CSLB General Building (B) — All Questions

A 25-horsepower, 460-volt, three-phase squirrel-cage motor has a table full-load current of 34 amperes. Using 75 degrees C terminations and copper, what is the minimum branch-circuit conductor size?

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

Single-motor branch-circuit conductors must carry at least 125 percent of the table full-load current: 34 x 1.25 = 42.5 A. In the 75 degrees C column, 10 AWG copper is 35 A and fails, while 8 AWG at 50 A is adequate. Sizes 6 and 4 AWG exceed the calculated minimum.2023 NEC §430.22

That same motor has a nameplate full-load current of 32 amperes and a marked service factor of 1.15. What is the maximum separate overload device rating?

• a.36.8 amperes
• b.40 amperes✓
• c.42.5 amperes
• d.32 amperes

Overload protection is based on the nameplate current, not the table value, and a service factor of 1.15 or greater permits 125 percent: 32 x 1.25 = 40 A. The 36.8-ampere answer applies the 115 percent factor used for motors with no marked service factor or temperature rise, and 42.5 amperes wrongly applies 125 percent to the table value of 34 A.2023 NEC §430.32(A)(1)

For that 34-ampere three-phase squirrel-cage motor, what is the maximum inverse-time circuit breaker permitted for branch-circuit short-circuit and ground-fault protection, using the next standard size allowance?

• a.90 amperes✓
• b.85 amperes
• c.100 amperes
• d.70 amperes

An inverse-time breaker is permitted at 250 percent of full-load current: 34 x 2.50 = 85 A, which is not a standard rating, so the next higher standard size of 90 amperes may be used. The 100-ampere answer skips a standard size, and 70 amperes corresponds to the 175 percent dual-element fuse calculation instead.2023 NEC Table 430.52(C)(1)

For the same 34-ampere motor, what is the maximum dual-element time-delay fuse permitted at the standard percentage?

• a.60 amperes✓
• b.70 amperes
• c.50 amperes
• d.90 amperes

Dual-element time-delay fuses are permitted at 175 percent of full-load current: 34 x 1.75 = 59.5 A, and because that is not a standard rating the next standard size of 60 amperes is used. The 90-ampere answer belongs to the inverse-time breaker at 250 percent, and 50 amperes falls below the calculated value.2023 NEC Table 430.52(C)(1)

A 5-horsepower, 230-volt single-phase motor has a table full-load current of 28 amperes. What is the minimum copper branch-circuit conductor at 75 degrees C terminations?

• a.10 AWG✓
• b.12 AWG
• c.8 AWG
• d.6 AWG

The conductor must carry 28 x 1.25 = 35 A. In the 75 degrees C column, 10 AWG copper is exactly 35 A and qualifies, while 12 AWG at 25 A does not. Note that 240.4(D) limits ordinary 10 AWG circuits to a 30-ampere device, but motor branch-circuit protection is sized under Article 430 instead.2023 NEC §430.22

A 30-horsepower, 460-volt three-phase motor has a table full-load current of 40 amperes. What is the minimum ampere rating of the disconnecting means?

• a.50 amperes
• b.40 amperes
• c.46 amperes✓
• d.60 amperes

The disconnecting means must have an ampere rating of at least 115 percent of the full-load current: 40 x 1.15 = 46 A, so a 60-ampere switch would be the practical selection. The 40-ampere answer omits the 115 percent factor entirely, and 50 or 60 amperes are available equipment ratings rather than the calculated minimum the question asks for.2023 NEC §430.110(A)

A feeder supplies a 25-horsepower motor (34 amperes) and a 10-horsepower motor (14 amperes), both 460 volts three phase. What minimum conductor ampacity is required?

• a.67.5 amperes
• b.60 amperes
• c.48 amperes
• d.56.5 amperes✓

Feeder conductors for several motors must carry 125 percent of the largest motor full-load current plus the sum of the other full-load currents: (34 x 1.25) + 14 = 42.5 + 14 = 56.5 A. The 48-ampere answer simply adds the two currents, and 67.5 amperes wrongly applies 125 percent to the total of both motors.2023 NEC §430.24

For that same two-motor feeder, if the largest branch-circuit protective device is 90 amperes, what is the maximum feeder overcurrent device rating?

• a.90 amperes
• b.104 amperes
• c.110 amperes
• d.100 amperes✓

The feeder device may not exceed the largest branch-circuit short-circuit and ground-fault protective device plus the sum of the full-load currents of the other motors: 90 + 14 = 104 A. Since 104 is not a standard rating and this is a maximum rather than a minimum, the next lower standard size of 100 amperes must be selected. Choosing 110 amperes would exceed the calculated ceiling.2023 NEC §430.62(A)

When sizing motor branch-circuit conductors and short-circuit protection, which current value must be used?

• a.The locked-rotor current from the nameplate code letter
• b.The motor nameplate full-load amperes
• c.The full-load current from Table 430.248 or 430.250✓
• d.The larger of nameplate current or table current

Conductor sizing, branch-circuit protection and disconnect rating are all based on the table values, which are conservative and independent of a particular manufacturer's nameplate. Nameplate current is used only for overload protection under 430.32, and locked-rotor current is used for withstand and disconnect horsepower comparisons, not for conductor sizing.2023 NEC §430.6(A)(1)

A continuous-duty motor over 1 horsepower is marked with a temperature rise of 40 degrees C but no service factor. What percentage of nameplate full-load current sets the maximum overload device?

• a.140 percent
• b.115 percent
• c.125 percent✓
• d.250 percent

Motors with a marked temperature rise not over 40 degrees C, like those with a service factor of 1.15 or greater, are permitted overload protection at 125 percent of nameplate current. The 115 percent figure applies to all other motors, 140 percent is the allowance when the initial setting will not carry the load, and 250 percent is a short-circuit protection value.2023 NEC §430.32(A)(1)

A continuous-duty motor over 1 horsepower has no marked service factor and no marked temperature rise. What is the maximum overload device rating as a percentage of nameplate full-load current?

• a.140 percent
• b.125 percent
• c.115 percent✓
• d.175 percent

Motors that carry neither a service factor of 1.15 or greater nor a marked temperature rise of 40 degrees C or less are limited to 115 percent of nameplate current, because there is less thermal margin available. The 125 percent value applies to the better-rated motors, 140 percent is the permitted increase when the motor will not start, and 175 percent is a fuse sizing percentage.2023 NEC §430.32(A)(1)

Where must a motor disconnecting means be located relative to the motor?

• a.Within 25 feet of the motor in all cases
• b.In sight from the motor location and the driven machinery, unless it is capable of being locked in the open position✓
• c.Anywhere on the same floor level as the motor
• d.In sight from the controller only, with no rule about the motor

A disconnect must be in sight from the motor and driven machinery so a technician working on the equipment can see that it is open; the alternative is a disconnect elsewhere that is capable of being individually locked in the open position with the locking means remaining in place. In sight means visible and not more than 50 feet away, and a separate rule requires a disconnect in sight from the controller.2023 NEC §430.102(B)

An air conditioning condensing unit nameplate reads minimum circuit ampacity 24.6 and maximum overcurrent protective device 40. What overcurrent device must be installed?

• a.A 30-ampere device, the next standard size above the minimum circuit ampacity
• b.A 25-ampere device to match the minimum circuit ampacity
• c.A 50-ampere device sized at 125 percent of the marked maximum
• d.A device not exceeding 40 amperes✓

The marked maximum overcurrent protective device value is a ceiling set by the equipment manufacturer and tested with the unit, so the installed device must not exceed 40 amperes; a smaller device is acceptable only if it will carry the starting current. The minimum circuit ampacity of 24.6 sizes the conductors, not the protective device, and nothing permits multiplying the marked maximum by another factor.2023 NEC §440.4(B)

A hermetic refrigerant motor-compressor has a rated-load current of 20 amperes. What is the maximum branch-circuit short-circuit and ground-fault protective device at the standard 175 percent figure?

• a.60 amperes
• b.25 amperes
• c.45 amperes
• d.35 amperes✓

Protection for a motor-compressor is sized at not more than 175 percent of the rated-load current: 20 x 1.75 = 35 A, which is a standard rating. Where that device still will not carry the starting current it may be increased, but never above 225 percent, which here would be 45 amperes. A 25-ampere device is below the calculated value and would nuisance trip on start.2023 NEC §440.22(A)

For that same 20-ampere rated-load compressor supplied alone, what minimum conductor ampacity is required?

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

Branch-circuit conductors for a single motor-compressor must have an ampacity of at least 125 percent of the rated-load current or branch-circuit selection current, whichever is greater: 20 x 1.25 = 25 A, which calls for 10 AWG copper. The 35-ampere figure is the 175 percent protective device calculation, and 20 amperes omits the required multiplier entirely.2023 NEC §440.32

A surface-mounted luminaire with a completely enclosed light source is installed in a clothes closet. What is the minimum clearance to the nearest point of the defined storage space?

• a.12 inches✓
• b.6 inches
• c.3 inches
• d.18 inches

Surface-mounted luminaires with a completely enclosed light source require 12 inches of clearance from the storage space, while recessed luminaires with an enclosed light source are permitted at 6 inches because the housing is set back into the ceiling or wall. Open incandescent lamps are not permitted in clothes closets at all.2023 NEC §410.16(C)

A recessed luminaire that is NOT identified for insulation contact must maintain what minimum clearance from thermal insulation?

• a.1 inch
• b.1/2 inch
• c.6 inches
• d.3 inches✓

A non-IC recessed luminaire must be spaced at least 3 inches from thermal insulation so heat can dissipate, and insulation must not be installed above it in a way that traps heat. The 1/2-inch dimension is the required clearance from combustible materials other than the mounting surface, which is a different measurement.2023 NEC §410.116

A generator nameplate current rating is 200 amperes. What minimum conductor ampacity is required from the generator terminals to the first overcurrent device?

• a.230 amperes✓
• b.200 amperes
• c.250 amperes
• d.160 amperes

Conductors from generator terminals to the first distribution device must have an ampacity of at least 115 percent of the nameplate current rating: 200 x 1.15 = 230 A, which corresponds to 4/0 copper at 75 degrees C. The 200-ampere answer omits the factor, 250 amperes uses 125 percent, and 160 amperes derates rather than uprates.2023 NEC §445.13(A)

A photovoltaic source circuit uses modules with a rated short-circuit current of 10 amperes. What minimum conductor ampacity is required before any temperature or conduit-fill adjustments?

• a.12.5 amperes
• b.15.6 amperes✓
• c.10 amperes
• d.20 amperes

The maximum circuit current is 125 percent of the rated short-circuit current, or 10 x 1.25 = 12.5 A, and the conductor must then be sized at 125 percent of that value without adjustment factors: 12.5 x 1.25 = 15.6 A. The 12.5-ampere answer stops after only the first multiplier, and 10 amperes ignores both.2023 NEC §690.8

A motor with a service factor of 1.16 will not start because the overload device trips, even though it is sized at 125 percent of nameplate current. To what maximum percentage may the overload device be increased?

• a.140 percent✓
• b.130 percent
• c.150 percent
• d.175 percent

Where the overload device selected at the basic percentage is not sufficient to start the motor or carry the load, motors with a service factor of 1.15 or greater or a marked temperature rise of 40 degrees C or less may be increased to 140 percent of nameplate current. The 130 percent ceiling applies to all other motors, and 175 percent is a fuse percentage for short-circuit protection.2023 NEC §430.32(C)

What is the maximum non-time-delay fuse permitted for branch-circuit short-circuit and ground-fault protection of a 34-ampere three-phase squirrel-cage motor, using the next standard size allowance?

• a.110 amperes✓
• b.100 amperes
• c.90 amperes
• d.125 amperes

Non-time-delay fuses are permitted at 300 percent of full-load current: 34 x 3.00 = 102 A, which is not a standard rating, so the next standard size of 110 amperes may be used. The 90-ampere answer belongs to the inverse-time breaker at 250 percent, and 125 amperes skips a standard size.2023 NEC Table 430.52(C)(1)

A 20-horsepower, 208-volt three-phase motor has a table full-load current of 59.4 amperes. What is the minimum copper branch-circuit conductor at 75 degrees C terminations?

• a.6 AWG
• b.4 AWG✓
• c.3 AWG
• d.8 AWG

The conductor must carry 59.4 x 1.25 = 74.25 A. In the 75 degrees C column, 6 AWG copper is 65 A and falls short, while 4 AWG copper at 85 A is adequate. Choosing 3 AWG is oversized for the minimum, and 8 AWG at 50 A is well below the requirement.2023 NEC §430.22