Voltage drop calculation for long feeder run

[ecurb]

Hi, first time on this web site forum, and my question is, What is the quick method of figuring out the voltage drop for a 150 ft. run for a 100 amp, 120/240 volt service for a new garage my friend is building? are #3's o.k. for this, or does he need to go to #2's?

 

[roger]

Re: Voltage drop calculation for long feeder run

Use this method.

Wire size equals (in CM); 2 X 12.9 x actual amps x distance / your maximum desired volts dropped. Then find the conductor in table 9 chapter 8.

It would look like this

2 x 12.9 x I x D
__________________

Volts Dropped

In reality you would not know the true load so # 2 would be the best choice.


Roger

[ July 13, 2004, 09:09 AM: Message edited by: roger ]

 

[dereckbc]

Re: Voltage drop calculation for long feeder run

Roger you can simplify by stating :

CM = (25.8 x I x D)/VD

Personally I use 22.2

 

[charlie b]

Re: Voltage drop calculation for long feeder run

You can also go to the home page of the Mike Holt website (button on the top left of this page). Look through the "free stuff," and you will find an excel spreadsheet that performs voltage drop calculations.

 

[ecurb]

Re: Voltage drop calculation for long feeder run

Thank you all for your replies. I am a second year apprentice with IBEW Local 134 in Chicago, and I can see that this web site will be of great value to me in the future, as it already has.
Ecurb is Bruce spelled backwards.

 

[ecurb]

Re: Voltage drop calculation for long feeder run

Whoops! I'm still a little confused. "CM" I take it is circular mills, which according to table 9 is 52620 for a #3 wire. Is 2 the # of conductors? The I(amps) and D(distance) are clear to me but the other #'s are not.
So 2? x 12.9? x 100amps x 150ft(or 150 x 2=300)/240 is the equation? Where does the CM come in?

 

[roger]

Re: Voltage drop calculation for long feeder run

Bruce, CM is "circular mills" and the 2 is for 2 conductors. Lets say you want to hold the voltage drop to no more than 3%

240 x .03 = 7.20 v

12.9 is just a standard for the constant of copper resistance. This number as Dereck pointed out is not accurate but is erred on the conservative side.

So if you were actually looking at a 100 amp load it would be; (we could shorten the calculation as Derck said, but lets go the long route this time)

2 x 12.9 x 100 x 150 = 387,000

387,000 / 7.2 = 53,750 circular mills.

Per table 8 this would be between #3 and #2 so the #2 would be the best choice.

Roger

 

[ecurb]

Re: Voltage drop calculation for long feeder run

Thank you Roger, that simplified things and helped me better understand the equation.
Bruce

 

[ryan_618]

Re: Voltage drop calculation for long feeder run

Am I the only one that uses Ohm's Law for single phase voltage drop?

 

[ecurb]

Re: Voltage drop calculation for long feeder run

Ryan, you know the problem, let's see your #'s. Ohm's law does not take into account all of the variables.
Gary B., I tried 3 times to download the voltage calculation spreadsheet, and each time my computer wigged out after I tried to downlaod the file. It may be corrupted or whatever. Is there another spot where I can get the info?

 

[ryan_618]

Re: Voltage drop calculation for long feeder run

Originally posted by ecurb:
Ryan, you know the problem, let's see your #'s. Ohm's law does not take into account all of the variables.

I=E/R.
Resistance of 300' of #3=.0735*100A=7.35VD
Resistance of 300' of #2=.0582*100A=5.82VD
....Pretty much the same as Roger's calculations.

 

[dereckbc]

Re: Voltage drop calculation for long feeder run

Originally posted by ryan_618:
Am I the only one that uses Ohm's Law for single phase voltage drop?

Sure when you got nothing better to do. Otherwise tables are quick, lazy, and get you where you are going.

 

[ryan_618]

Re: Voltage drop calculation for long feeder run

 

[ecurb]

Re: Voltage drop calculation for long feeder run

Ryan,

I guess you have the quick method down pat. I did the same calculations and came up with your answers. It does seem to be quicker and easier. Thanks,
Bruce

 

[ecurb]

Re: Voltage drop calculation for long feeder run

Oh yeah, I forgot to ask this question. The difference is not that great, but which is the proper column to use from Table 8, chap. 9. Is it the uncoated or coated category? I'm assuming by coated they mean insulated

 

[ryan_618]

Re: Voltage drop calculation for long feeder run

Hi Bruce. Actually, by coated they mean conductors that are "tinned". Very uncommon, other than on grounding electrode conductors for communications systems, at least from what I have seen.

 

[dereckbc]

Re: Voltage drop calculation for long feeder run

Originally posted by ryan_618:
Hi Bruce. Actually, by coated they mean conductors that are "tinned". Very uncommon, other than on grounding electrode conductors for communications systems, at least from what I have seen.

Actually Ryan some require them inside for all DC powered and ground circuits.

[ July 16, 2004, 05:16 PM: Message edited by: dereckbc ]

 

[ryan_618]

Re: Voltage drop calculation for long feeder run

Thanks Dereck. I figured this would be right up your alley!

 

[ecurb]

Re: Voltage drop calculation for long feeder run

Thanks again fellas. Have agreat weekend

 

[jcapp]

Re: Voltage drop calculation for long feeder run

I am a new user.
I having troulbe with VD.
K=12.6 L=100ft I=45 CM=#6--26,240
I am looking at a practice question, the method is:
VD=2KLI x 0.866 / CM. What is 0.866 does it have to do with 3 Phase