Long direct burial feed to transformer

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thunder15j

Member
Location
Cali
A customer is asking me to design and provide cost estimates for a direct burial 480 volt single phase feed to a transformer at the other end that will provide 120/240 single phase power to a building. The run is to be several thousand feet on this rural ranch setting. We are estimating the secondary load to be around 50 amps @ 120/240V. Voltage drop calulations are pretty cut and dried using one of several wire mfgs Vd charts. I realize that large conductors would be needed. A typical installation would be to run 2 480V current carrying conductors (l1,L2) and an equipment carrying conductor. At the transformer at the out building, a ground rod or similar electrode would be installed and that would be bonded to the nuetral of the secondary of the newly created 120/240V system.

However, is there a safe way of eliminating the EGC that would normally be run with the 480 volt feed (for cost issues)? Maybe an isolation transformer on the feed side. I know that under normal circumstances that simply driving a ground rod at the far end and bonding to secondary neutral will do little to clear a 480 volt fault if no grounded conductor is installed. The earth is not that great a conductor. Kind of like driving a ground rod attempting ground metal light poles to clear a fault-virtually useless (except for lightning).

I am not advocating eliminating the EGC. I normally run EGC with everthing, especially in the smaller sizes. Just wondering if a SAFE alternative would work. I pretty much already know the probable answer. We also might consider medium voltage (4160).
 

broadgage

Senior Member
Location
London, England
I do not believe that an EGC to the transformer could be safely omitted.
Under normal circumstances it would work without a EGC to the transformer, but would be potentialy very dangerous in the event of a fault.

Consider what would happen if the transformer developed an internal short between one end of the 480 volt winding and the steel case of the transformer.
The transformer case would become energised at about 277 volts (presuming that the 480 volts was obtained from a 277/480 volt system)
Some current would flow to the general mass of earth, via the ground rod , but probably not enough to open the OCPD on the 480 volt feeder.
With a ground rod resistance of say 25 ohms, only about 10 amps would flow, which would be unlikely to open the 480 volt breaker.
And remember that the EGC of the seperatly derived 120/240 volt system would be bonded to the transformer case, so it would not just be the transformer case that was energised at 277 volts, but all grounded items in the lower voltage system.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
It is what it is..... you should not take shortcuts to save money on the EGC.

Make sure you price using Al instead of copper. Higher voltage means smaller cable, but it also means more expensive equipment for the MV level, so you have to do trade off.

Since your going to be using a transformer, you could always get a transformer with a lower primary side, and that way you account for the VD, thereby use smaller cable. i.e. transformer with a 418V pri and 120/240 secondary. You can take a 15% voltage drop and still have full voltage at 120/240V. Or, you could order a transformer with additional taps, to get the voltage where you want it, i.e. +0, -6 x 2.5% (which is 15%).
 

topgone

Senior Member
A customer is asking me to design and provide cost estimates for a direct burial 480 volt single phase feed to a transformer at the other end that will provide 120/240 single phase power to a building. The run is to be several thousand feet on this rural ranch setting. We are estimating the secondary load to be around 50 amps @ 120/240V. Voltage drop calulations are pretty cut and dried using one of several wire mfgs Vd charts. I realize that large conductors would be needed. A typical installation would be to run 2 480V current carrying conductors (l1,L2) and an equipment carrying conductor. At the transformer at the out building, a ground rod or similar electrode would be installed and that would be bonded to the nuetral of the secondary of the newly created 120/240V system.

However, is there a safe way of eliminating the EGC that would normally be run with the 480 volt feed (for cost issues)? Maybe an isolation transformer on the feed side. I know that under normal circumstances that simply driving a ground rod at the far end and bonding to secondary neutral will do little to clear a 480 volt fault if no grounded conductor is installed. The earth is not that great a conductor. Kind of like driving a ground rod attempting ground metal light poles to clear a fault-virtually useless (except for lightning).

I am not advocating eliminating the EGC. I normally run EGC with everthing, especially in the smaller sizes. Just wondering if a SAFE alternative would work. I pretty much already know the probable answer. We also might consider medium voltage (4160).

How long is your run? "several thousand feet" doesn't mean much when computing!
 

thunder15j

Member
Location
Cali
Thnx for responses. The responses got the wheels turning, so here are a few related thoughts IMHO:

The idea of installing a tapped transformer at the far end of this proposed run (416 x120/240) would work well on a constant load such as a motor, although the voltage might be a little hot for a control circuit prior to startup. However, with a variable load-lighting, 120 volt convenience receptacles, etc., if and when the load is light, the voltage again might be on the high side.

The nuts and bolts of the circuit are: 6,000 foot run; 480 supply from 1600 amp board (single or three phase); 120/240 mixed load (single phase) estimated at 25 amps @ 480volt; run to direct buried (customer request-I like PVC conduit). Using a Vd online calculator from southwire (I can't get Mike's to run), it called for 750mcm Al wire to mantain a 3% Vd. It would be tempting to push the Vd id the run was economically viable, which I now feel it is not because of large wire sizing.

I know the NEC only calls for a #10 Cu EGC on a normal, shorter circuit (60 amp OCD). I also know, from gleaning knowledge from this site (thanks fellow e-men and e- w-men), that the EGC must also be oversized for voltage drop. I have seen other similiar installations with undersized ground wires that would not trip the OCD. Please guide me.

The electrical solution to this run, as suggested by another member, would be to consider medium voltage such as 4160. I'll have to get in touch with a wire mfg about their MV cables and also a high voltage contractor for that option.

Another option would be to tap into a closely located natural gas line that runs near the proposed project on the owners property. The customer could install a gas turbine to generate power close to the site. Anyone have any experience with such units? The ones that I have been around sure run on the quiet side. The customer will be running a gas line anyway to use at the new site.

(where's the #% spell checker?)
 

topgone

Senior Member
With 6,000 feet of line, I guess you are good to go with 4,160V or even 6.9kV if you have the equipment in that voltage range. Please bear in mind that if you have a load of 25A @ 480V, it will only be: 25 x (480/4160) = 2.88A @4160V; or 25 x 480/6900 =1.74A @6.9kV. VD is not a problem even with the smallest MV cables.

Please visit this site and try punching-in your numbers and compare. southwire
Hope that helps.
 

broadgage

Senior Member
Location
London, England
Generating the power locally MIGHT be an option if natural gas is available.
For permanent use, 2 generators would be required to allow for breakdowns and servicing without loss of supply.
It is likely that a battery and inverter would also be required for times of very low load, it would not make sense to run an engine for a single fridge or lamp.

Another option might to select the transformer tappings so as to give a slightly higher voltage than nominal. Perhaps 126/252 off load, would give a bit more margin than 120/240.

Most larger appliances will give acceptable performance at lower voltages, remember that many are listed for both 208 volt and 240 volt operation.
It might be worth installing a voltage regulator for lighting and small appliances, and accepting a rather variable voltage for larger loads.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
You might be able to get the local PCO to route a line there pretty cheap, or not. Some rural areas still have subsidized electrical lines for rural areas. be worth checking at least.

I would run as high a voltage out there as I could to reduce the current and thus the VD.

If you want to stay away from MV equipment and wiring 550V is not unheard of.

I don't think you can avoid running an EGC by code but I don't know what it gains you safety wise if you run an ungrounded circuit out there.

I bet AL UF is cheaper than CU, even upsized for being AL instead of CU.
 
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kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
I modeled it in ETAP and looking at some numbers, I think you could use a #2/0AWG Al, feeding a 15KVA, 440-120/240V transformer, then use the -2.5% tap on the transformer to adjust it to 120/240V on the LV side.


At least something along those lines. You would need to fine tune it based on your actual loads and what-not. But it certainly is going to be your cheapest alternative.
 

bob

Senior Member
Location
Alabama
I modeled it in ETAP and looking at some numbers, I think you could use a #2/0AWG Al, feeding a 15KVA, 440-120/240V transformer, then use the -2.5% tap on the transformer to adjust it to 120/240V on the LV side.

I don't know where king get his information from but I came up with either 600 or 750 kcm for about 3% at 25 amps 480 volts. Don't forget the EGC will have to be 750 or 600 kcm also. Another absurd result from table 250.122. If you used a 400 amp breaker with the 750 AL the EGC would be #3 cu instead of 750 AL. You can use the taps to compensate for the VD and maybe go to a smaller size conductor. It seem to me that the medium voltage circuit, 4160/2400 volts, would make more sense. I would be interested in the cost difference.
 
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BJ Conner

Senior Member
Location
97006
Rule of thumb.

Rule of thumb.

An old rule - 1 Kv= 1 mile. 12.47 Kv=12 Miles, 4.16 KV= 4 Miles and
0.48 Kv = 0.48Kv x 5,280= 2,535 Ft

It might be cheaper to use two transformers.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Apparently, someone is overlooking the fact that 3% VD is not a valid concern here when utilizing a transformer to step-down the voltage. The voltage drop comes out to around 8% for the smaller cable. So, instead of the standard 480-120/240V transformer, you'll notice that the transformer recommended is a 440-120/240V. The transformer taps are then used to fine tune the 120/240V.

People need to think outside the box, and forget the 3% and 5% fictitious VD "recommended" approach. The NEC is not a design guide, it is a safety manual, and it should only be used as it is intended.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Just out of curiousity.

If an ungrounded circuit was run out there, what benefit would there be to running an EGC with it? Other than code compliance (which by itself is enough IMO).
 

beanland

Senior Member
Location
Vancouver, WA
600V?

600V?

Square D and Acme make 600V to 120/240V transformers. We have done this where we have had long runs on airports to power remote equipment. Just an option.
 

Open Neutral

Senior Member
Location
Inside the Beltway
Occupation
Engineer
Consider it this way:

You can spend your $$ on copper rolled up around some iron; or stretched out flat in a ditch. [Well, Aluminum maybe but...]

The question is: which makes more sense in your specific case?

ps: If your PoCo is PG&E; don't even bother asking then to run it. They are taking my client for $45K for a 1400 ft 25KVA Open Delta feed; after we provide the ditch, duct & muletape.
 

broadgage

Senior Member
Location
London, England
Although not a code requirement, I suggest that about 3% is a reasonable figure in this case.
Remember that 3% is only the drop in the proposed feeder and NOT the total voltage drop from the source to the end of a branch circuit.

Presumably there will be some voltage drop between the 480 volt service, and the proposed feeder.
There would also be voltage drop in the transformer, and in the branch circuits of the building being supplied.
Consider the following
Drop from 480 volt service to proposed feeder-----------2%
Drop in 480 volt feeder-----------------------------------3%
Drop in transformer--------------------------------------2%
Drop in branch circuits supplied from transformer--------3%

That is a total of about 10% and verging on unacceptable, it would be only 108 volts on a nominal 120 volt circuit.
And that is presuming that the 480 volt service really is 480 volts, if it was 5% low, then the actual voltage at the point of use might be only about 102 volts.

Selecting taps on the transformer only helps to a limited extent, or the voltage will be excessive at no load.
I would consider setting the taps to give 126/252 off load, which with 10% voltage drop would give about 114 at full load.
Slightly smaller cable could be used to give say 5% drop in the feeder, or about 12% overall.
That would give 126 off load and 110 on full load.

All this presumes a steady voltage from the utility. A consistantly high or low voltage from the utility may be compensated for by proper selection of the transformer taps.
If however the utility service varies from 95% to 105% of the nominal, then the additional variable voltage drop in feeder etc makes the idea a non starter.

Worst case would be, no load and utility 5% high.
No load means no voltage drop in the feeder etc.
Therefore the voltage at the load will be 126 volts.

Full load and utility 5% low
Full load means 10% drop from 480 volt utility to 120 volt branch circuit.
Or only about 100 volts at the load, note that this cant be increased by selecting different transformer taps, since the off load voltage is already at the sensible maximum.

Options include accepting this or a greater variation for large appliances (many are listed for 208 volts as well) and using a voltage regulator for lighting and small appliances.

Alternatively reduce the load by use of natural gas or propane for water heating, space heating, and cooking.
 

thunder15j

Member
Location
Cali
A power line extension of hi-volts from our local utility, P, G & E, is around $10,000 per pole according to their new service dept. My guess of average length of distance between poles of around 500 feet (please advise on this figure; 500 ft is a nice round number) would lead to a pricey venture from old blue. 6000 feet/ 500 ft per pole= 12 poles. 12 @ 10,000 per pole= $120,000
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
A power line extension of hi-volts from our local utility, P, G & E, is around $10,000 per pole according to their new service dept. My guess of average length of distance between poles of around 500 feet (please advise on this figure; 500 ft is a nice round number) would lead to a pricey venture from old blue. 6000 feet/ 500 ft per pole= 12 poles. 12 @ 10,000 per pole= $120,000

That is only $20 a foot. Can you run UF of this size for $20 a foot?
 
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thunder15j

Member
Location
Cali
After consulting with a utiltity line-foreman, I was told that 250-300 ft was an average length between poles for 21K. (275'). So my 6000 ft run wold take (6000/275) 22 poles + 1 end pole. 23 poles @ 10K per pole = $230,000 for the pole line extension; $38 per foot.
 
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