CSLB Trade — Electrical (C-10) — All Questions
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115 questions
For a feeder/service neutral, the portion of the maximum unbalanced load that exceeds 200 A may generally be calculated at:
- a.80%
- b.50%
- c.100%
- d.70%✓
Per 220.61(B)(2), the neutral load in excess of 200 A may be taken at 70% (subject to the restrictions of 220.61(C), such as nonlinear loads). The first 200 A is at 100%; the other percentages do not match the code demand factor.2023 NEC §220.61
A 75 kVA, 208Y/120 V transformer has an impedance of 2%. Using the infinite-bus (worst-case) method, the approximate available fault current at the transformer secondary is about:
- a.10,400 A✓
- b.4,160 A
- c.2,080 A
- d.20,800 A
Secondary full-load current = 75,000 / (1.732 x 208) = 208 A; dividing by the per-unit impedance gives 208 / 0.02 = 10,400 A. Using the wrong impedance or forgetting to divide gives the incorrect values; this infinite-bus method assumes an unlimited primary source.
When circuit conductors are run in parallel in multiple raceways, the equipment grounding conductor in each raceway must be:
- a.Divided proportionally between raceways
- b.Sized in full, based on the circuit overcurrent device, in each raceway✓
- c.Omitted in all but one raceway
- d.One size smaller in each raceway
Per 250.122(F), where conductors are run in parallel in separate raceways or cables, a full-size equipment grounding conductor sized per Table 250.122 (based on the circuit OCPD) must be installed in each raceway. It cannot be divided, reduced, or omitted, because each raceway must carry a complete fault path.2023 NEC §250.122
A 208Y/120 V, 3-phase feeder supplies a balanced 50 kVA load. The approximate line current (and therefore the minimum conductor ampacity before continuous factors) is:
- a.120 A
- b.174 A
- c.139 A✓
- d.96 A
For a 3-phase load, I = VA / (1.732 x line voltage) = 50,000 / (1.732 x 208) = about 139 A. Dividing by voltage alone or using a single-phase formula gives the incorrect answers; the sqrt(3) factor is required for three-phase current.2023 NEC §215.2
For a 200 A service where the equipment and conductors are all rated for it, the conductor ampacity used for termination sizing is normally based on which temperature column?
- a.75 degrees C✓
- b.90 degrees C
- c.60 degrees C
- d.105 degrees C
Per 110.14(C), for circuits over 100 A (or conductors larger than 1 AWG), terminations are generally rated 75 degrees C, so the 75 degrees C column of Table 310.16 is used unless equipment is listed otherwise. The 90 degrees C column is only for derating starting points, and 60 degrees C applies to smaller circuits.2023 NEC §110.14
A panelboard must have a rating (busbar ampacity) not less than:
- a.The largest single branch circuit only
- b.The minimum feeder capacity required for the load served, as computed per Article 220✓
- c.50% of the service rating
- d.Twice the neutral load
Per 408.30, a panelboard must have a rating not less than the minimum feeder or service capacity required for the load computed under Article 220. Sizing only to a single branch circuit or an arbitrary fraction of the service would leave the busbar under-rated for the actual load.2023 NEC §408.30
Service-entrance conductors installed in a raceway in a wet location (such as underground or exposed to weather) must be:
- a.Any dry-rated conductor is acceptable
- b.Type TW without a jacket
- c.Listed and marked for wet locations, such as RHW, THW, THWN, or XHHW✓
- d.Bare copper only
Per 310.10(C), conductors in wet locations (including raceways underground or exposed to weather) must be a type listed for wet locations, such as RHW, THW, THWN, or XHHW. Dry-only types or bare conductors are not permitted where moisture is present.2023 NEC §310.10
A separately derived system (such as the secondary of a transformer feeding a distribution panel) must have a system bonding jumper connecting:
- a.The grounded conductor to the supply-side bonding jumper and equipment grounding conductor at the source or first disconnect✓
- b.The neutral to a branch-circuit breaker
- c.The two ungrounded phases together
- d.Only the metal raceway to earth
Per 250.30(A), a separately derived system requires a system bonding jumper connecting the grounded conductor to the supply-side bonding jumper and/or equipment grounding conductor at a single point (at the source or the first disconnect). Bonding phases together or omitting the grounded conductor connection would not establish the required fault path.2023 NEC §250.30
A detached garage is supplied by a feeder from the main dwelling. At the garage, the code generally requires:
- a.No grounding electrode because the feeder has an equipment grounding conductor
- b.A grounding electrode (or electrode system) to be installed and connected to the building disconnect, with the equipment grounding conductor run with the feeder✓
- c.Only a single ground rod with the neutral used as the ground
- d.The neutral must be bonded to the enclosure like at a service
Per 250.32, a building or structure supplied by a feeder must have a grounding electrode connected to the equipment grounding terminal at the disconnect, and an equipment grounding conductor must be run with the feeder while the neutral remains isolated. Re-bonding the neutral downstream (the old exception) is no longer generally permitted.2023 NEC §250.32
On a 4-wire delta system where the midpoint of one phase is grounded, the conductor with the higher voltage to ground (the high leg) must be durably and permanently marked, typically colored:
- a.Orange✓
- b.White
- c.Green
- d.Blue
Per 110.15, the phase conductor having the higher voltage to ground (about 208 V on a 120/240 V high-leg delta) must be marked by an outer finish that is orange or by other effective means at every point of connection. White identifies the grounded conductor and green identifies equipment grounding, so neither can mark the high leg.2023 NEC §110.15
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A service raceway enters a building from an underground distribution system. To prevent moisture and gases from entering, the raceway must be:
- a.Filled with concrete
- b.Sealed with an approved material (though a drainage provision may still be provided)✓
- c.Painted on the interior
- d.Left open for drainage
Per 230.8, a raceway entering from an underground distribution system must be sealed with an identified compound to block moisture and gases; spare or unused raceways must also be sealed. Leaving it open or filling with concrete does not meet the sealing requirement, and painting does not seal against gas migration.2023 NEC §230.8
Where multiple service disconnects are installed under the 2023 NEC, the general requirement (compared to older editions) is that they be:
- a.Operable by a maximum of six movements of the hand in one enclosure
- b.Located outdoors only
- c.Installed in separate enclosures, or in a listed assembly, rather than up to six switches in a single common enclosure✓
- d.Interlocked so only one can open at a time
The 2023 NEC changed the multiple-disconnect rule so that up to six service disconnects must be in separate enclosures (or part of a listed switchboard, switchgear, or metering-disconnect assembly), rather than as several breakers in one common enclosure. The old six-hand-movement allowance in a single cabinet is no longer the general rule.2023 NEC §230.71
When installing a surge-protective device (SPD), the best practice for the connecting conductors is to:
- a.Keep them as short and straight as practicable✓
- b.Make them as long as possible for flexibility
- c.Route them alongside the grounding electrode conductor for the full length
- d.Coil extra length behind the panel
SPD performance depends on low conductor impedance, so the leads should be kept as short and straight as practicable to minimize added voltage from lead inductance during a surge. Long or coiled leads raise the let-through voltage and reduce protection, defeating the purpose of the device.
In a standard dwelling load calculation, the general lighting and receptacle load demand factor applies 100% to:
- a.None of the load
- b.The first 3000 VA✓
- c.The first 10,000 VA
- d.All VA above 120,000
Per Table 220.42, the first 3000 VA of general lighting/receptacle load is taken at 100%, the portion from 3001 to 120,000 VA at 35%, and any remainder at 25%. The other choices misstate the thresholds of the demand-factor table.2023 NEC §220.42
A series-rated combination of overcurrent devices is permitted to protect equipment where the available fault current exceeds the interrupting rating of a downstream device, provided the combination is:
- a.Any two breakers from different manufacturers
- b.Rated only by adding the two interrupting ratings
- c.Tested and marked/listed as a recognized series combination, and the equipment is field-marked accordingly✓
- d.Field-assembled without documentation
Per 240.86, series ratings may be used only where the combination is listed and marked as a tested series combination (or engineered per 240.86(A)), and the end equipment is legibly field-marked to indicate the series-combination rating. Simply pairing arbitrary breakers or adding interrupting ratings is not permitted.2023 NEC §240.86