CSLB Class C-10 Electrical — Trade Exam Study Guide (2026)

This is a free sample chapter from the PrepPass CSLB C-10 Electrical study guide. The complete guide covers all six exam areas, includes 65 original practice questions with a full answer key, and comes with a free online practice-test companion at PrepPass.org/cslb-c10.

Independent study guide — not affiliated with, endorsed by, or produced by the CSLB, PSI, or the NFPA. Code figures are written to the 2023 NEC as adopted in the California Electrical Code (CEC); the NEC/CEC is amended on a cycle — verify the current adopted edition before you rely on any number. No copied exam content; all questions are original.

Chapter 1 — Planning, Plans & Estimating

Introduction

Before any wire is pulled, a C-10 contractor reads the electrical drawings, sizes the load, estimates materials and labor, and pulls the right permit. Everything downstream — the service size, the conductor ampacities, the bid price — flows from getting this planning stage right. This chapter covers plan reading, permits and inspection, the dwelling load calculation (Article 220), demand factors and estimating, and the code-cycle framework (NEC → California Electrical Code) that governs all electrical work in the state.

The load calculation is the single most transferable skill in this chapter: the number you produce here becomes the minimum service size in Chapter 2 and the conductor ampacity in Chapter 3. Learn it once, correctly, and three exam areas get easier.

Learning objectives

After this chapter you should be able to:

• Read a one-line diagram, a floor plan, and a panel schedule, and translate symbols into a material takeoff.
• State who may pull a permit and what the rough and final inspections verify.
• Compute a dwelling service load by both the standard method (Article 220, Part III) and the optional method (220.82).
• Apply the general lighting load (3 VA/ft²), the two small-appliance and one laundry circuits (1500 VA each), and the range demand table (220.55).
• Explain the three-year code cycle and how the California Electrical Code (CEC) relates to the model NEC.

Part A — Reading plans, one-line diagrams, and symbols

Construction documents communicate a design through a shared visual language, and the estimator who reads them fluently bids accurately.

• The one-line (single-line) diagram shows power flow from the service through the panelboards to the loads, using single strokes to represent whole conductor sets. It shows overcurrent devices, conductor sizes, and equipment ratings at a glance — it is the "electrical skeleton" of the job.
• The floor plan locates devices (receptacles, switches, luminaires, equipment) using standardized symbols, drawn to scale.
• The panel schedule lists every circuit in a panelboard with its breaker size, load description, and phase — this is what lets the estimator count home runs and devices.
• The symbol legend on each drawing defines what each symbol means, so the field crew installs exactly what the designer intended.
• Scale and dimensions: floor plans are drawn to scale, and the conductor lengths you need for takeoff and voltage-drop checks come from scaled horizontal runs plus the vertical rises (down from the ceiling, up from the slab) that a flat plan hides.

The estimator's job is to turn these documents into a quantity takeoff: a count of every device, box, fitting, and linear foot of wire and conduit.

Part B — Permits, plan review, and inspection

Most electrical installations require a permit from the local Authority Having Jurisdiction (AHJ) before work begins. The AHJ reviews the plans for code compliance and then inspects the work in stages.

• Who may pull the permit. Only a licensed contractor, or the property owner under the limited owner-builder exemption, may pull an electrical permit. A C-10 contractor pulls permits for the electrical scope.
• The AHJ enforces the adopted code (2023 NEC §90.4). It approves equipment, grants the permit, and — on any disputed point — its interpretation governs. When the plans and the inspector disagree, the inspector's call stands until formally appealed.
• Rough (rough-in) inspection happens after the wiring, boxes, and raceways are installed but before the walls are closed, so concealed work can be verified. Miss it and the wall has to come back open.
• Final inspection happens after the devices, panels, and equipment are complete and energized, to confirm the system is safe to place in service.
• Approved equipment must be listed and labeled and installed within the conditions of its listing and the manufacturer's instructions (2023 NEC §110.3(B)). "Follow the instructions that come in the box" is a code rule, not just good practice.

Exam trap — "approved" vs. "listed" vs. "labeled." These are three different words. Listed means a recognized testing lab (e.g., UL) evaluated the product. Labeled means it carries that lab's mark. Approved means acceptable to the AHJ. A product can be listed and labeled and still not be approved for a particular use. On the exam, "approved" always points back to the inspector.

Part C — Dwelling load calculations (Article 220)

Sizing a service or feeder begins with a load calculation. Article 220 gives two roads to the same destination — the minimum service size — and the exam tests both.

The standard method (Article 220, Part III) sums the loads and then applies tiered demand factors:

• General lighting load: dwelling general lighting and general-use receptacles are calculated at 3 volt-amperes per square foot of habitable area, using the outside dimensions of the dwelling (2023 NEC §220.12). Do not deduct for hallways or closets; do not include open porches, garages, or unused/unfinished spaces not adaptable for future use.
• Small-appliance and laundry circuits: include at least two 20-amp small-appliance branch circuits and one laundry circuit, each counted at 1500 VA in the calculation (2023 NEC §220.52).
• General lighting demand factor (Table 220.42): the first 3000 VA of the combined general-lighting + small-appliance + laundry load is taken at 100%, and the remainder up to 120,000 VA at 35%.
• Appliances: fastened-in-place appliances (water heater, disposal, dishwasher, etc.) are added; where there are four or more, a 75% demand factor may be applied to their nameplate total (2023 NEC §220.53).
• Range demand (Table 220.55): household cooking appliances over 1¾ (1.75) kW use the demand table, which reduces the connected nameplate load substantially — a single range up to 12 kW counts as just 8 kW of demand (Column C).
• Dryer: an electric clothes dryer is counted at 5000 VA or nameplate, whichever is larger (2023 NEC §220.54).

The optional method for dwellings (2023 NEC §220.82) trades the tiered tables for two simple percentages once the dwelling qualifies (it must be served by a single 120/240 V or 208Y/120 V set of 3-wire, 100 A or greater service):

• Take 100% of the first 10 kVA of the total connected load, and 40% of the remainder.
• Heating or air-conditioning load is added separately at the larger of the two, per §220.82(C).

Worked example — optional-method service size. A dwelling has a total connected load (general lighting at 3 VA/ft², the two small-appliance + one laundry circuits, range, dryer, water heater, dishwasher, disposal) that adds up to 48 kVA, plus a 10 kW central air-conditioner. Optional-method general load: first 10 kVA at 100% = 10 kVA; remaining 38 kVA at 40% = 15.2 kVA; subtotal = 25.2 kVA. Add the A/C at 100% (it is the larger of heat vs. cool here) = 10 kVA → total demand = 35.2 kVA. Minimum service current = 35,200 VA ÷ 240 V = 146.7 A → the next standard service is 150 A (a 200 A service is common for headroom). Source: 2023 NEC §220.82; standard ratings §240.6.

Exam trap — the range does not count at nameplate. A 12 kW range is a classic distractor: candidates plug 12,000 VA into the total. Column C of Table 220.55 caps one household range (≤ 12 kW) at 8 kW of demand. Using nameplate instead of the demand table oversizes the service and loses the point.

Part D — Demand factors and the neutral

A demand factor is the ratio of maximum demand to total connected load. It exists because not every load runs at full power at the same instant, so conductors and services can be sized economically instead of for a peak that never happens.

• The neutral demand. The feeder or service neutral carries only the unbalanced current between the two hot legs, so a further demand factor applies: the neutral is figured on the maximum unbalance, and the portion over 200 A may be taken at 70% (2023 NEC §220.61). A 240 V straight load (like a water heater) puts no current on the neutral at all.

Worked example — general-lighting demand. A 2,400 ft² dwelling. General lighting = 2,400 × 3 = 7,200 VA. Add two small-appliance circuits (2 × 1,500 = 3,000) and one laundry (1,500) = 4,500 VA. Combined = 11,700 VA. Apply Table 220.42: first 3,000 at 100% = 3,000; remaining 8,700 at 35% = 3,045. Net general-lighting demand = 6,045 VA. Source: 2023 NEC §§220.12, 220.52, Table 220.42.

Worked example — a complete standard-method service calculation. Size the minimum service for a 2,000 ft² dwelling with: an 8 kW nameplate range, a 4.5 kW water heater, a 1.2 kW dishwasher, a 0.9 kW disposal, a 5 kW dryer, and 9 kW of central air conditioning. 1. General lighting + small-appliance + laundry: (2,000 × 3) + (2 × 1,500) + 1,500 = 6,000 + 3,000 + 1,500 = 10,500 VA. Apply Table 220.42: first 3,000 @ 100% = 3,000; remaining 7,500 @ 35% = 2,625 → 5,625 VA. 2. Range (Table 220.55, Col. C, one unit ≤ 12 kW): 8,000 VA. 3. Dryer (§220.54): 5,000 VA nameplate ≥ the 5,000 VA floor → 5,000 VA. 4. Fixed appliances — water heater 4,500 + dishwasher 1,200 + disposal 900 = 6,600 VA. Only three fixed appliances (the range and dryer have their own rules), so no 75% factor (§220.53 needs four or more) → 6,600 VA. 5. Air conditioning (§220.60, the larger of heat vs. cool): 9,000 VA. Total demand = 5,625 + 8,000 + 5,000 + 6,600 + 9,000 = 34,225 VA. Service current = 34,225 ÷ 240 = 142.6 A → a 150 A service meets it. Note how the optional method (worked earlier) usually lands close — use whichever your problem specifies. Source: 2023 NEC §§220.12, 220.42, 220.53, 220.54, 220.55, 220.60.

Part E — The code cycle: NEC and the California Electrical Code

The National Electrical Code (NFPA 70) is revised on a three-year cycle by the NFPA. California does not enforce the model NEC directly; it adopts and amends it as Part 3 of Title 24 of the California Code of Regulations, publishing the California Electrical Code (CEC).

• Three-year cycle. NFPA 70 is republished every three years; California typically adopts the model code, with state-specific amendments, after each cycle.
• The CEC is the enforceable code. The California Electrical Code — not the raw NEC — is what the AHJ enforces statewide.
• The effective edition governs. Work must comply with the CEC edition in force on the permit date. The 2025 CEC (based on the 2023 NEC) took effect January 1, 2026; the prior 2022 CEC (based on the 2020 NEC) governed earlier permits. Which edition applies is set by the state and the AHJ — never by the contractor.
• Local amendments. Cities and counties may amend the CEC for local conditions (seismic, coastal, fire), so a careful contractor confirms amendments with each AHJ before bidding.

Exam trap — "which code applies?" The answer is never "the newest NEC." It is the CEC edition adopted and in force on the permit date, plus local amendments. This guide is written to the 2023 NEC as carried into the 2025 CEC; if you sit the exam under a different adopted edition, verify the current CEC edition — a handful of table values shift between cycles.