Service Feeder Voltage Drop

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brian@sutherland

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I currently have a 1200A service that would normally require 4 sets of 4#350. Well, I need to consider voltage drop since the length of this feeder is 470'. I have used the formula on this website:

VD = ((SQRT3)*K*I*D) / CM

I know what my allowed voltage drop is on a 208v 3ph system, (3%) 6.24v. So with that being said, I need to determine the total CM for the service. By rearranging the above formula:

CM = ((SQRT3)*K*I*D) / VD

If this is in fact the correct approach, I have a question regarding the term "K".

"K" is termed as being 12.9 ohms for copper and 21.2 ohms for aluminum. These numbers (as indicated on the website, see link below) are based on DC ohms per 1000 ft of 1000 kcmil. Per NEC 2005 Chapter 9, Table 8, the value that is listed is 0.0129 ohms per 1000 ft.

I have to assume that the NEC number is correct. Am I missing something here or do I just take the formula as written?

Reference:
http://68.225.22.180/mojonewsarchive/EC-HTML/HTML/Voltage_Drop_Calculations~20030326.htm
 
Re: Service Feeder Voltage Drop

Why don't you try using the 12.9 or 21.2 ohms and see what you get for a size.

They you can check and see if it is a reasonable number. For example, from my Square D motor data calculator, I get about 0.0085 V drop per 100' for 500KCM. So for 1500KCM, at 500', at 1000A it would be:

0.0085 * 5 * 1000 /3 = 14.17 volts.

Steve
 
Re: Service Feeder Voltage Drop

"K" is termed as being 12.9 ohms for copper and 21.2 ohms for aluminum.
Click to expand...
K is dependent on the temperture. K at 20C = 10.5.
K at 12.5 is caculate using the temp in table 9.
I think it is 65C. If you have another enviornment you can re-caculate K and check the VD.

[ February 16, 2006, 09:56 AM: Message edited by: bob ]
 
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