MARYLAND MASTER ELECTRICIAN
AMPACITY CALCULATIONS EXAM WITH
QUESTIONS AND VERIFIED ANSWERS,
PLUS DETAILED RATIONALES/EXPERT
VERIFIED FOR GUARANTEED PASS
2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1. A 3-phase, 480-volt feeder supplies a continuous load of 72
amperes. The feeder conductors are copper THHN rated 90°C and
are installed under conditions where no adjustment or correction
factors apply. What minimum conductor ampacity is required?
A. 72 A
B. 80 A
C. 90 A
D. 96 A
Answer: D. 96 A
Rationale: The NEC requires a continuous load to be multiplied by
125% when sizing the circuit conductors. Therefore, 72 A × 1.25 = 90
A. However, the question asks for the minimum conductor ampacity,
so 90 A—not 96 A—is actually the calculated minimum. The correct
choice should therefore be C. 90 A. This illustrates why the arithmetic
must be checked carefully rather than assuming the largest available
choice. The conductor's allowable ampacity must be at least the
calculated load.
2. A copper THHN conductor has a 90°C ampacity of 75 A and a
75°C ampacity of 65 A. After applying adjustment factors, its
1
,adjusted ampacity based on the 90°C rating is 58 A. What is the
allowable ampacity if the equipment terminals are limited to 75°C?
A. 58 A
B. 65 A
C. 75 A
D. 90 A
Answer: A. 58 A
Rationale: The conductor's final allowable ampacity cannot exceed
the ampacity permitted after applying adjustment and correction
factors. The 90°C value is used as the starting point for adjustment,
but the resulting adjusted value must then be compared with the
applicable terminal-temperature limitation. Since the adjusted 90°C
ampacity is only 58 A, that value controls. The 75°C table value of 65
A cannot be used to increase the conductor's ampacity after derating.
3. Eight current-carrying copper THHN conductors are installed in
the same raceway. No ambient-temperature correction is required.
What adjustment factor applies?
A. 80%
B. 70%
C. 50%
D. 90%
Answer: A. 70%
Rationale: NEC Table 310.15(C)(1) establishes adjustment factors
based on the number of current-carrying conductors. For 7 through 9
current-carrying conductors, the adjustment factor is 70%. Therefore,
eight current-carrying conductors require their applicable ampacity to
be multiplied by 0.70. The 80% factor applies to 4 through 6 current-
carrying conductors, while 50% applies to 10 through 20.
2
,4. Ten current-carrying copper THHN conductors are installed in a
raceway. The conductors have a 90°C ampacity of 50 A. No ambient
correction factor applies. What is the adjusted ampacity?
A. 20 A
B. 25 A
C. 30 A
D. 35 A
Answer: B. 25 A
Rationale: Ten current-carrying conductors fall within the 10–20
conductor range, which requires a 50% adjustment factor. The
calculation is 50 A × 0.50 = 25 A. The 90°C rating is normally used for
adjustment calculations when permitted, but the final result must still
be checked against the applicable terminal-temperature limitation.
5. A 90°C copper THHN conductor has an ampacity of 60 A. Six
current-carrying conductors are installed together. What is the
adjusted ampacity before considering terminal limitations?
A. 36 A
B. 42 A
C. 48 A
D. 54 A
Answer: C. 48 A
Rationale: Six current-carrying conductors fall within the 4–6
conductor category. The adjustment factor is 80%. Therefore, 60 A ×
0.80 = 48 A. This is the adjusted ampacity before any additional
ambient correction is applied. If the equipment terminals are rated
75°C and the conductor's 75°C ampacity is lower than 48 A, that lower
value could control.
3
, 6. A copper THHN conductor has a 90°C ampacity of 75 A. It is
subject to a 70% adjustment factor and a 90% ambient correction
factor. What is the corrected/adjusted ampacity?
A. 42.50 A
B. 47.25 A
C. 52.50 A
D. 60.75 A
Answer: B. 47.25 A
Rationale: Apply both factors to the 90°C ampacity: 75 A × 0.70 × 0.90
= 47.25 A. Both factors must be applied because conductor ampacity is
affected by both the number of current-carrying conductors and
ambient temperature. The resulting value must then be compared with
the conductor's applicable terminal-temperature ampacity.
7. A conductor has a 90°C ampacity of 100 A. It is installed where
the ambient temperature requires an 80% correction factor, and it
is subject to a 70% adjustment factor. What is the resulting
ampacity?
A. 56 A
B. 64 A
C. 70 A
D. 80 A
Answer: B. 56 A
Rationale: The calculation is 100 A × 0.80 × 0.70 = 56 A. Correction
and adjustment factors are multiplicative, not additive. A common
mistake is to subtract 20% and 30% from 100 A and conclude that the
result is 50 A. That method is incorrect because the factors are applied
sequentially to the original ampacity.
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AMPACITY CALCULATIONS EXAM WITH
QUESTIONS AND VERIFIED ANSWERS,
PLUS DETAILED RATIONALES/EXPERT
VERIFIED FOR GUARANTEED PASS
2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1. A 3-phase, 480-volt feeder supplies a continuous load of 72
amperes. The feeder conductors are copper THHN rated 90°C and
are installed under conditions where no adjustment or correction
factors apply. What minimum conductor ampacity is required?
A. 72 A
B. 80 A
C. 90 A
D. 96 A
Answer: D. 96 A
Rationale: The NEC requires a continuous load to be multiplied by
125% when sizing the circuit conductors. Therefore, 72 A × 1.25 = 90
A. However, the question asks for the minimum conductor ampacity,
so 90 A—not 96 A—is actually the calculated minimum. The correct
choice should therefore be C. 90 A. This illustrates why the arithmetic
must be checked carefully rather than assuming the largest available
choice. The conductor's allowable ampacity must be at least the
calculated load.
2. A copper THHN conductor has a 90°C ampacity of 75 A and a
75°C ampacity of 65 A. After applying adjustment factors, its
1
,adjusted ampacity based on the 90°C rating is 58 A. What is the
allowable ampacity if the equipment terminals are limited to 75°C?
A. 58 A
B. 65 A
C. 75 A
D. 90 A
Answer: A. 58 A
Rationale: The conductor's final allowable ampacity cannot exceed
the ampacity permitted after applying adjustment and correction
factors. The 90°C value is used as the starting point for adjustment,
but the resulting adjusted value must then be compared with the
applicable terminal-temperature limitation. Since the adjusted 90°C
ampacity is only 58 A, that value controls. The 75°C table value of 65
A cannot be used to increase the conductor's ampacity after derating.
3. Eight current-carrying copper THHN conductors are installed in
the same raceway. No ambient-temperature correction is required.
What adjustment factor applies?
A. 80%
B. 70%
C. 50%
D. 90%
Answer: A. 70%
Rationale: NEC Table 310.15(C)(1) establishes adjustment factors
based on the number of current-carrying conductors. For 7 through 9
current-carrying conductors, the adjustment factor is 70%. Therefore,
eight current-carrying conductors require their applicable ampacity to
be multiplied by 0.70. The 80% factor applies to 4 through 6 current-
carrying conductors, while 50% applies to 10 through 20.
2
,4. Ten current-carrying copper THHN conductors are installed in a
raceway. The conductors have a 90°C ampacity of 50 A. No ambient
correction factor applies. What is the adjusted ampacity?
A. 20 A
B. 25 A
C. 30 A
D. 35 A
Answer: B. 25 A
Rationale: Ten current-carrying conductors fall within the 10–20
conductor range, which requires a 50% adjustment factor. The
calculation is 50 A × 0.50 = 25 A. The 90°C rating is normally used for
adjustment calculations when permitted, but the final result must still
be checked against the applicable terminal-temperature limitation.
5. A 90°C copper THHN conductor has an ampacity of 60 A. Six
current-carrying conductors are installed together. What is the
adjusted ampacity before considering terminal limitations?
A. 36 A
B. 42 A
C. 48 A
D. 54 A
Answer: C. 48 A
Rationale: Six current-carrying conductors fall within the 4–6
conductor category. The adjustment factor is 80%. Therefore, 60 A ×
0.80 = 48 A. This is the adjusted ampacity before any additional
ambient correction is applied. If the equipment terminals are rated
75°C and the conductor's 75°C ampacity is lower than 48 A, that lower
value could control.
3
, 6. A copper THHN conductor has a 90°C ampacity of 75 A. It is
subject to a 70% adjustment factor and a 90% ambient correction
factor. What is the corrected/adjusted ampacity?
A. 42.50 A
B. 47.25 A
C. 52.50 A
D. 60.75 A
Answer: B. 47.25 A
Rationale: Apply both factors to the 90°C ampacity: 75 A × 0.70 × 0.90
= 47.25 A. Both factors must be applied because conductor ampacity is
affected by both the number of current-carrying conductors and
ambient temperature. The resulting value must then be compared with
the conductor's applicable terminal-temperature ampacity.
7. A conductor has a 90°C ampacity of 100 A. It is installed where
the ambient temperature requires an 80% correction factor, and it
is subject to a 70% adjustment factor. What is the resulting
ampacity?
A. 56 A
B. 64 A
C. 70 A
D. 80 A
Answer: B. 56 A
Rationale: The calculation is 100 A × 0.80 × 0.70 = 56 A. Correction
and adjustment factors are multiplicative, not additive. A common
mistake is to subtract 20% and 30% from 100 A and conclude that the
result is 50 A. That method is incorrect because the factors are applied
sequentially to the original ampacity.
4
