Voltage Drop Calculation

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Mike NY

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Could someone give the formula for voltage drop calculation. 600MCM, TW insulation, length is roughly 600', 400A, and 3 phase 208v. Having a major mind fart.
 
there are 2 formulas for voltage drop
single phase VD=2K*L*I/CM

3 phase VD= 1.732*K*L*I/CM

K=either 10.4 or 12.9 depending on who you believe

L= length of wire

I= current
CM=circular mils of wire
 
Mike NY said:
Could someone give the formula for voltage drop calculation. 600MCM, TW insulation, length is roughly 600', 400A, and 3 phase 208v. Having a major mind fart.
Click to expand...

E=IxZ
Where I=400 amps and Z=.040x.6 for copper in a PVC conduit for 600 feet

I come up with 9.6 volts drop to neutral or 8%.

Jim T
 
tlaidman said:
there are 2 formulas for voltage drop
single phase VD=2K*L*I/CM

3 phase VD= 1.732*K*L*I/CM

K=either 10.4 or 12.9 depending on who you believe

L= length of wire

I= current
CM=circular mils of wire
Click to expand...
or
1.732*R*I*L/1000 when K isn't given.
 
tlaidman said:
there are 2 formulas for voltage drop
single phase VD=2K*L*I/CM

3 phase VD= 1.732*K*L*I/CM

K=either 10.4 or 12.9 depending on who you believe

L= length of wire

I= current
CM=circular mils of wire
Click to expand...
The K value is determined by the type of material (copper or aluminum) and the temperature. the 10.4 (or 10.8?)value is probably based on 60 degrees C and is what we were taught years ago.

The NEC now uses 75 degrees C for the resistance values of Ch 9 Table 8, so that is what we base the K value on. Using 75 degrees C, the K value should be 12.9 for copper 21.2 for aluminum. These values are actually the resistance in ohms for a conductor of 1000 cmils, 1000 ft long at 75 degrees C.

-- note that the length of the wire is the distance ONE WAY using the formula method.
 
Last edited:
stevearne said:
240.4(B) allows the next standard size overcurrent device when sizing protection for under 800A
Click to expand...
When determining voltage drop you use actual or max conductor amperes... not the OCPD rating. 400A exceeds the ampacity of TW 600 kcmil copper conductors, even after ambient temperature correction to 21-25?C.
 
stevearne said:
The K value is determined by the type of material (copper or aluminum) and the temperature. the 10.4 (or 10.8?)value is probably based on 60 degrees C and is what we were taught years ago.

The NEC now uses 75 degrees C for the resistance values of Ch 9 Table 8, so that is what we base the K value on. Using 75 degrees C, the K value should be 12.9 for copper 21.2 for aluminum. These values are actually the resistance in ohms for a conductor of 1000 cmils, 1000 ft long at 75 degrees C.
Click to expand...
The problem with calculating VD that way is that the formula assumes the current on the conductor will cause the conductor's temperature to be 75?C. Most often, that is simply not so. There is no adjustment in the formula for the temperature of the conductor, no adjustment for the ambient temperature, and no adjustment for the power factor of the circuit. If we were to include these adjustments, the formula would look like this:

VDformula.gif
 
Smart $ said:
When determining voltage drop you use actual or max conductor amperes... not the OCPD rating. 400A exceeds the ampacity of TW 600 kcmil copper conductors, even after ambient temperature correction to 21-25?C.
Click to expand...
You are absolutely right that I = the actual load of the circuit for voltage drop. I should have looked back at the original post to see that 400A was the load.
 
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