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

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