CSLB General Building (B) — All Questions

A 208-volt, single-phase branch circuit supplies a continuous load of 24 amperes. What is the minimum standard overcurrent device rating permitted?

• a.30 amperes✓
• b.25 amperes
• c.24 amperes
• d.40 amperes

An overcurrent device supplying a continuous load must be rated at not less than 125 percent of that load: 24 A x 1.25 = 30 A, which is a standard rating in 240.6(A). The 24 A and 25 A choices ignore or under-apply the 125 percent factor, and 40 A jumps past the next standard size above the calculated value.2023 NEC §210.20(A)

For that same 24-ampere continuous load, what is the minimum size copper branch-circuit conductor, assuming 75 degrees C terminations and THWN insulation?

• a.12 AWG
• b.10 AWG✓
• c.8 AWG
• d.14 AWG

The conductor must carry at least 125 percent of the continuous load, or 30 A. In the 75 degrees C column of Table 310.16, 12 AWG copper is only 25 A, so it fails; 10 AWG copper is 35 A and is adequate, and 240.4(D) still allows a 30 A device on 10 AWG. Choosing 8 AWG is oversized for the minimum asked, and 14 AWG is limited to 15 A.2023 NEC §210.19(A)

Six current-carrying 12 AWG THHN copper conductors are installed in a single EMT run at a 30 degrees C ambient. What is the adjusted ampacity of each conductor before termination limits are applied?

• a.25 amperes
• b.20 amperes
• c.22.5 amperes
• d.24 amperes✓

Start in the 90 degrees C column of Table 310.16, where 12 AWG THHN is 30 A, then apply the adjustment factor for 4 through 6 current-carrying conductors, which is 80 percent: 30 A x 0.80 = 24 A. The 22.5 A answer wrongly starts from the 75 degrees C value of 25 A, 25 A applies no adjustment at all, and 20 A is simply the 60 degrees C table value.2023 NEC §310.15(C)(1)

Four current-carrying 8 AWG THWN-2 copper conductors run in a raceway through a 48 degrees C ambient. What is the adjusted ampacity of each conductor?

• a.36 amperes✓
• b.44 amperes
• c.40 amperes
• d.33 amperes

Begin with the 90 degrees C ampacity of 8 AWG copper, 55 A, then correct for the 46 to 50 degrees C ambient range at 0.82 and adjust for four current-carrying conductors at 0.80: 55 x 0.82 x 0.80 = 36 A. Answers of 40 A and 44 A apply only one of the two required factors, and 33 A comes from starting in the 75 degrees C column, which is not required when a 90 degrees C conductor is used as the derating base.2023 NEC §310.15

A 120-volt circuit carries 20 amperes through 12 AWG copper (6,530 circular mils) to a load 100 feet away. Using K = 12.9, what is the approximate voltage drop?

• a.10.5 volts
• b.3.9 volts
• c.5.2 volts
• d.7.9 volts✓

For a single-phase circuit, VD = (2 x K x I x L) / CM = (2 x 12.9 x 20 x 100) / 6,530 = 51,600 / 6,530, or about 7.9 volts. The 3.9-volt answer forgets that current travels out and back, so it omits the factor of 2; 5.2 volts and 10.5 volts do not correspond to any correct combination of the formula terms.2023 NEC Chapter 9, Table 8

A 480-volt, three-phase feeder carries 40 amperes 250 feet using 6 AWG copper (26,240 circular mils). Using K = 12.9, what is the approximate voltage drop?

• a.14.8 volts
• b.4.9 volts
• c.8.5 volts✓
• d.17.0 volts

For three-phase, VD = (1.732 x K x I x L) / CM = (1.732 x 12.9 x 40 x 250) / 26,240, or about 8.5 volts, roughly 1.8 percent of 480 volts. The 17.0-volt answer uses the single-phase factor of 2 instead of 1.732, 4.9 volts halves the run length, and 14.8 volts does not follow from the given values.2023 NEC Chapter 9, Table 8

A 120-volt branch circuit must carry 12 amperes 120 feet with voltage drop held to 3 percent. Using K = 12.9 for copper, what is the smallest conductor that meets the target?

• a.8 AWG
• b.14 AWG
• c.12 AWG
• d.10 AWG✓

Three percent of 120 volts is 3.6 volts, so the required area is CM = (2 x 12.9 x 12 x 120) / 3.6 = 37,152 / 3.6 = 10,320 circular mils. From Chapter 9, Table 8, 12 AWG has only 6,530 circular mils and fails, while 10 AWG has 10,380 circular mils and just satisfies the requirement. 8 AWG works electrically but is larger than the minimum asked for, and 14 AWG is far too small.

A feeder supplies 30 amperes of continuous load plus 50 amperes of noncontinuous load. What is the minimum standard overcurrent device rating?

• a.100 amperes
• b.80 amperes
• c.90 amperes✓
• d.110 amperes

The device must be sized for the noncontinuous load plus 125 percent of the continuous load: 50 + (30 x 1.25) = 87.5 A, and the next standard size in 240.6(A) is 90 A. The 80 A answer simply adds the loads without the 125 percent factor, and 100 A or 110 A skip past the next standard rating.2023 NEC §215.3

Which of the following is NOT a standard ampere rating for fuses and inverse-time circuit breakers?

• a.110 amperes
• b.45 amperes
• c.70 amperes
• d.55 amperes✓

The standard ratings listed in 240.6(A) include 45, 50, 60, 70, 80, 90, 100 and 110 amperes, but 55 amperes is not among them. Knowing this list matters because calculated loads are routinely rounded up to the next standard rating, and selecting a size that does not exist is a common exam trap.2023 NEC §240.6(A)

What is the minimum number of 20-ampere small-appliance branch circuits required for the kitchen, pantry, breakfast room and dining room receptacles of a dwelling unit?

• a.One
• b.Two✓
• c.Three
• d.Four

At least two 20-ampere small-appliance branch circuits must serve the receptacle outlets in those rooms, and those circuits may not supply other outlets. One circuit is below the minimum, while three or four exceed the code minimum and are design choices rather than requirements.2023 NEC §210.11(C)(1)

What is the minimum branch-circuit rating for the receptacle outlets serving a dwelling unit bathroom?

• a.15 amperes
• b.20 amperes✓
• c.25 amperes
• d.30 amperes

A dedicated 20-ampere branch circuit is required for bathroom receptacle outlets, and it may serve only bathroom receptacles unless it supplies a single bathroom entirely. A 15-ampere circuit is not permitted for this purpose, and 25 or 30 amperes are not used for 125-volt, 15- and 20-ampere receptacle circuits.2023 NEC §210.11(C)(3)

What is the allowable ampacity of 8 AWG copper Type NM cable?

• a.40 amperes✓
• b.55 amperes
• c.50 amperes
• d.45 amperes

The ampacity of Type NM cable must be taken from the 60 degrees C column of Table 310.16, where 8 AWG copper is 40 A, even though the conductors inside carry a 90 degrees C rating. The 50 A and 55 A choices come from the 75 degrees C and 90 degrees C columns, which may be used only as a starting point for derating, not as the final NM cable ampacity.2023 NEC §334.80

A feeder supplies a 100-ampere continuous load. Using 75 degrees C terminations and copper THWN, what is the minimum feeder conductor size?

• a.1 AWG✓
• b.2 AWG
• c.1/0 AWG
• d.3 AWG

The feeder conductor must have an ampacity of at least 100 x 1.25 = 125 A. In the 75 degrees C column, 2 AWG copper is 115 A and falls short, while 1 AWG copper is 130 A and satisfies the requirement. 1/0 AWG at 150 A is larger than necessary, and 3 AWG at 100 A does not cover the continuous-load multiplier.2023 NEC §215.2(A)(1)

Nine current-carrying 10 AWG THHN copper conductors share one raceway at a 30 degrees C ambient. What is the adjusted ampacity of each?

• a.35 amperes
• b.24.5 amperes
• c.26.3 amperes
• d.28 amperes✓

The 90 degrees C ampacity of 10 AWG copper is 40 A, and the adjustment factor for 7 through 9 current-carrying conductors is 70 percent: 40 x 0.70 = 28 A. The 26.3 A answer applies 70 percent to the 75 degrees C value, 24.5 A uses the 50 percent bracket that begins at 10 conductors, and 35 A ignores adjustment entirely.2023 NEC §310.15(C)(1)

A four-wire, three-phase, 208Y/120-volt feeder supplies electric-discharge lighting, and the major portion of the load is nonlinear. How many current-carrying conductors are counted for adjustment purposes?

• a.Two
• b.Four✓
• c.Three
• d.Five

When the major portion of the load is nonlinear, harmonic currents keep the neutral loaded even under balanced conditions, so the neutral counts as a current-carrying conductor along with the three phase conductors, giving four. If the load were linear the neutral would not be counted, giving three, and the equipment grounding conductor is never counted.2023 NEC §310.15(E)

Where must a multiwire branch circuit have a means to simultaneously disconnect all ungrounded conductors?

• a.Only where the circuit supplies more than one device on a single yoke
• b.At the first outlet box on the circuit
• c.At each utilization equipment location
• d.At the point where the branch circuit originates✓

Simultaneous disconnection of all ungrounded conductors is required at the point of origin, normally by handle ties or a multipole breaker in the panelboard, so no part of the shared neutral stays energized during service work. Disconnecting at the first outlet or at the equipment does not remove the hazard back at the panel, and the single-yoke rule is a separate requirement in 210.7.2023 NEC §210.4(B)

In a dwelling unit living room, no point measured horizontally along the floor line of any wall space may be more than what distance from a receptacle outlet?

• a.12 feet
• b.6 feet✓
• c.4 feet
• d.8 feet

The 6-foot rule means a 6-foot appliance cord can reach a receptacle from any point along the wall line, which works out to a maximum spacing of 12 feet between receptacles. The 12-foot answer confuses spacing between outlets with distance from any point, and 4 or 8 feet are not code values for general wall space.2023 NEC §210.52(A)(1)

In a dwelling unit, 125-volt receptacles installed within what distance of the outside edge of a sink require GFCI protection?

• a.5 feet
• b.3 feet
• c.6 feet✓
• d.10 feet

The measurement is 6 feet from the outside edge of the sink, taken along the shortest path a cord would follow without piercing a wall or partition. The 3-foot dimension belongs to other rules such as service conductor clearance from openable windows, and 5 or 10 feet are not used for the sink measurement.2023 NEC §210.8(A)

Arc-fault circuit-interrupter protection is required for 120-volt, single-phase, 15- and 20-ampere branch circuits supplying outlets in which dwelling unit areas?

• a.Kitchens, laundry areas, bedrooms and most other habitable rooms✓
• b.Bedrooms only
• c.Unfinished basements and garages only
• d.Bathrooms and outdoor areas only

The AFCI requirement now reaches nearly every habitable area of a dwelling, including kitchens and laundry areas, not just bedrooms as in older code cycles. Bathrooms, garages and unfinished basements are the spaces typically addressed by GFCI rules instead, so limiting AFCI to those areas reverses the two protection schemes.2023 NEC §210.12(A)

A 277-volt feeder supplies three continuous lighting loads of 12, 15 and 18 amperes. What is the minimum standard feeder overcurrent device rating?

• a.50 amperes
• b.45 amperes
• c.60 amperes✓
• d.70 amperes

Total the loads first: 12 + 15 + 18 = 45 A, all continuous, so the device must be at least 45 x 1.25 = 56.25 A and the next standard rating is 60 A. The 45 A answer omits the continuous-load factor, 50 A is below the calculated minimum, and 70 A overshoots the next standard size.2023 NEC §215.3

For that same 45-ampere continuous lighting feeder, what is the minimum copper conductor size at 75 degrees C terminations?

• a.6 AWG✓
• b.8 AWG
• c.4 AWG
• d.10 AWG

The conductor must carry 45 x 1.25 = 56.25 A. In the 75 degrees C column, 8 AWG copper is 50 A and is insufficient, while 6 AWG copper at 65 A meets the requirement. 4 AWG is oversized for the stated minimum, and 10 AWG at 35 A is far below the calculated ampacity.2023 NEC §215.2(A)(1)

What is the smallest conductor size generally permitted to be installed in parallel?

• a.250 kcmil
• b.2 AWG
• c.4/0 AWG
• d.1/0 AWG✓

Conductors of 1/0 AWG and larger may be run in parallel when the paralleled sets are the same length, material, size and insulation type and terminate in the same manner. Sizes smaller than 1/0, such as 2 AWG, are not permitted to be paralleled except in a few narrow special applications, and 4/0 or 250 kcmil are simply larger than the stated minimum.2023 NEC §310.10(G)

The rule permitting the next higher standard overcurrent device rating above the conductor ampacity may be applied only where that device rating does not exceed what value?

• a.400 amperes
• b.600 amperes
• c.1,000 amperes
• d.800 amperes✓

The round-up allowance stops at 800 amperes; above that rating the overcurrent device must not exceed the ampacity of the conductors it protects. The other values are not thresholds in 240.4(B), and choosing 600 A would needlessly restrict legitimate designs in the 601 to 800 A range.2023 NEC §240.4(B)

A 200-ampere feeder uses THHN copper conductors, but the equipment terminations are rated 75 degrees C. What is the minimum conductor size?

• a.4/0 AWG
• b.2/0 AWG
• c.3/0 AWG✓
• d.250 kcmil

Because the terminations are listed for 75 degrees C, the final ampacity must be taken from the 75 degrees C column even though THHN is a 90 degrees C insulation. In that column 2/0 copper is 175 A and fails, while 3/0 copper is 200 A and matches the device. Reading the 90 degrees C column would wrongly allow 2/0, and 4/0 or 250 kcmil exceed the minimum.2023 NEC §110.14(C)(1)(b)

Using the dwelling service and feeder conductor table, what is the minimum copper conductor size for a 200-ampere single-phase dwelling service?

• a.1/0 AWG
• b.3/0 AWG
• c.4/0 AWG
• d.2/0 AWG✓

For single-phase dwelling services and the main power feeder, conductors are permitted to be sized at 83 percent of the service rating, and the dwelling table lists 2/0 copper for a 200-ampere service. The 3/0 answer is what Table 310.16 alone would require without the dwelling allowance, and 1/0 is below what the dwelling table permits.2023 NEC §310.12

What is the allowable ampacity of 4/0 AWG copper THWN in the 75 degrees C column?

• a.195 amperes
• b.205 amperes
• c.260 amperes
• d.230 amperes✓

Table 310.16 lists 4/0 copper at 230 amperes in the 75 degrees C column. The 260-ampere figure is the 90 degrees C value and may be used only as a derating base, 195 amperes is the 60 degrees C value, and 205 amperes belongs to 3/0 copper at 90 degrees C rather than 4/0.2023 NEC Table 310.16

Twelve current-carrying 12 AWG THHN copper conductors run in one raceway through a 38 degrees C ambient. What is the adjusted ampacity of each conductor?

• a.15 amperes
• b.13.65 amperes✓
• c.12 amperes
• d.10.9 amperes

Start at the 90 degrees C ampacity of 30 A, apply the 36 to 40 degrees C correction factor of 0.91, then the 50 percent adjustment for 10 through 20 conductors: 30 x 0.91 x 0.50 = 13.65 A. Answers of 15 A and 12 A apply only one of the two factors, and 10.9 A uses the 45 percent bracket that begins at 21 conductors.2023 NEC §310.15

A 5,000-watt, 240-volt single-phase electric heater operates continuously. What is the minimum standard branch-circuit overcurrent device rating?

• a.30 amperes✓
• b.20 amperes
• c.25 amperes
• d.35 amperes

The load current is 5,000 / 240 = 20.8 A, and because the load is continuous the device must be rated at least 20.8 x 1.25 = 26 A, so the next standard rating is 30 A. The 20 A and 25 A answers either ignore the continuous factor or land below the calculated minimum, and 35 A is not the next standard size above 26 A.2023 NEC §210.20(A)

What is the line current of a balanced 45 kVA, three-phase, 208-volt load?

• a.216 amperes
• b.125 amperes✓
• c.72 amperes
• d.144 amperes

For a three-phase load, I = VA / (1.732 x E) = 45,000 / (1.732 x 208) = 45,000 / 360.3, or about 125 A. The 216-ampere answer divides by the voltage alone as if the load were single-phase, 72 amperes divides by three times the voltage, and 144 amperes does not follow from the formula.

On a 20-ampere branch circuit that supplies both lighting outlets and cord-and-plug-connected equipment not fastened in place, what is the maximum load the cord-and-plug equipment may impose?

• a.12 amperes
• b.20 amperes
• c.16 amperes✓
• d.15 amperes

Cord-and-plug-connected equipment that is not fastened in place is limited to 80 percent of the branch-circuit rating when other loads share the circuit: 20 x 0.80 = 16 A. The 20-ampere answer would let the equipment consume the entire circuit, and 12 or 15 amperes are not derived from the 80 percent rule.2023 NEC §210.23