480v 3 phase to 480/277v 3 phase

[winnie]

I want to argue about the applicability of 250.2(B)2...but that would really derail this thread. However thinking on the issue I came up with a potential 'gotya' in the current proposed solution.

Say the supply is a corner grounded delta, and the new delta-wye transformer is properly neutral point grounded. ?The voltage between supply conductors could be as high as 757V (going from one of the original ungrounded delta conductors to one of the newly derived wye conductors). The line-line voltage seen by the heaters will be 480V or 277V as intended, so this high 'apparent' shouldn't impact the function of the intentional circuits.

Where this voltage comes into play is for things like insulation: Will the insulation system in the machine and field wiring be happy with this voltage? Are there contactors in the machine that might have trouble with this voltage on adjacent poles? Do conductors land on terminal blocks designed for lower voltages?

If the supply is ungrounded, will a ground fault elsewhere in the plant cause transient damaging voltages in this machine?

Which simply goes to emphasize the importance of iceworm's questions.

-Jon

 

[LarryFine]

Say the supply is a corner grounded delta, and the new delta-wye transformer is properly neutral point grounded. ?The voltage between supply conductors could be as high as 757V (going from one of the original ungrounded delta conductors to one of the newly derived wye conductors).

The voltages between the primary and the secondary are not relevant. It won't matter about the voltage between either of two of the primary conductors and a secondary conductor (you're adding 480 and 277). Only the voltage between conductors and ground matter.

 

[winnie]

The voltages between the primary and the secondary are not relevant. It won't matter about the voltage between either of two of the primary conductors and a secondary conductor (you're adding 480 and 277). Only the voltage between conductors and ground matter.

I disagree. If you have 2 conductors adjacent to each other, then the voltage between conductors matters.

If you have two 'phases' sitting on adjacent terminals in a contactor on terminal block, then the voltage between those 'phases' matters.

-Jon

 

[kwired]

I disagree. If you have 2 conductors adjacent to each other, then the voltage between conductors matters.

If you have two 'phases' sitting on adjacent terminals in a contactor on terminal block, then the voltage between those 'phases' matters.

-Jon

I think the voltage you mentioned may not be right - there is a phase offset between primary and secondary that will have an impact, but I think it can easily be more than 600 volts also. But nothing is more than 600 volts to ground, and I could be wrong on how to look at this but you sort of have 600 volts of insulation plus 600 volts of insulation between two insulated conductors.

 

[LarryFine]

. . . you sort of have 600 volts of insulation plus 600 volts of insulation between two insulated conductors.

Exactly.

 

[Besoeker]

:thumbsup:
As long as:
1. The neutral point is left floating, since there is nothing to reference it to directly, and
2. The loads on the three legs must draw very close to identical current and each matched set of heaters must be switched on and off at the same time. Otherwise the voltage on some elements will be higher or lower than 277.

And are always on at the same time.

 

[winnie]

I think the voltage you mentioned may not be right - there is a phase offset between primary and secondary that will have an impact, but I think it can easily be more than 600 volts also. But nothing is more than 600 volts to ground, and I could be wrong on how to look at this but you sort of have 600 volts of insulation plus 600 volts of insulation between two insulated conductors.

I did the vector math graphically.

Depending on the particular phases in question, for the described situation (480V delta corner grounded and 480/277V wye neutral grounded) the voltages between conductors will be 203V, 277V, 342V, 480V, 663V and 757V. You have 7 conductors to consider (the 3 original, the 3 on the transformer secondary, and the grounded conductor). Lots of combinations. The worst case in this set is 757V

I see your point that the 757V will show up only between conductors, and therefore there will be two layers of 600V insulation. There will never be more than 480V to ground. I believe, however that 300.3(C)(1) requires that the insulation be equal or greater than the circuit voltage, which is the line-line measurement, not the line-ground measurement. (Insulated conductors could be pushed against uninsulated terminals, for example.)

And things like terminals and contactor legs are certainly rated for a maximum voltage between them in addition to a maximum voltage to ground.

-Jon

 

[kwired]

I did the vector math graphically.

Depending on the particular phases in question, for the described situation (480V delta corner grounded and 480/277V wye neutral grounded) the voltages between conductors will be 203V, 277V, 342V, 480V, 663V and 757V. You have 7 conductors to consider (the 3 original, the 3 on the transformer secondary, and the grounded conductor). Lots of combinations. The worst case in this set is 757V

I see your point that the 757V will show up only between conductors, and therefore there will be two layers of 600V insulation. There will never be more than 480V to ground. I believe, however that 300.3(C)(1) requires that the insulation be equal or greater than the circuit voltage, which is the line-line measurement, not the line-ground measurement. (Insulated conductors could be pushed against uninsulated terminals, for example.)

And things like terminals and contactor legs are certainly rated for a maximum voltage between them in addition to a maximum voltage to ground.

-Jon

Good information to chew on, IDK the answer. What is maximum voltage between any two conductors with 208/120 and 480/277 systems involved - that is pretty common. 480 corner ground is certainly not rare enough to say you would never run into it, there is a fair amount of it out there.


I seem to remember a thread one time asking about similar things involving a 831/480 volt wye possibility and whether one could use 600 volt insulation for that application. Don't remember what the replies were at the moment.

Add: I have heard before that with NEC changing 600 volts to 1000 volts for many things more recently we might eventually start seeing what is now 600 volt conductors being listed (maybe some changes to meet listing in some cases) as 1000 volt conductors.