COD E IN F O C U S BY CHARLES R. MILLER
There are 26 main sections in the first
two parts Article 110, a number of which
contain subsections. Part I contains
general provisions, and Part II contains
provisions for electrical systems rated
600 volts (V), nominal, or less.
Section 110. 26, Spaces About Electri-
cal Equipment, has six subsections (A
through F), and half of those are divided
into even more subsections. Although
110. 14 contains only three subsections,
it is no less important than 110. 26. The
three subsections in 110. 14 are (A) Terminals, (B) Splices and (C) Temperature
Limitations. The provisions in 110. 14(C)
are essential when sizing conductors and
determining maximum ampere ratings
for overcurrent protective devices.
As stated in 110. 14(C), the temperature
rating of a conductor shall not exceed the
lowest temperature rating of any connected termination, conductor or device.
Section 110. 14(C) is divided into provisions for equipment and provisions for
separate connectors. The determination
of equipment-termination provisions
shall be based on 110. 14(C)( 1)(a) or
(C)( 1)(b). Section 110. 14(C)( 1)(a) covers
circuits rated 100 amperes (A) or less or
marked for conductors 14 AWG through
1 AWG. Section 110. 14(C)( 1)(b) covers
circuits rated over 100A or marked for
conductors larger than 1 AWG.
Last month’s column covered the first
three of four provisions for circuits rated
100A or less or marked for conductors
14 AWG through 1 AWG. The discussion
continues this month.
The fourth provision in 110. 14(C)
( 1)(a) pertains to motors marked with
design letters B, C or D. Conductors
with an insulation rating of 75°C (167°F)
or higher can be used to supply these
motors as long as the ampacity of the
conductors does not exceed the 75°C
(167°F) ampacity. In Table 310.15(B)( 16),
the conductor temperature ratings are
60°C (140°F), 75°C (167°F) and 90°C
(194°F). If a 75°C conductor is installed
to supply power to a motor marked with
a design letter B, C or D, the conductor’s
ampacity can be based on the ampacity
shown in the 75°C column. This section
states conductors with an insulation rating of 90°C can also be installed, but the
ampacity shall not exceed the ampacity
in the 75°C column.
For example, conductors are needed
to supply power to a motor marked with
a design letter B. The terminations on the
opposite end from the motor will have a
temperature rating of 75°C. The motor
will be a 20-horsepower (hp) motor, and
ARTICLE 110 in the National Electrical Code (NEC), Requirements for Electrical
Installations, covers general requirements for the examination and approval,
installation and use, access to and spaces about electrical conductors and equipment;
enclosures intended for personnel entry; and tunnel installations [ 110. 1]. While
the scope mentions several topics covered in the article, it does not come close to
mentioning the vast number of topics in Article 110.
FIGURE 1 MOTORS MARKED WITH DESIGN LETTERS B, C OR D
The temperature rating associated with the ampacity of a conductor shall be selected
and coordinated so as not to exceed the lowest temperature rating of any connected
termination, conductor or device [ 110. 14(C)]. The fourth provision in 110. 14(C)( 1)(a)
pertains to motors marked with design letters B, C or D.
Conductors are needed to supply power to a motor marked with a design letter B. The
terminations on the opposite end from the motor will have a temperature rating of 75°C.
The motor will be a 20-hp motor, and the voltage supplying the motor will be 208V,
three-phase. The nameplate on the motor shows the motor will draw 51.6A when it is
connected to 208V, three-phase. What is the minimum size THHN conductors required to
supply power to his motor?
First, look in Table 430.250 to find the full-load current (FLC) of a 20-hp, 208V, three-phase motor.
FLC = 59.4A
Next, multiply the motor’s FLC by 125 percent.
59. 4 × 125% = 74. 25 = 74
Conductors supplying power to this motor shall have an ampacity of at least 74A.
This motor is marked with a design letter B.
General Installation Requirements, Part IX