Re: Voltage Drop

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Lightninarrester

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Re: Voltage Drop

Guys, please verify the following VD calculation for #2AWG 13.8KV cable in RMC conduit for 1200feet one way distance. Assume load of 100A.

Cable ampacity for Copper #2AWG 13.8KV is I=145A
R=0.2 from Table 9 of NEC.

by NEC formula, Vd= (1.732xLxRxI)/1000
Vd= (1.732x1200x0.2x100)/1000
= 41.568V
%Vd= 41.568V/13800 = 0.301%

[ August 04, 2005, 08:38 PM: Message edited by: Lightninarrester ]
 
Re: Re: Voltage Drop

If I use .201 ohms/kft. for #2 from table 8 chapter 9 I get:

41.776 v.
3.027%
 
Re: Re: Voltage Drop

I use my scientific calculator.

Oops. it is .3027%

I was trying to agree with you. :D
 
Re: Re: Voltage Drop

What are you doing? 100 amperes is close to 2500 kVA @ 13.8 kV so, I assume, you are not going to wire a garage out behind the house. :D
 
Re: Re: Voltage Drop

Originally posted by Lightninarrester:The calculator at Electrician.com is for max 480V.
Click to expand...
True, but irrelevant. It calculated the voltage drop at 40.4 volts. That is within "rounding error" of the 41.6 result shown here. Perhaps it used a different estimated value for the square root of 3, or more likely it used a slightly different value for the cable's resistance. But the voltage drop is close enough to call it correct.

The only thing that changes, when you talk about the 13,800 volt calculation, is the percent voltage drop. The calculator said that 40.4 volts was over 8% of a 480 volt system. Here again that is true, but irrelevant to our question. That same 40.4 volts would be .29% of a 13,800 system.

Do the following test with that calculator: Put in the numbers for this problem, except put in 480 volt three phase. Look at the voltage drop, in volts. Then change the input value to 208 volt, three phase. You will see that the voltage drop, in volts, is the same, but the percentage has gone up.
 
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