240 volt , 3 phase , need neutral

[kwired]

I wouldn't be too concerned about violating any listing - IMO the listing only means something when it left the factory. Once it is installed/used/etc. that listing starts to deteriorate even if you did not directly modify anything.

Now if they refuse to warranty something that was modified or not installed according to instructions, that deserves some consideration.

One could easily derive a "120 volt tap" in relation to one of the phase conductors though with an autotransformer, whether internal or external to the machine, you just need to get it to match up to the correct supply conductor that supplies the machine.

Better yet ask the manufacturer why they were too cheap to provide an internal control transformer as part of their machine:happyyes:

 

[big john]

It would be a violation if you actually grounded the center point. But you would be creating a derived neutral and that neutral will not be bonded to ground, since the hot leads are already ground referenced.

Good point. So it wouldn't fall under grounding transformers, just a regular autotransformer.

If there's nothing that would prohibit sizing it for the small control neutral, I think that's the way to go.

 

[Smart $]

Good point. So it wouldn't fall under grounding transformers, just a regular autotransformer.

If there's nothing that would prohibit sizing it for the small control neutral, I think that's the way to go.

I'm thinking 250.20(B) requires grounding the neutral.

 

[big john]

I'm thinking 250.20(B) requires grounding the neutral.

But like Golddigger pointed out, the system itself is already grounded. So then it would appear that using a floating zigzag is no different than using an autotransformer bank to buck voltage down for utilization purposes, the only distinction being it's being bucked completely to zero.

 

[Smart $]

But like Golddigger pointed out, the system itself is already grounded. So then it would appear that using a floating zigzag is no different than using an autotransformer bank to buck voltage down for utilization purposes, the only distinction being it's being bucked completely to zero.

I would love to debate the issue... ...but I believe there's another predominant issue which makes this one moot. And that is, this "new" neutral will not provide 120V to any of the 3 line conductors.

 

[big john]

...This "new" neutral will not provide 120V to any of the 3 line conductors.

Listen, stop derailing my brainstorms with valid objections. :angel:

 

[Smart $]

Listen, stop derailing my brainstorms with valid objections. :angel:

:rotflmao:

 

[kwired]

I'm thinking 250.20(B) requires grounding the neutral.

Only if there is a separately derived system, if it is an autotransformer you won't want to ground a second point within the already grounded system.

 

[GoldDigger]

I would love to debate the issue... ...but I believe there's another predominant issue which makes this one moot. And that is, this "new" neutral will not provide 120V to any of the 3 line conductors.

OK, some of us lost track of the high leg delta specification.
Even easier for that case, a simple 120-0-120 autotransformer between A and C will supply the neutral that equipment requires for control power.
(To me this is a good example of why the NEC should not have decided to call the center tap of a high leg delta a neutral. )

 

[mark32]

I would love to debate the issue... ...but I believe there's another predominant issue which makes this one moot. And that is, this "new" neutral will not provide 120V to any of the 3 line conductors.

I'm sure you're correct on this, but what's the concern with the "New" neutral not being able to provide 120v to the line conductors? It's only there to power the controls right?

 

[Smart $]

I'm sure you're correct on this, but what's the concern with the "New" neutral not being able to provide 120v to the line conductors? It's only there to power the controls right?

Correct,

...120V controls.

A 240V wye neutral will be 139V to line.

 

[Smart $]

OK, some of us lost track of the high leg delta specification.
Even easier for that case, a simple 120-0-120 autotransformer between A and C will supply the neutral that equipment requires for control power.
(To me this is a good example of why the NEC should not have decided to call the center tap of a high leg delta a neutral. )

Simple, yes.

Care to spec' a make/model with a cost and ease of installation comparable to a 240V PRI/120V SEC (any of several configurations)?

 

[kwired]

OK, some of us lost track of the high leg delta specification.
Even easier for that case, a simple 120-0-120 autotransformer between A and C will supply the neutral that equipment requires for control power.
(To me this is a good example of why the NEC should not have decided to call the center tap of a high leg delta a neutral. )

But if you connect it as an autotransformer and ground the center tap, you have a parallel back to the neutral of the main supply via EGC. IMO you need to leave that 120 volt tap ungrounded, which raises other NEC issues because it now should not be white or gray because it is not a grounded conductor.

Something else to think about- if you leave it ungrounded, the derived 120 volt lead has 120 volts to each supply lead even if one of them is the high leg of the supply system. If you ground the center tap and connect one supply lead to high leg - you let the smoke out of the windings.

 

[kwired]

Correct,

...120V controls.

A 240V wye neutral will be 139V to line.

240 volt wye system is not what the OP has though if you left the derived 120 volt lead ungrounded in an autotransformer configuration you still have a 120 volt lead between two 240 volt leads just like I described in my last post.

 

[big john]

...Care to spec' a make/model with a cost and ease of installation comparable to a 240V PRI/120V SEC (any of several configurations)?

I agree that a CPT would be the easiest solution here, but isn't one of the major sticking points that the installer risks voiding the manufacturer's warranty by modifying the control circuit to install one?

Putting in an autotransformer means a neutral could be derived outside the equipment and brought in. He could get a 1kVA 120:120 transformer for less than $200 and series-wire the windings.

 

[Smart $]

I agree that a CPT would be the easiest solution here, but isn't one of the major sticking points that the installer risks voiding the manufacturer's warranty by modifying the control circuit to install one?

Putting in an autotransformer means a neutral could be derived outside the equipment and brought in. He could get a 1kVA 120:120 transformer for less than $200 and series-wire the windings.

So who says the CPT cannot be external?

Using an isolation transformer connected as an autotransformer would work. IMO, cheating... and not the point I was attempting to make... but all's good. I would still prefer a bonded neutral.

 

[Smart $]

240 volt wye system is not what the OP has though if you left the derived 120 volt lead ungrounded in an autotransformer configuration you still have a 120 volt lead between two 240 volt leads just like I described in my last post.

The neutral point of a zig-zag would make it a 240V wye.

 

[big john]

So who says the CPT cannot be external...?

The OEM does, apparently:

...We talked with the vendor about installing one and they are "freaking out " saying that would violate their UL listing...

If they're that particular, I don't see any way of installing a CPT that would make them happy. Mind you, if they're that particular I can also imagine them complaining about any derived neutral. :roll:

 

[Smart $]

The OEM does, apparently: If they're that particular, I don't see any way of installing a CPT that would make them happy. Mind you, if they're that particular I can also imagine them complaining about any derived neutral. :roll:

I believe that comment was regarding putting a CPT inside their cabinet... not outside. They don't have any say if the CPT is external to their cabinet.

 

[big john]

...They don't have any say if the CPT is external to their cabinet.

But since the existing design uses one of the three phase conductors to power their controls, installing the CPT would mean modifying the design. Personally, that's likely what I would've done. But I can also understand the risk of voiding the warranty.