120/240V 1P-3W, 100A Service to Very Remote Building 2

[milwaukeebob]

Greetings everyone, I have a question regarding a fairly straightforward but odd situation/request that I've not seen in my plant days. The majority of my experience is in plant generation with fairly limited cable lengths and standard configurations.

In short, I have a customer who needs standard 120/240, 1P/3W, 100A service at a small building that is very remote. Actually, they have defined the absolute maximum need including future growth at approximately 60A. Here's the complication - the local power company is only able to provide them a 120/240, 1P/3W, 200A service drop to a meter panel that is approximately 1 mile from the location of the building. In general, the customer has proposed stepping this voltage up to from 240V to 600V with a 1P xfmr (only utilizing the 2 hot legs) at the point of the service drop and then stepping the voltage down with another 600V-120/240 1P xfmr at the building end. On the secondary side of this xfmr at the building would be a 1P-3W, 100A panelboard with the appropriate branch circuit breakers.

For this long cable run between the service drop and the remote building, I am familiar with voltage-drop concerns and up-sizing the cables/EGC's accordingly as well as required primary/secondary OCP devices for the xfmrs. The problem is one of the electricians has signaled that they cannot only utilize the two (2) hot legs (1P) at the service drop and then re-establish that system neutral on the secondary of the 600V - 120/240V xfmr at the remote building. I am unsure of the details of his concerns because I haven't spoken to him yet, but I've seen this arrangement with other voltages like 480V systems over shorter runs.

I'd appreciate feedback on this. Am I missing something? Thanks in advance

 

[jhnblgr]

sounds like you should have a discussion with the utility about running a high voltage line this is not atypical to have a private line off with a meter on the end and transformer. I think the customer has a miscommunication with the utility. Even if they have an issue with a distant meter they can install a meter off the high voltage lines at the tie in.

 

[waross]

Ask your electrician to check out the code section on separately derived systems.
The 120/240 Volt secondary of the step down transformer is a separately derived system.
Install the normal grounding electrode.
Bond all equipment to the ground electrode as is normally done.
Install one connection between the neutral and the grounding system.
That may be to install or leave in place the green screw in the 100 Amp panel.
The question remaining is whether to add a second ground connection directly from the transformer neutral terminal, in addition to the ground to neutral connection in the panel.
I would check with the local inspector for his interpretation.
I would add that connection but be ready to remove it if the inspector didn't like it.

 

[dpc]

I didn't run any numbers, but technically there is no issue with a step-up transformer feeding a step-down transformer to reduce voltage drop. The step-down transformer will establish a neutral at the center tap of the secondary and that is grounded at the transformer. This is pretty commonly done. If the electrician is expressing concerns, I'd find a different electrician.

It would make more sense to run at the utility's medium voltage and install a transformer at the location of the load. But that's likely a commercial issue, not a technical one.

 

[waross]

I would consider 2300 Volts or 4160 Volts but that may be ruled out by regulatory issues.

 

[milwaukeebob]

Thanks dpc and waross. Always a pleasure to read all your responses to various questions/issues posted on eng-tips. You guys have made me a better engineer over the years.

I agree with both of you that medium voltage would be the way to go but, as indicated, there are commercial issues. Thanks to jhnblgr as well. Unfortunately, as I understand this is not a possibility but I will ask again.

I've run the numbers and the only other thing I was going to suggest to the customer is that both transformers, especially the step-down at the building have adequate minus (-) primary taps to adjust the secondary voltage accordingly. In addition, I will recommend all compression lug connections at primary fused disconnects and at the xfmrs. I hate mechanical lugs for many reasons but in this case higher resistance and more incremental voltage drop is my concern.

​

 

[stevenal]

There should be no problem with the system proposed. I've seen problems using 120/240 winding with a neutral connection used as a step up transformer. The result is unbalanced current and voltage, and possible overloading. All balanced out with the neutral wire removed.
Have you found a source for the 600 V transformers? We ended up using 7200, since they were a stock item used by utilities. Less voltage drop also. AHJ approved also.

 

[waross]

Thanks dpc and waross.

dpc Knows what he is talking about.

 

[waross]

Hi Steven.
7200 Volts is good technically, for industrial.
For domestic, many areas have an administrative line at 600 Volts.
Not all contractors nor electricians have the qualifications to work on voltages above 600 Volts.
Many areas require a trained, qualified maintenance staff for installations at voltages above 600 Volts.
The 600 Volt limit is regulatory, not technical.

 

[stevenal]

Waross,
The project I was involved with was residential and subject to the USA NEC. As I indicated, it was approved by the AHJ. The customer knew that not all electricians will work on the 7200 V portion and decided to proceed anyway. He did the trenching and cable laying himself, and found a lineman to make up the elbows.

My main point goes back to the original question. The two hot leg connection is not only not a problem, the alternative very likely would cause a problem.

 

[bacon4life]

Some utilities offer 240 V/480 V 1P 3W service for street lights, which is typical used for street lights in my region.

 

[waross]

The two hot leg connection is not only not a problem, the alternative very likely would cause a problem.

Yes. I agree completely.

 

[oldfieldguy]

I used two approaches.

For a metering building a thousand feed away from our nearest MCC, I chose to run single-phase 480 volts with a 480-240/120 transformer at the building.

For another measurement building a mile down a pipeline right-of-way I ran a 7200 volt buried cable to a 15 kVA transformer that supplied 240/120 to the building.

 

[LionelHutz]

Around here, it's common for the rural customers to put in their own poles and overhead line for the utility to feed directly off the power line at the road. I believe the transformer can either be leased or purchased, but it's possible the utility provides it on the customer pole instead of at the road. You can also trench in a line and install your own pad mount transformer and they'll connect that at the road as well.

This utility should have documentation on the service types and installations they will support and the requirements for them. I'd try to get that info first before deciding what to do.