480v to 240v 3ph tranformer x4 tap
- Thread starter Scali
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- New Jersey
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- Journeyman Electrician
According to 250.20 and 250.21 it sounds like it's your choice. I would ground it.
kwired
Electron manager
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- NE Nebraska
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- EC
To add to Rob's post, if you are not utilizing a "neutral" for any loads, you wouldn't have to ground the X4 terminal.
That would leave you with two other choices, corner ground it, or use as an ungrounded system - in which case ground detection/indication equipment would be needed.
That would leave you with two other choices, corner ground it, or use as an ungrounded system - in which case ground detection/indication equipment would be needed.
After doing a lot of research, the best option is to ground the x4 to the case. (Why you ask?) The X4 will create a good path for a ground fault!(Going back into the electrical loop per say) And would primarily cancel the fault. (Hence Rob) Ground it!
Your Thoughts,
Your Thoughts,
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Actually, if you have an X4 terminal (midpoint of one winding), you are required to ground it for a 300V or less secondary. It does not readily appear so after reading the requirements of 250.20(B)(3)... but it is required under (B)(1) because you can limit the voltage to ground on two of the ungrounded conductors to not more than 150 volts. YMMV w/AHJ
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- New Jersey
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- Journeyman Electrician
I agree, since the phase voltage is 240 volts your voltage to ground would be less than 150 volts because of 250.20(B)(1). So for a 240 volt system the grounding is not optional.
kwired
Electron manager
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- NE Nebraska
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- EC
One line is 208 volts to ground though. But since you can reduce at least one line to less then 150 to ground the way it is worded may still mean you must ground it in a way to result in the lesser voltage to ground?
As I said above it may mean you must ground it for the least voltage to ground possible if you can get below 150 in any way.
One scenario comes to mind though that I would like some opinions for: Say you have an existing 240 volt corner grounded system and the source transformer fails. Must you replace it with a three wire output transformer or could you use a 4 wire and not utilize the X4 terminal? How about if it were a utility transformer and they replaced it (for any reason)? When they build pole top transformer banks they generally have 120/240 single phase units they build them with, they just don't use any of the center taps if building a corner ground system. I know they don't have to follow NEC, but you still have a system that is capable of being grounded so that there is at least one point below 150 volts to ground.
If you can not utilize the replacement with an X4 terminal, then that means you either need a different transformer or must change existing wiring of the formerly grounded phase if it is now going to be ungrounded. I'd rather see the X4 left floating then to find out someone missed a few places that need an overcurrent device added where there wasn't one before.
meternerd
Senior Member
- Location
- Athol, ID
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- retired water & electric utility electrician, meter/relay tech
[How about if it were a utility transformer and they replaced it (for any reason)? When they build pole top transformer banks they generally have 120/240 single phase units they build them with, they just don't use any of the center taps if building a corner ground system. I know they don't have to follow NEC, but you still have a system that is capable of being grounded so that there is at least one point below 150 volts to ground.]
If it is a utility bank, they'd replace it same for same. Otherwise they'd have to have the customer install a new meter panel, or at least modify the existing (not likely, due to UL, etc.) because they use different meters. 3W Delta uses Form 12S (5 jaw). 4W Delta uses Form 15S (7 jaw).
If it is a utility bank, they'd replace it same for same. Otherwise they'd have to have the customer install a new meter panel, or at least modify the existing (not likely, due to UL, etc.) because they use different meters. 3W Delta uses Form 12S (5 jaw). 4W Delta uses Form 15S (7 jaw).
I believe as long as the system is grounded you are compliant. The requirement is that it be grounded rather than ungrounded... not how it is grounded when grounded. Now if you use X4 as a system conductor, then it would have to be grounded.
kwired
Electron manager
- Location
- NE Nebraska
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- EC
One real life instance, but yet not same example - pole top transformers - increase in capacity needed and a higher capacity padmount was installed. The part not the same is in this case both before and after was high leg system, but say it had been corner ground originaly and padmount were installed with option to use an X4 terminal as a grounded conductor or leave it float and ground a phase?? The question here is does NEC say you must ground that X4 terminal just because doing so can give you less voltage to ground?
kwired
Electron manager
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- NE Nebraska
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- EC
My question was not grounded vs ungrounded, it was whether or not NEC requires you to use X4 if it is present as a grounded conductor, because it can allow for less voltage to ground. If the answer is yes then it is against NEC to corner ground any system unless there is no possible way to connect it for less voltage to ground - and creates potential problem in a situation where you may have a need to change a transformer where there is existing corner ground.
I would like to think this is not the NEC intent to force such changes when such an incident would come up but the way it is worded could certainly be interpreted that way.
I thought I answered that question... :slaphead:
I understand your concern, but if owner decides they want a corner grounded system for whatever reason, or replace an existing corner grounded system, I don't see how 250.20(B) has any effect, even if the transformer has an X4 terminal. It's about not letting the system go ungrounded.
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texie
Senior Member
- Location
- Fort Collins, Colorado
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- Electrician, Contractor, Inspector
That would be compliant. As long as the POCO is delivering a corner ground and the customer has no control. This also would apply the other way around. For example, the customer can't just decide if he wants an ungrounded (for example 480 volt) by not bringing a grounded conductor to the service and bonding it. If the POCO has the supply grounded at ANY point on the secondary then it is a grounded system and must be installed as such.
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- Placerville, CA, USA
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- Retired PV System Designer
As I read it, the voltage to ground does not matter at all (except maybe to the motivation of the CMP?) because (3) requires the grounding totally independent of any other provisions or voltages.
The center tap of one phase of a 480 delta, providing a 240/480 high leg system would have to be grounded too.
But only kicks in if the center tap is used for a circuit conductor. We're talking about where there is no line-to-neutral load.
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- Placerville, CA, USA
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- Retired PV System Designer
True. But IMO (1) only kicks in when all of the phase to neutral voltages are less than 150 volts, and would not apply to a 240/120 high leg.
You can build an MWBC which contains only phase lines that are less than 150V to ground, but that alone does not trigger (1).
- Location
- New Jersey
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- Journeyman Electrician
I agree, the factor that was overlooked was that the Delta secondary would have a high leg of 208 volts to ground making 250.20(B)(1) not applicable.
That's one interpretation. The requirement does not say all ungrounded conductors. It can just as easily mean any ungrounded conductors.
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- Placerville, CA, USA
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- Retired PV System Designer
In normal English speech and writing, "the ungrounded conductors" means exactly "all of the ungrounded conductors". Otherwise it would read " some of...." or "one or more of... ".
Well that's one viewpoint. Can you cite authoritative documentation that agrees? We also have to take into consideration the Code is a mix of normal and technical writing. I say lacking descriptors to the contrary, it can be either.
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