Neutral required? Conflicting opinions.

[sfav8r]

A friend of mine lives in a rural area. He has to run from the POCO transformer about 1,000' to his house. The voltage at the pole is 600v. The design engineer says he needs to run 3 wires from the 600v transformer to the step-down transformer. Since this is a 1,000 run, the extra wire is a cost factor.

I don''t see the need for the third wire. Cant he run two 750kcm cables from the 600v transformer to the step-down transformer, then ground the tap at the load side and run it to the house?

I hope I'm being clear. I've been up for 20 hours and I'm can barely add and subtract right now

Any code references or other material to forward to the engineer would be appreciated.

Thanks

 

[jeff43222]

I'm no expert on the subject, but doesn't there need to be a path back to the PoCo? Are you proposing the neutral for all those 120V circuits be connected to ground and nothing else?

 

[LarryFine]

How many ungrounded conductors will there be in the 1000' run, one or two? Unless one of the 600v conductors is grounded, there will need to be a system ground run with the two circuit conductors.

Unless I'm missing something, the POCO has to run a ground to the last transformer, and its secondary service neutral, too.

Afterthought: might it not be more economical (and efficient) to run the primary voltage all the way to the house transformer? (Unless 600v IS the primary voltage, but that's kinda low.) The wire would be a lot smaller.

 

[iwire]

First am I understanding this correctly.

The plan is to receive power from the POCO at 600 volts single phase run this 1000' to the home to a step down transformer that will give you a 240/120 service at the house?

Your thought is that only two 'hots' are needed as the neutral will not be used at the transformer?

It would work but it would be unsafe and not meet code.

As an example if you are supplying a 240 volt appliance would you run just the two hots or would you bring two hots and a grounding means?

If you do not bring the grounded conductor and bond it to the enclosure the enclosure may become energized if there is a fault.

All I am getting at is on a service the grounded conductor takes the place of an EGC and must be run for safety reasons.

The code requirement is 250.24(B)

Grounded Conductor Brought to Service Equipment. Where an ac system operating at less than 1000 volts is grounded at any point, the grounded conductor(s) shall be run to each service disconnecting means and shall be bonded to each disconnecting means enclosure. The grounded conductor(s) shall be installed in accordance with 250.24(B)(1) through (B)(3).

But your post leaves out a bunch.

I assume the customer will own the 600 to 240/120 transformer?

There will have to be overcurrent protection and a disconect on the supply side of that transformer which will be your service disconect.

The load side of the transformer will be treated as a separately derived system.

 

[hardworkingstiff]

I'm curious as to which PoCo will supply 600-volts. Around here they only will supply 120/240 single-phase.

A transformer from primary to 600-volts is a special order non stock item for them and they don't care to be responsible for maintaining it.

 

[sc57ford]

Also curious as to why 750KcMIL, at 600v the voltage drop should not be all that great unless the customer has a large motor starting and stopping often. 750KcMIL is good for 535A, 90deg, XHHW in conduit, and 885 in free air. Just curious.

 

[winnie]

I see no reason to run a neutral in this application. If this is a _grounded_ system, then the grounded conductor must be run, but this grounded conductor need not be the neutral.

-Jon

 

[LarryFine]

I see no reason to run a neutral in this application. If this is a _grounded_ system, then the grounded conductor must be run, but this grounded conductor need not be the neutral.

Jon, are you saying that an EGC need not be run with the 600v conductors?

 

[iwire]

I see no reason to run a neutral in this application. If this is a _grounded_ system, then the grounded conductor must be run, but this grounded conductor need not be the neutral.

-Jon

Winne if the 600 volts is coming right from the power company than this is a service.

A grounded system service must have the grounded conductor brought with it, there is no 'EGC' to use.

 

[sfav8r]

First am I understanding this correctly.

The plan is to receive power from the POCO at 600 volts single phase run this 1000' to the home to a step down transformer that will give you a 240/120 service at the house?

Your thought is that only two 'hots' are needed as the neutral will not be used at the transformer?

It would work but it would be unsafe and not meet code.


The code requirement is 250.24(B)

But your post leaves out a bunch.

I assume the customer will own the 600 to 240/120 transformer?

There will have to be overcurrent protection and a disconect on the supply side of that transformer which will be your service disconect.

The load side of the transformer will be treated as a separately derived system.

To tidy up a few things. The voltage at the POCO is higher (I don't know but it think it's 1.7KV) There will be a 600V transformer installed on the same pole. That transformer will feed a step-down transformer 1,000 feet away.

Running the full voltage the 1,000 feet was discussed and it was decided against. I wasn't part of that discussion.

My thought was this. The 600V transformer can feed the 240/120 transforemer 1,000 feet away with just two wires. In response to jeff43222, This IS the path back to the POCO. At the destination, there will be three wires that leave the 240/120 transformer. The two phases and the neutral which will be off the center tap and will be grounded at that transformer.

I have never done or even been ivolved in an installation like this, so there is a good liklihood I am wrong. It just got me curious and I was offering my 2-cents to my friend who is doing the job.

As an interesting side note, his neighbor in this rural area is an electrician with the local water district and he mentioned that they run similar installations with just two wires all the time. However, it is possible that they are only using 240 and not 120. They also are likely to have different regs. to follow.

Finally, I have not looked into the wire sizing at all, that's just what was on the plans.

 

[iwire]

My thought was this. The 600V transformer can feed the 240/120 transformer 1,000 feet away with just two wires.

Yes it would operate but it would not be safe.

In response to jeff43222, This IS the path back to the POCO. At the destination, there will be three wires that leave the 240/120 transformer.

That is 'a' path not an 'effective fault path'

The two phases and the neutral which will be off the center tap and will be grounded at that transformer.

That must be done but that will not be an effective fault path back to the source (the power company)

As an interesting side note, his neighbor in this rural area is an electrician with the local water district and he mentioned that they run similar installations with just two wires all the time.

That could be done if one of those two conductors is a grounded conductor.

You would have to know how the power company is supplying the 600 volts.

 

[don_resqcapt19]

The 600V transformer can feed the 240/120 transforemer 1,000 feet away with just two wires.

Unless the secondary of the transformer has a grounded conductor, you will have to install an EGC between the two transformers.
Don

 

[charlie b]

I must need another cup of coffee, because I think I am missing this one entirely.

As I see it, the service disconnecting means (and the point of demarcation between the POCO and the user) is at or near the 17kV - 600V transformer. From that point, it is not necessary to run a 1000 foot neutral to the 600v ? 120/240V transformer. That is because the neutral is not being used by that transformer's primary. But it is necessary to bring an EGC for all that 1000 foot distance. The return path for a fault should be through an EGC, not through a neutral.

I don't see a difference between this situation and a building that has a 480 volt service, from which point a feeder to a 480V ? 120/208V transformer would need an EGC but not need a neutral. What am I missing? Do I have the wrong point for the POCO/User interface?

 

[LarryFine]

To tidy up a few things. The voltage at the POCO is higher (I don't know but it think it's 1.7KV) There will be a 600V transformer installed on the same pole. That transformer will feed a step-down transformer 1,000 feet away.

To me, that's a silly extra cost. Why have a transformer that isn't necessary? Are you sure it's not 17Kv? The higher the primary voltage is, the smaller the conductor can be, and the lower the voltage drop would be over the 1000' run.

Running the full voltage the 1,000 feet was discussed and it was decided against. I wasn't part of that discussion.

I'd re-open the discussion if possible. There's every reason to do it that way, and no disadvantage that I can see. Insulation is cheaper than conductor. It would only require a single hot wire and a system grounded conductor.

My thought was this. The 600V transformer can feed the 240/120 transforemer 1,000 feet away with just two wires. In response to jeff43222, This IS the path back to the POCO.

What happens if the 600-to-240/120 transformer develops, lets say, an internal short to the can? The earth would be the only fault path back to the source.

At the destination, there will be three wires that leave the 240/120 transformer. The two phases and the neutral which will be off the center tap and will be grounded at that transformer.

That part is normal, and could be done without the extra intermediate voltage.

I have never done or even been ivolved in an installation like this, so there is a good liklihood I am wrong. It just got me curious and I was offering my 2-cents to my friend who is doing the job.

Ask your friend if he wants an extra dime's or so worth of opinions, too. You could always print this discussion out for him to read.

As an interesting side note, his neighbor in this rural area is an electrician with the local water district and he mentioned that they run similar installations with just two wires all the time. However, it is possible that they are only using 240 and not 120.

The secondary configuration has no impact on this discussion. What matters is the configuration of the 1000' run. I would really like to know the objection to running the primary to the house.

Finally, I have not looked into the wire sizing at all, that's just what was on the plans.

Wire size should be specified. too.

 

[iwire]

Unless the secondary of the transformer has a grounded conductor, you will have to install an EGC between the two transformers.
Don

Don I have never seen a power company provide an EGC.

 

[iwire]

As I see it, the service disconnecting means (and the point of demarcation between the POCO and the user) is at or near the 17kV - 600V transformer.

Why would I put a service disconnect at that end?

We need more info but I see it this way.

The power company is providing a 600 volt service, from the service point to the 600-240/120 step down transformer are customer owned 1000' long service conductors and do not require a disconnecting means until they get to the structure served.

That being the case their will be no EGC to bring along, it will have to be the power company's grounded conductor.

But maybe I see the job differently than the rest of you.

 

[iwire]

I'd re-open the discussion if possible. There's every reason to do it that way, and no disadvantage that I can see. Insulation is cheaper than conductor. It would only require a single hot wire and a system grounded conductor.

I can think of a reason.

These lines are customer owned and maintained.

Few electricians are qualified to work above 600 volts and those that are cost more.

The installation cost might be less but IMO the long term service will cost more.

Who is the home owner going to call when the terminations let go on the high voltage side of their expensive high voltage transformer goes?

 

[LarryFine]

As I see it, the service disconnecting means (and the point of demarcation between the POCO and the user) is at or near the 17kV - 600V transformer.

I agree, if the transformer and 600v lines are customer owned/supplied.

From that point, it is not necessary to run a 1000 foot neutral to the 600v ? 120/240V transformer. That is because the neutral is not being used by that transformer's primary. But it is necessary to bring an EGC for all that 1000 foot distance. The return path for a fault should be through an EGC, not through a neutral.

Agreed wholeheartedly.

I don't see a difference between this situation and a building that has a 480 volt service, from which point a feeder to a 480V ? 120/208V transformer would need an EGC but not need a neutral.

Wouldn't this normally be done only if there are also 480v loads present? Otherwise, you could use just the 208/120 and skip the extra step-down. Or you could bring the primary voltage directly to the 208/120v transformer(s).

The main advantage of using a voltage higher than the end-use voltage is for reduced current and conductor size and voltage drop. That's why I feel that the 600v section is wasteful and redundant.

The advantage of the run being 600v and not 240 is minmal, and more expensive, than running the system primary all the way. I don't see the advantage of this setup.

 

[LarryFine]

Who is the home owner going to call when the terminations let go on the high voltage side of their expensive high voltage transformer goes?

To me, that's another reason to have the primary run all the way. I'd rather have the POCO be responsible for as much of the run as possible.

 

[charlie b]

Larry, I was referring to a large building with a 480 volt "backbone" distribution system. I suppose that such a system is likely to have 480 volt loads, but that would not change the situation. A feeder to a 480 volt MCC does not need a neutral, if all the motor loads are 3-phase.

Bob, I agree that this situation needs a bit of clarification. I am not certain where the utility stops and the customer starts.