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#70400 - 10/07/06 02:34 AM how to find voltage drop wire size
lite bulb Offline

Registered: 02/12/06
Posts: 77
Loc: wilm,de.
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.

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#70401 - 10/07/06 04:18 AM Re: how to find voltage drop wire size
electure Offline


Registered: 12/24/00
Posts: 4229
Loc: Fullerton, CA USA
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:
It will compute up to 1200 amps

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

#70402 - 10/07/06 01:45 PM Re: how to find voltage drop wire size
HotLine1 Offline


Registered: 04/03/02
Posts: 6833
Loc: Brick, NJ USA
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!!


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

#70403 - 10/08/06 10:13 AM Re: how to find voltage drop wire size
earlydean Offline

Registered: 12/22/03
Posts: 749
Loc: Griswold, CT, USA
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.

#70404 - 10/08/06 04:48 PM Re: how to find voltage drop wire size
Bob Offline

Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
If you are not able to make this caculation,
this project may be way over your head

#70405 - 10/12/06 06:00 PM Re: how to find voltage drop wire size
ScubaDan Offline

Registered: 08/23/06
Posts: 21
Loc: Sacramento, CA, USA
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.

#70406 - 10/13/06 03:53 AM Re: how to find voltage drop wire size
SteveFehr Offline

Registered: 03/19/05
Posts: 1192
Loc: Chesapeake, VA
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).]

#70407 - 10/13/06 08:26 AM Re: how to find voltage drop wire size
ExpressQuote Offline

Registered: 04/27/06
Posts: 83
Loc: BC, Canada
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...

#70408 - 10/13/06 10:03 AM Re: how to find voltage drop wire size
winnie Offline

Registered: 09/15/03
Posts: 652
Loc: boston, ma
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.


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.


#70409 - 10/13/06 12:13 PM Re: how to find voltage drop wire size
earlydean Offline

Registered: 12/22/03
Posts: 749
Loc: Griswold, CT, USA

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.

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