,Q1 DEMONSTRATE MASTERY OF CORE CONCEPTS
A commercial service rated 1200 A, 277/480 V, 3-phase is fed by a single utility
transformer with a 5.75% impedance. Available fault current at the service
terminals is 42 kA. The contractor proposes a 1200 A fusible switch with Class L
fuses marked 200 kAIC. Under the NEC, which statement best characterizes the
compliance of this design?
A. Compliant, because the fuse AIC rating exceeds the available fault current by more than 4:1.
B. Noncompliant, because the switch must be series-rated with a listed combination to achieve
200 kAIC. CORRECT
C. Compliant, because Class L fuses are always self-protecting and require no equipment listing.
D. Noncompliant, because the available fault current exceeds the 10 kA interrupting rating of the
switch.
RATIONALE: Equipment AIC ratings must be based on listed series combinations or the
equipment's own rating, not the fuse rating alone; the switch must be listed for the available fault
current. Class L fuses are current-limiting but do not automatically raise the switch's rating
without a listed combination. Options A, C, and D misapply the concept of interrupting rating and
series ratings.
Q2 DEMONSTRATE MASTERY OF CORE CONCEPTS
A dwelling unit has a 200 A, 120/240 V service. The calculated load is 180 A. The
utility requires a minimum 3/0 AWG copper overhead service drop with 2/0 AWG
neutral. The contractor plans to install a 200 A panel with a 200 A main breaker.
Which condition would most likely require an increase in the service conductor
size?
A. The calculated load increases to 195 A after adding a 7.2 kW range.
B. The ambient temperature at the service drop exceeds 30°C. CORRECT
C. The service drop length exceeds 100 ft.
D. The utility mandates a minimum 4/0 AWG for all 200 A services.
RATIONALE: Ampacity adjustment for ambient temperature above 30°C requires derating
conductors per NEC Table 310.15(B)(2)(a). Calculated load increases within the breaker rating
do not necessarily require larger conductors, and length alone does not affect ampacity. Utility
requirements may dictate a larger size but are not an NEC condition.
Page 2
,Q3 DEMONSTRATE MASTERY OF CORE CONCEPTS
A 480 V, 3-phase, 4-wire system serves a 100 hp, 460 V, 3-phase induction motor
with a nameplate FLA of 124 A. The motor is started with a reduced-voltage
autotransformer starter at 65% tap. Which NEC requirement applies to the motor
branch-circuit short-circuit and ground-fault protection?
A. The branch-circuit protection must be sized at 250% of the motor FLA.
B. The branch-circuit protection must be sized at 175% of the motor FLA.
C. The branch-circuit protection must be selected from Table 430.52 based on the motor type
and may be increased if it trips. CORRECT
D. The branch-circuit protection must be a listed combination starter with a 200 kAIC rating.
RATIONALE: NEC 430.52 requires branch-circuit short-circuit and ground-fault protection to be
selected from Table 430.52, with permissible increases if the device trips during starting. Options
A and B are specific percentages for inverse-time breakers and dual-element fuses, respectively,
but not universally applicable. Option D addresses equipment rating, not branch-circuit protection
sizing.
Q4 DEMONSTRATE MASTERY OF CORE CONCEPTS
A 120/208 V, 3-phase, 4-wire panelboard supplies a 30 A, 3-phase, 208 V
continuous load and a 20 A, 120 V noncontinuous load on phase A. The neutral is
not reduced. What is the minimum size THHN copper feeder conductor required?
A. 8 AWG
B. 6 AWG CORRECT
C. 4 AWG
D. 3 AWG
RATIONALE: The continuous load must be multiplied by 125% (30 A × 1.25 = 37.5 A), plus the
noncontinuous load of 20 A, totaling 57.5 A on phase A. A 6 AWG THHN copper conductor has
an ampacity of 65 A at 30°C, which meets the requirement. 8 AWG is rated 50 A and is
insufficient; 4 AWG and 3 AWG are larger than necessary.
Page 3
, Q5 DEMONSTRATE MASTERY OF CORE CONCEPTS
A 100 A, 240 V feeder supplies a 40 A continuous load and a 50 A noncontinuous
load. The feeder is installed in a raceway with three current-carrying conductors in
an ambient temperature of 40°C. What is the minimum ampacity of the conductor
after applying correction factors?
A. 90 A
B. 100 A
C. 110 A CORRECT
D. 125 A
RATIONALE: The calculated load is 40 A × 1.25 + 50 A = 100 A. For three current-carrying
conductors, no adjustment is needed, but ambient temperature correction at 40°C for 90°C
conductors is 0.91. The required ampacity before correction is 100 A / 0.91 = 109.9 A, so a
conductor with at least 110 A ampacity is needed. Options A, B, and D do not account for the
correction factor.
Q6 DEMONSTRATE MASTERY OF CORE CONCEPTS
A 3-phase, 4-wire, 480Y/277 V system serves a 100 A continuous load and a 50 A
noncontinuous load. The neutral carries only nonlinear loads and is considered a
current-carrying conductor. How many current-carrying conductors are in the
raceway if the feeder also contains an equipment grounding conductor?
A. 3
B. 4 CORRECT
C. 5
D. 6
RATIONALE: The neutral is counted as a current-carrying conductor because it carries nonlinear
loads (harmonics). Thus, the three phase conductors plus the neutral equal four current-carrying
conductors. The equipment grounding conductor is not counted. Options A, C, and D incorrectly
include or exclude the neutral or EGC.
Page 4
A commercial service rated 1200 A, 277/480 V, 3-phase is fed by a single utility
transformer with a 5.75% impedance. Available fault current at the service
terminals is 42 kA. The contractor proposes a 1200 A fusible switch with Class L
fuses marked 200 kAIC. Under the NEC, which statement best characterizes the
compliance of this design?
A. Compliant, because the fuse AIC rating exceeds the available fault current by more than 4:1.
B. Noncompliant, because the switch must be series-rated with a listed combination to achieve
200 kAIC. CORRECT
C. Compliant, because Class L fuses are always self-protecting and require no equipment listing.
D. Noncompliant, because the available fault current exceeds the 10 kA interrupting rating of the
switch.
RATIONALE: Equipment AIC ratings must be based on listed series combinations or the
equipment's own rating, not the fuse rating alone; the switch must be listed for the available fault
current. Class L fuses are current-limiting but do not automatically raise the switch's rating
without a listed combination. Options A, C, and D misapply the concept of interrupting rating and
series ratings.
Q2 DEMONSTRATE MASTERY OF CORE CONCEPTS
A dwelling unit has a 200 A, 120/240 V service. The calculated load is 180 A. The
utility requires a minimum 3/0 AWG copper overhead service drop with 2/0 AWG
neutral. The contractor plans to install a 200 A panel with a 200 A main breaker.
Which condition would most likely require an increase in the service conductor
size?
A. The calculated load increases to 195 A after adding a 7.2 kW range.
B. The ambient temperature at the service drop exceeds 30°C. CORRECT
C. The service drop length exceeds 100 ft.
D. The utility mandates a minimum 4/0 AWG for all 200 A services.
RATIONALE: Ampacity adjustment for ambient temperature above 30°C requires derating
conductors per NEC Table 310.15(B)(2)(a). Calculated load increases within the breaker rating
do not necessarily require larger conductors, and length alone does not affect ampacity. Utility
requirements may dictate a larger size but are not an NEC condition.
Page 2
,Q3 DEMONSTRATE MASTERY OF CORE CONCEPTS
A 480 V, 3-phase, 4-wire system serves a 100 hp, 460 V, 3-phase induction motor
with a nameplate FLA of 124 A. The motor is started with a reduced-voltage
autotransformer starter at 65% tap. Which NEC requirement applies to the motor
branch-circuit short-circuit and ground-fault protection?
A. The branch-circuit protection must be sized at 250% of the motor FLA.
B. The branch-circuit protection must be sized at 175% of the motor FLA.
C. The branch-circuit protection must be selected from Table 430.52 based on the motor type
and may be increased if it trips. CORRECT
D. The branch-circuit protection must be a listed combination starter with a 200 kAIC rating.
RATIONALE: NEC 430.52 requires branch-circuit short-circuit and ground-fault protection to be
selected from Table 430.52, with permissible increases if the device trips during starting. Options
A and B are specific percentages for inverse-time breakers and dual-element fuses, respectively,
but not universally applicable. Option D addresses equipment rating, not branch-circuit protection
sizing.
Q4 DEMONSTRATE MASTERY OF CORE CONCEPTS
A 120/208 V, 3-phase, 4-wire panelboard supplies a 30 A, 3-phase, 208 V
continuous load and a 20 A, 120 V noncontinuous load on phase A. The neutral is
not reduced. What is the minimum size THHN copper feeder conductor required?
A. 8 AWG
B. 6 AWG CORRECT
C. 4 AWG
D. 3 AWG
RATIONALE: The continuous load must be multiplied by 125% (30 A × 1.25 = 37.5 A), plus the
noncontinuous load of 20 A, totaling 57.5 A on phase A. A 6 AWG THHN copper conductor has
an ampacity of 65 A at 30°C, which meets the requirement. 8 AWG is rated 50 A and is
insufficient; 4 AWG and 3 AWG are larger than necessary.
Page 3
, Q5 DEMONSTRATE MASTERY OF CORE CONCEPTS
A 100 A, 240 V feeder supplies a 40 A continuous load and a 50 A noncontinuous
load. The feeder is installed in a raceway with three current-carrying conductors in
an ambient temperature of 40°C. What is the minimum ampacity of the conductor
after applying correction factors?
A. 90 A
B. 100 A
C. 110 A CORRECT
D. 125 A
RATIONALE: The calculated load is 40 A × 1.25 + 50 A = 100 A. For three current-carrying
conductors, no adjustment is needed, but ambient temperature correction at 40°C for 90°C
conductors is 0.91. The required ampacity before correction is 100 A / 0.91 = 109.9 A, so a
conductor with at least 110 A ampacity is needed. Options A, B, and D do not account for the
correction factor.
Q6 DEMONSTRATE MASTERY OF CORE CONCEPTS
A 3-phase, 4-wire, 480Y/277 V system serves a 100 A continuous load and a 50 A
noncontinuous load. The neutral carries only nonlinear loads and is considered a
current-carrying conductor. How many current-carrying conductors are in the
raceway if the feeder also contains an equipment grounding conductor?
A. 3
B. 4 CORRECT
C. 5
D. 6
RATIONALE: The neutral is counted as a current-carrying conductor because it carries nonlinear
loads (harmonics). Thus, the three phase conductors plus the neutral equal four current-carrying
conductors. The equipment grounding conductor is not counted. Options A, C, and D incorrectly
include or exclude the neutral or EGC.
Page 4
