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"A closer
look at voltage drop"
With
trilingual summary
In any
electrical system, current flow results in a reduction or drop in voltage from
source to load, because of the impedance in the intervening conductors. In the
United States, standards say little about voltage drop. The requirements of ANSI
C84.1, such as 480 volts at the source of a circuit in which minimum load
voltage is 440, allow for voltage drops of 8 to 10 percent. The National
Electrical Code recommends (but does not require) maximum voltage drop of 5
percent. Such limits are often far exceeded during motor starting when load
current is 5 to 8 times normal. Motor terminal voltage must then be determined
accurately to make sure motor torque is adequate for acceleration.
At low values of current, circuit impedance can be considered as resistance
only. In a-c circuits, however, inductive reactance is also present. This
becomes significant as conductor size increases to 10 mm or more. Because
conductor ampacity also varies with size, some electricians mistakenly assume
that ampacity and voltage drop calculations are directly related. However,
inductance does not affect ampacity, and temperature influences only resistance.
Because voltage drops across resistance and reactance are out of phase with each
other, circuit voltages cannot be calculated by simple addition or subtraction.
The resistive component of voltage drop will be in phase with current; the
reactive component will be 90 degrees out of phase.
Most literature explains voltage drop calculation using a phasor diagram in
which current is presumed known. The drop is then calculated and subtracted from
the source voltage. In practice, however, the source voltage is the known
quantity; current is calculated based on total circuit impedance including the
load, and load voltage is derived from that.
Whatever the method, impedance of both sides of the circuit must be accounted
for. Therefore, if conductor impedance is determined for the length of circuit
from source to load, that figure must be doubled (or multiplied by 1.732 in a
three-phase circuit) to calculate voltage drop.
From “A
closer look at voltage drop,"
published in the
Electrical Apparatus November 2007 issue
. Visit our online webstore to
order
copies. © 2007
Barks Publications, Inc. All Rights Reserved.
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