3Ph 480V Corner Grounded Delta connection to a 3-Ph 480V/240V delta-delta Transformer

[enterprisenx]

Hello

I need a 240V 3 phase source to supply a new motor. This is a specialty case for our site as all of our motors are 480V. Our electric supply is 480V B-Phase Ground (Corner Grounded) Delta. Let's assume I have to use a 230V 3 phase motor. If I buy a square d transformer like Square D 3T5F transformer, I am trying to understand if I need to ground my B phase on either side of the transformer or both. So the incoming lines are 480V - A, 0V - B, 480V - C, and the outgoing phases need to supply 240V only (hence delta). Does the B phase on the 240V secondary need to be grounded? The 480V primary side already has a grounded phase, but do I ground strap the H2 (B phase) terminal to the ground terminal? Wouldn't that potentially cause circulating ground currents by grounding again at at the primary side back to the MCC (origin of power source)?

Thanks for the feedback!

 

[david luchini]

Hello

I need a 240V 3 phase source to supply a new motor. This is a specialty case for our site as all of our motors are 480V. Our electric supply is 480V B-Phase Ground (Corner Grounded) Delta. Let's assume I have to use a 230V 3 phase motor. If I buy a square d transformer like Square D 3T5F transformer, I am trying to understand if I need to ground my B phase on either side of the transformer or both. So the incoming lines are 480V - A, 0V - B, 480V - C, and the outgoing phases need to supply 240V only (hence delta). Does the B phase on the 240V secondary need to be grounded? The 480V primary side already has a grounded phase, but do I ground strap the H2 (B phase) terminal to the ground terminal? Wouldn't that potentially cause circulating ground currents by grounding again at at the primary side back to the MCC (origin of power source)?

Thanks for the feedback!

The secondary of the transformer is not required to be grounded, but if it ungrounded, it would require Ground Detectors per 250.21(B).

It would probably be simpler to ground the secondary. This would be done in accordance with 250.30(A). You would not ground strap the H2 terminal to the ground terminal in the transformer.

 

[texie]

The secondary of the transformer is not required to be grounded, but if it ungrounded, it would require Ground Detectors per 250.21(B).

It would probably be simpler to ground the secondary. This would be done in accordance with 250.30(A). You would not ground strap the H2 terminal to the ground terminal in the transformer.

I agree, no grounding of the 480 side would be desired or allowed. I wonder if the 240 delta side of this transformer is actually a 240 delta with a center tap neutral as many are made this way. If that is the case then there would be no option but to ground the 240 side as you would be required to do so since you would have less than 150 volts to ground. But even if this is not the case and it is straight delta and no center tap, as you noted an ungrounded 240 side would lead to other issues.

 

[kwired]

Hello

I need a 240V 3 phase source to supply a new motor. This is a specialty case for our site as all of our motors are 480V. Our electric supply is 480V B-Phase Ground (Corner Grounded) Delta. Let's assume I have to use a 230V 3 phase motor. If I buy a square d transformer like Square D 3T5F transformer, I am trying to understand if I need to ground my B phase on either side of the transformer or both. So the incoming lines are 480V - A, 0V - B, 480V - C, and the outgoing phases need to supply 240V only (hence delta). Does the B phase on the 240V secondary need to be grounded? The 480V primary side already has a grounded phase, but do I ground strap the H2 (B phase) terminal to the ground terminal? Wouldn't that potentially cause circulating ground currents by grounding again at at the primary side back to the MCC (origin of power source)?

Thanks for the feedback!

That corner grounded delta system still needs equipment grounding conductors run beyond the service disconnecting means, you do not bond any equipment to the current carrying B phase at equipment you run separate EGC like you do for other voltage systems.The grounded phase is a "grounded conductor" that carries current just like the neutral of a typical 120/240 is also a current carrying conductor. You separate into "grounded conductor" and "equipment grounding conductor" after service equipment or first disconnect of separately derived systems to prevent having non intended metal components end up carrying current that is supposed to be on the grounded conductor.

I agree, no grounding of the 480 side would be desired or allowed. I wonder if the 240 delta side of this transformer is actually a 240 delta with a center tap neutral as many are made this way. If that is the case then there would be no option but to ground the 240 side as you would be required to do so since you would have less than 150 volts to ground. But even if this is not the case and it is straight delta and no center tap, as you noted an ungrounded 240 side would lead to other issues.

Even if there is center tap available on that 240 volt secondary, you don't have to use it if you are not supplying any loads that utilize it. If you were to build this transformer out of three single phase units - you very well have three center taps available - you can only ground one of them or you will have undesirable current. If you do utilize that center tap - that is the point of the system NEC wants to be grounded, if you don't utilize it you can ground any other point of the system, but only one point.

 

[enterprisenx]

3Ph 480V Corner Grounded Delta connection to a 3-Ph 480V/240V delta-delta Transformer

That corner grounded delta system still needs equipment grounding conductors run beyond the service disconnecting means, you do not bond any equipment to the current carrying B phase at equipment you run separate EGC like you do for other voltage systems.The grounded phase is a "grounded conductor" that carries current just like the neutral of a typical 120/240 is also a current carrying conductor. You separate into "grounded conductor" and "equipment grounding conductor" after service equipment or first disconnect of separately derived systems to prevent having non intended metal components end up carrying current that is supposed to be on the grounded conductor.


Even if there is center tap available on that 240 volt secondary, you don't have to use it if you are not supplying any loads that utilize it. If you were to build this transformer out of three single phase units - you very well have three center taps available - you can only ground one of them or you will have undesirable current. If you do utilize that center tap - that is the point of the system NEC wants to be grounded, if you don't utilize it you can ground any other point of the system, but only one point.

This transformer does have a center tap for 120V loads but we don't have any. We are only serving a 3 phase 230V (240V) motor. Would you say then we are in agreement that none of the primary or secondary phases should be connected to the transformer ground terminal? This means the transformer enclosure will be grounded via the grounding lug and an EGC will run to the motor grounding terminal only.

The part I am confused about is how the b-phase ground affects or not affects the secondary side. So the primary is H1 to GRD - 480V ; H2 to GRD - 0V ; H3 to GRD-480V. But phase to phase is 480V, so that means all three secondaries (phase to phase) X1-X2, X1-X3, X2-X3 are 240V. But are the secondary phase to ground are 240V or is X2 to ground 0V due to the primary H2 to ground being 0V?

Thanks!

 

[Jraef]

If you don't corner ground the secondary, the phase-ground voltage reference is floating so any measurement will change based on capacitance.

But to reiterate, you MUST either ground one corner, OR add ground fault monitoring on that secondary. Corner grounding is a LOT less expensive.

Also, if by chance this new machine has a VFD, delta is a poor choice. You can buy 240Y137V transformers to avoid the issues with delta systems, then you ground the Y point and the NEC is satisfied too.

 

[Besoeker]

I agree, no grounding of the 480 side would be desired or allowed.

If it's being fed from a corner grounded delta isn't one phase grounded already?

 

[MTW]

Your primary side system is already grounded, so you do not need any grounding straps on the primary connections. Land the three lines as normal and put your EGC to the frame.

The secondary is a separately derived system, it's lacking a metallic connection ground reference to the primary side system. The only reference it will have to ground is thru capacitance of the windings and conductors, as was stated earlier. And that capacitive coupling will vary depending on site conditions and the connected loads. It will do nothing for any fault currents you have on the secondary side. Fault currents need to be able to get back to that secondary transformer coil.

So grounding the separately derived secondary side of the delta coil is the best option, short of an expensive ground fault monitoring system.
Your delta coil can be either corner grounded, or center tap grounded. Corner grounding necessitates the use of different wiring methods for the grounded phase as well as special breakers that are not slash rated as are most standard breakers. 120/240 or 277/480 Full delta rated breakers are harder to get and more expensive.

If it were my choice I would be grounding the center tap, which results in a hi leg delta system. Its more common than corner grounded system, requires no special wiring methods, knowledge or breakers. But as mentioned earlier by Jaref, center tapped delta is not a good choice for VFD loads, due to the lack of symmetry on the output. Most VFD's want symmetry on the input due to the way that the MOV protection is configured internally. If you have elecro-mechanical controls, the center tapped hi leg delta is no problem for motor loads.

Also note that a separately derived transformer output coil requires grounding similar to a service. It is normally not enough to ground it to the EGC that supplies the primary side. It is required, that it is bonded to the building grounding electrode system of the main service. That could mean the building steel or the water service entrance, or the main grounding electrode buss in the switchboard. Ground rods by themselves are not a code accepted solution.

 

[MyCleveland]

If you don't corner ground the secondary, the phase-ground voltage reference is floating so any measurement will change based on capacitance.

But to reiterate, you MUST either ground one corner, OR add ground fault monitoring on that secondary. Corner grounding is a LOT less expensive.

Also, if by chance this new machine has a VFD, delta is a poor choice. You can buy 240Y137V transformers to avoid the issues with delta systems, then you ground the Y point and the NEC is satisfied too.

Jraef
Could you expand on this statement or point me towards an article for review please.

 

[jumper]

If it were my choice I would be grounding the center tap, which results in a hi leg delta system. Its more common than corner grounded system, requires no special wiring methods, knowledge or breakers.

Actually you do need to pay attention to breakers.

While most people know that you do not put a SP breaker on the high leg, many 2P breakers are slashed rated 120/240V. On a delta high leg system one would need a fully rated 2P 240V breaker if using B phase, the 208V high leg, for single phase 240V 2P load.

I have never seen a 3P slash rated 120/240V breaker.

 

[kwired]

This transformer does have a center tap for 120V loads but we don't have any. We are only serving a 3 phase 230V (240V) motor. Would you say then we are in agreement that none of the primary or secondary phases should be connected to the transformer ground terminal? This means the transformer enclosure will be grounded via the grounding lug and an EGC will run to the motor grounding terminal only.

The part I am confused about is how the b-phase ground affects or not affects the secondary side. So the primary is H1 to GRD - 480V ; H2 to GRD - 0V ; H3 to GRD-480V. But phase to phase is 480V, so that means all three secondaries (phase to phase) X1-X2, X1-X3, X2-X3 are 240V. But are the secondary phase to ground are 240V or is X2 to ground 0V due to the primary H2 to ground being 0V?

Thanks!

You have a "separately derived system"

Secondary conductors are totally isolated from the primary - except through what ever is grounded, which can be anything - though codes have preferences on what gets grounded in some situations.


The primary windings don't care what or if any of the supply conductors are grounded, it just want to see ~480 volts between each input line.

Primary and secondary are separate systems, both may have a ground reference, but that is all that is in common, and it is common in same way that POCO's primary and even their transmission systems also have a reference to ground.

You typically never will have a situation where you utilize voltage or even care what it is between a primary and a secondary point.

 

[MTW]

Actually you do need to pay attention to breakers.

While most people know that you do not put a SP breaker on the high leg, many 2P breakers are slashed rated 120/240V. On a delta high leg system one would need a fully rated 2P 240V breaker if using B phase, the 208V high leg, for single phase 240V 2P load.

I have never seen a 3P slash rated 120/240V breaker.

Agreed, I was thinking of the OP use as one 3P motor feeder, and no other loads.

What is common here is to put in a 1Φ panel for all 1Φ loads, thereby excluding the B phase from the panel, and using normal slash rated breakers at will. This gives the ability to use slash rated 2P's if required.

Then put in a 3Φ panel for equipment loads. That won't prevent someone from using a slash rated 2P breaker in the 3Φ panel, and most 3P breakers are full voltage rated, and can be used for a 2P circuit, without obtaining a full rated 2P unit. If the neutral line is omitted, from the panel, it further discourages someone from using a 2P unit for a 1Φ 3W circuit.

Relevant sections;
240.85 Applications. A circuit breaker with a straight voltage
rating, such as 240V or 480V, shall be permitted to be
applied in a circuit in which the nominal voltage between
any two conductors does not exceed the circuit breaker’s
voltage rating. A two-pole circuit breaker shall not be used
for protecting a 3-phase, corner-grounded delta circuit unless
the circuit breaker is marked 1φ–3φ to indicate such
suitability.
A circuit breaker with a slash rating, such as 120/240V
or 480Y/277V, shall be permitted to be applied in a solidly
grounded circuit where the nominal voltage of any conductor
to ground does not exceed the lower of the two values
of the circuit breaker’s voltage rating and the nominal voltage
between any two conductors does not exceed the higher
value of the circuit breaker’s voltage rating.

Informational Note: Proper application of molded case circuit
breakers on 3-phase systems, other than solidly
grounded wye, particularly on corner grounded delta systems,
considers the circuit breakers’ individual pole interrupting
capability.

240.15 (B) (3) Ungrounded Conductors.
3-Phase and 2-Phase Systems. For line-to-line loads
in 4-wire, 3-phase systems or 5-wire, 2-phase systems, individual
single-pole circuit breakers rated 120/240 volts ac
with identified handle ties shall be permitted as the protection
for each ungrounded conductor, if the systems have a
grounded neutral point and the voltage to ground does not
exceed 120 volts.

 

[kwired]

Agreed, I was thinking of the OP use as one 3P motor feeder, and no other loads.

What is common here is to put in a 1Φ panel for all 1Φ loads, thereby excluding the B phase from the panel, and using normal slash rated breakers at will. This gives the ability to use slash rated 2P's if required.

Then put in a 3Φ panel for equipment loads. That won't prevent someone from using a slash rated 2P breaker in the 3Φ panel, and most 3P breakers are full voltage rated, and can be used for a 2P circuit, without obtaining a full rated 2P unit. If the neutral line is omitted, from the panel, it further discourages someone from using a 2P unit for a 1Φ 3W circuit.

Relevant sections;
240.85 Applications. A circuit breaker with a straight voltage
rating, such as 240V or 480V, shall be permitted to be
applied in a circuit in which the nominal voltage between
any two conductors does not exceed the circuit breaker’s
voltage rating. A two-pole circuit breaker shall not be used
for protecting a 3-phase, corner-grounded delta circuit unless
the circuit breaker is marked 1φ–3φ to indicate such
suitability.
A circuit breaker with a slash rating, such as 120/240V
or 480Y/277V, shall be permitted to be applied in a solidly
grounded circuit where the nominal voltage of any conductor
to ground does not exceed the lower of the two values
of the circuit breaker’s voltage rating and the nominal voltage
between any two conductors does not exceed the higher
value of the circuit breaker’s voltage rating.

Informational Note: Proper application of molded case circuit
breakers on 3-phase systems, other than solidly
grounded wye, particularly on corner grounded delta systems,
considers the circuit breakers’ individual pole interrupting
capability.

240.15 (B) (3) Ungrounded Conductors.
3-Phase and 2-Phase Systems. For line-to-line loads
in 4-wire, 3-phase systems or 5-wire, 2-phase systems, individual
single-pole circuit breakers rated 120/240 volts ac
with identified handle ties shall be permitted as the protection
for each ungrounded conductor, if the systems have a
grounded neutral point and the voltage to ground does not
exceed 120 volts.

You can use slash rated breakers in a three phase panel, you just can't connect them to the high leg.

If you have a 240 corner ground system you can't use 120/240 slash rate breakers period.

 

[JFletcher]

If I may ask a dumb question here;

Wouldnt a wye secondary be cheaper/easier than corner grounded delta? I have a hard time buying that a motor that would work off a 3kva xfmr HAS to wired 240V...

 

[kwired]

If I may ask a dumb question here;

Wouldnt a wye secondary be cheaper/easier than corner grounded delta? I have a hard time buying that a motor that would work off a 3kva xfmr HAS to wired 240V...

General purpose motor is very likely to be 230/460.

OEM motor on some equipment could be single voltage only.

 

[Jraef]

The other side of your point is valid though, at 3kVA the HP must be small and a LOT of small HP motors will be labeled as 208-230V. The OP should double check that motor.

 

[Jraef]

Jraef
Could you expand on this statement or point me towards an article for review please.

So read this, starting with the section on transformer configurations at page 29, then read the section on Surge Suppressors and Common Mode Filters on page 51.
http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf

These are mostly generic guidelines that apply to ALL modern VFDs. In the case of A-B, they give you a way (in their manuals) to easily disconnect the ground reference point of the MOVs and Common Mode capacitors so that they don't blow up and ruin the drive, but you must also understand that by removing it, you also remove some of the protection that offered.

 

[MyCleveland]

So read this, starting with the section on transformer configurations at page 29, then read the section on Surge Suppressors and Common Mode Filters on page 51.
http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf

These are mostly generic guidelines that apply to ALL modern VFDs. In the case of A-B, they give you a way (in their manuals) to easily disconnect the ground reference point of the MOVs and Common Mode capacitors so that they don't blow up and ruin the drive, but you must also understand that by removing it, you also remove some of the protection that offered.

Thank you....I will start reading through the material.