CSLB General Building (B) — All Questions

A service uses 3/0 AWG copper ungrounded service-entrance conductors. What is the minimum size copper grounding electrode conductor?

• a.4 AWG✓
• b.6 AWG
• c.2 AWG
• d.1/0 AWG

Table 250.66 groups 2/0 and 3/0 copper service conductors together and calls for a 4 AWG copper grounding electrode conductor. The 6 AWG row applies to service conductors of 1 or 1/0 AWG, 2 AWG copper is required only above 3/0 through 350 kcmil, and 1/0 applies to service conductors above 350 through 600 kcmil.2023 NEC Table 250.66

A service is supplied by two parallel sets of 250 kcmil copper conductors per phase. What is the minimum size copper grounding electrode conductor?

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

With parallel conductors the equivalent area is the sum of the areas in one phase: 2 x 250 kcmil = 500 kcmil, which falls in the over 350 through 600 kcmil row and requires 1/0 copper. Using 2 AWG treats a single 250 kcmil conductor as the whole service, and 3/0 corresponds to services above 1,100 kcmil.2023 NEC Table 250.66

Where the grounding electrode conductor connects only to a driven ground rod, what is the largest size it is required to be?

• a.2 AWG copper
• b.4 AWG copper
• c.6 AWG copper✓
• d.1/0 AWG copper

A rod, pipe or plate electrode has a limited earth contact area, so the conductor to it is never required to be larger than 6 AWG copper regardless of what Table 250.66 would otherwise demand. The 4 AWG limit applies to a concrete-encased electrode, and larger sizes are not required for any rod connection.2023 NEC §250.66(A)

Where the grounding electrode conductor connects only to a concrete-encased electrode, what is the largest size it is required to be?

• a.2 AWG copper
• b.6 AWG copper
• c.4 AWG copper✓
• d.1/0 AWG copper

The conductor to a concrete-encased electrode is never required to be larger than 4 AWG copper. The 6 AWG cap belongs to rod, pipe and plate electrodes, and 2 AWG or 1/0 would only come from Table 250.66 before this limiting rule is applied.2023 NEC §250.66(B)

What is the minimum size copper equipment grounding conductor for a circuit protected at 60 amperes?

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

Table 250.122 is entered by the rating of the overcurrent device ahead of the circuit, and the 60-ampere row calls for 10 AWG copper. 12 AWG is listed for 20 amperes, 8 AWG for 100 amperes, and 6 AWG for 200 amperes, so all three are the wrong rows.2023 NEC Table 250.122

A 200-ampere feeder requires what minimum size copper equipment grounding conductor?

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

The 200-ampere row of Table 250.122 specifies 6 AWG copper. 8 AWG belongs to 100 amperes, 4 AWG to 300 amperes and 3 AWG to 400 amperes, so those choices come from reading adjacent rows.2023 NEC Table 250.122

What is the minimum size copper equipment grounding conductor for a 400-ampere feeder?

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

Table 250.122 lists 3 AWG copper at 400 amperes. 4 AWG is the 300-ampere value, 2 AWG the 500-ampere value and 1 AWG the 600-ampere value, so all three are one row away from the correct entry.2023 NEC Table 250.122

A 100-ampere feeder originally sized with 3 AWG copper (52,620 circular mils) is upsized to 1/0 AWG copper (105,600 circular mils) for voltage drop. What is the minimum equipment grounding conductor?

• a.4 AWG copper✓
• b.8 AWG copper
• c.6 AWG copper
• d.2 AWG copper

When ungrounded conductors are increased in size, the equipment grounding conductor must be increased in the same proportion by circular-mil area. The ratio is 105,600 / 52,620 = 2.0, and the base 100-ampere equipment grounding conductor of 8 AWG (16,510 circular mils) doubles to about 33,100 circular mils; 6 AWG has only 26,240 circular mils, so 4 AWG at 41,740 circular mils is the smallest that qualifies. Leaving it at 8 AWG ignores the proportional rule.2023 NEC §250.122(B)

A service is supplied by a single set of 500 kcmil copper ungrounded conductors. What is the minimum size copper main bonding jumper?

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

The main bonding jumper is sized from the same table used for the grounded conductor and supply-side bonding jumpers, and the over 350 through 600 kcmil row calls for 1/0 copper. The 2 AWG and 4 AWG answers come from smaller service conductor rows, and 3/0 applies to much larger services.2023 NEC §250.28(D)(1)

A service uses two parallel sets of 750 kcmil copper per phase. Using the 12.5 percent rule, what is the minimum copper main bonding jumper?

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

Where the ungrounded service conductors exceed 1,100 kcmil of copper, the main bonding jumper must be at least 12.5 percent of that area: 2 x 750 = 1,500 kcmil, and 1,500 x 0.125 = 187.5 kcmil. The smallest standard conductor with at least that area is 4/0 AWG at 211.6 kcmil; 3/0 at 167.8 kcmil falls short and 250 kcmil is larger than the calculated minimum.2023 NEC §250.28(D)(2)

Service-entrance conductors supply 68 amperes of continuous load plus 120 amperes of noncontinuous load. What minimum ampacity must the conductors have?

• a.205 amperes✓
• b.188 amperes
• c.235 amperes
• d.225 amperes

Service-entrance conductors follow the same rule as feeders: noncontinuous load plus 125 percent of continuous load, or 120 + (68 x 1.25) = 120 + 85 = 205 A. The 188-ampere answer simply adds the loads, while 225 and 235 amperes apply the 125 percent factor to the entire load instead of only the continuous portion.2023 NEC §230.42(A)(1)

Using the dwelling conductor table, what is the minimum copper conductor size for a 400-ampere single-phase dwelling service?

• a.600 kcmil
• b.500 kcmil
• c.350 kcmil
• d.400 kcmil✓

The dwelling service and feeder table lists 400 kcmil copper for a 400-ampere single-phase dwelling service, reflecting the 83 percent allowance. Table 310.16 alone would push you toward 600 kcmil, and 350 kcmil is the table value for a 350-ampere service.2023 NEC §310.12

A 45 kVA, three-phase transformer has a 480-volt primary. If only primary overcurrent protection is provided at 125 percent, what is the maximum standard primary device rating?

• a.90 amperes
• b.60 amperes
• c.80 amperes
• d.70 amperes✓

Primary full-load current is 45,000 / (1.732 x 480) = 54.1 A, and 54.1 x 1.25 = 67.6 A. Because the calculated value does not correspond to a standard rating, the next higher standard size of 70 amperes is permitted. The 60-ampere answer rounds down and would nuisance trip, while 80 and 90 amperes skip past the next standard size.2023 NEC §450.3(B)

What is the secondary full-load current of a 75 kVA transformer with a 208Y/120-volt, three-phase secondary?

• a.120 amperes
• b.361 amperes
• c.208 amperes✓
• d.104 amperes

Secondary current is I = VA / (1.732 x E) = 75,000 / (1.732 x 208) = 75,000 / 360.3, or about 208 amperes. The 361-ampere answer divides by the voltage alone, 120 amperes uses the phase voltage incorrectly, and 104 amperes is roughly half the correct value.

What is the minimum length of a rod-type grounding electrode?

• a.10 feet
• b.6 feet
• c.8 feet✓
• d.5 feet

Rod and pipe electrodes must be at least 8 feet long and must have at least 8 feet in contact with the soil once installed. The 10-foot value is associated with the minimum earth contact required of a metal underground water pipe electrode, and 5 or 6 feet fall short of the rod requirement.2023 NEC §250.52(A)(5)

Where two rod electrodes are installed to make up the grounding electrode system, what is the minimum spacing between them?

• a.3 feet
• b.6 feet✓
• c.8 feet
• d.10 feet

Rods that supplement one another must be separated by at least 6 feet so their earth shells do not overlap and defeat the purpose of the second rod. The 8-foot figure is the minimum rod length rather than spacing, and 3 or 10 feet are not the code minimum.2023 NEC §250.53(A)(3)

A single ground rod does not have to be supplemented by a second electrode if its resistance to earth does not exceed what value?

• a.5 ohms
• b.25 ohms✓
• c.100 ohms
• d.1 ohm

A single rod, pipe or plate electrode must be supplemented unless it is shown to have a resistance to earth of 25 ohms or less. Values such as 5 ohms or 1 ohm come from specialized standards for communications and lightning systems, not from this general requirement, and 100 ohms is far too high.2023 NEC §250.53(A)(2)

A metal underground water pipe qualifies as a grounding electrode only if it is in direct contact with the earth for at least what length?

• a.20 feet
• b.10 feet✓
• c.8 feet
• d.5 feet

The metal underground water pipe electrode must have 10 feet or more in direct contact with the earth, including any metal well casing bonded to it. The 20-foot value belongs to the concrete-encased electrode, and 5 or 8 feet do not meet the requirement.2023 NEC §250.52(A)(1)

A concrete-encased electrode may be formed by at least 20 feet of which of the following?

• a.Bare 4 AWG copper conductor encased in the concrete footing✓
• b.Bare 6 AWG copper conductor encased in the concrete footing
• c.Insulated 4 AWG copper conductor run through the footing
• d.Bare 8 AWG copper conductor encased in the concrete footing

A concrete-encased electrode consists of at least 20 feet of bare copper not smaller than 4 AWG, or at least 20 feet of one-half inch or larger electrically conductive reinforcing bar, encased in at least 2 inches of concrete near the bottom of a footing in direct contact with earth. Smaller conductors do not qualify, and the copper must be bare so it can couple to the concrete.2023 NEC §250.52(A)(3)

When a metal underground water pipe is used as a grounding electrode, what else is required?

• a.It must be bonded to the gas piping system as the supplement
• b.No supplement is required if the pipe is longer than 20 feet
• c.It must be supplemented by an additional electrode of a type identified in 250.52✓
• d.It may be used only if the resistance to earth is 25 ohms or less

A water pipe electrode is never allowed to stand alone because plastic repairs or dielectric fittings can silently disconnect it from earth, so it must always be supplemented by another qualifying electrode such as a rod or a concrete-encased electrode. Pipe length does not remove the requirement, gas piping is bonded but is not permitted as an electrode, and the 25-ohm test applies to rod, pipe and plate electrodes.2023 NEC §250.53(D)(2)

What is the maximum number of service disconnecting means permitted for a single set of service-entrance conductors?

• a.Eight
• b.Two
• c.Four
• d.Six✓

Up to six service disconnects are permitted, but current code requires them to be in separate enclosures or in a listed assembly with individual disconnects, each marked as a service disconnect. Two and four are below the ceiling but are not the code maximum, and eight exceeds the limit.2023 NEC §230.71

What is the minimum service disconnecting means rating for a one-family dwelling?

• a.60 amperes
• b.100 amperes✓
• c.125 amperes
• d.200 amperes

A one-family dwelling service disconnect must be rated at least 100 amperes, 3-wire. The 60-ampere minimum applies to certain limited installations and older code cycles, while 125 and 200 amperes are common design choices but exceed the code minimum.2023 NEC §230.79(C)

What is the minimum clearance above a residential driveway for overhead service conductors limited to 150 volts to ground?

• a.18 feet
• b.10 feet
• c.12 feet✓
• d.15 feet

Residential driveways and commercial areas not subject to truck traffic require 12 feet of clearance. The 10-foot clearance applies only above finished grade, sidewalks and platforms accessible to pedestrians, and 18 feet is required over public streets, alleys and roads.2023 NEC §230.24(B)

Overhead service conductors must maintain what minimum clearance from a window that is designed to be opened?

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

Service conductors must keep 3 feet of clearance from windows designed to open, doors, porches, balconies, stairs and similar locations. Conductors run above the top level of a window are considered out of reach and are exempt, and the other distances do not appear in this rule.2023 NEC §230.9(A)

Ground-fault protection of equipment is required for a solidly grounded wye service with more than 150 volts to ground when the disconnect is rated at least what value?

• a.800 amperes
• b.1,000 amperes✓
• c.600 amperes
• d.1,200 amperes

The threshold is a service disconnect rated 1,000 amperes or more on a solidly grounded wye system of more than 150 volts to ground but not exceeding 1,000 volts phase-to-phase, which describes a typical 480Y/277-volt service. The 1,200-ampere figure is the maximum permitted setting of the ground-fault protection, not the threshold at which it is required.2023 NEC §230.95

How must a panelboard be protected against overcurrent?

• a.By an overcurrent device rated at 125 percent of the panelboard rating
• b.By the sum of the branch-circuit devices installed in it
• c.By an overcurrent device having a rating not greater than the panelboard rating✓
• d.No separate protection is required if the feeder conductors are properly sized

Each panelboard must be protected by an overcurrent device rated no higher than the panelboard rating, located either in the panelboard itself as a main or ahead of it in the feeder. Summing the branch devices routinely exceeds the panelboard rating because of diversity, and no code rule allows protecting a panelboard at 125 percent of its rating.2023 NEC §408.36

On the load side of the service disconnecting means, what is the general rule for using the grounded neutral conductor to ground equipment enclosures?

• a.It is permitted only in dwelling units
• b.It is permitted if the neutral is at least 6 AWG copper
• c.It is permitted in any panelboard fed from the service
• d.It is not permitted; a separate equipment grounding conductor must be used✓

Downstream of the service disconnect the neutral and the equipment grounding conductor must be kept separate so that normal load current does not travel on enclosures and raceways. Bonding them together at a subpanel creates parallel neutral paths and objectionable current, and neither conductor size nor occupancy type changes this general prohibition.2023 NEC §250.142(B)

A dry-type transformer rated 112.5 kVA or less is installed indoors. What is the minimum separation from combustible material unless it is separated by a fire-resistant heat-insulating barrier?

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

Dry-type transformers of 112.5 kVA or less must be at least 12 inches from combustible material unless separated by a fire-resistant, heat-insulating barrier or unless the unit is completely enclosed except for ventilating openings. The smaller clearances listed belong to other rules such as recessed luminaire and insulation spacing, not to transformer installation.2023 NEC §450.21(A)