4.4 Basic Electricity, Ohm's Law & Electrical Power for HVAC

Alabama HVAC Contractor License Exam

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1 Alabama HACR Laws, Rules & Licensing

1.1 Alabama HACR Board Authority, Licensing Pathways & Experience Requirements 1.2 Contractor Bonding, Insurance, Fees & Annual Renewal 1.3 Permits, Local Jurisdictions, Inspections & Disciplinary Actions

2 Contractor Business, Contracts, Estimating & Alabama Lien Law

2.1 Business Organization, Risk Management & Project Administration 2.2 Construction Contracts, Change Orders & Warranties 2.3 Alabama Mechanics' Lien Law & Financial Management 2.4 Construction Estimating, Markup & Bid Preparation 2.5 Project Management, Scheduling & Field Supervision 2.6 Risk Management, Contractor Insurance & Surety Bonds

3 Safety Standards, OSHA & EPA Section 608

3.1 OSHA Construction Safety (29 CFR 1926) for HVAC Contractors 3.2 EPA Section 608 Refrigerant Regulations & Recovery Requirements 3.3 Refrigerant Management, Leak Repair Rules & A2L Transition

4 General Technical Knowledge & Thermodynamics

4.1 Principles of Heat Transfer, Temperature & Pressure 4.2 The Mechanical Refrigeration Cycle & Refrigerant Properties 4.3 Psychrometrics & Air Properties for Humid Climates 4.4 Basic Electricity, Ohm's Law & Electrical Power for HVAC

5 Piping Systems, Refrigerant Lines & Fuel Gas Piping

5.1 Refrigerant Piping Sizing, Materials & Installation 5.2 Fuel Gas Piping Sizing & Installation (IRC Chapter 24) 5.3 Condensate Drain Systems & Trapping

6 System Sizing & ACCA Manual J Load Calculations

6.1 ACCA Manual J Fundamentals & Design Conditions 6.2 Envelope Loads: Conduction, Radiation & Fenestration 6.3 Infiltration, Ventilation & Internal Heat Gains

7 Duct System Design & ACCA Manual D

7.1 Airflow Principles, Static Pressure & Fan Performance 7.2 ACCA Manual D Duct Sizing & Friction Rate 7.3 Duct Materials, Installation Standards & Air Distribution

8 Heating & Cooling Equipment Operation

8.1 Heat Pump Systems & Supplemental Heating 8.2 Gas Furnaces & Combustion Systems 8.3 Air Conditioning Units, Compressors & Metering Devices

9 Mechanical Codes, Electrical Applications & Maintenance Diagnostics

9.1 Combustion Air & Venting Systems (IRC Chapter 24) 9.2 Electrical Applications for HVAC (NFPA 70 / NEC Article 440) 9.3 Preventative Maintenance, Troubleshooting & Plan Reading

10 Employment Law, Payroll Taxes & Contractor Finance

10.1 Federal & Alabama Labor Laws for HVAC Employers 10.2 Payroll, Employment Taxes & Worker Classification 10.3 Job Costing, Break-Even Analysis & Cash Flow Forecasting

11 Preventative Maintenance, Service Procedures & Commissioning

11.1 Preventative Maintenance Programs & Service Agreements 11.2 Filter, Coil & Drain Maintenance for Alabama's Humid Climate 11.3 Evacuation, Dehydration & Refrigerant Charging Procedures 11.4 Airflow Verification, Static Pressure Testing & Commissioning

12 Electrical Applications & Construction Documents

12.1 Electric Motors, Compressors & Motor Protection 12.2 Transformers, Control Circuits & Low-Voltage Wiring 12.3 Branch Circuits, Conductors, Grounding & Disconnects 12.4 Construction Documents, Specifications & Submittals

4.4 Basic Electricity, Ohm's Law & Electrical Power for HVAC

Key Takeaways Explain This Topic AI

Ohm's Law states that voltage equals current times resistance, and the three power forms P = VI, P = I squared R, and P = V squared / R all follow from it.

Electric resistance heat converts energy at 3,412 Btu per hour for every kilowatt, which is why a 15 kW strip bank produces 51,180 Btu/h and draws 62.5 A at 240 volts.

Series circuits share one current and divide voltage; parallel circuits share one voltage and divide current, which is why every load in an HVAC control circuit is wired in parallel.

Three-phase power is calculated as the square root of 3 times line voltage times line current times power factor, roughly 1.732 times the single-phase product.

NEMA limits motor voltage unbalance to 1 percent, and a 2 percent unbalance can raise winding temperature enough to shorten motor life sharply.

Last updated: September 2026

4.4 Basic Electricity, Ohm's Law & Electrical Power for HVAC

Note

Why this belongs in general knowledge : Roughly one exam question in ten on the Alabama HAC examination is electrical, and the ten Electrical Applications items assume the fundamentals in this section are already fluent. Every strip-heat calculation, capacitor test, motor amperage check, and minimum circuit ampacity problem reduces to Ohm's Law and the power formulas.

The Four Quantities

• Quantity Symbol Unit Water analogy Meter
• Voltage (electromotive force) E or V volt (V) Pressure Voltmeter, in parallel
• Current I ampere (A) Flow rate Ammeter or clamp meter, in series or around one conductor
• Resistance R ohm Pipe friction Ohmmeter, on a de-energized circuit only
• Power P watt (W) Work performed Wattmeter or calculated

Ohm's Law E = I × R I = E R R = E I E = I \times R \qquad I = \frac{E}{R} \qquad R = \frac{E}{I} E = I × R I = R E ​ R = I E ​

The Power Wheel P = E × I P = I 2 × R P = E 2 R P = E \times I \qquad P = I^2 \times R \qquad P = \frac{E^2}{R} P = E × I P = I 2 × R P = R E 2 ​

Worked example : a 240-volt electric heating element measures 9.6 ohms.

Current: I = 240 / 9.6 = 25 A I = 240 / 9.6 = 25\text{ A} I = 240/9.6 = 25 A

Power: P = 240 × 25 = 6,000 W = 6 kW P = 240 \times 25 = 6{,}000\text{ W} = 6\text{ kW} P = 240 × 25 = 6 , 000 W = 6 kW

Heat output: 6 kW × 3,412 = 20,472 Btu/h 6\text{ kW} \times 3{,}412 = 20{,}472\text{ Btu/h} 6 kW × 3 , 412 = 20 , 472 Btu/h

The constant 3,412 Btu/h per kilowatt is worth memorizing; it converts any electric resistance heat rating to a thermal load and appears repeatedly on the exam.

• Series and Parallel Circuits
• Series

One path. The same current flows through every component, resistances add, and voltage divides in proportion to resistance. R T = R 1 + R 2 + R 3 I T = I 1 = I 2 = I 3 R_T = R_1 + R_2 + R_3 \qquad I_T = I_1 = I_2 = I_3 R T ​ = R 1 ​ + R 2 ​ + R 3 ​ I T ​ = I 1 ​ = I 2 ​ = I 3 ​

Safety devices are wired in series with the load precisely so that opening any one of them stops all current - the high-limit switch, rollout switch, pressure switch, and door switch on a furnace form a series safety string.

Parallel

Multiple paths. Every branch sees the same voltage, currents add, and total resistance is always less than the smallest branch resistance . 1 R T = 1 R 1 + 1 R 2 + 1 R 3 I T = I 1 + I 2 + I 3 \frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} \qquad I_T = I_1 + I_2 + I_3 R T ​ 1 ​ = R 1 ​ 1 ​ + R 2 ​ 1 ​ + R 3 ​ 1 ​ I T ​ = I 1 ​ + I 2 ​ + I 3 ​

Loads - contactor coils, transformers, blower motors, condenser fan motors - are wired in parallel so that each receives full voltage and any one can operate independently.

Two-resistor shortcut : R T = R 1 × R 2 R 1 + R 2 R_T = \dfrac{R_1 \times R_2}{R_1 + R_2} R T ​ = R 1 ​ + R 2 ​ R 1 ​ × R 2 ​ ​ . Two 20-ohm branches in parallel give 400 / 40 = 10 400 / 40 = 10 400/40 = 10 ohms.

Alternating Current, Phase and Power Factor

North American utility power alternates at 60 hertz - the sine wave completes 60 full cycles per second, reversing polarity 120 times.

Meters read root-mean-square (RMS) values. A nominal 240 V RMS sine wave has a peak of 240 × 1.414 = 339 240 \times 1.414 = 339 240 × 1.414 = 339 V, which is why a 240-volt circuit stresses insulation to roughly 340 volts.

Single-phase residential service is normally 120/240 V; three-phase commercial service is commonly 208Y/120 V, 480Y/277 V, or 240 V delta.

Power in AC Circuits

• Quantity Formula (single phase) Formula (three phase)
• Apparent power (VA) E × I E \times I E × I 1.732 × E L × I L 1.732 \times E_L \times I_L 1.732 × E L ​ × I L ​

True power (watts) E × I × P F E \times I \times PF E × I × PF 1.732 × E L × I L × P F 1.732 \times E_L \times I_L \times PF 1.732 × E L ​ × I L ​ × PF

Power factor P F = watts volt-amperes PF = \dfrac{\text{watts}}{\text{volt-amperes}} PF = volt-amperes watts ​ same

Purely resistive loads such as electric heat have a power factor of 1.0, so watts equal volt-amperes. Motors are inductive: current lags voltage and the power factor falls to roughly 0.80 to 0.90, so a motor drawing 10 A at 240 V consumes 2,400 VA but only about 2,040 watts of true power.

Three-phase example : a 480-volt, three-phase compressor draws 22 A at a 0.85 power factor. P = 1.732 × 480 × 22 × 0.85 = 15,549 watts ≈ 15.5 kW P = 1.732 \times 480 \times 22 \times 0.85 = 15{,}549\text{ watts} \approx 15.5\text{ kW} P = 1.732 × 480 × 22 × 0.85 = 15 , 549 watts ≈ 15.5 kW

Voltage Unbalance: A Motor Killer

Three-phase voltage unbalance produces disproportionate current unbalance and rapid winding heating. % Unbalance = Maximum deviation from average voltage Average voltage × 100 \%\text{ Unbalance} = \frac{\text{Maximum deviation from average voltage}}{\text{Average voltage}} \times 100 % Unbalance = Average voltage Maximum deviation from average voltage ​ × 100

Worked example : measured line voltages are 462, 476, and 469 V. The average is ( 462 + 476 + 469 ) / 3 = 469 (462 + 476 + 469)/3 = 469 ( 462 + 476 + 469 ) /3 = 469 V. The maximum deviation is 476 − 469 = 7 476 - 469 = 7 476 − 469 = 7 V. Unbalance is 7 / 469 × 100 = 1.49 % 7 / 469 \times 100 = 1.49\% 7/469 × 100 = 1.49% .

NEMA recommends operating motors at no more than 1% voltage unbalance .

Current unbalance runs roughly 6 to 10 times the voltage unbalance percentage.

At 2% voltage unbalance a motor should be derated; at 5% it should not be operated at all.

Measuring Safely

Voltage is measured in parallel with the circuit, under power. Resistance is measured only with power off and with capacitors discharged - an ohmmeter applied to a live circuit is destroyed and can injure the technician.

A clamp ammeter reads the magnetic field around a single conductor. Clamping both conductors of a 240-volt circuit reads zero, because the opposing fields cancel.

Meter category (CAT) ratings describe how much transient energy the meter can survive: CAT II at the receptacle, CAT III at the distribution panel and fixed equipment, CAT IV at the service entrance and utility connection. Use a CAT III 600 V or better meter at HVAC disconnects and panels, and confirm the leads carry the same rating as the meter.

Always apply the live-dead-live test: prove the meter on a known live source, test the target circuit, then prove the meter again.