Chapter 3 Introduction to Basic Electrical Circuit Materials 59
Underwriters Laboratories Inc.
Underwriters Laboratories Inc. is an independent, not-
for-profit organization that conducts tests for the interest of
public safety. If UL tests equipment and finds that it satis-
fies minimal safety design standards, the equipment is
awarded their seal of approval. The UL seal of approval,
shown in Figure 3-10, is a recognized industry standard
that proves the product has been independently tested and
approved as a safe product. It also ensures the products
being produced will continually meet the minimum safety
standards. There are over 20 billion UL seals on over
19,000 different types of products.
last device connected in that circuit. A maximum of a 3%
voltage drop is permissible by NEC standards. A formula
for calculating the size conductor needed to stay within
the 3% limit is below.
CMA is the area in circular mils, K is the material con-
stant, I is the amperage, L is the length of wire, and VD is
the voltage drop.
Example: First, decide the type of material to be
used in wiring the circuit, copper or aluminum. The
material constant (K) for copper is 12. The material
constant for aluminum is 18. This is an NEC stan-
dard based on the resistance of the material. To find
the size of a copper conductor needed for a 10-amp
load located 150 feet from the circuit breaker panel,
we multiply the material constant times the amper-
age times the length (K I L). Remember, the
length is twice the distance from the panel. Next,
divide by the permissible 3% voltage drop. The
voltage in this case is 120 volts. Three percent of
120 volts is 3.6 volts.
This formula is very handy when trying to determine
a conductor size that will not cause a significant voltage
drop. Excessive voltage drops prevent electrical equip-
ment from operating properly.
Review Questions for Section 3.1
1. A half inch is equal to __________ mils.
2. The cross-sectional area of a conductor is measured
in __________.
3. The unit measure for conductance is __________.
4. What is the circular mil area of No. 22 copper
wire?
5. List four factors that affect conductor resistance.
6. What is the resistance value of 500 feet of No. 4
copper conductor?
36000
3.6
10 300
3.6
CMA
=12
CMA =
CMA = 10,000 (or a No. 10 copper conductor is needed)
K I L
VD
CMA =
Figure 3-10. The Underwriters Laboratories Inc. seal of
approval.
Occupational Safety and Health
Administration
The Occupational Safety and Health Administration
(OSHA) is part of the U.S. Department of Labor and is
mainly responsible for worker safety. OSHA conducts
on-site inspections of industries that have a reportedly
high incidence of accidents. OSHA has the authority to
inspect and fine industries that do not meet minimal
safety standards. For example, OSHA can inspect a facil-
ity to ensure all workers are using UL-approved, noncon-
ductive stepladders for electrical work or ensure worker
areas have proper ventilation to remove any hazardous
fumes. Be aware that OSHA has adopted the NEC stan-
dards. OSHA is not limited to electrical work, but it is
responsible for the safety of all industries, such as agri-
culture, automobile, fishing, manufacturing, and ware-
house operations.
Conductor voltage drop
The National Electrical Code has set a maximum
voltage drop standard for branch circuits. A branch
circuit is the wiring from an electrical circuit panel to the