Bonding Delta to Delta transformers

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On a 480 delta to 240 delta dry type transformer, I know we have to ground one of the secondary phase conductors to establish a ground reference for short circuit and overcurrent devices.

My question is, If the transformer comes with a 5% neutral, do we bond the secondary there or is there not enough current carrying capability do to it's reduced size?
 

don_resqcapt19

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krsparky,
On a 480 delta to 240 delta dry type transformer, I know we have to ground one of the secondary phase conductors to establish a ground reference for short circuit and overcurrent devices.
There is no code rule that requires one of the secondary conductors to be bonded on a 240 volt 3 wire delta system.
What is a 5% neutral?
Don
 
I was always under the impression that article 250-30 Grounding seperately derived systems goverened the grounding of a transformer regardless whether the secondary is a wye or a delta.

If you do not ground a winding from the secondary, you have no reference for a breaker or fuse to open on short circuit. I'm sure you have noticed that on a corner grounded delta service one of your phase conductors reads "0" volts to ground.

A 5% neutral is provided with all delta to delta transformers to provide a light to be lit at 120 volts in the event that there is no other 120 volt source where your transformer is. It only has the current carrying capability of 5% of the secondary rating.

My question, is whether or not that 5% percent is suitable for the other phases to clear a fault .
 

don_resqcapt19

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I was always under the impression that article 250-30 Grounding seperately derived systems goverened the grounding of a transformer regardless whether the secondary is a wye or a delta.
There is nothing in 250.30 that tells you what types of systems must be grounded. That section only tells you what to do when you have a grounded SDS. 250.20(D) tells you what systems must be grounded and a 3 phase, 3 wire, 240 volt delta system is not one of them. You are permitted to ground it, but you are not required to.
If you do not ground a winding from the secondary, you have no reference for a breaker or fuse to open on short circuit. I'm sure you have noticed that on a corner grounded delta service one of your phase conductors reads "0" volts to ground.
Sure, but there is no code requirement to connect the 240 volt delta system as a corner ground. If you leave it ungrounded, the first ground fault does nothing. There is no real current flow or any other type of problem. If you have a second ground fault, then you have about the same thing as with the first ground fault on a corner grounded system, except that there will be equipment damage and arcing at both ground fault points and the impeadance of the faults may act to limit the current flow that will in turn slow or prevent the operation of the OCPD.
A 5% neutral is provided with all delta to delta transformers to provide a light to be lit at 120 volts in the event that there is no other 120 volt source where your transformer is. It only has the current carrying capability of 5% of the secondary rating.
Actully that is somewhat rare. It is not provided with all delta/delta transformers. It requires a center tap on one of the phase windings. This would be a 3 phase, 4 wire delta and the center tap grounded. If you have this system and you ground one of the phase conductors in addtion to the center tap conductor, you will let the factory installed smoke out of the transformer.
Don
 

Smart $

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don_resqcapt19 said:
A 5% neutral is provided with all delta to delta transformers to provide a light to be lit at 120 volts in the event that there is no other 120 volt source where your transformer is. It only has the current carrying capability of 5% of the secondary rating.

Actully that is somewhat rare. It is not provided with all delta/delta transformers. It requires a center tap on one of the phase windings. This would be a 3 phase, 4 wire delta and the center tap grounded. If you have this system and you ground one of the phase conductors in addtion to the center tap conductor, you will let the factory installed smoke out of the transformer.
Don
I'm of the impression the 5% neutral connection need not be grounded if it is not used. Is this true?

Additionally, it seems that 250.21 requires ground detectors to be installed if the topic transformer is not grounded.

How should one size a bonding jumper when connected as a 4-wire system, center-tap grounded, if the neutral is only capable of 5% of the secondary rating? Under certain circumstances, wouldn't the 5% tap react like a fuse and...
...let the factory installed smoke out...
???
 

infinity

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Smart,

What don is saying:

If you have this system and you ground one of the phase conductors in addtion to the center tap conductor, you will let the factory installed smoke out of the transformer.

is that if the transformer is already center tap grounded and you ground one phase as in a corner grounded system once you turn it on, out comes the smoke. You would have shorted out the center tapped winding.
 

iwire

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Everything electrical or electronic comes from the factory with some smoke packed inside. It's a bad thing to let the smoke out. When you let the smoke out the device usually won't work any more.
 
I guess I am not going to get an actual answer to my specific question. I guess I'll move on to the factory.

I thought this forum would provide valuable insight, not turn into an ego session.
 

infinity

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krsparky said:
I guess I am not going to get an actual answer to my specific question. I guess I'll move on to the factory.

I thought this forum would provide valuable insight, not turn into an ego session.


The problem is you need to provide more information on a transformer with a 5% neutral. Who actually makes these? I would guess that many of us here have never seen one. And for the record, your ego comment was little out of line.
 

don_resqcapt19

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I guess I am not going to get an actual answer to my specific question. I guess I'll move on to the factory.
You got the answer...if you have no 120 volt loads there is no code requirement to provide a grounded system. You are permitted to corner ground the system if you want to. If you have 120 volt loads you are required to ground the center tap, once you do this you cannot ground a phase conductor as that will result in a short circuit.
If you ground the center tap, the OCPD on the phase conductors should prevent transformer damage in the event of a phase to ground fault.
Don
 

don_resqcapt19

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Trevor,
provide more information on a transformer with a 5% neutral.
This would be three phase transformer with a center tap on one winding, and the neutral load on these types of transformers is often limted to 5% of the transformer capacity. The following is from page 40 of this SquareD document.
Q. Why is there a 5% limit for single phase loading on Delta-Delta connected transformers with 240/120 center tapped secondaries?
A. Delta-Delta connected transformers are intended to supply balanced three-phase loads, such as motors. Unbalanced loading will cause a circulating current to flow in the windings. This additional current is like a “hidden” load within transformer windings and can severely de-rate or even overload the transformer. Adding a center tap on the secondary for a combination of a 240V three-phase and 120V singlephase loads will create an unbalance. The amount of unbalance is limited to 5% to prevent excessive circulating currents.
Where the power company provides a 120/240 high leg service, they use 2 or 3 individual transformers with the one that has the center tap sized larger than the other(s) to get around this 5% limitation.
Don
 

don_resqcapt19

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Smart,
Additionally, it seems that 250.21 requires ground detectors to be installed if the topic transformer is not grounded.
I don't think so. This would be an installation made per section 250.21(4). The requirement for ground detection only applies to installations made per 250.21(3).
Don
 

Smart $

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don_resqcapt19 said:
Smart,

I don't think so. This would be an installation made per section 250.21(4). The requirement for ground detection only applies to installations made per 250.21(3).
Don
Think again... please.

2005 NEC:
250.21 Alternating-Current Systems of 50 Volts to 1000 Volts Not Required to Be Grounded.
...
...
...
...
...
Where an alternating-current system is not grounded as permitted in 250.21(1) through (4), ground detectors shall be installed on the system.
 
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Smart $

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infinity said:
Smart,

What don is saying:
If you have this system and you ground one of the phase conductors in addtion to the center tap conductor, you will let the factory installed smoke out of the transformer.
is that if the transformer is already center tap grounded and you ground one phase as in a corner grounded system once you turn it on, out comes the smoke. You would have shorted out the center tapped winding.
I got that part... already knew it prior to Don's post... though I have seen cross-terminated, parallel secondary conductors not smoke a 12kV to 480 transformer and still wonder why.

I was thinking along the line of a partial ground fault on a 3? circuit, not enough to trip the breaker but enough to exceed the 5% rating on the tap. The ground fault current returns to the transformer via the bonding jumper and the transformer would see it as a high current 120V load. Would this situation smoke the tranformer?
 
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dereckbc

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Krsparky relax, we are trying to help you. As explained there is no requirement to have a grounded system. If you have a 3-wire delta system there are some additional rules that have to be followed like ground fault detection systems to set off an alarm in the event one phase happens to be faulted to ground. The whole point of a delta system is to keep a system running where outages cannot be tolerated like process control. If you go and ground the center tap, it is no longer a delta, it is a grounded wye system.

As Don tried to explain a system does not have to be referenced to ground in order for OCPD to operate. In a delta system if one phase is faulted, nothing happens, everything keeps churning like normal except an alarm goes off to alert maintenance personnel. Once and if the second fault occurs, then the OCPD operates.
 

Smart $

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dereckbc said:
...If you go and ground the center tap, it is no longer a delta, it is a grounded wye system.

...
That is false info. It would become a delta 3?, 4-wire system.
 
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roger

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Smart $ said:
Think again... please.

Where an alternating-current system is not grounded as permitted in 250.21(1) through (4), ground detectors shall be installed on the system.

2005 NEC:

Just to clarify things, this wording is exclusive to the 2005 NEC, those using earlier versions will not see this.

Roger
 
derekbc,

I understand all of that. My application is in a brand new building. The 240 delta transformer is providing power for a pump testing station. The need for short circuit protection is of paramount importance. Leaving the delta open and ungrounded is not an option.

I am simply trying to acertain whether or not the reduced neutral tap is sufficient for clearing a large fault on the other three phases. I am pretty certain at this point the answer is yes.

I am trying to simplify the installation by creating a 4 wire secondary as I am afraid future maintenance personell may be confused by the ungrounded 3 wire system
 

infinity

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krsparky said:
derekbc,

I understand all of that. My application is in a brand new building. The 240 delta transformer is providing power for a pump testing station. The need for short circuit protection is of paramount importance. Leaving the delta open and ungrounded is not an option.

I am simply trying to acertain whether or not the reduced neutral tap is sufficient for clearing a large fault on the other three phases. I am pretty certain at this point the answer is yes.

I am trying to simplify the installation by creating a 4 wire secondary as I am afraid future maintenance personell may be confused by the ungrounded 3 wire system


You bring up an interesting question. The link that Don provided explains the 5% loading limit on the center tapped neutral but doesn't specifically answer your question. Couldn't a 3 wire corner grounded Delta transformer be used instead of the 4 wire Delta?
 
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