64 ELECTRICALCONTRACTOR | SEP. 14 | WWW.ECMAG.COM
COD E IN F O C U S BY CHARLES R. MILLER
For the purpose of Article 424,
heating equipment shall include
heating cable, unit heaters, boilers,
central systems, or other approved fixed
electric space-heating equipment, but
it shall not apply to process heating
and room air conditioning. The section
referenced in Table 210.2 that covers
branch circuits is 424.3. In accordance
with 424.3(B), fixed electric space-heating equipment and motors shall be
considered continuous-load. As defined
in Article 100, a continuous load is a load
Sizing Conductors, Part XL
Fixed electric space-heating equipment
ARTICLE 210 IN THE NATIONAL ELECTRICAL CODE (NEC) covers branch
circuits, except when they supply only motor loads, which Article 430 covers. When
sizing conductors for branch circuits, it is necessary to comply with the provisions
in Article 210. It may also be necessary to check for applicable provisions in other
articles. As 210.2 states, branch circuits shall comply with Article 210 and with the
applicable provisions of other articles of this Code. The first table in Article 210 is
helpful in finding branch-circuit provisions in other parts of the Code. The provisions for branch circuits supplying equipment listed in Table 210.2—Specific-Purpose
Branch Circuits—amend, or supplement, the provisions in Article 210. While the list
in Table 210.2 is helpful in determining if it is necessary to look in other parts of the
Code, it is not all-inclusive. Just because a certain specific-purpose branch circuit is
not covered in Table 210.2 does not mean it is not covered. Unlike some equipment—
such as appliances in Article 422, which is not listed—fixed electric space-heating
equipment is listed in Table 210.2 and covered in Article 424.
FIGURE 1 FIXED ELECTRIC SPACE-HEATING EQUIPMENT
In accordance with 424.3(B), fixed electric space-heating equipment and motors shall be considered continuous-load.
A wall-mounted heater will be installed, and the nameplate shows
1,500W at 120V. The branch-circuit conductors supplying this heater
will be THHN copper and will be installed in a raceway. What is the
minimum size conductor to supply this heater, and what minimum
ampere rating is required for the overcurrent device (a breaker)?
The voltage drop will not exceed the NEC recommendation. All
terminations will be rated 75°C. The maximum ambient temperature
will be 30°C, and there will not be more than three current-carrying
conductors in the raceway.
Find the current draw (amperes) by dividing watts by volts.
1,500 ÷ 120 = 12.5A
As 424.3(B) states, fixed electric space-heating equipment and
motors shall be considered a continuous load. When sizing branch-circuit conductors, the minimum branch-circuit conductor size
shall have an allowable ampacity not less than 125 percent of the
continuous load [210.19(A)( 1)]. When sizing overcurrent protection,
the rating of the overcurrent device shall not be less than 125
percent of the continuous load [210.20(A)].
12. 5 × 125% = 15. 6 = 16A
The breaker’s rating must be at least 16A. In accordance with
240.6(A), the next standard rating higher than 16A is 20A.
Check Table 310.15(B)( 16) for a conductor that has an ampacity of
at least 16A.
14 AWG (in the 75°C column) = 20A
Although a 14 AWG conductor has an allowable ampacity of 20A,
installing an overcurrent device rated higher than 15A to protect
the conductor (for this installation) is not permitted. See 240.4(D)
( 3). Because the rating of the breaker is 20A, the minimum size
conductor is 12 AWG.
12 AWG (in the 75°C column) = 25A
Fixed electric space-heating equipment Table 310.15(B)( 16)
Temperature Rating of Conductor
60°C ( 140°F) 75°C (167°F) 90°C (194°F)
T W, UF
Types RHW, THHW,
THW, THWN, XHHW,
Types TBS, SA, SIS, FEP,
FEPB, MI, RHH, RHW- 2,
THHN, THHW, THW- 2,
THWN- 2, USE- 2, XHH,
XHHW, XHHW- 2, ZW- 2
** Refer to 240.4(D) for conductor overcurrent protection limitations.
The minimum rating for the overcurrent device is 20A. The minimum
size conductor required to supply this heater is 12 AWG.
1,500 W at 120V
Assume space heating watts are equivalent to volt-amperes.
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