Practice: ohm's law and power
Each problem uses new numbers, so you practice the method rather than memorizing an answer.
Pick the answer: sizes and ratings come from your code book.
Worked solution
Formula
| Symbol | Meaning | Unit |
|---|---|---|
| E | Voltage (electromotive force) | V |
| I | Current | A |
| R | Resistance | Ω |
| P | Power | W |
Worked example
A resistive heating element measures 17.6 Ω and is connected to a 208 V circuit. How much current does it draw?
- Ohm's law solved for current: I = E ÷ R.
- I = 208 V ÷ 17.6 Ω = 11.82 A.
Answer: 11.82 A
Ohm's law and power, explained
Ohm's law ties together the three basic quantities of a circuit. Voltage (E) pushes current (I) through a resistance (R), and E = I × R. If you know any two, you can find the third: I = E ÷ R and R = E ÷ I.
Power (P) is the rate at which a load turns electrical energy into heat or work. P = E × I, and by putting Ohm's law into that formula you also get P = I² × R and P = E² ÷ R. The power wheel is just these 12 formulas drawn in a circle, three for each of E, I, R and P.
These formulas are exact for resistive loads such as heaters, water heater elements and incandescent lamps. A resistive load keeps the same resistance when the voltage changes, so its wattage changes with the square of the voltage. Motors and other inductive loads also need power factor; see the power factor topic.
How to solve it on the exam
- Write down the two values the question gives you and the one it asks for.
- Pick the formula that uses exactly those values: E = I × R, I = E ÷ R, R = E ÷ I, P = E × I, P = I² × R or P = E² ÷ R.
- Convert units first: kW to W (× 1,000), kilohms to ohms, milliamps to amps.
- If a resistive load runs at a voltage other than its rating, find its resistance from the nameplate first, because the resistance does not change.
- Check that the answer makes sense: more voltage across the same resistance means more current and more power.
Common mistakes
- Using P = E × I on a motor or other inductive load without the power factor. These formulas are exact only for resistive loads.
- Assuming a heater keeps its nameplate wattage at a lower voltage. Power changes with the square of the voltage.
- Forgetting to square the current in P = I² × R.
- Mixing kW and W, or kilohms and ohms, in the same formula.
- Turning a formula upside down, such as R = I ÷ E instead of R = E ÷ I.
Exam tips
- Draw the power wheel on the notepad you get at the exam before you start the calculations part. Then each question is just picking the right slice.
- Keep intermediate results in your calculator's memory instead of rounding them, so your answer lines up with the choices.
Ohm's law and power calculator
Questions people ask
What is Ohm's law?
Ohm's law says the voltage across a resistance equals the current through it times the resistance: E = I × R. Rearranged, I = E ÷ R and R = E ÷ I.
What is the power wheel?
The power wheel is a chart of 12 formulas that link voltage (E), current (I), resistance (R) and power (P). Each quarter solves for one of the four values from any two of the others.
Why does a 240 V heater put out less heat at 208 V?
Its resistance stays the same, so its power drops with the square of the voltage. At 208 V a 240 V heater gives about 75% of its rated watts, because (208 ÷ 240)² = 0.751.
Is Ohm's law in the NEC?
No. Ohm's law is basic electrical theory, not a code rule. The NEC gives values you use with it, such as the conductor resistances in Chapter 9, Table 8 and the nominal voltages in 120.5 (220.5(A) in the 2023 NEC).
Sources
About the tables: The exam is open book. These problems cite the 2026 NEC section and table so you practice finding them in your own book. We never print NEC text or tables.