I've been reading some previous posts on backfeeding transformers and noticed a number of comments indicating it is wrong to tie in the primary neutral to the transformer when doing so. Can someone please explain why? My concern is I was told by my boss (my dad) to use the neutral even though it wouldn't be used on a job I did recently and I have another I'll be installing tomorrow. If this is a problem I'd like to correct the previous installation and do this one correctly the first time. Thank you.
backfeeding transformers
- Thread starter spider2
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jcormack
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- Pennsylvania
Re: backfeeding transformers
When the primary of a transformer is of a Wye configuration (ireguardless of of normal or "backfed") the center point (or neutral is generally *NOT* connected to earth ground. This is not done for many reasons, circulating currents on imbalance, harmonic transferance, short circuit contribution to primary system ground faults to name a few. But perhaps the biggest misconception is this is required to provide a neutral for the secondary - if the transformer is connected Wye-delta, grounding the center of the Wye on the primary side *Does Not* provide a ground path for a secondary fault. Power transformers are typically installed as Delta-Wye, or Delta-Delta for a reason. When using Wye-Wye or Wye-Delta do not ground the primary Wye's center.
When the primary of a transformer is of a Wye configuration (ireguardless of of normal or "backfed") the center point (or neutral is generally *NOT* connected to earth ground. This is not done for many reasons, circulating currents on imbalance, harmonic transferance, short circuit contribution to primary system ground faults to name a few. But perhaps the biggest misconception is this is required to provide a neutral for the secondary - if the transformer is connected Wye-delta, grounding the center of the Wye on the primary side *Does Not* provide a ground path for a secondary fault. Power transformers are typically installed as Delta-Wye, or Delta-Delta for a reason. When using Wye-Wye or Wye-Delta do not ground the primary Wye's center.
- Location
- Bremerton, Washington
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Re: backfeeding transformers
This question has come up before, my comment while it may work to reverse a 480 x 120/208, it would be violation of the listing unless it is listed for that use.
Also mgf such as Square D make a listed step up transforme...but its a lot more expensive.
This question has come up before, my comment while it may work to reverse a 480 x 120/208, it would be violation of the listing unless it is listed for that use.
Also mgf such as Square D make a listed step up transforme...but its a lot more expensive.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: backfeeding transformers
quote
Power transformers are typically installed as Delta-Wye, or Delta-Delta for a reason. When using Wye-Wye or Wye-Delta do not ground the primary Wye's center.
Many power transformers, particularly medium voltage, are often installed wye-wye, and the primary neutral must be grounded to avoid excessive secondary voltages. This is not common in reverse fed dry type transformers, but when encountered, must be followed.
If the primary neutral of a wye-delta is grounded, as in a reverse feed transformer, it will tend to balance the panel on the 208 side, and will often trip the 208 breaker even with no load on the xfmr.
I agree with no neutral on a wye-delta, but not on a wye-wye.
Jim T
quote
Power transformers are typically installed as Delta-Wye, or Delta-Delta for a reason. When using Wye-Wye or Wye-Delta do not ground the primary Wye's center.
Many power transformers, particularly medium voltage, are often installed wye-wye, and the primary neutral must be grounded to avoid excessive secondary voltages. This is not common in reverse fed dry type transformers, but when encountered, must be followed.
If the primary neutral of a wye-delta is grounded, as in a reverse feed transformer, it will tend to balance the panel on the 208 side, and will often trip the 208 breaker even with no load on the xfmr.
I agree with no neutral on a wye-delta, but not on a wye-wye.
Jim T
Re: backfeeding transformers
Interesting.
Maybe I have been missing something here but as an application engineer for one of the larger DTDT transformer manufacturers we never had a policy or a reason why you couldn't use a 480-208y/120 transformer as a step up. We always instructed that the grounding jumper (strap) to the enclosure from the XO of the Y must be removed simply connecting to the X1, X2, and X3 while bring the EGC and bonding it to the enclosure. The 480v delta would be addressed as one would a delta separate source and corner grounded unless there were a good reason why not to which would be addressed a would be done when applying an ungrounded delta.
With regard to OCPDs, they are applied in the same way and proportions as for step down. Inrush currents are also proportionate. If they were 12x the FLA of the HV side they would be 12x the FLA of the LV side.
The only other issue is that the taps would still be on the HV side and would have to be looked at backwards, i.e. (2)2-1/2 FCAN and (4)2-1/2FCBN would be (4) and (2) where you loose (2)FCBN taps only being able to compensate for 5% below normal votage and not 7-1/2 or 10%.
I look at it as being dependent upon how the NP reads. If it marked primary and secondary or simply showing HV and LV. On would expect the primary to be the input whether HV or LV and the secondary the output. If so in all likelyhood using a transformer that is marked as such would violate the NEC.
If simply marked with the voltages just because we expect a primary side to be HV doesn't mean that we can't define the primary as being the LV side and the secondary the HV side.
Besides what I have from my from my frmer employs I also have information which is available on both the ACME and Magnetek/Jefferson DTDT sites which all state the same thing, that they will provide the "correct NP voltage." The only two cautions are:
the delta-wye now creates a wye-delta and not to use the X0 which is only addressed by Jefferson. However, Jefferson sill doesn't address unbonding the grounding jumper to the enclosure which still must be done.
On 1kva and smaller transformers the turns ratio are such that the secondardy will have a higher voltage than the NP at NL but will drop when loaded. What this means is that the HV output of the transformer will often have a significantly lower voltage than shown the NP when "reversed feed."
Interesting.
Maybe I have been missing something here but as an application engineer for one of the larger DTDT transformer manufacturers we never had a policy or a reason why you couldn't use a 480-208y/120 transformer as a step up. We always instructed that the grounding jumper (strap) to the enclosure from the XO of the Y must be removed simply connecting to the X1, X2, and X3 while bring the EGC and bonding it to the enclosure. The 480v delta would be addressed as one would a delta separate source and corner grounded unless there were a good reason why not to which would be addressed a would be done when applying an ungrounded delta.
With regard to OCPDs, they are applied in the same way and proportions as for step down. Inrush currents are also proportionate. If they were 12x the FLA of the HV side they would be 12x the FLA of the LV side.
The only other issue is that the taps would still be on the HV side and would have to be looked at backwards, i.e. (2)2-1/2 FCAN and (4)2-1/2FCBN would be (4) and (2) where you loose (2)FCBN taps only being able to compensate for 5% below normal votage and not 7-1/2 or 10%.
I look at it as being dependent upon how the NP reads. If it marked primary and secondary or simply showing HV and LV. On would expect the primary to be the input whether HV or LV and the secondary the output. If so in all likelyhood using a transformer that is marked as such would violate the NEC.
If simply marked with the voltages just because we expect a primary side to be HV doesn't mean that we can't define the primary as being the LV side and the secondary the HV side.
Besides what I have from my from my frmer employs I also have information which is available on both the ACME and Magnetek/Jefferson DTDT sites which all state the same thing, that they will provide the "correct NP voltage." The only two cautions are:
the delta-wye now creates a wye-delta and not to use the X0 which is only addressed by Jefferson. However, Jefferson sill doesn't address unbonding the grounding jumper to the enclosure which still must be done.
On 1kva and smaller transformers the turns ratio are such that the secondardy will have a higher voltage than the NP at NL but will drop when loaded. What this means is that the HV output of the transformer will often have a significantly lower voltage than shown the NP when "reversed feed."
- Location
- Wisconsin
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- PE (Retired) - Power Systems
Re: backfeeding transformers
Tom,
I have never seen any documentation from any legitmate source that says backfeeding a "standard", 15KVA or larger, transformer is a violation of any listing or manufacturers instructions.
By the same token, I have always heard of problems and warnings against backfeeding smaller, especially "control" and T-connected, and specialty transformers.
Of course, some manufacturers may have prohibitions against backfeeding so it is always important to confirm the backfeeding with the specific transformer manufacturer.
Tom,
I have never seen any documentation from any legitmate source that says backfeeding a "standard", 15KVA or larger, transformer is a violation of any listing or manufacturers instructions.
By the same token, I have always heard of problems and warnings against backfeeding smaller, especially "control" and T-connected, and specialty transformers.
Of course, some manufacturers may have prohibitions against backfeeding so it is always important to confirm the backfeeding with the specific transformer manufacturer.
Re: backfeeding transformers
Speaking of "T" connected -
You may find it interesting that many of the "plain Jane" 3-phase 480-208/120 dry xfmrs in the smaller KVA ratings (3, 6, 9, even 15) are really "T" connected not D-Y.
You may have to study the diagram closely to see that since the terminal #'s are what we are used to:H1,H2,H3 - X1,X2,X3,X0.
Speaking of "T" connected -
You may find it interesting that many of the "plain Jane" 3-phase 480-208/120 dry xfmrs in the smaller KVA ratings (3, 6, 9, even 15) are really "T" connected not D-Y.
You may have to study the diagram closely to see that since the terminal #'s are what we are used to:H1,H2,H3 - X1,X2,X3,X0.
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