SKILL: SOLVING PERCENT PROBLEMS
Voltage Drop
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A voltage drop occurs when current passes through a wire or an
electrical device. The higher the resistance of a wire, the greater
the voltage drop. Too much voltage drop can cause the device
to overheat.
The National Electric Code—a document of guidelines for electricians—states that the voltage drop across a wire should be no
more than 5 percent of the voltage from the voltage source. To find
5 percent of a number, you can set up the calculation as a proportion.
Example
The lighting in a hotel includes many fixtures that will be
arranged on long wires. The electrician needs to know the maximum voltage drop allowed in order to choose the proper wire.
The circuit will use a voltage source of 120 V. What is 5% of 120?
(1) Write the problem as a proportion.
voltage drop
perce
nt
voltage
100
(2) Substitute.
voltage drop
5
120
100
(3) Calculate and simplify.
voltage drop
5
p 120 p 120
120
100
voltage drop 6
ANSWER The maximum voltage drop in the wire is 6 V.
Use the proportion to answer the following questions.
1. If the voltage source is increased to 277 V, what is the maximum
voltage drop in the wire?
2. To be on the safe side, the electrician decided to find a wire
with a voltage drop that is 3 percent of the voltage from the
voltage source. What is the voltage drop in the wire?
CHALLENGE A student wants to hang a string of lights outside and connect it to an extension cord. The voltage drop
across the extension cord is 3.1 V. The outlet supplies 240 V.
Does the voltage drop in the extension cord meet the code
guidelines?
The many lights in this spectacular display
in Kobe, Japan, produce a large voltage
drop. The appropriate type of wire must
be used to supply its current.
388 Unit 3: Electricity and Magnetism