three phase tranformer for specific equipment.

Noahrks

Member
Location
Conroe,Tx USA
I have a three phase 480v primary step down to 120/240v secondary transformer. I have XO at secondary for my neutral if needed. In my application I don't need a neutral for the three phase machine I am feeding. I have an EGC coming in and out to secondary of course they come together in the transformer and bond metal casing along with a ground rod. Do I bring XO down and bond with all my EGC even if I don't need it on the secondary side? it doesn't feed anything else other than the machine.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
It may be a bit of a fine line since you don't need the neutral,but it appears 250.20(B)(1) requires you to ground it.
In any event, if you don't choose to ground it, 250.21(B) would require ground detectors. On the surface, baring any mitigating circumstances, it appears the simplest
action would be to ground XO.
 

Smart $

Esteemed Member
Location
Ohio
BTW, is it really marked X0?

IIRC, 240/120V 3? 4W secondary center-tap terminal is marked X4.
 

Noahrks

Member
Location
Conroe,Tx USA
Thank you Angie for the reply I am going to ground it, just for peace of mind. smart$ yes it is marked xo I recall seeing that on several transformers I have worked on, although I have seen X4 on some also. Maybe different manufacturers label them differently.
 

kwired

Electron manager
Location
NE Nebraska
It may be a bit of a fine line since you don't need the neutral,but it appears 250.20(B)(1) requires you to ground it.
In any event, if you don't choose to ground it, 250.21(B) would require ground detectors. On the surface, baring any mitigating circumstances, it appears the simplest
action would be to ground XO.
I have to agree with the fine line comment. I wish this would be changed to clarify if the intent is to use that tap as a ground irregardless or give us a choice to corner ground it if we have no use for that center tap. For one thing if it must be used irregardless it would prevent that transformer from being a code compliant replacement for an existing corner grounded transformer, though there doesn't appear to really be much harm in otherwise using it.

Noahrks - outside of this mentioned complication corner grounding is another option.
 

bobby ocampo

Senior Member
It may be a bit of a fine line since you don't need the neutral,but it appears 250.20(B)(1) requires you to ground it.
In any event, if you don't choose to ground it, 250.21(B) would require ground detectors. On the surface, baring any mitigating circumstances, it appears the simplest
action would be to ground XO.
Having the center tap solidly grounded will have an advantage of a controlled transient over voltage in your electrical system. Maximum potential will be 208volts on a single line to ground fault.

For operation of you OCPD you need to have equipment grounding conductor (green) to be bonded with the center tap connected to ground.
 

bobby ocampo

Senior Member
I have a three phase 480v primary step down to 120/240v secondary transformer. I have XO at secondary for my neutral if needed. In my application I don't need a neutral for the three phase machine I am feeding. I have an EGC coming in and out to secondary of course they come together in the transformer and bond metal casing along with a ground rod. Do I bring XO down and bond with all my EGC even if I don't need it on the secondary side? it doesn't feed anything else other than the machine.
How many 3phase machine will you load in the transformer?

If you will load one or two machines only there is an advantage if you will float the neutral and provide indicator on a single line to ground fault. There will be continuity of service on a single line to ground fault for ungrounded system. Your load may be able to continue to operate and you have control on when to shut it down after finishing the operation. See IEEE 142-2007

You will still need to bond the metal casing of the transformer as your equipment grounding conductor (green or bare)
 

jrannis

Senior Member
I have a three phase 480v primary step down to 120/240v secondary transformer. I have XO at secondary for my neutral if needed. In my application I don't need a neutral for the three phase machine I am feeding. I have an EGC coming in and out to secondary of course they come together in the transformer and bond metal casing along with a ground rod. Do I bring XO down and bond with all my EGC even if I don't need it on the secondary side? it doesn't feed anything else other than the machine.
Interesting to know that a delta/delta transformer has an XO. I'm sure the OP knows for sure what the voltages are.
Is this an off the shelf transformer or did you order is specific for that voltage?
I remember using a delta/delta several years ago and my only option on that job was to corner ground it.
 

bobby ocampo

Senior Member
Interesting to know that a delta/delta transformer has an XO. I'm sure the OP knows for sure what the voltages are.
Is this an off the shelf transformer or did you order is specific for that voltage?
I remember using a delta/delta several years ago and my only option on that job was to corner ground it.
Delta-Delta transformer has an option of center tap in one phase. Center tap grounded is an option in IEEE 142-2007 Chapter 1 System grounding
 

kingpb

Senior Member
I have a three phase 480v primary step down to 120/240v secondary transformer. I have XO at secondary for my neutral if needed. In my application I don't need a neutral for the three phase machine I am feeding.
I'll tell you straight out, there is no way you are going to feed a three phase machine with that voltage set-up.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
120/240 three wire is single phase.
240/120 four wire is high leg delta.
Any other shorter descriptions are ambiguous
These are the IEEE/ANSI definitions. You can make the case that the NEC wants, but does not require, this methodology based on the informational note to Article 100 definition of Voltage, Nominal.
However in the POCO world, it seems they prefer to use 120/240V
 

kwired

Electron manager
Location
NE Nebraska
Because 120/240V is single phase.

Now if you would have said 240/120V, I think it would have been clear that you are talking about a high leg delta, which is in fact a 3-phase system.
And the OP may not have used proper terminology but still made is fairly clear he had a three phase system. I kind of had a feeling you were saying it is impossible based on misuse or misunderstanding from the start and not because he actually had something where it is not possible.
 

kingpb

Senior Member
And the OP may not have used proper terminology but still made is fairly clear he had a three phase system. I kind of had a feeling you were saying it is impossible based on misuse or misunderstanding from the start and not because he actually had something where it is not possible.
I understand there is a lot of misuse in terminology, and we have become accustomed to making assumptions and reading between the lines. But here is a simple scenario to illustrate the importance of being accurate:

We both receive a set of drawings to bid on, and the load list has a bunch of loads on it that requires say 20 - xfmrs to be purchased that are 480-120/240V. So, we both send it to the estimators to bid it, you say to the estimator, oh BTW I think those transformers are 3-phase; I on the other hand, bid it as written. Your price is too high and I am awarded the job.

The day after the award, I send in an RFI and ask for confirmation that the loads are single phase, and the response comes back saying they are all three phase. So, I issue a change order to the Owner and he says no way am I paying for that. My response is that according to ANSI/IEEE C84.1 the information provided clearly says single phase (480- 120/240V) but the RFI confirms three phase is required. I get my change order and ironically my bid with CO is actually higher than your original price. But I got the work and made more money. Why? Because I did not read between the lines, or assume I new what they were talking about. I simply used the industry recognized standards to go by to win my case.

If it's 220V it's 220V, not 220 or 221 whatever it takes!

Cheers!
 

kwired

Electron manager
Location
NE Nebraska
I understand there is a lot of misuse in terminology, and we have become accustomed to making assumptions and reading between the lines. But here is a simple scenario to illustrate the importance of being accurate:

We both receive a set of drawings to bid on, and the load list has a bunch of loads on it that requires say 20 - xfmrs to be purchased that are 480-120/240V. So, we both send it to the estimators to bid it, you say to the estimator, oh BTW I think those transformers are 3-phase; I on the other hand, bid it as written. Your price is too high and I am awarded the job.

The day after the award, I send in an RFI and ask for confirmation that the loads are single phase, and the response comes back saying they are all three phase. So, I issue a change order to the Owner and he says no way am I paying for that. My response is that according to ANSI/IEEE C84.1 the information provided clearly says single phase (480- 120/240V) but the RFI confirms three phase is required. I get my change order and ironically my bid with CO is actually higher than your original price. But I got the work and made more money. Why? Because I did not read between the lines, or assume I new what they were talking about. I simply used the industry recognized standards to go by to win my case.

If it's 220V it's 220V, not 220 or 221 whatever it takes!

Cheers!
Quite often there are other clues that would indicate it is single or three phase - like three phase switchboards, panelboards, controllers, data from the load itself... But in the absence of that it would be a high priority to me to find out for certain what is needed, as it seems like this would leave you dead in the water pretty quickly should you be wrong with it.
 
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