Electrical Calculations for the California General B Contractor Exam

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12 questions · Audio-based · Study on the go

This content is produced by Pass The CSLB, an independent educational channel. I am not a licensed contractor, attorney, or engineer — this is exam preparation material only, based on publicly available CSLB study resources. Nothing here constitutes legal, professional, or engineering advice. Exam content is set by PSI and the CSLB and may change — always verify current requirements against official CSLB materials. I cannot guarantee any exam outcome. Now let's get into it.

This episode covers electrical calculations — specifically the ones that official CSLB preparation resources consistently identify as essential material for the General B exam. I am going to teach you Ohm's Law, Watt's Law, the 80% continuous load rule, the 125% conductor sizing formula, voltage drop limits at 3% and 5%, the 1,500 VA kitchen circuit baseline, demand factors, and the 1.732 multiplier for three-phase power systems. Every number I just listed must live in your memory on test day. This exam is strictly closed-book with a calculator provided by the testing center. The formulas are not provided. You bring them in your head, or you guess. Let's build them.

##CHAPTER_1##

I want to start somewhere that most exam prep courses never go — the history of why these units exist — because I have found that when you know who invented something and why they nearly failed, the formulas become memorable in a way that pure repetition never achieves.

The four units you will work with throughout this episode — volts, amperes, ohms, and watts — were named after four different men from four different countries working in roughly the same century. They never fully collaborated. They were each solving a different piece of the same puzzle without knowing the others had the remaining pieces.

Alessandro Volta was Italian, born in 1745. He invented the voltaic pile — the world's first battery — in 1799. Before Volta, there was no reliable way to produce a steady, controllable electrical current. Scientists could generate sparks, but they could not sustain a flow long enough to study it. Volta changed that. His voltaic pile gave every researcher who came after him the one thing they needed most: a predictable source of current to experiment with. The volt is named after him.

André-Marie Ampère was French, born in 1775. In the 1820s, Ampère demonstrated mathematically and experimentally that electrical current flowing through a wire produces a magnetic force around that wire. This relationship between electricity and magnetism became the foundation of electric motors, generators, and transformers — essentially everything in modern construction. The unit of electrical current — the ampere, shortened to amp — carries his name.

Georg Simon Ohm was German, born in 1789. He was a schoolteacher — not a celebrated researcher, not a man of wealth or institutional backing — just a schoolteacher who spent years investigating the relationship between voltage, current, and resistance using the most basic equipment available. In 1827, he published his findings in a book. The scientific establishment largely dismissed him. The German Minister of Education reportedly declared his work unworthy of a serious scientist. Ohm was so demoralized by the rejection that he resigned his teaching post. For 15 years, his discovery sat largely ignored and unrecognized.

Then in 1841, the Royal Society of London awarded him the Copley Medal — one of the highest scientific honors in the world at that time. Four years later, he finally received a university professorship. He died in 1854, finally recognized. The ohm, the unit of electrical resistance, was named after him in 1861 — more than 3 decades after he nearly gave up entirely.

I tell you that story because I want you to understand that the formula you are about to put on a flashcard survived its own creator almost abandoning it. That formula is real, it is tested, and a schoolteacher nearly lost it to discouragement.

James Watt was Scottish, born in 1736. Here is the twist — Watt never worked with electricity. His famous achievement was improving the steam engine in the 1760s and 1770s, and he invented the concept of horsepower as a sales tool to help mine operators understand how much work his machines could do compared to horses. But Watt established a framework for thinking about power as a rate of doing work over time — not just an amount of energy, but how fast that energy is delivered. Later scientists applied that framework to electrical systems. The watt — the unit of electrical power — was formally named in his honor in 1882. That was 63 years after James Watt died. He never heard the word used in an electrical context once in his life.

I find that genuinely remarkable. Every time you see the letter W in a power calculation, that is a Scottish steam engineer who has been gone since 1819. Every time you see the Greek letter omega for resistance, that is a German schoolteacher who almost disappeared from history. The units are not abstract symbols. They are the names of real people who were right about things the world was not ready to hear yet.

Now let me show you how these four quantities relate to each other mathematically.

Picture two triangles side by side. Both triangles operate by the same rule. To find a missing variable, you cover it with your thumb. If the variable you covered was at the top of the triangle, multiply the two bottom variables together. If the variable you covered was on the bottom, divide the top variable by the other bottom variable. That is the complete operating instruction for both triangles.

The first triangle is Watt's Law. Power — measured in watts — sits at the top. Voltage and current sit at the bottom. Here is the mnemonic I use for this one: Pigs Eat Insects. P on top, E and I on the bottom. Cover P and you multiply E times I. Cover E and you divide P by I. Cover I and you divide P by E.

The second triangle is Ohm's Law. Voltage sits at the top. Current and resistance sit at the bottom. My mnemonic for this one: Villains Ignore Rules. V on top, I and R on the bottom. Cover V and you multiply I times R. Cover I and you divide V by R. Cover R and you divide V by I.

A portable compressor on a job site is rated at 1,440 watts on a 120-volt outlet. Current draw: Watt's Law, cover I, divide P by E. 1,440 / 120 = 12 amps.

Reverse it: 12 amps on 120 volts, find wattage. Cover P, multiply E times I. 120 × 12 = 1,440 watts.

Ohm's Law scenario: a circuit reads 120 volts at 18 amps. What is the resistance? Cover R. Divide V by I. 120 / 18 = 6.67 ohms.

##CHAPTER_2##

Here is where pure math becomes insufficient and code knowledge becomes the deciding factor.

A standard thermal-magnetic circuit breaker contains a bimetallic strip — two different metals bonded together — through which the circuit current passes. As current flows, that strip heats up. The two metals expand at different rates, causing the strip to bend. If the current is high enough and sustained long enough, the strip bends far enough to trip the breaker and open the circuit. This is not instantaneous. It is cumulative.

The electrical code defines a continuous load as any load where the maximum current is expected to flow for 3 hours or more. Space heaters. Parking structure lighting. Commercial refrigeration. Exhaust fans running all day. When a load runs continuously at exactly a breaker's rated amperage, the thermal buildup inside that breaker is real, sustained, and builds toward the trip threshold over time.

The trigger number: a standard overcurrent protective device under continuous load conditions may only be loaded to 80% of its rated capacity. A 20-amp breaker can carry a maximum of 16 amps continuously. A 30-amp breaker: 24 amps. A 15-amp breaker: 12 amps. A 40-amp breaker: 32 amps.

I put a reference chart on screen showing common breaker sizes alongside their 80% continuous load limits and equivalent wattage at 120V and 240V. Notice how wattage at 240V is exactly double that at 120V — same amperage, double voltage, double power.

The inverse of this rule: if you know the continuous load and need to select a minimum breaker size, multiply the load by 1.25. 16 amps × 1.25 = 20-amp minimum breaker. One divided by 0.80 equals 1.25. Same rule, two directions. 80% going in — maximum load on an existing breaker. 1.25 going out — multiplier for sizing a new breaker.

##CHAPTER_3##

Most real circuits carry a combination of continuous and noncontinuous loads. The minimum conductor ampacity equals 100% of the noncontinuous load plus 125% of the continuous load.

The 1.25 multiplier applies only to the continuous portion. The noncontinuous portion is counted at full face value with no multiplier. This is exactly where the exam builds its wrong answers.

Worked example: a circuit supplies a continuous exhaust fan load of 14 amps and a noncontinuous receptacle load of 8 amps. Minimum conductor ampacity? Continuous: 14 × 1.25 = 17.5 amps. Noncontinuous: 8 × 1.0 = 8 amps. Total: 17.5 + 8 = 25.5 amps.

Wrong answers you will see: 22 amps (added without the multiplier), 27.5 amps (applied 1.25 to both loads), and variations from arithmetic errors. These are not random — they are the exact results of specific, predictable mistakes. Know the trap.

##CHAPTER_4##

Voltage drop is about distance, and distance is something a general contractor deals with on every project.

Picture a garden hose connected to a spigot at full pressure. A 10-ft hose delivers nearly full pressure at the nozzle. A 200-ft hose loses pressure along the way. Wire works identically. Every foot of copper conductor has measurable resistance. As current travels, voltage is consumed overcoming that resistance. The load at the far end receives less voltage than the panel sent.

When a motor receives lower voltage than designed, it draws additional current to maintain torque. More current means more heat. Heat degrades winding insulation. Eventually the motor fails — a warranty issue, a safety issue, and an angry client.

Code limits: voltage drop on any single branch circuit must not exceed 3% of source voltage. Combined voltage drop on feeders plus branch circuits must not exceed 5% total.

Calculation: multiply source voltage by 0.03 to find maximum allowable drop. Subtract from source voltage to find minimum terminal voltage.

120V circuit: 120 × 0.03 = 3.6V drop. Minimum terminal voltage: 116.4V. 240V circuit: 240 × 0.03 = 7.2V drop. Minimum terminal voltage: 232.8V.

Memorize those results directly. Having 116.4V and 232.8V ready before you touch the calculator saves time and mental energy on a timed exam.

##CHAPTER_5##

When kitchen small appliance circuits must be included in a service load calculation, the code mandates a statutory minimum of 1,500 VA per 2-wire small appliance branch circuit serving kitchen, pantry, or dining room countertops — regardless of what Watt's Law produces.

Two circuits: 2 × 1,500 = 3,000 VA. Four circuits: 4 × 1,500 = 6,000 VA. That is the number for the service calculation. Not the physics. The code minimum.

The reason: kitchens are unpredictable load environments. Toaster, microwave, coffee maker, and blender running simultaneously is a realistic scenario. Rather than enumerate every combination, the code sets a floor. The exam exploits the fact that physics-first thinking (20A × 120V = 2,400 VA per circuit) produces a number that will appear as a wrong answer choice.

##CHAPTER_6##

Demand factors acknowledge that not everything in a building runs simultaneously. Certain load categories can be reduced mathematically when computing total service requirements — preventing expensive oversizing of electrical services.

However, electric space heating receives no demand factor reduction. When outdoor temperatures hit seasonal extremes, every heater runs at full capacity simultaneously. The code mandates 100% of nameplate capacity for electric heating loads in all service calculations. No reduction. No probability discount.

If the nameplate says 12 kW, the service calculation includes 12 kW. The exam will offer 75%, 80%, and other percentages as options for heating loads. Every one of them is wrong. 100%, every time.

##CHAPTER_7##

In the 1880s, Thomas Edison had built his empire on DC power. Nikola Tesla — a Serbian-American inventor who had briefly worked for Edison before a bitter falling out — partnered with George Westinghouse to promote alternating current. The decisive physics advantage of AC: you can step voltage up with a transformer, transmit it hundreds of miles with minimal loss, and step it back down at the destination. Edison's DC system required a generating station every mile. Tesla and Westinghouse's AC system could power a region from a single facility.

Edison fought back publicly, arranging demonstrations where animals were electrocuted with AC power to associate it with danger in the public mind. Physics won anyway. Westinghouse's AC system powered the 1893 Chicago World's Fair — widely recognized as the moment AC permanently defeated DC.

Three-phase AC uses three separate waves, each offset by 120 degrees. The combined power delivery is smoother and more efficient than single-phase — which is why every commercial building runs on it.

The mathematical consequence of that 120-degree offset: to calculate power in a three-phase system, you must multiply by the square root of 3, which is approximately 1.732. The formula: P = 1.732 × V × I × PF. At a power factor of 1.0: P = 1.732 × V × I.

I put the single-phase versus three-phase comparison on screen here. Look at both formulas side by side, the constant used in each, and the common voltages. The trigger for the testing room: three-phase — multiply by 1.732. Single-phase — do not.

##CHAPTER_8##

Understanding why the 80% rule exists is valuable. But what saves you at the testing desk is having 8 specific numbers memorized so completely that when you read a scenario, you know within seconds which number applies.

Eight trigger numbers. Let me read them.

1.25 — the multiplier for continuous loads when sizing conductors. Apply only to the continuous portion, then add the noncontinuous load at 100%.

0.80 — the maximum loading percentage for a breaker under continuous load. Breaker rating × 0.80 = maximum continuous load allowed.

3% — maximum voltage drop on a single branch circuit. At 120V: 3.6V drop, minimum terminal voltage 116.4V. At 240V: 7.2V drop, minimum terminal voltage 232.8V.

5% — maximum combined voltage drop on feeders plus branch circuits together, from source panel to farthest outlet.

1,500 VA — the minimum value per 2-wire small appliance branch circuit in kitchens for service load calculations. Use this number, not Watt's Law.

100% — the demand factor for electric space heating and continuous refrigeration. Full nameplate, no reduction, always.

1.732 — the square root of 3, required in every three-phase power calculation.

3 hours — the time threshold defining a continuous load, which triggers both the 1.25 conductor sizing rule and the 0.80 breaker loading limit.

I call these the Eight Pillars. Forget one and the question built on it becomes a guess. Keep all eight and the electrical section becomes a strength on your exam.

One final note: official preparation resources consistently note that the General B exam integrates blueprint reading with math. You may be handed laminated construction drawings and asked to count fixtures, read panel schedules, and verify conductor sizing. The math is exactly what was covered today. The added skill is extracting the right numbers from a drawing quickly. Practice reading electrical plan drawings and identifying circuit and panel schedule data.

Here is what I want you to do right now. There is an audio practice quiz built specifically for this episode — every concept I covered today has questions attached to it. It is audio-based, meaning the questions are read aloud and you answer by tapping on your phone. That format was designed for people studying on the go — driving to a job site, eating lunch in the truck. Go to the description below this video. You will see a link that says PassTheCSLB. Tap it. It will take you straight there.

If anything I covered today raised a question — drop it in the comments below. I read every one and I respond.

And if you are not subscribed yet, do it now. Every trade, every code section, every calculation domain you need for the General B exam has its own episode in this series. Subscribe and stay on track. I will see you in the next one.