I'm up against a situation where a client needs to get 33Kw,120/240 that is roughly 900'away from his house to a new single phase service from a meterpole.Because of voltage drop considerations i am considering the installation of 2 transformers; 1 at the beginning and 1 at end of the line.Going first from 240-480at the meterpole and then from 480-120/240 at the house.Are there any code related problems with this scenario?Thanks in advance for any input you might have

Sounds like a good plan and you may find you can go to 600 volts for the same money to further reduce the wire size.

Thanks iwire,i'll look into the possibility of 600Volt run even so i have to admit that 600Volt on a residential property kinda scare me

VD using 500 kcm is 3% or 7 volts L to L.
Check using 2 37.7 kva 240/7200 volt using
#2 URD cable. Probably be much cheaper.No reason to be afraid. Installed correctly it is safe.

[This message has been edited by Bob (edited 08-14-2005).]

[This message has been edited by Bob (edited 08-14-2005).]

Wow
Talk about upping the ante!
Just for the purpose of study i'll check into this scenario as well-not sure what the county inspection department would think of such an installation,even if properly done!!

The only problem I see with going above 600 volts is in the terminating of the underground cables.

Only a small percentage of electricians know the procedures or products needed above 600 volts.

I would not worry about the safety aspects, the power company regularly runs 13.8 KV under ground to pad mount transformers on peoples front yards. Installed correctly it is safe.

Stepping up 240V to 480 or 600V, running 900 feet, and then stepping down to 240V strikes me as a very bad idea.

What you save on voltage drop in the conductors, you lose on the impedance of the two additional transformers in the circuit. Say you size the transformers right to load, and they each have a 2.5% impedance. This would be a 5% voltage drop without even considering the voltage drop in the service (feeder) conductors. Once you factor in the losses associated with oversizing the transformers, and the cost of the transformers, simply running fat conductors starts looking much more attractive.

Unless I totally misunderstand transformer impedance (won't be the first time).

Seems to me that the 'right' way to do this is to bring the primary distribution voltage underground onto the property, and place the transformer near the house as Bob (not iwire, who is also a Bob) suggests. Not taking 240V and stepping up to 7200V, but simply starting with whatever the local distribution voltage is, and stepping that down to 240V near the point of load.

Or simply design for the start for the voltage drop, eg. dividing loads up onto separate feeders, one with large allowed voltage drop, one with small allowed voltage drop.

-Jon

It seems that if the power companies all over the world transport energy this way it should really not be that bad an idea.
I have not got the data to know how long it would take my client to recover the savings in energy losses through transformers as opposed to energy losses and added cost of conductors to be used on a 240volt system.This would be good for a really long calculation

I'm getting really tempted at doing the 7200V method.
One of my contacts here does the Hi Voltage splicing here and knows all the ins and outs of making it work safely.
Problem remains to see if the AHJ would go along with this

Fair enough. Not a bad idea, but one which I belive, when you do the necessary calculations, will prove to be more expensive, and less effective than simply running oversized conductors.

You are quite correct, power companies do this all the time; it is the standard method of moving power around. But this is not without a price, the power companies are always trading off one factor for another to find the most effective (usually cheapest) method of delivering the necessary power.

I would really appreciate some teaching on all of the issues that would go into this calculation. I can make guesses, but it is over my head in terms of experience.

I know transformers have impedance, and that this will result in the secondary voltage dropping as load increases, so the transformers themselves introduce 'voltage drop'. But transformers have taps, so that you can set the no load voltage to be a touch higher, giving the target voltage under load. So with a transformer set you can to some extent 'zero out' your voltage drop. You will still see a dynamic change in voltage with load, but you've now placed that range of voltages _around_ your target voltage, rather than simply below it. I also don't know what sort of impedance to expect for single phase transformers of this size.

Transformer impedance is different from resistive voltage drop because inductive impedance doesn't actually consume real power...so if you are consuming 30kVA with a 5% inductive 'voltage drop', your power costs will be less than if you were consuming 30kVA with a 5% resistive voltage drop. But on the other side of this energy savings a totally unloaded transformer will still consume some power. This means that even when you client turns off _everything_, they will still be consuming (and paying for) electricity. Figuring out the overall energy use over time will not be a trivial calculation.

Curious to see what the 'optimal' design here is.

-Jon

Its going to boil down to up front cost versus flexibility and down the road expense.
I MUCH APPRECIATE YOUR THINKING
Hope someone here can give us more insight into this area because after all we want to give our customers relevant and accurate advice in our area of expertise.

I think over sizing your feeder conductors is going to be more economical than using transformers. 500 kcmil CU will probably cost less that 2 transformers and the smaller wire between them. If you do decided to use transformers I would stick with either 480 or 600. Medium voltage cable and transformers are not cheap and you will still have losses from the transformers.

Is this feeder supplying the dwelling or a separate structure from the dwelling? What large loads are included in the 33 KW?

Curt

Arial is not an option?

I would be curious about the cost of an arial run with 6 poles over underground all that distance.

Am I missing something here, or are you trying to figure in a cost of having a service brought to a new home?

In that case, why doesn't the POCO provide you with the transformer, after you install the poles and wiring at your cost. I have seen this done in the past, where the POCO did provide the transformer,at no cost, but left it up to the hoemowner to install poles and wiring for it.

(Have also seen the POCO charge for the transformer too. But at $1600 for the transformer, it wasn't a bad deal)


Dnk......

(editted to add statement)

[This message has been edited by Dnkldorf (edited 08-14-2005).]

First off,this does supply power to a new single family residence with all the goodies,from AC,waterpumps ,waterheater,electric cooking appliances and and general lighting and appliances.
33kw reflects code calcs using allowed demand factors.
Overhead would be both ugly and likely not that much less money than underground.
Even going 480 resulted in a feeder of 2 #3/0 CU to keep voltage drop within the 3%

Just trying to clarify, there are 3 different transformer options:
1) Step up low voltage to higher low voltage, step down on other side of run (240 to 600 then 600 to 240)
2) Step up low voltage to medium voltage, step down on the other side (240 to 7200 then 7200 to 240)
3) Stay at utility distribution voltage to pad-mount near home, then step down to 240V

IMHO choice 3) is the most common solution for a long main feed. If (for whatever reason) this is not an option, _then_ I am curious which of the other options is optimal. If choice 3) is available, then I can't see any of the other options being viable unless the utility is charging a tremendous amount.

-Jon

Unfortunately option 3 is not available.
Power company supply voltage is 120/240 single phase to a meterpole located 100' from power company power pole.
From there we have to take it all the way to the new single family residence.
So we really need to see wether we want to stay at the supplied voltage or bump it up at the beginning of the run only to bring it back down at residence.
Its definitly cheaper to do transformers than using 3 #600MCM cu wires to stay within the code recommended 3%drop limitation.
Long term costs are a lot more difficult to predict

Many POCO's aren't adverse to running an HV feeder onto private property, provided they have access and easement for the transformer. Most will allow you to do the conduit and pad to thier spec's, then pull, terminate and place the transformer themselves.

This way, you're only dropping voltage once, as opposed to back to back transformers. Only draw-back is living up to thier spec's, and possible markers, and access to thier equipment by them.

That way you're not metering a transformer either....

The other option is buying stock in copper...

There is no code requirement for voltage drop only a recommendation. In reality the load calculated using article 220 is overkill. This feeder will probably not have more than 20 amps on it 90% of the time. You need to remember that the transformers are going to have losses 24 hrs a day even with no load on them which is going to cost the homeowner $$. The only time voltage drop on the wire will result in any wasted energy is when it is heavily loaded which will be almost never. The main consideration for voltage drop of this feeder is to reduce flicker when heavy loads are turned on. Even if using transformers results in a slightly lower initial installation costs (I’m not sure it will) using large wire will be cheaper in the long term.

Curt

I get a 2.3% drop using 500kcmil CU at 137 amps in a 25C (77F) ambient.

The POCO here has made some very airtight deals with the PUC and their charge for bringing the power close to the residence is just ridiculous considering exactly some of the issues you're mentioning,their spec,easements and exorbitant charges for equipment and installations they perform themselves.
So here we go trying to minimize the hurt for our customer

Point well taken,Curt
I actually used 156Amps to get an even comparison with the 37.5KW transformers i was planning to use.
It will be interesting to see how all this plays out

Curt's point about the reality of low current flow most of the time is important. Usually the load (and voltage drop) would be much less than what you see at 150A. But his other point is critical. When things like the heat pump kick in, the short duration starting current will be much more than the full running current; even if you sized for 3% VD at 200A, the lights would still flicker every time a large motor kicks on.

What about the following approach:

From the pole to the house, run two sets of 2/0 conductors. Run the first set of conductors to a 150A panel, and place all of the 240V loads on it (heaters, heat pumps, range, etc). Run the second set of conductors to a 50A panel, and place all of the lighting and general purpose receptacles on this feeder. Make sure that all of the heavy loads are rated for operation on 208V as well as 240V.

This doesn't get rid of the losses in the wire; it just makes them less noticeable; the lights won't flicker when the heat pump kicks in, and the flicker in power to the hot water heater just won't matter. Presumably you could reduce the size of the neutral on the 'power' feeder...a pair of 2/0 conductors has roughly the same cross section as a single 250 kcmil conductor, so this essentially cuts your copper requirements in half.

The major code issue is that you are now running _two_ feeders to a detached structure. I think that you could get away with this as 'different characteristics' for the two feeders, and if both panels are located in the same place in the house, then the general concept of a single location for the main disconnect is met.

-Jon

Your points are well taken.I figurre that the LRA on a 5 ton AC unit would be around 110Amps+.The chore of making sure that we dont have an excessive voltage drop at that kind of a massive hit is just another reason why the transformer approach seemed sensible.
The 2 feeder approach certainly has its advantages if i can get the building department to go along with it.Will have to calc it all through and see how i could present it.
Thanks for your input!

"""Power company supply voltage is 120/240 single phase to a meterpole located 100' from power company power pole.""""


That's the factor I didn't catch in your first post.

Arial would make no sense then.

I "assumed" they may feed you with the HV to the last pole at your project..


Dnk.......

When evaluating low voltage transformers for this type of loading consider the newer NEMA TP-1 designs.
According to a Square D publication "... the TP-1 energy efficient unit has less energy loss at 35% loading than the [normal] low temperature rise units. The low rise units have less losses at 100% loading. The conclusion is that the low temperature rise units show more savings at higher loading. Again, statistics show that the average loading of a transformer is typically 35%..." A NEMA TP-1 37.5KVA single phase unit is designed to have an effciency of 98.2% at 35% load.

I was faced with a similar situation, only my run is about 400 feet. The POCO didn't want to hear anything about running poles down the customers drivway. Oh well.

Ultimately, I drew the same conclusion as Curt. The load is going to be minimal most of the time, and I was helped by the fact that the homeowner is going to be using gas for heating and appliances, and he will be installing 12 SEER or better AC.

Rhino, Just out of curiosity, where are you doing this job in the SD area? When I visit there, I am under the impression that no home is more than 30' from a POCO transformer. So you must be working out in the desert or Rancho Santa Fe to have a 900 foot run!

Ever thought about getting the power company to do a 1-ph primary meter installation? You could then run their primary voltage to your own padmount. Granted you would have to pay for an energized transformer, but all of your voltage drop and especially your flicker problems would be solved without having to mess around with the insane requirements of your power company.

BTW, does the power company still think it is the 1920's or 30's? Sounds like some of the worst customer service I've heard of. I know the power companies I work/worked for would never be that bad.

Wes

I'm doing this job in the east county area of San Diego,some of the lots there are gigantic (5 acre minimum size for single family residence is not rare)and as land values go up in the city more and more people are going out to the more remote and,lets say difficult areas.
The electric company will allow running a High voltage feed and a pad mounted transformer(supplied by them)to the tune of something like $40000.00.Not a really good deal for the home owner.

BTW
Thanks for the input!
Am i mistaken or has there been some recent(within last couple years )changes in the way transformers are supposed to be made anyway that mandates maximum efficiency at 35% load by all manufacturers?
Appreciate any constructive feedback.

Rhino said:

I'm doing this job in the east county area of San Diego,some of the lots there are gigantic (5 acre minimum size for single family residence is not rare)and as land values go up in the city more and more people are going out to the more remote and,lets say difficult areas.
The electric company will allow running a High voltage feed and a pad mounted transformer(supplied by them)to the tune of something like $40000.00.Not a really good deal for the home owner.

Wow. I know labor is a bit more expensive in PRK, but geez.

Assuming you can get a primary meter set-up, I don't see any reason that you couldn't get my suggestion done for $10k or less barring difficult trenching conditions. A padmount will run you about a $1-1.5k. ~$3/ft on 1/0AXN-J primary cable. Another $3/ft for a 100' 4/0UTA service entrance and say $500 for a kit for the elbows, bushings, secondary bars, etc. That may leave you another ~$5k for labor and trenching, not counting what it would normally cost you to wire a new house.

Wes

[This message has been edited by Partywaggin (edited 08-17-2005).]

Hey Party
Yeah i sure wish that it was i who could do all of the setup.
Not so-the electric company will not allow that,they'll only have us do trench conduit backfill and pad and then they come in and do the easy part and charge an arm and a leg.
Dont ask me how they were able to to negotiate such terms with the PUC.
Anyway,Thanks for your input!