July 12, 2013

How to calculate voltage drop?

How to calculate voltage drop:
In situations where the circuit conductors span large distances, the voltage drop is calculated. If the voltage drop is too great, the circuit conductor must be increased to maintain the current between the points. The calculations for a single-phase circuit and a three-phase circuit differ slightly.

Single-phase voltage drop calculation:

Three-phase voltage drop calculation:

OR 3øVD = (SQRT(3)*L*R*I)/1000
VD = Voltage drop (conductor temp of 75°C) in volts
VD% = Percentage of voltage drop (VD ÷ source voltage x 100). It is this value that is commonly called "voltage drop" and is cited in the NEC 215.2(A)(4) and throughout the NEC.
L = One-way length of the circuit's feeder (in feet)
R = Resistance factor per NEC Chapter 9, Table 8, in ohm/kft
I = Load current (in amperes)
Source voltage = The voltage of the branch circuit at the source of power. Typically the source voltage is either 120, 208, 240, 277, or 480 V.

Related Link: How much voltage drop is Acceptable?


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