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Is the Southwire Voltage drop calculator wrong?

Merry Christmas
M

MattEng

Member
Location
Illinois
Occupation
Electrical PE
One example here, and maybe someone can just point out where my math's wrong but I cannot for the life of me get the same answer as Southwire's calculator

#2 Aluminum 480V run of 510 feet for 90 amps. Setting Southwire to power factor of 1 and commercial 75, it gives an answer of 5.61%. 5.61% of 480V = 26.93Volts dropped

Mathwise, (.32 (ohms per 1000 feet from table 9 NEC) * 510 feet)/1000 feet gives resistance of .1632 ohms.

.1632O * 90A = 14.688Vl-n. Multiply by square root of 3 to bring back to L-L world and it's 25.4 Volts dropped. 25.4/480 = 3.9%


Not the biggest difference there, but I am getting bigger differences on higher power runs. Is this a known issue that southwire's calculator isn't correct?
 
M

MattEng

Member
Location
Illinois
Occupation
Electrical PE
Elect117 said:
give all the parameters used on southwire?
Click to expand...
System:
Mode: Commercial 75
Power Factor: 1
Current: AC
Units: Feet
Phase: 3
Conductor: Aluminum
Instalation: Cable, Conduit (non steel) and direct burial

Input:
Calculator: Voltage Drop
Conductor Size: 2
Length of cable run: 510
Current at end of cable run: 90
Parallel sets: 1

Gives an answer of 5.61% voltage drop, with engineering information of 90Amps rated, .3190 ohms resistance, .0450 ohms reactance, and 1 power factor

I get 3.9% drop. I'm aware that's still above 3% drop, but I'm more wondering how Southwire is coming to it's conclusion.
 
M

MattEng

Member
Location
Illinois
Occupation
Electrical PE
wwhitney said:
25.4/480 = 5.3%, which is closer but still differs from 5.61%.

Cheers, Wayne
Click to expand...
Wow. I really did typo at the very very end. And multiple times as I kept checking

Still different and that bothers me significantly, but not nearly as much
 
E

Elect117

Senior Member
Location
California
Occupation
Engineer E.E. P.E.
My guess is it is a rounding issue that shows in the form of ~1.5V on a 480V circuit. Which is close enough to be usable. I couldn't tell you which one is more correct since we don't have access to their coding of the calculator.

They might also be taking into account mutual inductance or some form of it in their calculation. To which nobody would do by hand willingly. lol

Who knows.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Voltage drop needs to consider the length of the 'out' conductor and also the 'return' one for single phase circuits. The Southwire calculator does this automatically. You need to do this with manual calculations.

Could this ne the difference?
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
jim dungar said:
Voltage drop needs to consider the length of the 'out' conductor and also the 'return' one for single phase circuits. The Southwire calculator does this automatically. You need to do this with manual calculations.

Could this be the difference?
Click to expand...
No, the Southwire calculator has a choice for single phase or 3 phase. Also, that would make a difference of 2/sqrt(3), which is not the particular difference we are seeing.

Cheers, Wayne
 
M

MattEng

Member
Location
Illinois
Occupation
Electrical PE
So I actually found where the difference is coming from. If I change the conduit type on southwire to "single conductor in free air" Southwire gives the exact percentages I calculate by hand. This makes sense, as I'm only looking at the resistance and reactance (if using a power factor and getting the angles in the math) of the cable itself for my calculations of voltage drop.

Then the number changes depending on if steel or non steel conduit or three phase overhead is selected. So Southwire is doing something in their calculation with the magnetic field surrounding the cables impacting the resistance of neighboring cables even with a power factor of 1. And that's the bit of the calculation I'm not doing with the hand and calculator method.

I just don't know what Southwire is doing there as we don't have source code as far as I'm aware.
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
MattEng said:
If I change the conduit type on southwire to "single conductor in free air" Southwire gives the exact percentages I calculate by hand. . . .
Then the number changes depending on if steel or non steel conduit or three phase overhead is selected.
Click to expand...
The calculator's output for reactance changes based on the above. For #2 Alu, Commercial 75C the resistance appears to always be 0.319 ohms/kft (it uses the nominal temperature to adjust the resistance without any effort to calculate the operating temperature). For single conductor in free air, the reactance is 0. While for "Cable, Conduit (non-steel), Direct Burial" the reactance is 0.045 ohms/kft.

For power factor 0.9 and "Cable, etc", the calculator's voltage drop output matches what you get from the Zeff formula in Note 2 to Table 9 in Chapter 9 of the NEC, based on the above numbers. But for power factors 0, 0.8 or 1.0, it does not. Whereas for "Free Air" (reactance 0, the voltage drop is linear in power factor), the calculator's results match for any power factor.

My guess is a bug in the calculator.

Cheers, Wayne
 
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