480V Single Phase with Grounded Leg

[jbt260]

Thank you all for your input! I have to maintain the existing lighting circuit as is. Each light pole is bonded to earth and to the grounded leg of the 480V 2 wire circuit. There is probably about a 1/2 mile of total circuit feet. The POCO will only provide 480/277V to the building. I had to derive a 480V 2 wire system with a grounded leg from this service. I originally tried to feed the circuit with two phases from the 480/277V system, not knowing that one of the hots was grounded at each pole. You can guess what happened when the circuit was energized !!! Since this original post, I have found a 50KVA 480-480 single phase transformer. I believe this will solve my problems.

Thanks again!

 

[LarryFine]

. . . I have found a 50KVA 480-480 single phase transformer.

That's the only solution.

One more thing to pay attention to: which phase has the greatest load, particularly at night, so you supply your transformer from the other two phases.

 

[winnie]

An isolation transformer is not the only possible solution, but it is the most straightforward solution.

In addition to the isolation approaches, there are a couple of 'buck-boost' or similar non-isolated transformer connections that you might be able to use.

For example, if you can get a 277V:208V single phase transformer, then you could connect it in a boost configuration (277V applied to the 277V coil, 480V taken across _both_ 277Vand 203V coils in series) and get 480V single phase at 50KVA using only a 21KVA transformer. I doubt that these transformers exist, but maybe they are common for street lighting applications, and I don't know about them because I don't do street lighting In addition to the uncommon transformer, this also has the problem of putting all the load on a single phase. Note that this gives an output voltage of 485V.

Another possible approach is to use a 30KVA 277V:277V single phase transformer; drive the primary with one phase and put the secondary in series with another phase. If you get the phasing right, then you get a 480V output. (Get the phasing wrong, and you get 277V) This puts the load equally on 2 phases, just like using a 480V:480V transformer, just with a smaller transformer and lower losses.

The third approach that could work is to use a pair of 480V:120V transformers. Supply one transformer phase A to B, the other transformer phase A to C, and 'stack' the 120V outputs in series with phase A. The far end of the stack will be at 485V, with some amount of loading on all 3 phases. You would need 2 10KVA transformers. A standard 30KVA 480V delta : 208V/120V wye transformer could be used in this fashion to provide your 50KVA two wire 480V.

-Jon

 

[benaround]

Here is my take on this,

There was a rest area with just lights ( no buildings ) and the POCO did the lighting job.

Now, there is a new building at the rest area, and they want the 'lights' tied into the new

premise wiring system. The problem is that there is not an EGC run to the pole lights, or

between poles either ( 410.20, 410.21 ). This lighting job is no longer under the POCO, it

is under the NEC, and should be installed to the NEC. JMO.

 

[Cavie]

Here is my take on this,

There was a rest area with just lights ( no buildings ) and the POCO did the lighting job.

Now, there is a new building at the rest area, and they want the 'lights' tied into the new

premise wiring system. The problem is that there is not an EGC run to the pole lights, or

between poles either ( 410.20, 410.21 ). This lighting job is no longer under the POCO, it

is under the NEC, and should be installed to the NEC. JMO.

Let me explain this one more time. Very Common POCO street lighting. 480v 2 wire system. 1 black wire 480v to ground. 1 white wire Neutral. 1 green wire EGC. Green wire runs from service to pole to pole to pole. It is connected to the Pole, the light fixture and a 20' ground rod at each pole.

 

[LarryFine]

In addition to the isolation approaches, there are a couple of 'buck-boost' or similar non-isolated transformer connections that you might be able to use.

Problem: If the source supply is grounded (and we heard it is), a non-isolated circuit will not be happy with a grounded circuit conductor.

 

[winnie]

Problem: If the source supply is grounded (and we heard it is), a non-isolated circuit will not be happy with a grounded circuit conductor.

I believe that I correctly dealt with that: with these approaches: you keep the same grounded conductor. In a 480/277Y system, one leg of a 277V circuit is grounded. If you now do something to boost the other leg to 480V, the grounded leg remains unchanged.

-Jon

 

[iwire]

Let me explain this one more time. Very Common POCO street lighting. 480v 2 wire system. 1 black wire 480v to ground. 1 white wire Neutral. 1 green wire EGC. Green wire runs from service to pole to pole to pole. It is connected to the Pole, the light fixture and a 20' ground rod at each pole.

Does this sound to you like there is a green?

I have to re-feed an existing roadway lighting circuit that is 480V single phase with a grounded leg. The leg is grounded at every light pole.

It does not to me and if this is no longer under the control of the utility it sounds like a violation.

And Cavie .......... the crow is not tasting to good.

 

[dbuckley]

A tfmr manufacturer came up with a solution for a 50KVA, 480V-480V, single phase transformation. They suggest using two 25KVA, 240x480V Primary, 120/240V Secondary, single phase transformers. You wire the primaries in parallel for 480V. You wire the secondaries in series for 480V. See attached diagram. Any thoughts are welcome !

As long as the manufacturer of the trannies says their trannies are suitable for this arrangement then it is fine and on their head.

But.... the issue that springs to mind for me is one of insulation of the transformer and/or their terminations; in the diagram assume the left hand transformer X4 will be grounded, so the terminal X1 of the right hand transformer will have 480V on it with respect to ground, which is exactly what you want. However when the transformer was designed it was presumably designed with a maximum terminal to ground voltage of 240V. You're doubling the voltage normally seen. So as long as the manufacturer signs off on this, then thats Ok.

In the world of street lighting, the arrangement of one (and I'll use a variation of the UK term here) combined neutral and ground conductor and one hot conductor is commonly seen practice in many juristictions around the world, in fact I gaze at street lights wired this way every time I'm stuck in a several particular traffic jams. But such things are usually the preserve of some authority, and this circuit arrangement is generally not permitted outside of utility or similar usage, and thus I think that when Bob suggests it isn't an NEC acceptable arrangement, I suspect he is right. Thus there is a regulatory hurdle to this problem; the technical solution of a correct transformer arrangement has been correct since post 1.

 

[jbt260]

But.... the issue that springs to mind for me is one of insulation of the transformer and/or their terminations; in the diagram assume the left hand transformer X4 will be grounded, so the terminal X1 of the right hand transformer will have 480V on it with respect to ground, which is exactly what you want. However when the transformer was designed it was presumably designed with a maximum terminal to ground voltage of 240V. You're doubling the voltage normally seen. So as long as the manufacturer signs off on this, then thats Ok.

This is a very good point. I checked with the xfmr manuf and they said the xfmrs are insulated to 600V.

 

[don_resqcapt19]

What edition of the code applies to this installation? If this is under the 2005 code or earlier, you could rebond the grounded conductor at each structure (pole) and not use an EGC. If this is under the 2008 code, you must install an EGC with the circuit conductors.

 

[benaround]

What edition of the code applies to this installation? If this is under the 2005 code or earlier, you could rebond the grounded conductor at each structure (pole) and not use an EGC. If this is under the 2008 code, you must install an EGC with the circuit conductors.

Don, (2005) You can do this to a single branch circuit ? Is the reference 250.32(B)(2) ?

On a seperate note, an open neutral connection on this setup would be nasty, 480 to grd

 

[iwire]

Don, (2005) You can do this to a single branch circuit ? Is the reference 250.32(B)(2) ?

Great point.

On a separate note, an open neutral connection on this setup would be nasty, 480 to grd

I don't even what to think about it, I have only gotten hit with 480 once and it was between two spots on the back of one hand .......... hurt like hell.

 

[don_resqcapt19]

Frank,
I don't see anything in 250.32(B)(2) (2005 code) that says you can't use that provision for a single branch circuit.

 

[iwire]

OK I should know better than to disagree with Don without at least looking it up. :grin:

I thought 250.32(B) applied to feeders only.

 

[hurk27]

OK I should know better than to disagree with Don without at least looking it up. :grin:

I thought 250.32(B) applied to feeders only.

Nope

250.32(A)Where two or more buildings or structures are supplied from a common ac service by a feeder(s) or branch circuit(s)

Still a very bad option IMO

 

[hurk27]

And to also include, that these are probably long runs, and the voltage drop of the grounded conductor will be on each and every pole, and ground rods will do nothing for this.

 

[jghrist]

Only if you are starting from a 480Y/277V system.

A single phase 480:480 transformer, like the OP asked about, could be used to create a 480 2-wire secondary which could then be treated the same as any other 2-wire system.

But the OP asked about serving this from a 480/277Y system. If there are other loads at 277V, then this can't be done. If there are no 277V loads, you could leave the transformer neutral ungrounded and ground a phase. I don't know if this would be code compliant or if the multigrounding of the grounded phase would be code compliant.

 

[don_resqcapt19]

But the OP asked about serving this from a 480/277Y system. If there are other loads at 277V, then this can't be done. If there are no 277V loads, you could leave the transformer neutral ungrounded and ground a phase. I don't know if this would be code compliant or if the multigrounding of the grounded phase would be code compliant.

Adding a transformer to get single phase grounded 480 to the existing 480/277 system would not be a problem.