By JAMES E. McALISTER
Straightedge is used to determine IR drop in conductors where high currents are to be carried over long runs.
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IN many high-current applications, the voltage drop in current-carrying wires themselves can be quite significant. Even in lower current applications, especially when long wires are used, voltage drops can be an important factor.
The equation used to calculate the voltage drop can be expressed as:
E= R x I x L
where: E= voltage drop in millivolts
R= wire resistance, in ohms per 1000 feet
I = current in amperes
L= length in feet
Even though the wire resistance per 1000 feet can be found in most electrical handbooks, looking up the resistance when several wire sizes are involved can become a nuisance. The nomogram shown here allows the voltage drop in wires to be calculated with a minimum of effort.
Example of Use
Calculate the voltage drop in 100 feet of #10 wire carrying 10 amperes.
First, draw a line from the desired wire gauge (#10 on the AWG scale) through the current the wire is carrying (10 on the Current scale). This line should be extended until it crosses the turning scale. From this point a line should be drawn through the wire length (100').The intersection of this line with the Voltage-Drop scale gives the voltage drop in the wire (1 volt). If two wires of the same gauge are paralleled to carry current, a single equivalent gauge can be found for computational purposes by subtracting 3 from the original wire-gauge number. If two #16 wires are paralleled, for example, a #13 should be used in the calculations of the voltage drop.
Similarly, 6 should be subtracted for four wires in parallel, and 9 should be subtracted for eight wires in parallel.
This useful characteristic is a consequence of the arrangement of wire tables.
Also see: More on the SCHEIBER SYSTEM