Stepping up voltage for voltage drop.

[LISHAJI]

I want to provide a 60A 1? 120/240V supply to a pavilion with ordinary lighting loads. This pavilion is about 1500ft from a 200A 1? 120/240V loadcenter. I am considering using (2) 240 x 480V to 120/240V transformer to step up and then step down at the pavilion to reduce feeder conductor sizes. I would also, additionally, use the taps at HV side to compensate for any voltage drop. Attached is a wiring diagram of the transformer.

My question is, if I would connect the center taps of both 120/240V and 240x480V to ground, will there be an issue?.

In the wiring diagram this would be X2, X3 at LV side and 5, 10 at HV side all connected together at the step up transformer; and X2, X3 at LV side and 4, 9 at HV side all connected together at the step down transformer.

 

[Jraef]

I want to provide a 60A 1? 120/240V supply to a pavilion with ordinary lighting loads. This pavilion is about 1500ft from a 200A 1? 120/240V loadcenter. I am considering using (2) 240 x 480V to 120/240V transformer to step up and then step down at the pavilion to reduce feeder conductor sizes. I would also, additionally, use the taps at HV side to compensate for any voltage drop. Attached is a wiring diagram of the transformer.

My question is, if I would connect the center taps of both 120/240V and 240x480V to ground, will there be an issue?.

In the wiring diagram this would be X2, X3 at LV side and 5, 10 at HV side all connected together at the step up transformer; and X2, X3 at LV side and 4, 9 at HV side all connected together at the step down transformer.

How about this; explain why you think that would be necessary for each side?

 

[hurk27]

I would not be using the taps to over come a voltage drop as when the load changes so will the voltage at the other end, and this could lead to over voltage equipment when the load is light, you only use taps when the load is fairly fixed, the correct way is to size the 1500 foot run for the 30 amp load to keep the VD at a minimum percentage.

As far as the grounding goes, you never ground the primary side of the transformer (X2-X3), you run an EGC to the transformer case and to the tap jumper and then run this EGC with the secondaries to the other transformers case and it's secondary center tap (X2-X3) which is your derived neutral for your panel at the pavilion, at the pavilion you must install a grounding electrode system just like at any out-building.

Also note if you plan to install the transformer at the pavilion inside it, then I would use a 600 volt rated fused disconnect at the first transformer then a 600 volt disconnect at the pavilion as using a 480/277 rated main breaker would be costly, come off a the transformer to a small panel with a main breaker and your done. like 5% or lower, I get for copper that a #3 AWG will give you a 4.6% VD, if you want to go aluminum then a #1awg would give you 4.75% VD.

 

[LISHAJI]

@Wayne .
The reason I want to ground the high voltage is for ground fault protection. I would be running EGC to connect both the transformers. Without grounding the HV, the question will remain, with just EGC run, would there be a ground fault current? I agree I need to install a fused disconnect either at the supply end or pavilion on the 480V. I am dropping the idea of using tap for voltage drop as it is load dependent, instead would use a bigger conductor.

 

[LISHAJI]

How about this; explain why you think that would be necessary for each side?

I would be using 120V lighting and receptacle loads at the pavilion.

 

[kwired]

@Wayne .
The reason I want to ground the high voltage is for ground fault protection. I would be running EGC to connect both the transformers. Without grounding the HV, the question will remain, with just EGC run, would there be a ground fault current? I agree I need to install a fused disconnect either at the supply end or pavilion on the 480V. I am dropping the idea of using tap for voltage drop as it is load dependent, instead would use a bigger conductor.

The supply side transformer has 240 volt primary, you connect a EGC from the 120/240 system to the case to protect from faults on the primary. The 480 volt secondary can be grounded at any point, but most would choose the center tap in order to have less voltage to ground. This will give the secondary low impedance to the EGC should the secondary have a ground fault. That same grounded conductor is only connected to the case of the load side transformer and not to the midpoint of it's primary winding, and of course to have 120/240 as final secondary voltage the midpoint of that winding must be the point that is grounded.


As mentioned don't use voltage adjustment taps for voltage drop compensation - when lightly loaded you will have overvoltage.

Might want to look at the cost difference between larger conductors and using the transformers, plus all associated costs - differences in raceway sizes, extra switching/other gear and decide which way is worth it. Also remember transformers will have losses and those add up over time.

 

[LISHAJI]

Thanks for the help, that sums it all.

The supply side transformer has 240 volt primary, you connect a EGC from the 120/240 system to the case to protect from faults on the primary. The 480 volt secondary can be grounded at any point, but most would choose the center tap in order to have less voltage to ground. This will give the secondary low impedance to the EGC should the secondary have a ground fault. That same grounded conductor is only connected to the case of the load side transformer and not to the midpoint of it's primary winding, and of course to have 120/240 as final secondary voltage the midpoint of that winding must be the point that is grounded.


As mentioned don't use voltage adjustment taps for voltage drop compensation - when lightly loaded you will have overvoltage.

Might want to look at the cost difference between larger conductors and using the transformers, plus all associated costs - differences in raceway sizes, extra switching/other gear and decide which way is worth it. Also remember transformers will have losses and those add up over time.

 

[hurk27]

@Wayne .
The reason I want to ground the high voltage is for ground fault protection. I would be running EGC to connect both the transformers. Without grounding the HV, the question will remain, with just EGC run, would there be a ground fault current? I agree I need to install a fused disconnect either at the supply end or pavilion on the 480V. I am dropping the idea of using tap for voltage drop as it is load dependent, instead would use a bigger conductor.

A ground fault on the secondary side of a transformer will only show as line to line current on the primary side, current only wants to return to source and each transformer is a separate source, a ground fault on the secondary will not cause current to flow in the EGC on the primary side, this is why we never bond the center tap on the primary side no matter if it is being used as a step up or step down, yes we have to run a EGC with the primary but we only connect it to the case of the transformer and to the center tap on the secondary as required by code.

 

[LISHAJI]

@wayne
What I meant was if the HV side is not grounded, a single line fault won't produce any fault current. Its only when both the lines are faulted to ground, it would cause a fault current and then would be reflected as line current on the other side of the transformer. So during a single line ground fault (say to the transformer enclosure) is there a safety issue with the transformer energised with HV?

On the other hand if I ground the HV, then a single line ground fault would produce enough fault current to clear the fault. However I am not sure whether safety is any better in either cases, excepting in one case there is 480V L-L and in the second case it is 240 L-G.

A ground fault on the secondary side of a transformer will only show as line to line current on the primary side, current only wants to return to source and each transformer is a separate source, a ground fault on the secondary will not cause current to flow in the EGC on the primary side, this is why we never bond the center tap on the primary side no matter if it is being used as a step up or step down, yes we have to run a EGC with the primary but we only connect it to the case of the transformer and to the center tap on the secondary as required by code.

 

[kwired]

@wayne
What I meant was if the HV side is not grounded, a single line fault won't produce any fault current. Its only when both the lines are faulted to ground, it would cause a fault current and then would be reflected as line current on the other side of the transformer. So during a single line ground fault (say to the transformer enclosure) is there a safety issue with the transformer energised with HV?

On the other hand if I ground the HV, then a single line ground fault would produce enough fault current to clear the fault. However I am not sure whether safety is any better in either cases, excepting in one case there is 480V L-L and in the second case it is 240 L-G.

Reading through 250.20, I don't think this system would be required to be grounded, but if it is not grounded 250.21(B) does require ground detectors to be used. Equipment grounding conductors and grounding electrodes are still required even if the system is ungrounded, so the only real advantage of going ungrounded is that you don't have immediate shut down because of opening overcurrent protection when a ground fault occurs. But if you have a fault indicated and disregard the indicator you basically end up with a grounded system and will have interruption if a ground fault occurs on the other line.

 

[hurk27]

@wayne
What I meant was if the HV side is not grounded, a single line fault won't produce any fault current. Its only when both the lines are faulted to ground, it would cause a fault current and then would be reflected as line current on the other side of the transformer. So during a single line ground fault (say to the transformer enclosure) is there a safety issue with the transformer energised with HV?

On the other hand if I ground the HV, then a single line ground fault would produce enough fault current to clear the fault. However I am not sure whether safety is any better in either cases, excepting in one case there is 480V L-L and in the second case it is 240 L-G.

Yes but in your first post you stated you wanted to ground both center taps (X2-X3 and 4-9 on the HV side) on the same transformer which is why I am trying to point out that the grounding of the X2-X3 on the step up transformer or the 4-9 center tap on the step down transformer is totally unnecessary, by only grounding the secondaries (4-9 on the step up, and X2-X3 on the step down) will provide all the fault current path you can ever get, a fault on the 480v lines to ground will only go back to the secondaries of the step up transformer, it will not flow on a EGC connected to the X2-X3 back to the panel, this is because a transformer is isolating and the primary and secondary are isolated from each other, a fault on the 480v lines does not want to return back to the service grounding system, it is only returning to the center tap (4-9) on the secondaries, the same goes for the step down transformer, you only ground the X2-X3 center tap on the secondaries, again for the same reason above there is no reason to ever ground the primary (4-9) on the step down transformer.

Another point is if this was a common core three phase WYE and you grounding the X0 of the primary of the transformer back to the supply grounding and it is also a WYE supply you would have all kinds of circulating currents that would cause overheating of the EGC and the transformer, the NEC does not require the primary of a transformer to be grounded, the reason is the conductors feeding a primary already have a referance to the supply conductors at the suply end, the same goes for the 480v conductors between the two transformers, if you connect the EGC run with these conductors to the center tap on the secondary, you now have just given this EGC a referance to the 480v conductors and a fault will cause current to flow, at the step down transformer you will do this again by connecting an EGC to the X2-X3 on this transformer, not the 120/240 secondaries have a referance to these conductors and this gives them a fault path.

Think of it this way, a ground fault is only a fault between the center tap and one of the phase conductors (hot's), if you were to short one of the hots to the center tap on a secondary of a transformer don't you think you would have a fault?, now attach a EGC to this center tap and short a hot to this EGC do you think current would not flow and open a breaker? yes it will without any EGC to the primary.

Here is how all transformers are run, the EGC run to the step up transformer is sized for the 60 amps supplying the first transformer, it is connected to the transformer case and a jumper is made to the center tap on the secondaries (4-9 Tap) then a EGC is run from this tap with the 480v secondaries sized for the 30 amps, to the step down transformer, it is connected to this transformers case and a jumper is made to the secondary center tap (X2-X3) then an EGC is run from this tap with the secondaries to the load whether it is a breaker panel or just a load, in no case is there any reason to ever connect any EGC to the primary of either transformer.


As was stated there are other requirements in the NEC for grounding electrodes at each SDS (transformer) as well as other requirements, maybe someone could find a graphic that shows the above.

I think this is what Jraef was questioning in post 2 also.

 

[LISHAJI]

Yes but in your first post you stated you wanted to ground both center taps (X2-X3 and 4-9 on the HV side) on the same transformer which is why I am trying to point out that the grounding of the X2-X3 on the step up transformer or the 4-9 center tap on the step down transformer is totally unnecessary, by only grounding the secondaries (4-9 on the step up, and X2-X3 on the step down) will provide all the fault current path you can ever get, a fault on the 480v lines to ground will only go back to the secondaries of the step up transformer, it will not flow on a EGC connected to the X2-X3 back to the panel, this is because a transformer is isolating and the primary and secondary are isolated from each other, a fault on the 480v lines does not want to return back to the service grounding system, it is only returning to the center tap (4-9) on the secondaries, the same goes for the step down transformer, you only ground the X2-X3 center tap on the secondaries, again for the same reason above there is no reason to ever ground the primary (4-9) on the step down transformer.

Another point is if this was a common core three phase WYE and you grounding the X0 of the primary of the transformer back to the supply grounding and it is also a WYE supply you would have all kinds of circulating currents that would cause overheating of the EGC and the transformer, the NEC does not require the primary of a transformer to be grounded, the reason is the conductors feeding a primary already have a referance to the supply conductors at the suply end, the same goes for the 480v conductors between the two transformers, if you connect the EGC run with these conductors to the center tap on the secondary, you now have just given this EGC a referance to the 480v conductors and a fault will cause current to flow, at the step down transformer you will do this again by connecting an EGC to the X2-X3 on this transformer, not the 120/240 secondaries have a referance to these conductors and this gives them a fault path.

Think of it this way, a ground fault is only a fault between the center tap and one of the phase conductors (hot's), if you were to short one of the hots to the center tap on a secondary of a transformer don't you think you would have a fault?, now attach a EGC to this center tap and short a hot to this EGC do you think current would not flow and open a breaker? yes it will without any EGC to the primary.

Here is how all transformers are run, the EGC run to the step up transformer is sized for the 60 amps supplying the first transformer, it is connected to the transformer case and a jumper is made to the center tap on the secondaries (4-9 Tap) then a EGC is run from this tap with the 480v secondaries sized for the 30 amps, to the step down transformer, it is connected to this transformers case and a jumper is made to the secondary center tap (X2-X3) then an EGC is run from this tap with the secondaries to the load whether it is a breaker panel or just a load, in no case is there any reason to ever connect any EGC to the primary of either transformer.


As was stated there are other requirements in the NEC for grounding electrodes at each SDS (transformer) as well as other requirements, maybe someone could find a graphic that shows the above.

I think this is what Jraef was questioning in post 2 also.

Perfect! Thanks