Voltage Drop

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bob

Senior Member
Location
Alabama
eeeee said:
Where in the NEC is the K factor table for AC voltage drop calculations-3 phase?

Thanks,
I don't think the NEC makes reference to K. It varies with temperature.

R = C x (K/A)1000 = ohms per 1000 ft.
C = 1.02 for stranded conductors and 1 for solid.
A = cross setional area in cmil
K = 10.575 at 25C and about 12.5 at 65C. In your area it might be zero.
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
I've been using this formula for K.

K = R2 * CM

R2 = R1[1 + TCR(T2 - 75) From: NEC Tbl.8 Note 2.
TCR = Temperature Coefficient of Resistance, cu = 0.00323, al = 0.00330
T2 = new temperature, usually 60 or 90
CM = circular mils

Except for percentage adjustments in Tbl.8, Note 3, this formula does not differentiate between stranded and solid AWG. Tbl.8, Note 1, seems to assume normal-stranded wire for the chosen TCR's, unless differences between strands and solids was considered negligible.

To incorporate Bob's formula correctly for solid AWG, it looks like I could multiply my result above by 0.98 ?
 
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