Current calculation for a high leg delta connection

[S.herman]

It is generally accepted that a single phase load should not be connected between the neutral conductor and the high leg of a delta transformer connection with one transformer center tapped to provide 120 volts to neutral. Does anyone know how to calculate the current if, for some reason a load should be connected between the high leg and neutral? Assume, for example, that the hight leg is phase C. If Phase A to Neutral is 50 amps, phase B to neutral is 80 amps, and phase C to neutral is 90 amps, what would be the total neutral current?

 

[kwired]

It is generally accepted that a single phase load should not be connected between the neutral conductor and the high leg of a delta transformer connection with one transformer center tapped to provide 120 volts to neutral. Does anyone know how to calculate the current if, for some reason a load should be connected between the high leg and neutral? Assume, for example, that the hight leg is phase C. If Phase A to Neutral is 50 amps, phase B to neutral is 80 amps, and phase C to neutral is 90 amps, what would be the total neutral current?

It, isn't generally accepted that you should connect a load in this manner. For one thing if using circuit breakers on a 120/240 delta you won't find single pole breakers rated for 208 or 240 volts, unless you went into 277/480 series panels/breakers.

Outside of that - current flow I would assume depends on if it is open or closed delta, at least how it flows within the windings.

Closed delta the neutral off a high leg load would split two ways to get back to the high leg output of the transformer.

Open delta it would all have to flow through the closed end.

The other two legs neutral currents do cancel one another just like a center tapped single phase system.

I don't know the math needed to figure the net current on a common conductor to all lines though. I can see it possibly being different for open vs closed delta though. Never really had a formal advanced math class that would help understand this better.

 

[Phil Corso]

It is generally accepted that a single phase load should not be connected between the neutral conductor and the high leg of a delta transformer connection with one transformer center tapped to provide 120 volts to neutral. Does anyone know how to calculate the current if, for some reason a load should be connected between the high leg and neutral? Assume, for example, that the high leg is phase C. If Phase A to Neutral is 50 amps, phase B to neutral is 80 amps, and phase C to neutral is 90 amps, what would be the total neutral current?

S. Herman...
Your ampere distribution is unclear to me! Are you asking, "what impact does a 208V load, connected between the 'N' terminal and the Hi-Leg terminal 'C', have on the currents carried in all three legs"?

Regards, Phil Corso

 

[GoldDigger]

S. Herman...
Your ampere distribution is unclear to me! Are you asking, "what impact does a 208V load, connected between the 'N' terminal and the Hi-Leg terminal 'C', have on the currents carried in all three legs"?

Regards, Phil Corso

More significant, IMHO, is the effect on the current in the windings, including the half-winding to neutral in the case of an open delta. The current in the other two legs is not going to change visibly since the oddball return path currents through the windings to the center tap do not appear on the phase/leg wires outside the transformer.

 

[Tony S]

Work out the voltage first √V_bc²-V_bn²

Base everything on that.

The derived current in each leg can only add to the existing current. The problem comes with the current in B→N as it could exceed the windings rating.

 

[Tony S]

BTW neutral in this case will be 80-50/2=15A

 

[mivey]

It is generally accepted that a single phase load should not be connected between the neutral conductor and the high leg of a delta transformer connection with one transformer center tapped to provide 120 volts to neutral. Does anyone know how to calculate the current if, for some reason a load should be connected between the high leg and neutral? Assume, for example, that the hight leg is phase C. If Phase A to Neutral is 50 amps, phase B to neutral is 80 amps, and phase C to neutral is 90 amps, what would be the total neutral current?

You need phase angles but assuming unity power factor and uniform, balanced voltages and L-N loads you get 94.87A:

50<0d + 80<180d + 90<90d = 94.87<108.43d