i am putting out to bid a 1600 amp 120/208 3 ph service, and with the price of cu. i want to see what the savings would be in alm. but i need help on finding the voltage drop wire size for alm. and cu., the run is 300' underground in pvc.

To properly compute the voltage drop, you would need to know the actual load on the service. The voltage drop at 500Amps for instance, would be much less than it would be at 1000Amps.

Try this:
http://www.elec-toolbox.com/calculators/voltdrop.htm
It will compute up to 1200 amps




[This message has been edited by electure (edited 10-07-2006).]

LB:
Normally, a 1600 amp, 120/208, 4 wire, would fall out like this:
4-4" PVC
4 sets of 600mcm THHN/THWN Cu
MCB 1600 amps

500's don't fly as 380x4=1520 amps
Largset MOCP would be 1500 amps IF avail.

Now, AL would start at 750MCM

As to VD calcs, the linked calculator does not provide input for parallels.

Basic VD calcs are in American Electricians Handbook, & are not that tough to do.

BTW, is the job 'design build'?? or do you have a 'spec' you're trying to re-engineer.
Basically, be careful!!

John

PS, if you can't locate the formulas, drop me an e-mail

Just use NEC, Chapter 9, Table 8 and my handy formula (based on Ohm's Law):

3 phase VD = 1.73 x I x R x L / 1,000

where VD is the voltage drop
I is the current in amperes
R is the resistance per 1000 feet found in Table 8 for your wire size and type
L is the length of one conductor

I like this one because you don't have to worry about remembering "k"

If you need single phase, then substitute "2" for "1.73". If you want an even closer estimate of VD, then use Table 9, which also considers the inductive reactance of the conductors coupled with the type conduit.

Litebulb
If you are not able to make this caculation,
this project may be way over your head

Question to all contributors of this thread...

How can you aquire a correct voltage drop without considerations to ambient temperature, raceway type, and power factor?

Answer for those who are stumped...
You can't.

ScubaDan, he stated it's in PVC. Without knowing anything else, it's standard to assume 75C. PF would have to be taken into account, but an assumed PF of 1.0 is always going to be safe. Honestly, a PF of .85-.9 is probably not going to make a difference in the cable size used, but it could be taken into account if the result is borderline and he really wants to justify using a smaller kcmil cable.


[This message has been edited by SteveFehr (edited 10-13-2006).]

I haven't taken the time to look at the whole scenario too closely, but if that is the distance and you really need that much current, would it not be worthwhile to look at bumping up the voltage to reduce the wire size and the bump it back down at the building?

Just curious...

During discussions of long services, the idea of using higher voltages often comes up. And it isn't a bad idea; this is how long distance power distribution is done. My understanding of the _theory_ (no, I've not done one of these installations, and I am sure there are corrections to be made to the below :

You have to consider the cost and losses of the transformers, and additionally you must remember the _impedance_ of the transformers. Each transformer has its own 'built in' voltage drop.

You can use the transformers to compensate for voltage drop, by appropriately adjusting the transformer taps; however this sort of adjustment is good for one current level only. Since the voltage drop _changes_ as the load changes, adjusting the transformer taps will not help with things like light flicker with large loads, or other problems associated with the change in voltage drop as the load changes. In power distribution systems, transformers have automatic tap changing hardware to regulate the output voltage.

Roughly:

1) if you can carry the primary voltage closer to the building, and put the transformer closer to the building, you will probably be better off in terms of voltage drop. You have the same transformers, and so the same transformer impedance, just arranged for better resistive losses in the conductors.
2) if you can get your supply at higher voltage, and then step down as needed, you _may_ be better off. This is especially true if you have loads that can run directly at the higher voltages. For example, you may be better off with a 480V supply, running loads directly at 480V, and then having smaller 120/208V panels from transformers.
3) if you take your low voltage service, step it up to a higher intermediate voltage, and then step it down again, you are shooting yourself in the foot. The impedance of the transformers (3 of them chained together) will more than make up for the improved voltage drop.

I know that 'voltage regulating transformers' exist at 120V, but I believe that these are ferro-resonant type transformers that work by keeping the core saturated; since the saturation doesn't change much with input voltage, the output is stabilized relative to the input.

Do voltage regulating tap changing autotransformers exist for these sort of situations? It would seem to me that a very small autotransformer could compensate for voltage drop on a very large service, small meaning a transformer of perhaps 5% the service KVA.

-Jon

ScubaDan,

Check out Table 9 of Chapter 9 to obtain impedance per 1,000 feet based on type of conduit. See my prior post for the formula, if you don't know Ohm's Law.
Power factor is a different concern.
Ambient temperature is a concern, best solved by the lower portion of Table 310.16, after all, because what we are looking for is the correct wire size to operate our equipment.

Before you get too far in this, you might check around for availabity of the feeder size Aluminum. Factories are quoting up to 26 weeks for delivery.

Hi Steve,

Actually, a PF of 1.00 using 2KIL is the most liberal, not consevative, of the PF values to use. The lower the PF factor, the more the voltage drop will rise and this could easily mandate a larger conductor size.

Using V = IR, R incumbases all factors contributing to the circuit resistance. One of the major factors is PF along with ambient temperature.

Check out IEEE Std 141 exact voltage drop formula or just download Volts from ECN's store to see how PF and ambient temperature affect voltage drop.

I guess that depends on whether or not you start with kW or kVA when you assume PF=1!

Ambient temperature will certainly have an impact on what the conductor temperature is, but in the end, it's only the conductor temperature that matters.


[This message has been edited by SteveFehr (edited 10-13-2006).]

Hi Steve,

"I guess that depends on whether or not you start with kW or kVA when you assume PF=1!"

Power factor is a independant measure of the cosine of the phase angle offset. If you look at the power triangle, the bottom leg is real, true or working power(P) in units of watts, kW or HP. The hypotenuse (top line) is apparent power(Pa) in units of VA or kVA, the verticle leg is Imaginary or Reactive power in units of VAR or kVAR. Using a little trig will derive the angle of reactive power or theta. The cosine of this angle is the circuit's power factor.


"Ambient temperature will certainly have an impact on what the conductor temperature is, but in the end, it's only the conductor temperature that matters."

The conductor's temperature is the accumulation of conductor heat from it's resistance and ambient temperature.

As a brief note, I am the author of Volts. Since Volts' download is free for 10-days you can try this problem as well as other scenarios to view the results.

Looking over the above ramblings and studying the original post, some of us have missed the subtleties..."I am putting out to bid a service..." apparently lite bulb ain't bidding on it, he's looking for bids and help in writing the specs. Bob hit the nail on the head, I think. And the wisdom of hauling all those amperes 300 feet seems lacking when at this point the whole installation (& how do I know?) is still on the drawing board. If lite bulb is designing the service, put the transformer closer to the switchboard, buddy. Let the POCO save you the voltage drop. And a building that requires that level of 208 might be better served by spec'ing the prime movers at 277/480 and setting a much smaller dry type xfmr for the small amount of convenience receptacles and computer equipment. But I have no way of knowing the occupancy and what kind of equipment is to be in the building, so this is all shooting in the dark. But there are more efficient ways of getting power to the building. Some people and POCO's are inflexible. Recently I overheard a HO in a supply house asking about "what size wire do I need to run secondary 800 feet from the transformer to feed a 200 amp service..." I guarantee that a single pipe with primary cable will cost less than 1/8 of a mile of 250/250/4-0+ G Al...

Electrical design is not a dart board, it is Physics + Economics+ NEC. Wal-Mart has evolved to where their services are multiples of 600 & 800 amp 277/480 V services, panelboards that don't require ground fault protection that you would need if the stores used a bigger single switchboard. Typically, they use a 800 A. PB for HVAC,another one for single pole lighting circuits, a third one for balers and other machine loads and this third one also feeds several dry type 120/208 xfmrs feeding PB's for snack bars and office receptacles, etc. I am sure that there have ben a few changes to the above with the advent of the Supercenters, but the basic scheme is to have the highest voltage to reduce voltage drop and break it up into less than 1,000 amp packages to keep from having expensive gfi switchboards...And btw, we used .9 pf in all our load calc's.

[This message has been edited by Almost Fried (edited 10-14-2006).]

[This message has been edited by Almost Fried (edited 10-14-2006).]

[This message has been edited by Almost Fried (edited 10-15-2006).]