voltage drop
- Thread starter dakotalon
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bphgravity
Senior Member
- Location
- Florida
Re: voltage drop
VD = I x R
or
VD = 2 x K x D x I / cm
Where K = R x cm / 1000 , where R = resistance of the conductor at 1000' as found in Table 8 and cm = circular mil as found in table 8.
D = distance to the load from the OCD
I = Load of equipment in amps
VD = I x R
or
VD = 2 x K x D x I / cm
Where K = R x cm / 1000 , where R = resistance of the conductor at 1000' as found in Table 8 and cm = circular mil as found in table 8.
D = distance to the load from the OCD
I = Load of equipment in amps
charlie
Senior Member
- Location
- Indianapolis
Re: voltage drop
The concept of voltage drop is very basic. Regardless of the formula that you use, it is a modification of E = IR. The formula that is given in the NEC Handbook is:
Vd = 2RLI / 1000
Vd = voltage drop (in volts)
R = resistance of the conductor per 1000 feet
I = amperes
2 = both ways, going out to the load and back to the source
L = length of the run
1000 = gets rid of the 1000 feet on the conductor resistance
In order to change the formula to 3 phase, you must multiply by the Sin of 60 degrees, i.e. sq.rt.3/2. This changes the formula to:
Vd = sq.rt.3 RLI/1000
The concept and formulae above are based on direct current, not alternating current. If calculations must be accurate, reactance must be accounted for as well as the power factor of the load and the resistance of the circuit to alternating current. The question is, how important is the accuracy of your calculations?
There is no difference between Mike Holt's method and the NEC Handbook's method if you use the exact value of K.
The concept of voltage drop is very basic. Regardless of the formula that you use, it is a modification of E = IR. The formula that is given in the NEC Handbook is:
Vd = 2RLI / 1000
Vd = voltage drop (in volts)
R = resistance of the conductor per 1000 feet
I = amperes
2 = both ways, going out to the load and back to the source
L = length of the run
1000 = gets rid of the 1000 feet on the conductor resistance
In order to change the formula to 3 phase, you must multiply by the Sin of 60 degrees, i.e. sq.rt.3/2. This changes the formula to:
Vd = sq.rt.3 RLI/1000
The concept and formulae above are based on direct current, not alternating current. If calculations must be accurate, reactance must be accounted for as well as the power factor of the load and the resistance of the circuit to alternating current. The question is, how important is the accuracy of your calculations?
There is no difference between Mike Holt's method and the NEC Handbook's method if you use the exact value of K.
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