480VAC Question

No, you do not need to pull a neutral. When you ground one side of the control transformer secondary, that grounded secondary side becomes your control circuit neutral.

On a 480 Volt, 3 Phase, Ungrounded System, phase to phase or phase to ground is 480 volts.

All you said is OK except "Phase to Ground is 480V":

If the system is Ungrounded, you have no Voltage to ground!

Conrol Circuit: If the transformer is of Isolating type(no links to primary like in an Auto transformer). I say that because the PO seems very uncompetent to be handling this issue. Maybe he will start learning or take a course.

Amendment to the foregoing: For 480, phase to ground is 480/√3 ≈ 277 volts.

Amendment to your amendment: He said 480V ungrounded systems, i.e. delta. 277V is the phase to Neutral voltage on a 4 wire Wye (Star) system, and assuming the neutral is grounded, it would be the phase to ground voltage as well.

The control transformer shown in this schematic drawing is very common to the low voltage motor starters used in North America for the starter control and motor space heater supply. The primary of the control transformer is 480 V (line to line) and the secondary is 120 V. The transformer doesn't care about whether its primary supply is line to line or line to neutral. For the low voltage motor starter, the motor space heater (if the motor is => 15 HP) is supplied from the same control transformer. Most cases, this control transformer is CVT (constant voltage transformer – a type of ferroressonant transformer) for maintaining the constant control voltage supply so that the starter doesn't malfunction due to the voltage dip caused by starting of other motors in the vicinity.

Also see these recent postings related to this:

http://cr4.globalspec.com/thread/53729 (the heater mentioned in this question seems to be the motor space heater)

http://cr4.globalspec.com/thread/53686

- MS

If nothing in the system is grounded, couldn't the voltage to ground be arbitrary, so long as the phase differences remained in relation to each other?

Yes, but...

If the system is not grounded, it is not automatically a "delta" system. In fact, in many instances where a "neutral" is not required by the load, the transformer will be a delta/wye configuration and the star point of the secondary will not be brought out of the transformer nor will it be grounded.

In this case, the transformer may be considered as an ungrounded delta system, and under the Canadian code, would require ground fault indication equipment be installed on the secondary. I assume the NEC is similar in this regard?

Phase to phase is 480V (not 208V)

You don't need a Neutral for the transformer or else (The mains Neutral is not needed here)

The secondary is isolated from Primary in your circuit. Therefore, one of the secondary outputs can be grounded and called Neutral if the equipment controlled and supplied by it requires that (for actuating the protection devices mainly).

Finally, you seem to lack a lot on the subject: either consult a local competent electrical engineer if you are doing any repairs or project, OR get a course inthe subject.

My Apologies -

Clarification to my post:

480 Volt - 3 Phase - 3 WIRE - UNGROUNDED - DELTA - System

will result in volt meter readings as follows:

Phase to Phase = 480 Volts

Phase to Ground = 480 Volts

Grounding of one of the control transformer secondary conductors was recommended for proper operation of secondary interface devices/systems.

"Ungrounded Delta, 3 wires system"!

How can you get 480V between Phase and Ground?

Phase -Phase 480V is OK but not phase-ground 480V ==> means one of the phases has a ground leak or ?? peculiar statement/situation....You have a fault somwhere??

KJK/USA,

You persist in saying: Phase to Ground = 480 Volts, for an ungrounded three-phase delta supply. The actual voltage from any one phase wire to ground is not inherently determined. In practice it can vary from 0 to 480 volts and can also be higher. The actual voltage will be determined by the amount and balance of current flow, capacitance between the conductors and ground, internal resistance of the meter doing the measurement, mutual inductance with nearby wires, and other unpredictable factors.

If all circuit factors are equal and the meter's internal resistance is quite high compared to the capacitive charging of the conductors, then the voltage phase to ground will tend towards the middle (nominally 277 volts), which is the same as you would see in a properly grounded wye system. Since these factors are seldom equal, then the voltage you measure is unpredictable (in advance), and can easily be different from one moment to another (because of changes in the connected loads, etc.).

Thanks--John M.

You don't need to pull a neutral based on the diagram you've shown. As a matter of fact, the two feeders were connected before the connections and from the line side of the delta-wye transformer given in the diagram and not after the transformer. There would be in no way the two loads would be in relation to each other and so do not pull the neutral wire of this transformer and connect it to the two phases given. Electrical disturbance shall arise if a reversal would be implemented.

Regards,

Odie