CSLB Trade — Electrical (C-10) — All Questions

A 200 A service uses 250 kcmil copper ungrounded service-entrance conductors. Per Table 250.66, what is the minimum copper grounding electrode conductor (GEC)?

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

In Table 250.66 the row 'over 3/0 through 350 kcmil' requires a 2 AWG copper GEC, so 250 kcmil falls here. 4 AWG serves only 2/0-3/0, 6 AWG serves 1 or 1/0, and 1/0 is not required until conductors exceed 350 kcmil.2023 NEC §250.66

The GEC from a service runs only to a driven ground rod (rod/pipe electrode). What is the largest copper GEC required for this connection?

• a.4 AWG
• b.2 AWG
• c.It must match Table 250.66 sizing
• d.6 AWG✓

Per 250.66(A), the portion of the GEC that is the sole connection to a rod, pipe, or plate electrode need not be larger than 6 AWG copper. Table 250.66 sizes larger conductors, but the rod exception caps it at 6 AWG; 4 AWG and 2 AWG exceed what the rod can use.2023 NEC §250.66(A)

The only grounding electrode is a concrete-encased electrode (Ufer). What is the largest copper GEC that must be run to it?

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

Per 250.66(B), where the GEC connects solely to a concrete-encased electrode, it need not be larger than 4 AWG copper. 6 AWG is the cap only for rod/pipe/plate electrodes; 2 AWG and 1/0 exceed the concrete-encased allowance.2023 NEC §250.66(B)

A feeder is protected by a 100 A overcurrent device. Per Table 250.122, what is the minimum copper equipment grounding conductor (EGC)?

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

Table 250.122 requires an 8 AWG copper EGC for a 100 A overcurrent device. 10 AWG serves up to 60 A, 6 AWG is for 200 A, and 4 AWG is for 300 A.2023 NEC §250.122

What is the minimum copper EGC for a circuit protected by a 200 A breaker, per Table 250.122?

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

Table 250.122 lists 6 AWG copper for a 200 A device. 8 AWG serves only up to 100 A; 4 AWG is for 300 A and 3 AWG for 400 A.2023 NEC §250.122

What is the minimum length that a ground rod electrode must be in contact with the soil?

• a.10 ft (3.0 m)
• b.6 ft (1.8 m)
• c.8 ft (2.44 m)✓
• d.4 ft (1.2 m)

Section 250.53(G) requires that a rod be installed so at least 8 ft (2.44 m) of length is in contact with soil. The other lengths do not meet the code minimum for a driven rod electrode.2023 NEC §250.53(G)

A single ground rod is installed. Under what condition is a supplemental electrode NOT required?

• a.The rod is copper-clad
• b.The rod's resistance to earth is 25 ohms or less✓
• c.The rod is 10 ft long
• d.A GEC of 6 AWG is used

Per 250.53(A)(2) Exception, a single rod, pipe, or plate electrode with a resistance to earth of 25 ohms or less needs no supplemental electrode. Rod length, cladding, and GEC size do not exempt the second-electrode requirement.2023 NEC §250.53(A)(2)

A ground ring is used as a grounding electrode. What is the minimum size and length required?

• a.4 AWG copper, at least 20 ft
• b.2 AWG copper, at least 20 ft✓
• c.2 AWG copper, at least 10 ft
• d.6 AWG copper, at least 10 ft

Section 250.52(A)(4) requires a ground ring of at least 2 AWG bare copper encircling the building for a length of not less than 20 ft. Smaller conductors or shorter lengths do not qualify as a ground ring electrode.2023 NEC §250.52(A)(4)

Service-entrance conductors are 3/0 AWG copper. Per Table 250.102(C)(1), what is the minimum copper main bonding jumper?

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

The main bonding jumper is sized from Table 250.102(C)(1) based on the service conductors; for 2/0-3/0 copper it is 4 AWG copper. 6 AWG and 8 AWG are too small, and 2 AWG is not required until conductors exceed 3/0.2023 NEC §250.28

When the grounded (neutral) service conductor is run to the service disconnect, its minimum size is based on which table?

• a.Table 250.102(C)(1), sized from the ungrounded service conductors✓
• b.Table 310.16 only
• c.Table 250.122, sized from the OCPD
• d.Table 250.66, sized from the GEC

Per 250.24(C), the grounded conductor brought to the service must not be smaller than required by Table 250.102(C)(1), based on the largest ungrounded service conductor. It is not sized from the OCPD (that is the EGC in 250.122) nor from the GEC table.2023 NEC §250.24(C)

Ungrounded conductors are increased in size for voltage drop. What must happen to the equipment grounding conductor?

• a.No EGC is required
• b.It is increased proportionally to the ungrounded conductor's increase in area✓
• c.It is increased to match the neutral
• d.It stays the same as Table 250.122

Per 250.122(B), when ungrounded conductors are upsized (e.g., for voltage drop), the EGC must be increased proportionally by the same ratio in circular-mil area. It cannot simply remain at the base Table 250.122 size, and it is never matched to the neutral.2023 NEC §250.122(B)

What is the general rule for the metal water piping system inside a building served by an electrical service?

• a.It must be used as the sole grounding electrode
• b.It must be bonded, sized per Table 250.102(C)(1)✓
• c.It must be left isolated from the grounding system
• d.It may only be grounded through a water heater

Per 250.104(A), interior metal water piping must be bonded to the service grounded conductor/enclosure or GEC, with the bonding jumper sized from Table 250.102(C)(1). Isolation is prohibited, and water pipe alone cannot be the sole electrode.2023 NEC §250.104(A)

What is the function of the main bonding jumper at a service?

• a.It connects two ground rods together
• b.It replaces the grounding electrode conductor
• c.It bonds the neutral to the water pipe
• d.It connects the grounded (neutral) conductor to the service equipment enclosure✓

The main bonding jumper connects the grounded (neutral) conductor to the metal service disconnect enclosure/equipment grounding path at the service, per 250.24 and 250.28. It is not a rod-to-rod connection, does not replace the GEC, and is not the water-pipe bond.2023 NEC §250.28

Where two rod electrodes are used to meet the supplemental-electrode rule, what is the minimum spacing between them?

• a.10 ft (3.0 m)
• b.4 ft (1.2 m)
• c.6 ft (1.8 m)✓
• d.3 ft (0.9 m)

Per 250.53(A)(3), where multiple rod, pipe, or plate electrodes are used, they must be at least 6 ft (1.8 m) apart. Closer spacings reduce the benefit of the second electrode and do not satisfy the code.2023 NEC §250.53(A)(3)

Using the 75 C column of Table 310.16, what is the allowable ampacity of a 3/0 AWG copper conductor?

• a.150 A
• b.230 A
• c.200 A✓
• d.175 A

Table 310.16 lists 200 A for 3/0 copper at 75 C. 175 A is 2/0 copper, 230 A is 4/0 copper, and 150 A is 1/0 copper at 75 C.2023 NEC §310.16

What is the 75 C ampacity of a 4/0 AWG copper conductor per Table 310.16?

• a.200 A
• b.255 A
• c.195 A
• d.230 A✓

Table 310.16 gives 230 A for 4/0 copper at 75 C. 255 A is 250 kcmil, 200 A is 3/0, and 195 A is the 60 C value for 4/0 copper.2023 NEC §310.16

At the 75 C column of Table 310.16, what is the ampacity of a 1/0 AWG copper conductor?

• a.170 A
• b.175 A
• c.130 A
• d.150 A✓

Table 310.16 lists 150 A for 1/0 copper at 75 C. 130 A is 1 AWG, 170 A is the 90 C value for 1/0, and 175 A is 2/0 copper at 75 C.2023 NEC §310.16

A 4/0 AWG copper THHN conductor (90 C, base 260 A) is in an ambient of 45 C. Using the 0.87 correction factor, what is the corrected ampacity?

• a.260 A
• b.195 A
• c.226 A✓
• d.247 A

260 A x 0.87 = 226 A after applying the 41-45 C temperature-correction factor from Table 310.15(B) to the 90 C column. 260 A ignores correction, and the other values use wrong factors.2023 NEC §310.15(B)

Six current-carrying conductors are installed in a single raceway. What adjustment factor applies to their ampacity?

• a.70%
• b.100%
• c.50%
• d.80%✓

Table 310.15(C)(1) applies an 80% adjustment factor for 4 to 6 current-carrying conductors. 70% is for 7-9, 50% is for 10-20, and no adjustment (100%) applies only to 3 or fewer.2023 NEC §310.15(C)(1)

How many current-carrying conductors can be in a raceway before an adjustment (fill derating) factor must be applied?

• a.More than 4
• b.More than 3✓
• c.More than 9
• d.More than 6

Per 310.15(C)(1), adjustment factors apply when there are more than 3 current-carrying conductors in a raceway or cable. The first threshold (4-6 conductors) triggers 80%; the higher counts have their own factors.2023 NEC §310.15(C)(1)

For equipment terminals on a circuit rated 100 A or less (not marked otherwise), which temperature column must be used to size conductors?

• a.90 C column
• b.60 C column✓
• c.The highest column of the conductor insulation
• d.75 C column always

Per 110.14(C)(1), for circuits rated 100 A or less or 14-1 AWG conductors, terminations are limited to the 60 C column unless the equipment is listed for higher. The 90 C column may be used only for correction/adjustment, never for final termination sizing.2023 NEC §110.14(C)

A feeder supplies a continuous load of 176 A. What minimum conductor ampacity (before derating) is required?

• a.200 A
• b.141 A
• c.220 A✓
• d.176 A

Per 215.2(A)(1)/210.19, feeder conductors must be sized at 125% of the continuous load: 176 A x 1.25 = 220 A. Using 176 A ignores the 125% factor, and the other values misapply it.2023 NEC §215.2

Six 3 AWG THHN copper conductors (90 C ampacity 110 A) are in one raceway at 30 C ambient. After the fill adjustment, what is the derated ampacity?

• a.100 A
• b.77 A
• c.110 A
• d.88 A✓

With 4-6 current-carrying conductors the 80% adjustment applies: 110 A x 0.80 = 88 A. No temperature correction is needed at 30 C. 77 A wrongly uses 70%, and 110 A/100 A ignore the fill adjustment.2023 NEC §310.15(C)(1)

Using the 60 C column of Table 310.16, what is the ampacity of a 6 AWG copper conductor?

• a.75 A
• b.50 A
• c.55 A✓
• d.65 A

Table 310.16 lists 55 A for 6 AWG copper at 60 C. 65 A is the 75 C value, 75 A is the 90 C value, and 50 A is 8 AWG copper at 75 C.2023 NEC §310.16

What is the maximum overcurrent protection allowed for a 12 AWG copper conductor under the small-conductor rule?

• a.20 A✓
• b.30 A
• c.25 A
• d.15 A

Per 240.4(D), 12 AWG copper is limited to 20 A regardless of its higher table ampacity. 15 A is the 14 AWG limit, and 25 A or 30 A exceed the small-conductor cap for 12 AWG.2023 NEC §240.4(D)

Which of the following is NOT a standard ampere rating for a fuse or inverse-time breaker per 240.6(A)?

• a.45 A
• b.110 A
• c.130 A✓
• d.70 A

The standard ratings in 240.6(A) include 45, 70, and 110 A, but 130 A is not listed (the next standard sizes are 125 A and 150 A). Knowing the standard list is essential for applying the next-size-up rule.2023 NEC §240.6(A)

A conductor has an allowable ampacity of 115 A that does not match a standard fuse size. The load is 112 A. What is the largest standard OCPD permitted?

• a.115 A
• b.125 A✓
• c.150 A
• d.110 A

Per 240.4(B), when the conductor ampacity does not correspond to a standard rating and is 800 A or less, the next higher standard device (125 A) is permitted. 115 A is not a standard size, 110 A is unnecessarily lower, and 150 A skips past the next standard.2023 NEC §240.4(B)

For circuits rated over 800 A, what is the rule for the overcurrent device relative to conductor ampacity?

• a.The OCPD rating must not exceed the conductor ampacity✓
• b.There is no limit
• c.It may be twice the ampacity
• d.Next higher standard size is allowed

Per 240.4(C), where the circuit is over 800 A, the ampacity of the conductors must be equal to or greater than the OCPD rating; the next-size-up allowance of 240.4(B) does not apply. Doubling or 'no limit' are never correct.2023 NEC §240.4(C)

A service uses 3/0 copper conductors at 75 C (200 A ampacity) with a 200 A load. What is the correct standard OCPD?

• a.175 A
• b.225 A
• c.200 A✓
• d.250 A

The conductor ampacity (200 A) exactly matches a standard OCPD rating, so a 200 A device is used - the next-size-up rule of 240.4(B) does not apply when the ampacity equals a standard size. 175 A undersizes for the 200 A load, and 225/250 A overprotect the conductor.2023 NEC §240.6(A)

As a general rule, where must overcurrent protection for a conductor be located?

• a.Anywhere along the run
• b.At the point where the conductor receives its supply✓
• c.At the load end of the conductor
• d.Within 25 ft of any panel

Per 240.21, overcurrent devices must generally be placed where the conductor receives its supply (its origin). Tap rules are specific exceptions; there is no general rule allowing load-end or arbitrary placement.2023 NEC §240.21

Why is a service disconnect required to be readily accessible and located near the point of entrance of the service conductors?

• a.To improve power factor
• b.To meet minimum ampacity
• c.To reduce voltage drop
• d.To allow quick de-energizing of the premises in an emergency✓

A readily accessible service disconnect near the entrance lets responders quickly de-energize the building and limits the length of unprotected service-entrance conductors inside. Voltage drop, power factor, and ampacity are unrelated to the disconnect-location rule.

A branch circuit carries a continuous load of 80 A. What minimum OCPD rating is required?

• a.64 A
• b.80 A
• c.90 A
• d.100 A✓

Per 210.20(A), the OCPD must be at least 125% of the continuous load: 80 A x 1.25 = 100 A. 80 A ignores the 125% factor, 90 A is insufficient, and 64 A wrongly reduces the load.2023 NEC §210.20(A)

Using the standard 3 VA/sq ft general lighting unit load, what is the general lighting load for a 2,000 sq ft dwelling (before demand factors)?

• a.4,500 VA
• b.3,000 VA
• c.6,000 VA✓
• d.2,000 VA

Dwelling general lighting is computed at 3 VA per sq ft: 2,000 x 3 = 6,000 VA. The other choices misapply or omit the 3 VA/sq ft factor.

A dwelling has two required small-appliance branch circuits and one laundry circuit. What is the total calculated load for these before demand factors?

• a.1,500 VA
• b.3,000 VA
• c.6,000 VA
• d.4,500 VA✓

Per 220.52, each small-appliance and laundry circuit is figured at 1,500 VA: (2 x 1,500) + 1,500 = 4,500 VA. 3,000 VA omits the laundry circuit and 1,500 VA counts only one.2023 NEC §220.52

A dwelling has one 12 kW electric range. Using Table 220.55 Column C, what is the demand load?

• a.6 kW
• b.12 kW
• c.8 kW✓
• d.9.6 kW

Column C of Table 220.55 gives 8 kW of demand for a single range not over 12 kW. 12 kW ignores the demand factor, and the other values misread the column.2023 NEC §220.55

Two 12 kW electric ranges are in a two-unit dwelling. Using Table 220.55 Column C, what is the total demand?

• a.24 kW
• b.11 kW✓
• c.8 kW
• d.16 kW

Column C gives 11 kW for two ranges (not over 12 kW each). 8 kW is the value for one range, 16 kW doubles the single-range value, and 24 kW ignores demand entirely.2023 NEC §220.55

A single household electric clothes dryer is rated 4 kW. What value is used for the service/feeder demand calculation?

• a.3,000 VA
• b.4,000 VA
• c.2,000 VA
• d.5,000 VA✓

Per 220.54, each dryer is calculated at 5,000 VA or the nameplate rating, whichever is larger. Since 4 kW is below the 5,000 VA minimum, 5,000 VA is used; 4,000 VA and the lower values ignore the minimum.2023 NEC §220.54

A range has a Column C demand of 8 kW. What is the maximum neutral demand load for that range?

• a.8 kW
• b.4 kW
• c.2.8 kW
• d.5.6 kW✓

Per 220.61(B)(1), the feeder/service neutral load for a range may be taken at 70% of the ungrounded demand: 8 kW x 0.70 = 5.6 kW. 8 kW ignores the 70% allowance, and the other values misapply it.2023 NEC §220.61

Under the optional dwelling calculation, how are the general loads (other than heating/AC) treated?

• a.First 10 kVA at 100%, remainder at 40%✓
• b.All at 100%
• c.All at 40%
• d.First 3 kVA at 100%, remainder at 35%

Per 220.82(B), the sum of general loads is taken at 100% for the first 10 kVA and 40% for the remainder. The 3 kVA/35% split belongs to the standard method, not the optional calculation.2023 NEC §220.82(B)

A dwelling's general lighting plus small-appliance/laundry load totals 13,500 VA. Using the standard demand factors (first 3,000 VA at 100%, remainder at 35%), what is the demand?

• a.6,675 VA✓
• b.13,500 VA
• c.4,725 VA
• d.3,000 VA

First 3,000 VA at 100% = 3,000 VA; remaining 10,500 VA at 35% = 3,675 VA; total = 6,675 VA. 13,500 VA ignores demand factors, and the other values misapply them.

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

• a.125 A
• b.150 A
• c.100 A✓
• d.60 A

Per 230.79(C), a one-family dwelling service disconnect must be rated at least 100 A, 3-wire. 60 A is below the minimum, and 125 A/150 A exceed the required minimum.2023 NEC §230.79(C)

The optional feeder/service calculation of 220.84 for a multifamily dwelling may be used only when which condition is met?

• a.Each unit exceeds 2,000 sq ft
• b.Only one meter serves the building
• c.The building is over 3 stories
• d.There are at least 3 units and no dwelling is supplied by more than one feeder✓

Per 220.84, the optional multifamily method applies when there are 3 or more dwelling units, no unit is supplied by more than one feeder, and each unit has electric cooking plus electric heat or AC. Story count, unit size, and meter count are not the qualifying conditions.2023 NEC §220.84

A dwelling has 10 kW of electric heat and a 5 kW air conditioner that never run at the same time. What load is included in the calculation?

• a.5 kW (the smaller)
• b.7.5 kW (average)
• c.10 kW (the larger)✓
• d.15 kW (both)

Per 220.60, for noncoincident loads only the larger of the two is included: 10 kW. Adding both (15 kW), averaging, or using the smaller misapplies the noncoincident-load rule.2023 NEC §220.60

Service-entrance conductors must supply a computed load of 160 A continuous. What minimum conductor ampacity is required per 230.42?

• a.160 A
• b.128 A
• c.200 A✓
• d.175 A

Per 230.42(A), service conductors must have an ampacity of at least 125% of continuous load: 160 A x 1.25 = 200 A. 160 A ignores the factor, 128 A reduces the load, and 175 A is insufficient.2023 NEC §230.42

What is the full-load secondary current of a 45 kVA, 3-phase, 208 V transformer?

• a.125 A✓
• b.94 A
• c.108 A
• d.216 A

I = 45,000 / (1.732 x 208) = 125 A. The formula uses line-to-line voltage times the square root of 3; the other values drop the sqrt-3 factor or use single-phase math.2023 NEC §450.3

What is the full-load primary current of a 75 kVA, 3-phase, 480 V transformer?

• a.125 A
• b.72 A
• c.90 A✓
• d.156 A

I = 75,000 / (1.732 x 480) = 90.2 A, so about 90 A. The other choices omit the sqrt-3 factor or use the wrong voltage.2023 NEC §450.3

For a transformer over 1000 V or with primary-only protection under 450.3(B), where primary current is 9 A or more, the primary OCPD generally may not exceed what percentage of primary full-load current?

• a.125%✓
• b.167%
• c.300%
• d.250%

Table 450.3(B) permits primary-only protection at not more than 125% of rated primary current when that current is 9 A or more. 167%/250%/300% apply to higher-current or combined primary-and-secondary arrangements, not the basic 125% primary-only case.2023 NEC §450.3(B)

A transformer has a primary full-load current of 40 A and uses primary-only overcurrent protection. What is the maximum standard primary OCPD (125% rule)?

• a.40 A
• b.50 A✓
• c.45 A
• d.60 A

40 A x 125% = 50 A, which is a standard size, so 50 A is the maximum per Table 450.3(B). 40 A ignores the 125% factor, while 45 A undersizes and 60 A exceeds the calculated maximum.2023 NEC §450.3(B)

Conductors are connected to a transformer secondary without an OCPD at the secondary terminals. What set of rules governs their length and sizing?

• a.The transformer secondary/tap rules of 240.21(C)✓
• b.The small-conductor rule 240.4(D)
• c.The service-entrance rules of Article 230
• d.There are no restrictions

Per 240.21(C), transformer secondary conductors without secondary protection must follow specific tap rules limiting length (e.g., 10 ft or 25 ft) and minimum ampacity. Article 230 covers services, and 240.4(D) covers small conductors; unrestricted runs are never permitted.2023 NEC §240.21(C)

Why must ventilating openings of a dry-type transformer be kept clear of walls and obstructions?

• a.To meet grounding rules
• b.To improve power factor
• c.To reduce audible noise
• d.To dissipate heat and prevent overheating✓

Per 450.9, transformer ventilation must not be blocked so that heat generated by losses can dissipate and the transformer does not overheat. Noise, grounding, and power factor are governed by other rules and are not the purpose of the ventilation clearance.2023 NEC §450.9