V lights connected on A and C phase of a 3 PH 240/120 Delta hi-leg system I should get 0 amps measured on the neutral correct?
What about an open delta?
Is there any phase angle magic that would make some unbalanced current perhaps with fluorescent ballasts or LED power supplies?
Thanks in advance.
 

Hv&Lv

Senior Member
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Engineer/Technician
An open delta is a single phase transformer with an extra pot that has one coil in parallel, the other coil ungrounded which is the high leg.
the voltage phasor looks like a ”T”

your lights are on opposing coils 180 degrees apart.

it’s only when all three currents are flowing in a three phase something that the currents shift to 120 apart, assuming no single phase loads are shifting them further...
 

LarryFine

Master Electrician Electric Contractor Richmond VA
If I have 2 equal banks of 120V lights connected on A and C phase of a 3 PH 240/120 Delta hi-leg system I should get 0 amps measured on the neutral correct?
What about an open delta?
Is there any phase angle magic that would make some unbalanced current perhaps with fluorescent ballasts or LED power supplies?
Thanks in advance.
The A-N-C section of a 3-phase delta system is identical to a 1-phase system. I-dentical!

In fact, the open/high-leg delta began as a modification of existing 1-phase services.

They function as two independent superimposed systems and can be calculated that way.

Much more info available on request.
 

GoldDigger

Moderator
Staff member
Open Delta and high leg Delta are two different characteristics.
A two-out-of-three wye circuit (120/208) will always have current on the neutral.
Calling it 120/240 Delta pretty much makes it high leg, but that can be closed (three pot) or open (two pot) delta.
 

tortuga

Senior Member
Location
(44.057116, -123.103394)
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field supervisor
Thanks all, Its a huge old industrial building with lots of combined circuits in junction boxes, scant labeling and changing color codes.
We were there to replace the lighting with LED.
I ended up spending a few extra hours sorting out neutrals. I was still getting some odd measurements on a what should be a balanced lighting system. However was able to get it from 3 circuits on one neutral to two so at least the neutral is properly protected.
 

GoldDigger

Moderator
Staff member
same characteristics, one is able to carry more three phase loads
What I am trying to say is that in the broader scope, not limited only to systems having a 120/240 component, you can have high leg deltas (center tap grounded) and non-high leg deltas (either wye grounded, corner grounded or ungrounded) and that is one binary choice. Within that same larger population you can also have open deltas and closed deltas, and that is an independent binary choice to be made.
 

Hv&Lv

Senior Member
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Engineer/Technician
Oh.
Corner grounded... you run into that a lot?? Doesn’t it play havoc with gfi’s?
 

GoldDigger

Moderator
Staff member
Actually, as long as the electronics of the GFCI can properly derive power from the connected wires, and as long as the grounded conductor (corner) is kept separate from the grounding conductor (EGC) as with any other voltage scheme, then a GFCI which senses the sum of the currents through the two ungrounded conductors and one grounded conductor should work just fine. And if you only have single phase loads from one of the hot conductors to the grounded conductor (not a neutral) then single phase two wire GFCIs should work just fine.
 

Hv&Lv

Senior Member
Location
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Occupation
Engineer/Technician
We stopped building them some time back from complaints about gfi’s not working. One in particular was an asphalt plant with a three phase mdp gfi.
now we just center ground them
 
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