ECN Forum Forums Code Related Discussion NEC & other Code issues A really long mainfeeder

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).]