, Question 1
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.
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.
Correct Answer: B - Noncompliant, because the switch must be
series-rated with a listed combination to achieve 200 kAIC.
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.
Question 2
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?
Page 2
, 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.
C. The service drop length exceeds 100 ft.
D. The utility mandates a minimum 4/0 AWG for all 200 A services.
Correct Answer: B - The ambient temperature at the service drop
exceeds 30°C.
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.
Question 3
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.
D. The branch-circuit protection must be a listed combination starter with
a 200 kAIC rating.
Correct Answer: C - The branch-circuit protection must be
selected from Table 430.52 based on the motor type and may be
increased if it trips.
Page 3
, 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.
Question 4
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
C. 4 AWG
D. 3 AWG
Correct Answer: B - 6 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.
Question 5
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
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.
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.
Correct Answer: B - Noncompliant, because the switch must be
series-rated with a listed combination to achieve 200 kAIC.
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.
Question 2
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?
Page 2
, 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.
C. The service drop length exceeds 100 ft.
D. The utility mandates a minimum 4/0 AWG for all 200 A services.
Correct Answer: B - The ambient temperature at the service drop
exceeds 30°C.
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.
Question 3
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.
D. The branch-circuit protection must be a listed combination starter with
a 200 kAIC rating.
Correct Answer: C - The branch-circuit protection must be
selected from Table 430.52 based on the motor type and may be
increased if it trips.
Page 3
, 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.
Question 4
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
C. 4 AWG
D. 3 AWG
Correct Answer: B - 6 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.
Question 5
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
Page 4
