I Am Currently On A Job With A Small Retail Space Which Has Only A 120/208 Service. The Hvac Contractor Mistakenly Installed A 480 V Heating Unit In The Space. The Gc Decided It Would Be Cheaper For Us (the Electricians) To Install A Transformer Than To Have The Heating Unit Changed Out To 208. That's All Fine And Well. So Our For-man Ordered A 480 V Delta To 120/208 Wye Transformer And Had Us Wire It Backwards. That's Also All Fine And Well Except That He Said All We Needed To Do Was To Run Our 208 To The "secondary", Our 480 From The "primary" And Leave Xo Un-bonded. It Works Of Course But I Feel We Should Have Changed The Wiring So That The 480 Side Would Be Wye And The 208 Side Would Be Delta. It Seems To Me That Having A Delta Secondary Leaves You With An Isolated System With No Reference To Ground. I Argued With My For-man About It But He Insisted It Would Be Okay. So We Wired It Like The Man Said. The Amazing Thing Is That When It Was All Done And We Fired It Up, I Checked Voltage From Each Phase To Ground And Got 277. Someone Please Explain To Me How This Is Possible. And If A Wire To The Heating Unit Were To Short To Ground - Would It Blow A Fuse?
Delta Secondary
- Thread starter ROCK POP
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JohnJ0906
Senior Member
- Location
- Baltimore, MD
If I understand, the 480 side is not grounded?
Sounds all OK except for not grounding/bonding on the secondary (480) side.
By the way, capitalizing every word makes it hard to read. This is sometimes caused by leaving cap lock on.
Welcome to the forum! :smile:
Sounds all OK except for not grounding/bonding on the secondary (480) side.
By the way, capitalizing every word makes it hard to read. This is sometimes caused by leaving cap lock on.
Welcome to the forum! :smile:
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Not bonding the primary XO is fine but you cannot rewire a transformer and change it's configuration from Delta to Wye. The absence of intentionally grounding the secondary (delta side) would require the use of a ground detector. Look at 250.21(4).
brian john
Senior Member
- Location
- Leesburg, VA
Due to winding ratios YOU CANNOT reconnect a 480 delta to 208/120 wye to 208 delta to 480/277 wye.
the above 480-208/120 transformer is 4 to 1, 480 VAC per winding gives you 120 VAC
at 208 delta to 480/277 is .75 to 1 or 208 VAC per winding to 277 VAC
He should have ordered the correct transformer.
the above 480-208/120 transformer is 4 to 1, 480 VAC per winding gives you 120 VAC
at 208 delta to 480/277 is .75 to 1 or 208 VAC per winding to 277 VAC
He should have ordered the correct transformer.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
What the others have said above.
As you have described things, you have an ungrounded delta secondary. A ground fault on the secondary side will not result in significant current flow, and will not trip a breaker.
When you measured your system, you found 277V to ground. This is because an 'ungrounded' system is really better described as a 'capacitively grounded' system; while there is no metallic electrical connection to ground, all of the wiring is in proximity to grounded metal, and each wire forms a capacitor with that metal. There also may be very slight leakage currents through the various insulators. This capacitive coupling is enough to balance the supply terminals relative to ground. If you had used a low impedance meter such as a 'wiggy' to measure your terminal voltage, then you would probably have gotten something less than 277V.
It is also possible that there is a derived neutral point in the HVAC equipment that is grounded. This would be an improper ground to neutral bond, but it could force the terminal voltages to the correct value.
It _is_ possible to rewire a transformer, but to get the voltages that you want (208V delta input, 480/277V wye output) you would actually need to have the coils changed. This is not a field modification, and for a small dry type transformer this would be more expensive that a new transformer.
It is possible to add a separate device called a 'zig-zag' transformer to the system. This is an autotransformer used to derive a neutral point from a delta system. You could then ground this derived neutral point and have the equivalent of a grounded wye system.
You can also 'corner ground' a delta secondary. This involves intentionally connection one of the phase terminal to ground. This provides all of the benefits of a solidly grounded system, however it requires 480V rated breakers (rather than 480/277V 'slash' rated breakers), and can be quite confusing if it is not expected!
-Jon
As you have described things, you have an ungrounded delta secondary. A ground fault on the secondary side will not result in significant current flow, and will not trip a breaker.
When you measured your system, you found 277V to ground. This is because an 'ungrounded' system is really better described as a 'capacitively grounded' system; while there is no metallic electrical connection to ground, all of the wiring is in proximity to grounded metal, and each wire forms a capacitor with that metal. There also may be very slight leakage currents through the various insulators. This capacitive coupling is enough to balance the supply terminals relative to ground. If you had used a low impedance meter such as a 'wiggy' to measure your terminal voltage, then you would probably have gotten something less than 277V.
It is also possible that there is a derived neutral point in the HVAC equipment that is grounded. This would be an improper ground to neutral bond, but it could force the terminal voltages to the correct value.
It _is_ possible to rewire a transformer, but to get the voltages that you want (208V delta input, 480/277V wye output) you would actually need to have the coils changed. This is not a field modification, and for a small dry type transformer this would be more expensive that a new transformer.
It is possible to add a separate device called a 'zig-zag' transformer to the system. This is an autotransformer used to derive a neutral point from a delta system. You could then ground this derived neutral point and have the equivalent of a grounded wye system.
You can also 'corner ground' a delta secondary. This involves intentionally connection one of the phase terminal to ground. This provides all of the benefits of a solidly grounded system, however it requires 480V rated breakers (rather than 480/277V 'slash' rated breakers), and can be quite confusing if it is not expected!
-Jon
JohnJ0906
Senior Member
- Location
- Baltimore, MD
ROCK POP said:
You would still have 480v 3ph. Corner grounding does not change this.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
ROCK POP said:
As John said, corner grounding does not reduce the number of phases you have. But any protective devices on the 480V need to be rated for a "grounded B" phase system. This is usually not a problem with fusible equipment, however it is with circuit breakers.
I agree that this type of commerical application may have been better suited using a 208-480Y/277 step up transformer.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
True, but there is the "simultaneoulsy open all lines" advantage of the breaker, whereas a fused disco must open, but not fuse, the bonded phase.jim dungar said:
- Location
- Illinois
- Occupation
- retired electrician
Larry,
Don
Why would the disconnect have to open the grounded conductor?
Don
cripple
Senior Member
- Location
- Santa Fe, New Mexico
Transformer
Transformer
This is from Square D
Back Feeding Transformers
Occasionally, someone has an urgent need for a non-stock step-up transformer.
The natural inclination is to grab an in-stock step-down unit and to operate it in reverse. Except for small (less than 3 KVA models, which have compensated windings to provide rated voltage at rated load), if permitted by local codes, then it is generally OK to reverse feed a transformer but there are several precautions that should be considered. To illustrate, lets assume that a standard 9T23B3874 step-down transformer rated 75 KVA/3-phase/60 HZ with a 480 VAC Delta Primary with (6) 2-1/2% FC2A4BN voltage taps and a 208Y/120 VAC Secondary is to be operated step-up; that is, 208 VAC input to 480 VAC output.
The installer may discover that the primary protector, having been properly selected and applied per Article 450 of the National Electrical Code, nevertheless, trips or blows when he attempts to energize the reverse operated transformer. This phenomenon can occur because the low impedance winding (the 208Y/120 VAC one that was intended by design to be the secondary winding) now serves as the primary and the value of the magnetizing inrush current (Mag-I) is actually much greater than expected. The Mag-I experienced when energizing transformers is similar to the inrush current associated with motor starting. The primary and secondary full load amps of the subject transformer are 90 Amps @ 480 VAC and 208 Amps @ 208 VAC respectively.
When connected step-down and energized at 480 VAC, the maximum peak inrush current is approximately 990 Amps or 11 times the rated 90 Amp primary winding full load current. But when connected step-up and energized at 208 VAC, the maximum peak inrush can reach 7700 Amps or 37 times the rated 208 Amp secondary winding full load current.
Normally, the taps on the primary can be used to compensate for instances where the voltage of the source is not the same as the rated voltage of the transformer. When the normal secondary is energized to serve as the primary, there are no adjustment taps. So if the voltage of the source is higher than the rated voltage of the transformer?s normal secondary, it will be over-excited resulting in higher than rated core loss and exciting current, this is generally not a serious concern unless the over-voltage exceeds 5%.
When the secondary (WYE) of a DELTA-WYE transformer is energized, instead of the primary (DELTA), the neutral should not be connected to ground or to the enclosure. An unbalanced source might circulate current in the transformer DELTA and cause over-heating. Also the impedance to ground of the transformer might be lower than the system ground impedance and excessive current to ground might over-heat the transformer. Unbalanced conditions could cause a voltage to ground to appear on the enclosure if it was connected to the neutral.
Small transformers, rated less than 3 KVA, are compensated for voltage regulation to ensure that they will provide rated voltage at rated load. This means that the output voltage will be 5 to 10% lower than rated if the winding are reverse connected. The transformer was originally designed for primary and secondary as stated in the nameplate, which is validated by the UL mark. If the transformer is reverse-fed or used in an application other than what it was originally intended for, make sure you consult your local codes and UL standards.
Transformer
This is from Square D
Back Feeding Transformers
Occasionally, someone has an urgent need for a non-stock step-up transformer.
The natural inclination is to grab an in-stock step-down unit and to operate it in reverse. Except for small (less than 3 KVA models, which have compensated windings to provide rated voltage at rated load), if permitted by local codes, then it is generally OK to reverse feed a transformer but there are several precautions that should be considered. To illustrate, lets assume that a standard 9T23B3874 step-down transformer rated 75 KVA/3-phase/60 HZ with a 480 VAC Delta Primary with (6) 2-1/2% FC2A4BN voltage taps and a 208Y/120 VAC Secondary is to be operated step-up; that is, 208 VAC input to 480 VAC output.
The installer may discover that the primary protector, having been properly selected and applied per Article 450 of the National Electrical Code, nevertheless, trips or blows when he attempts to energize the reverse operated transformer. This phenomenon can occur because the low impedance winding (the 208Y/120 VAC one that was intended by design to be the secondary winding) now serves as the primary and the value of the magnetizing inrush current (Mag-I) is actually much greater than expected. The Mag-I experienced when energizing transformers is similar to the inrush current associated with motor starting. The primary and secondary full load amps of the subject transformer are 90 Amps @ 480 VAC and 208 Amps @ 208 VAC respectively.
When connected step-down and energized at 480 VAC, the maximum peak inrush current is approximately 990 Amps or 11 times the rated 90 Amp primary winding full load current. But when connected step-up and energized at 208 VAC, the maximum peak inrush can reach 7700 Amps or 37 times the rated 208 Amp secondary winding full load current.
Normally, the taps on the primary can be used to compensate for instances where the voltage of the source is not the same as the rated voltage of the transformer. When the normal secondary is energized to serve as the primary, there are no adjustment taps. So if the voltage of the source is higher than the rated voltage of the transformer?s normal secondary, it will be over-excited resulting in higher than rated core loss and exciting current, this is generally not a serious concern unless the over-voltage exceeds 5%.
When the secondary (WYE) of a DELTA-WYE transformer is energized, instead of the primary (DELTA), the neutral should not be connected to ground or to the enclosure. An unbalanced source might circulate current in the transformer DELTA and cause over-heating. Also the impedance to ground of the transformer might be lower than the system ground impedance and excessive current to ground might over-heat the transformer. Unbalanced conditions could cause a voltage to ground to appear on the enclosure if it was connected to the neutral.
Small transformers, rated less than 3 KVA, are compensated for voltage regulation to ensure that they will provide rated voltage at rated load. This means that the output voltage will be 5 to 10% lower than rated if the winding are reverse connected. The transformer was originally designed for primary and secondary as stated in the nameplate, which is validated by the UL mark. If the transformer is reverse-fed or used in an application other than what it was originally intended for, make sure you consult your local codes and UL standards.
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