CODE IN FOCUS
where the maximum current is expected
to continue for three hours or more.
Because 424.3(B) specifies that fixed
electric space-heating equipment and
motors shall be considered continuous-load, it is not necessary to determine if
the maximum current will be expected
to continue for three hours or more.
For example, a wall-mounted heater
is to be installed. The nameplate on the
heater shows 1,500 watts (W) at 120 volts
(V). The branch-circuit conductors sup-
plying power to this electric heater will
be THHN copper, and the conductors
will be installed in a raceway. The voltage
drop will not exceed the NEC recom-
mendation. All of the 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. What is the
minimum size conductor required to
supply this heater and what minimum
ampere rating is required for the over-
current device, which will be a breaker?
First, calculate this heater’s current
draw. Since watts and voltage are given,
divide the nameplate watts by the nameplate volts to find the amperes (A). For
the purpose of this article, assume heating watts equivalent to volt-amperes
(VA). This heater has a current draw of
12.5A ( 1,500 ÷ 120 = 12. 5). Be careful at
this point, and don’t select a 15A branch
circuit, because, as 424.3(B) states, fixed
electric space-heating equipment 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 overcurrent device rating shall not be less than 125 percent
of the continuous load [210.20(A)]. The
branch-circuit rating for this heater
shall be at least 16A ( 12. 5 × 125% = 15. 6 =
16). Therefore, the rating of the breaker
must be at least 16A. In accordance with
240.6(A), this is not a standard rating.
The next standard rating higher than 16A
is 20A. The conductor must also have a
rating of at least 16A. In accordance with
the 75°C column of Table 310.15(B)( 16),
a 14 AWG conductor is good for 20A.
However, a 14 AWG conductor is one of
the conductors with a double asterisk,
which references a footnote at the bottom of Table 310.15(B)( 16) that states to
see 240.4(D) for conductor overcurrent
protection limitations. Unless specifically permitted in 240.4(E) or (G), the
overcurrent protection shall not exceed
15A for 14 AWG copper [240.4(D)( 3)].
Because the rating of the breaker is 20A,
the minimum size conductor is 12 AWG
(see Figure 1).
In accordance with 424.3(A), individual branch circuits shall be permitted
to supply any volt-ampere or wattage
FIGURE 2 INDIVIDUAL BRANCH CIRCUITS FOR FIXED ELECTRIC SPACE-HEATING EQUIPMENT
Individual branch circuits shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for
which they are rated [424.3(A)].
The wall-mounted heater will be installed in a commercial occupancy.
The nameplate shows 4,000W at 240V, single-phase. The branch-circuit
conductors will be THHN copper and will be installed in a raceway.
What is the minimum size conductor required for 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.
4,000 ÷ 240 = 16.67A
Because this is a continuous load, multiply the load by 125 percent.
16. 67 × 125% = 20. 8 = 21A
The rating of the breaker must be at least 21A. In accordance with
240.6(A), the next standard rating higher than 21A is 25A.
Check Table 310.15(B)( 16) for a conductor with an ampacity of at least 21A.
12 AWG (in the 75°C column) = 25A
Although a 12 AWG conductor has an allowable ampacity of 25A,
installing an overcurrent device rated higher than 20A to protect the
conductor (for this installation) is not permitted. See 240.4(D)( 5). Because
the rating of the breaker is 25A, the minimum size conductor is 10 AWG.
10 AWG (in the 75°C column) = 35A
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 25A. The minimum size conductor required to supply this heater is 10 AWG.
4,000 W at 240V
Assume space-heating watts are equivalent to volt-amperes.
Since the minimum size conductor is 10 AWG, it would be permissible to install a breaker rated 30A unless the instructions included in the
listing or labeling of the heater specify that the maximum rating shall be 25A.
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