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BUS-APPL73-02 · Lesson 2 · 12 clock hours · theory

DC and AC Circuit Theory Applied to Appliances

Objective. Students will apply Ohm's Law and series/parallel circuit rules to calculate expected voltage, current, and resistance values in appliance circuits and explain the difference between 120 V and 240 V residential supply configurations.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a credentialed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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DC and AC Circuit Theory Applied to Appliances

Every appliance—whether a dishwasher, electric dryer, or microwave—is an electrical circuit. Before you can diagnose a fault, you need to predict what the normal voltages, currents, and resistances should be. That prediction comes from two foundational tools: Ohm's Law and the rules for series and parallel circuits.

Ohm's Law and the Power Formula

Ohm's Law states that voltage, current, and resistance are always related by:

> V = I × R

where V is voltage in volts (V), I is current in amperes (A), and R is resistance in ohms (Ω). Rearranged: I = V ÷ R and R = V ÷ I.

The power formula connects electrical power to voltage and current:

> P = V × I

Power is measured in watts (W). Because V = IR, you can also write P = I²R or P = V²÷R. Appliance nameplates list wattage and voltage, so you can solve for current even before you touch a meter.

Example: A dryer heating element is rated 240 V / 5,400 W. Current = P ÷ V = 5,400 ÷ 240 = 22.5 A. That single number tells you what size wire and breaker the circuit needs.

Series Circuits

In a series circuit, all components share one current path. The same current flows through every component, and the voltages across each component add up to the supply voltage:

> V_total = V₁ + V₂ + V₃ …

Total resistance is simply the sum of all resistances: R_total = R₁ + R₂ + R₃ …

In appliances, a thermostat, a safety thermal fuse, and a heating element are often wired in series. If any one of them opens (breaks), current stops everywhere in that path—the entire function goes dead. This is a critical diagnostic clue: no heat can mean a failed element or a failed thermostat or a blown thermal fuse.

Parallel Circuits

In a parallel circuit, each branch has its own path back to the source. Every branch sees the full supply voltage. The total current is the sum of the branch currents:

> I_total = I₁ + I₂ + I₃ …

Total resistance is lower than any individual branch: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ …

In a household circuit panel, all outlets and appliance circuits are wired in parallel. An electric range may also have its surface burner elements wired in parallel so that one failed element does not knock out the others.

120 V vs. 240 V Residential Supply

North American residential service delivers a 120/240 V split-phase supply. The service panel receives three conductors from the utility transformer: two ungrounded (hot) legs and one grounded neutral. Each hot leg is 120 V to neutral. Because the two legs are 180° out of phase with each other, measuring from hot leg to hot leg gives 240 V.

A 240 V appliance that reads only 120 V on one leg is a common fault: one hot leg is open at the breaker, the connector, or inside the appliance terminal block.

Fuse and Breaker Sizing — The 125% Rule

Standard wiring practice requires that a branch circuit breaker or fuse protecting a continuous load be rated at no less than 125% of the load's full-load current. A continuous load is one expected to operate for three hours or more.

> Minimum breaker size = Full-load current × 1.25

Always round up to the next standard breaker size (standard sizes include 15, 20, 25, 30, 40, 50 A, and others). Undersizing causes nuisance trips; oversizing removes protection from the wiring.

Measuring Safely

Before measuring resistance or continuity, de-energize the appliance and unplug it (or complete lockout/tagout if it is hard-wired). Measuring resistance on a live circuit will damage your meter and can injure you. When measuring live voltage, keep one hand in your pocket or behind your back to reduce the path current takes through your body. Wear safety glasses whenever working near energized terminals. Use a meter rated for the voltage class of the circuit you are testing. Never probe a 240 V terminal with a meter rated only for 150 V CAT II or lower.

Putting It Together on the Bench

When you pick up a service call, start with the nameplate: note the supply voltage and wattage (or current). Calculate expected current and resistance before you probe anything. Then measure and compare. A heater element that reads infinite resistance is open. One that reads near zero is shorted. A 30% deviation from the calculated value in a resistive load is worth investigating further. These habits—predict, then measure—are the foundation of efficient appliance diagnostics.

Key terms8

Ohm's LawThe relationship V = IR, stating that the voltage across a conductor equals the current through it multiplied by its resistance.
Series circuitA circuit in which all components share a single current path; the same current flows through every component, and individual voltages add up to the supply voltage.
Parallel circuitA circuit in which components are connected across the same two points so each branch sees the full supply voltage; total current equals the sum of branch currents.
Split-phase supplyThe standard North American residential electrical service that provides 120 V from either hot leg to neutral and 240 V between the two hot legs.
Power formulaP = VI; electrical power in watts equals voltage in volts multiplied by current in amperes.
125% ruleStandard wiring practice requirement that a branch circuit overcurrent device protecting a continuous load be rated at no less than 125% of the load's full-load current.
Voltage dropThe reduction in voltage measured across a component or conductor as current flows through its resistance, calculated as V = IR.
Nameplate ratingThe manufacturer-specified electrical data (voltage, wattage, current, frequency) marked on an appliance; used as the baseline for diagnostic calculations.

Worked examples4

1. A dishwasher heating element has a measured resistance of 16 Ω and is connected to a 120 V supply. What current does it draw, and what is its power consumption?
  1. Identify the known values: V = 120 V, R = 16 Ω.
  2. Apply Ohm's Law to find current: I = V ÷ R = 120 ÷ 16 = 7.5 A.
  3. Apply the power formula: P = V × I = 120 × 7.5 = 900 W.
Answer: The element draws 7.5 A and consumes 900 W.
2. An electric dryer nameplate reads 240 V / 5,400 W. What is the full-load current, and what is the minimum standard breaker size using the 125% rule?
  1. Find full-load current using the power formula rearranged: I = P ÷ V = 5,400 ÷ 240 = 22.5 A.
  2. Apply the 125% rule: Minimum breaker size = 22.5 × 1.25 = 28.125 A.
  3. Round up to the next standard breaker size above 28.125 A, which is 30 A.
Answer: Full-load current is 22.5 A; the minimum standard breaker size is 30 A.
3. Two resistors, R₁ = 40 Ω (representing a switch contact and wiring) and R₂ = 160 Ω (representing a heating element), are wired in series across a 6 V battery pack. What is the voltage drop across each component?
  1. Find total resistance: R_total = R₁ + R₂ = 40 + 160 = 200 Ω.
  2. Find circuit current: I = V ÷ R_total = 6 ÷ 200 = 0.03 A.
  3. Find voltage drop across R₁: V₁ = I × R₁ = 0.03 × 40 = 1.2 V.
  4. Find voltage drop across R₂: V₂ = I × R₂ = 0.03 × 160 = 4.8 V.
  5. Verify: V₁ + V₂ = 1.2 + 4.8 = 6 V ✓
Answer: Voltage drop across R₁ (switch) = 1.2 V; voltage drop across R₂ (element) = 4.8 V.
4. A refrigerator defrost heater (R = 50 Ω) and a fan motor winding (R = 200 Ω) are wired in parallel across 120 V. What is the total current drawn from the supply?
  1. Find current through the defrost heater: I₁ = V ÷ R₁ = 120 ÷ 50 = 2.4 A.
  2. Find current through the fan motor: I₂ = V ÷ R₂ = 120 ÷ 200 = 0.6 A.
  3. Find total current: I_total = I₁ + I₂ = 2.4 + 0.6 = 3.0 A.
Answer: The circuit draws a total of 3.0 A from the 120 V supply.

The math in this lesson1

Learners apply I = V ÷ R and P = V × I to find 7.5 A and 900 W for a dishwasher element.
A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor lecture with whiteboard examples: Ohm's Law (V = IR), power formula (P = VI), series vs. parallel behavior; students work ten practice problems drawn from real appliance spec sheets—for example, calculating heater element current draw from resistance and supply voltage. · Circuit simulation exercise using a DC breadboard kit: students build a series circuit (two resistors representing a switch and a heating element), measure voltage drop across each component, and record results in a data table; they then add a parallel branch and observe how current distribution changes. · Worksheet: students are given the nameplate data from a residential electric dryer (240 V, rated wattage) and a dishwasher heating element resistance value; they calculate expected current, identify the correct fuse or breaker size using the 125% rule from standard wiring practice, and explain why a 240 V circuit requires two hot legs.
Materials: Whiteboard and markers, DC breadboard kits with resistors and jumper wires, 6 V battery packs, digital multimeters, appliance nameplate data sheets (dryer, dishwasher, range), Ohm's Law reference cards, student calculation worksheets
Check: Students submit the ten Ohm's Law problems and the dryer/dishwasher worksheet; pass requires correct answers on at least 8 of 10 calculation problems and correct identification of both fuse sizes with written justification — 80% score.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Electrical fundamentals / PSA Certified Appliance Professional
1. A technician measures 18 Ω across a dryer heating element and the supply is 240 V. What current does the element draw?
Electrical fundamentals / PSA Certified Appliance Professional
2. In a series circuit containing a thermal fuse, a thermostat, and a heating element, the thermal fuse opens. What happens?
Electrical fundamentals / PSA Certified Appliance Professional
3. A residential electric range requires a 240 V circuit. Which of the following correctly describes how 240 V is obtained from a standard North American split-phase service panel?
Electrical fundamentals / PSA Certified Appliance Professional
4. An electric dryer draws 24 A at full load. Using the 125% rule, what is the minimum overcurrent device rating, and which standard size should be selected?
Safety / PSA Certified Appliance Professional
5. A technician is about to measure the resistance of a dishwasher heating element. What must be done before taking the measurement?
Electrical fundamentals / PSA Certified Appliance Professional
6. Two appliance loads are connected in parallel across 120 V. Load A draws 5 A and Load B draws 3 A. What is the total current from the source?
Electrical diagnostics / PSA Certified Appliance Professional
7. A technician finds that an electric range surface burner operates normally but the oven bake element does not heat. The supply is 240 V. After measuring, one hot leg reads 120 V to neutral and the other hot leg reads 0 V to neutral. What is the most likely cause?
Electrical fundamentals / PSA Certified Appliance Professional
8. Which statement best explains why a higher-resistance component in a series circuit has a larger voltage drop than a lower-resistance component?
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