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

CSLB lists six topics under this section: existing system evaluation, plan and specification interpretation, electrical calculations, code requirements, material selection, and energy production and storage project planning. This is the largest section of the exam, and it is the one with the most arithmetic. Every later chapter leans on it: the load you calculate here sizes the service in Chapter 2, and the material you select here is what you install, label, test, and replace in Chapters 3 and 4.

Learning objectives

After this chapter you should be able to:

• Evaluate an existing electrical system for capacity, condition, and compliance before quoting a change to it.
• Read a one-line diagram, floor plan, panel schedule, and symbol legend, and turn them into a quantity takeoff.
• Calculate a dwelling load by the standard method and size the service from it.
• Use Ohm's law, the power formulas, and the voltage-drop formula without a reference sheet.
• Explain which code governs a job in California and who decides.
• Choose materials by listing, rating, location, and manufacturer's instructions, and explain why a replacement breaker is chosen by frame, AIC rating, and ampacity.
• Build a bid from a takeoff, including waste, labor, burden, overhead, profit, discounts, and sales tax.

1.1 Existing system evaluation

Concept. Before you touch an existing system, you find out what is there, what condition it is in, how much capacity is left, and which of the old conditions you are now responsible for. A quote written without that survey is a guess about someone else's wiring.

Why it matters for the exam. CSLB names this as its own bullet. Questions in this area describe a building, a symptom, or a proposed addition and ask what you check first, what you may reuse, or what the new work triggers.

The walk-through

Work from the utility to the loads.

1. Service and metering. Find the service size on the main breaker and the service-entrance conductors, the meter socket condition, the service disconnect location, and the grounding electrode connections. Confirm the service disconnect is where the code wants it: readily accessible, outside, or inside nearest the point of entry, and not in a bathroom.
2. Panelboards. Check the panel's rating and its listed breaker types, open knockouts and missing blanks, double-tapped breakers, burned or discolored terminals, a missing directory, and any evidence that breakers were swapped for the wrong kind. Panel capacity is measured two ways: spaces and ampacity of the bus, and you need both.
3. Branch circuits. Identify the wiring method in each area: nonmetallic-sheathed cable, metal-clad cable, conduit with conductors, or old knob-and-tube. Identify the conductor size against the breaker that protects it.
4. Devices and boxes. Look for ungrounded receptacles, missing cover plates, overfilled boxes, and unsupported fixtures.
5. Protection that is absent. Compare what is installed with what a new circuit in that room would require today: ground-fault protection, arc-fault protection, tamper-resistant receptacles.
6. Load. Add up the connected loads or calculate the demand (see 1.3) and compare it with the service rating.

What the new work triggers

The California Electrical Code is written so that some new rules apply only when you change something. These are the ones to know.

• Modified, replaced, or extended branch circuits in living areas need arc-fault protection. The California text of 210.12(E) says that if branch-circuit wiring for the dwelling areas listed in 210.12(B), the dormitory areas in (C), or the other occupancies in (D) is modified, replaced, or extended, the circuit must be protected by a listed combination-type AFCI means, or by a listed outlet branch-circuit AFCI at the first receptacle of the existing circuit. There is an exception: no AFCI is required where an existing circuit is extended by not more than 6 feet and no additional outlets or devices (other than splicing devices) are added; conductors inside an enclosure are not counted in the 6 feet.
• Replacing a receptacle brings today's receptacle rules with it. City of Menifee's plan-check handout, based on the 2022 CEC, states that 125-volt, 15- and 20-ampere receptacles installed or replaced in dwelling units shall be listed tamper-resistant, and that receptacles installed or replaced in wet locations must be listed weather-resistant with an in-use cover. City of St. Paul's 2023 NEC checklist says the same for replacement tamper-resistant and weather-resistant receptacles.
• Replacing service equipment brings the emergency disconnect with it. City of St. Paul's 2023 NEC checklist for dwelling services states that replacing the service panelboard or disconnect requires installing an emergency disconnect (230.85(C)), even for the same ampacity, and that the emergency disconnect must have a short-circuit current rating at least equal to the available fault current.
• Reconditioned panelboards are not permitted. The California text of 408.2(A) says reconditioned panelboards shall not be permitted. (Reconditioned switchboards and switchgear are permitted.) Equipment damaged by fire, products of combustion, corrosive influences, or water must be specifically evaluated by its manufacturer or a qualified testing laboratory before returning to service.
• Single-family homes and duplexes need panel space for electrification. In the California text of 408.2(B), panelboards serving an individual dwelling unit in a single-family building (one or two dwellings) shall be provided with circuit breaker spaces for heat pump water heaters, heat pump space heaters, electric cooktops, and electric clothes dryers as specified in the California Energy Code, Section 150.0(n), (t), (u), and (v). The sections that follow (422.3, 440.3, and 706.10) require the dedicated circuits, receptacles, and raceways for those appliances and for energy storage. See Chapter 6 for the energy-storage piece.

Knob-and-tube wiring

Knob-and-tube wiring is old, ungrounded, and was designed to dissipate heat into open air. The California text of 394.12 forbids concealed knob-and-tube in commercial garages, theaters and similar locations, motion picture studios, hazardous (classified) locations, and in hollow spaces of walls, ceilings, and attics where the space is insulated with loose, rolled, or foamed-in-place insulation that envelops the conductors. A California amendment for housing (marked HCD 1 and HCD 2) creates a path to insulate around knob-and-tube that is still in service, but only if all of the following happen:

1. The wiring is surveyed by a California-licensed electrical contractor, who certifies that it is in good condition with no deterioration, improper overcurrent protection, or improper connections or splices. Repairs, alterations, or extensions require permits and inspection.
2. The certification form is filed with the AHJ and a copy goes to the property owner.
3. All accessible areas where insulation surrounds the wiring are posted with a visible caution notice printed in both English and Spanish.
4. The insulation is noncombustible as defined in the California Building Code.
5. The insulation contains no electrically conductive material.
6. The AHJ may still require permits and inspections for the insulation work.

Trap: "licensed electrical contractor" in that exception. The survey and certification are done by a California-licensed electrical contractor, not by the insulation contractor.

Available fault current and the equipment you reuse

Every device that opens a circuit has an interrupting rating, and the rating must be at least the fault current available at its terminals. You will see this stated three ways: AIC (amperes interrupting capacity) on a breaker, short-circuit current rating (SCCR) on equipment, and "ratings sufficient for ... available fault current" in the code's disconnect rules (for example the source disconnect rule in 705.20, which requires ratings sufficient for the maximum circuit current, available fault current, and voltage at the terminals). An old panel in a building whose utility transformer has been replaced with a larger one can end up with breakers that are underrated for the fault current now available. This is one of the checks an existing-system evaluation is for.

1.2 Plan and specification interpretation

Concept. Construction documents are a contract in drawing form. Drawings show what and where; specifications state quality, brand, method, and performance. You price what both documents together require.

The drawing set

Sheet type	What it tells you
Site plan	Service location, utility connection, underground runs, site lighting
Floor plan	Location of every device, luminaire, panel, and piece of equipment, drawn to scale
One-line (single-line) diagram	Power flow from the service through panelboards to loads, with overcurrent device ratings, conductor sizes, and equipment ratings
Panel schedule	Each circuit's breaker size, load, and phase
Riser diagram	Vertical routing between floors
Details and legends	Symbols, mounting heights, special installation notes
Specifications	Materials, methods, testing, closeout
Addenda	Written changes issued during bidding; they override the sheet they change

Read the symbol legend first. The same symbol can mean different things on two sets of drawings, and the legend on the set in your hands is the one that counts.

Reading a panel schedule

A panel schedule lists circuit number, description, breaker size, and phase. Use it three ways:

1. Count home runs. Each circuit is a run back to the panel. Missing one means missing the wire and the labor.
2. Check phase balance. Add the load on each phase leg. A schedule with 14 kVA on phase A and 6 kVA on phase B is out of balance, and the feeder neutral and the service sizing both feel it.
3. Check spare capacity. Count spare spaces and spare capacity against any planned future load.

Scale and length

Plans are drawn to scale. A scale of 1/4 inch = 1 foot means one inch on the drawing is 4 feet in the building.

Worked example 1.1: measuring a run. A circuit on a 1/4"-scale plan measures 5 1/2 inches in horizontal run from the panel to the first box. The panel is mounted so the cable drops 6 feet from the ceiling and rises 4 feet to the box.

• Horizontal: 5.5 in x 4 ft/in = 22 ft
• Vertical: 6 ft + 4 ft = 10 ft
• Total: 32 ft before waste

A flat plan hides vertical rises. Always add them.

Specification traps

• Addenda change the bid. An addendum issued after your first look can swap a luminaire type or add a circuit. Write the addendum numbers on your bid.
• "Or equal." A spec that names a manufacturer and says "or equal" lets you substitute equal listed equipment, but the engineer decides what is equal, so your price assumes approval.
• General notes. Notes like "all conduit EMT unless noted" override your habit.
• Conflicts between the drawings and the specifications. The general conditions of the contract usually state which governs. When they do not, ask in writing before the bid is due, and price the more demanding requirement until you get an answer.

1.3 Electrical calculations

The exam will ask you to calculate. A calculator is provided (CSLB, form 13E-10), but no reference sheet. This section covers the formulas you must carry in your head, then the dwelling load calculation.

Ohm's law and power

For a simple resistive circuit:

• E = I x R (volts = amperes x ohms)
• P = E x I (watts = volts x amperes)
• Combining them: P = I^2 x R and P = E^2 / R

For alternating current with reactive loads, the product of volts and amperes is volt-amperes (VA), not watts. Power factor relates the two: watts = VA x power factor.

Worked example 1.2: heater current. A resistive 1,500-watt heater on a 120-volt circuit: I = P / E = 1,500 / 120 = 12.5 A.

Worked example 1.3: series resistance. Three 4-ohm resistors in series across 24 V: total R = 12 ohms, so I = 24 / 12 = 2 A, and the voltage across each is 2 x 4 = 8 V.

Worked example 1.4: parallel resistance. Two resistors, 6 ohms and 3 ohms, in parallel: R = (6 x 3) / (6 + 3) = 2 ohms. The combined resistance is always smaller than the smallest branch.

Single-phase and three-phase

• Single-phase: VA = V x I, so I = VA / V.
• Three-phase: VA = 1.732 x V(line-to-line) x I, so I = VA / (1.732 x V).

Worked example 1.5: three-phase current. A 45 kVA three-phase load at 208 volts line-to-line: I = 45,000 / (1.732 x 208) = 45,000 / 360.3 = 124.9 A.

Voltage drop

A conductor has resistance, so current through it drops some voltage along the way. The drop is calculated from the round-trip length because current goes out on one conductor and returns on another.

Single-phase voltage drop = (2 x K x I x L) / CM

where K is the resistivity constant of the conductor material in ohm-circular-mils per foot (your problem will state it), I is the load current in amperes, L is the one-way length in feet, and CM is the conductor's cross-sectional area in circular mils.

Worked example 1.6: voltage drop. A 120-volt circuit carries 16 A to a load 150 feet away (one way) on a conductor of 10,380 circular mils, with K = 12.9.

• VD = (2 x 12.9 x 16 x 150) / 10,380
• Numerator: 2 x 12.9 = 25.8; 25.8 x 16 = 412.8; 412.8 x 150 = 61,920
• VD = 61,920 / 10,380 = 5.97 volts
• Percent = 5.97 / 120 x 100 = 4.97%

The load at the far end sees about 114 volts. The code's voltage-drop guidance is a recommendation in an informational note, not a mandatory rule; look up the exact percentages in your code book (210.19 and 215.2). A larger conductor, a shorter run, or a higher voltage is the fix.

The dwelling load calculation: standard method

A load calculation answers one question: how big must the service or feeder be? Each AHJ supplies a worksheet that follows the Article 220 steps. The steps below are the ones appearing on the City of Los Angeles, City of Yucaipa, Ventura County, and City of Dublin dwelling worksheets named in this chapter's sources.

1. General lighting. Habitable floor area in square feet x 3 VA/ft^2 (220.41 in the 2023 NEC, as cited on the City of St. Paul's 2023 NEC checklist; worksheets written to earlier editions cite 220.12).
2. Small-appliance branch circuits. At least two circuits at 1,500 VA each (220.52(A)).
3. Laundry branch circuit. One circuit at 1,500 VA (220.52(B)).
4. Total of lines 1 to 3.
5. Demand on that total (Table 220.42). The first 3,000 VA at 100%, the remainder at 35%.
6. Fixed appliances (disposal, dishwasher, water heater, and similar): add them up. If there are four or more, the total may be taken at 75% (220.53). The worksheets list typical values to use when nameplate data is not available.
7. Electric clothes dryer. 5,000 VA or the nameplate rating, whichever is larger (220.54).
8. Cooking equipment. Taken from Table 220.55 (Article 220 demand table for household cooking equipment). The City of Los Angeles worksheet enters the range at 8,000 VA or nameplate; the table's exact demand values depend on the number and rating of appliances, so look up the row in your code book rather than relying on a memorized figure.
9. Air conditioning versus heating. Noncoincident loads (220.60): include only the larger of the two, since they do not run at the same time.
10. Largest motor. Add 25% of the largest motor (430.24, as noted on the worksheets).
11. Other loads, such as an electric vehicle charger, spa, or pool equipment, at nameplate.
12. Total VA, then divide by the service voltage (240 volts on the City of Los Angeles worksheet) to get amperes, and choose a service rating at or above the result.

Worked example 1.7: standard-method service for a 2,000 ft^2 dwelling. Loads: a 4.5 kW water heater, 1.2 kW dishwasher, 0.9 kW disposal, a 5 kW dryer, 9 kW of central air conditioning (no electric heat), and a range for which the problem tells you to use 8,000 VA of demand. The problem identifies 1,200 VA as the largest motor load.

1. General lighting: 2,000 x 3 = 6,000 VA
2. Small-appliance: 2 x 1,500 = 3,000 VA
3. Laundry: 1,500 VA
4. Total: 6,000 + 3,000 + 1,500 = 10,500 VA
5. Demand: first 3,000 at 100% = 3,000; remaining 7,500 x 0.35 = 2,625; net = 5,625 VA
6. Fixed appliances: 4,500 + 1,200 + 900 = 6,600 VA. Three appliances, so no 75% factor: 6,600 VA
7. Dryer: 5,000 VA or nameplate; nameplate is 5,000: 5,000 VA
8. Range (given): 8,000 VA
9. Air conditioning: 9,000 VA (no heat, so nothing to compare)
10. 25% of largest motor: 0.25 x 1,200 = 300 VA

Total: 5,625 + 6,600 + 5,000 + 8,000 + 9,000 + 300 = 34,525 VA

Current at 240 V: 34,525 / 240 = 143.9 A. Choose the next standard service rating above 143.9 A; confirm the standard ratings in your code book.

Existing dwellings: the optional method for additions

The City of Dublin's worksheet applies a different optional calculation for existing dwellings under 220.83. It takes the first 8,000 VA at 100% and the remainder at 40%, adding air conditioning at 100% of nameplate. The worksheet is the source for those percentages. The optional method for a new dwelling is a separate section with its own breakpoints; the figure you see quoted for it should be checked in your code book before you rely on it. Exam trap: do not mix the two sections' breakpoints.

Worked example 1.8: existing-dwelling addition. An existing home is adding a room. After totaling the connected load of all general lighting, appliances, and the addition, the total comes to 38,000 VA (no air conditioning being added).

• First 8,000 VA at 100% = 8,000
• Remainder: 30,000 x 0.40 = 12,000
• Demand = 20,000 VA; at 240 V that is 20,000 / 240 = 83.3 A

Lighting circuit count

City of St. Paul's 2023 NEC rough-in checklist gives a quick method for the minimum number of lighting circuits: area served x 3 VA/ft^2, then divide by the circuit's VA (15 A x 120 V = 1,800 VA; 20 A x 120 V = 2,400 VA). A 15-amp circuit therefore covers 1,800 / 3 = 600 ft^2; a 20-amp circuit covers 2,400 / 3 = 800 ft^2. The checklist notes these are bare minimums and the circuit may not provide enough capacity.

Continuous loads

A continuous load is one where the maximum current is expected to continue for a long period (the exact duration is in the Article 100 definition; look it up in your code book). The code sizes the overcurrent device at 125% of the continuous load plus 100% of the noncontinuous load (210.20). The Texas Department of Transportation's design manual states the 125% rule for lighting loads, which are continuous.