Short answer: Ohm’s law says voltage equals current times resistance, E = I × R. Power is P = E × I, which also gives P = I² × R and P = E² ÷ R. In a series circuit the current is the same everywhere and the resistances add. In a parallel circuit the voltage is the same across every branch and the branch currents add. Most theory questions are one of these ideas plus careful arithmetic.
PSI lists electrical theory in two subject areas of the Texas Journeyman exam: Definitions, Theory and Plans has 3 scored NEC Knowledge questions, and Calculations and Theory has 2 scored Calculations questions. PSI does not say how many cover Ohm’s law alone, and the same formulas sit inside many other calculations. The Texas Journeyman Practice Pack has 1,000 original questions and 10 timed mock exams in the real two-part format, with step-by-step solutions for every calculation. Try free practice questions without signing up. Get the full pack for US$39 with one payment.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words. Check your work against your instructor’s method and follow your supervisor on the job.
The four formulas you use every day
| Find | Formula | Units |
|---|---|---|
| Voltage | E = I × R | volts |
| Current | I = E ÷ R | amperes |
| Resistance | R = E ÷ I | ohms |
| Power | P = E × I = I² × R = E² ÷ R | watts |
Pick the form that uses the two values the question gives you. If you know power and voltage, current is I = P ÷ E. If you know power and voltage and need resistance, R = E² ÷ P.
These formulas work directly for ideal resistive loads. Real heaters and incandescent lamps change resistance somewhat as they heat up. Motors, transformers and electronic loads add reactance and power factor, which the AC single-phase lesson covers.
Series circuits
In a series circuit there is one path for current.
- Current is the same through every part.
- Total resistance is the sum: R total = R1 + R2 + R3.
- Voltage drops add up to the source voltage. Each drop is I × R for that part.
- One open part stops the whole circuit.
Worked example. Three resistors of 10, 20 and 30 ohms are in series across 120 V.
- R total = 10 + 20 + 30 = 60 ohms.
- I = 120 ÷ 60 = 2 A, the same through each resistor.
- Drops: 2 × 10 = 20 V, 2 × 20 = 40 V, 2 × 30 = 60 V. They add to 120 V.
- Total power = 120 × 2 = 240 W.
Parallel circuits
In a parallel circuit each branch connects across the same two points.
- Voltage is the same across every branch.
- Branch currents add to the total current.
- Total resistance is less than the smallest branch. For two branches, R total = (R1 × R2) ÷ (R1 + R2).
- One open branch does not stop the others. That is why the receptacles and lights in a building are wired in parallel.
Worked example. A 20 ohm and a 30 ohm load are in parallel across 120 V.
- I1 = 120 ÷ 20 = 6 A. I2 = 120 ÷ 30 = 4 A.
- Total current = 6 + 4 = 10 A.
- R total = 120 ÷ 10 = 12 ohms. Check: (20 × 30) ÷ (20 + 30) = 600 ÷ 50 = 12 ohms.
- Total power = 120 × 10 = 1,200 W.
Worked example: a heater on the wrong voltage
A resistance heater is rated 4,800 W at 240 V. It is connected to 208 V. What is its output?
- Assume the element’s resistance stays constant for this calculation: R = 240² ÷ 4,800 = 57,600 ÷ 4,800 = 12 ohms.
- At 208 V: P = 208² ÷ 12 = 43,264 ÷ 12 = 3,605 W.
- Shortcut: power changes with the square of the voltage. (208 ÷ 240)² = 0.751, and 4,800 × 0.751 = about 3,605 W.
The voltage is about 13% lower, but the calculated power is about 25% lower. Exam questions that change the voltage on a resistive load usually expect you to hold resistance constant.
Mistakes that cost points
- Holding power constant when the voltage changes. For a constant-resistance load, power changes with the square of the voltage.
- Adding parallel resistances. Add currents in parallel, not resistances.
- Mixing kilowatts and watts. Convert to watts before you divide by volts.
- Using 120 V for a 240 V load. Read which voltage the load is connected across.
To check your numbers, use our Ohm’s law and power calculator and series and parallel calculator.
Check yourself
- A 1,500 W heater runs on 120 V. What current does it draw? 12.5 A, from I = P ÷ E.
- Two 10 ohm resistors are in parallel. What is the total resistance? 5 ohms.
- In a series circuit, which value is the same through every part? The current.
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Practice calculations with step-by-step lessons
Texas Journeyman Electrician Exam Prep 2026 NEC (paperback, 327 pages). 486 practice questions: 316 chapter questions across 20 chapters plus two full mock exams of 85 questions each, 59 NEC Knowledge in 130 minutes and 26 Calculations in 110 minutes. Includes calculation lessons and code lookup paths, and NEC questions come with article, section or table references.
FAQ
What is Ohm’s law in simple terms?
Voltage equals current times resistance. If you know any two of voltage, current and resistance, you can find the third.
What are the power formulas?
P = E × I, P = I² × R and P = E² ÷ R. All three give the same answer for a resistive load. Use the one that matches the values you have.
Why does a 240 V heater put out less heat on 208 V?
Assuming its resistance stays constant, power falls with the square of the voltage. A 4,800 W, 240 V heater gives about 3,605 W on 208 V.
Is Ohm’s law on the Texas Journeyman exam?
PSI lists electrical theory in Definitions, Theory and Plans (3 scored NEC Knowledge questions) and Calculations and Theory (2 scored Calculations questions). PSI does not say which formulas individual questions use, but these formulas are the base for load and conductor calculations.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
