For electrical exam calculations, start with I = V ÷ R for a resistive circuit, I = VA ÷ V for single phase, and I = VA ÷ (1.732 × V) for a balanced three-phase load using line-to-line voltage. First identify the circuit and units. Then calculate the current before applying NEC conductor or protection rules.
The formulas you’ll use most
Use V for volts, I for amperes, R for ohms, P for watts and S for volt-amperes. Power factor, PF, is a decimal. The AC power formulas below assume sinusoidal voltage and current. The three-phase formulas also require a balanced load.
| Find | Formula | Use it for |
|---|---|---|
| Current from voltage and resistance | I = V ÷ R | DC or a purely resistive circuit |
| Resistive power | P = V × I = I² × R = V² ÷ R | DC or a purely resistive AC load |
| Single-phase apparent power | S = V × I | Volt-amperes |
| Single-phase real power | P = V × I × PF | Watts |
| Balanced three-phase current from kVA | I = (kVA × 1,000) ÷ (√3 × VLL) | Line current, using line-to-line voltage |
| Balanced three-phase current from watts | I = P ÷ (√3 × VLL × PF) | Line current, using electrical input watts |
Use approximately 1.732 for √3. VLL means line-to-line voltage. In a 208Y/120-volt system, that is 208 volts, not 120.
Watch: volts, amps, ohms and watts
Review the electrical terms with Victor Ring, then work the two examples below. Keep track of the units before choosing a formula.
Electrical Key Concepts Intro for NEC Exam Prep | JCR ProductionsWatch JCR’s lesson on YouTube ↗
Example 1: a resistance load
A 240-volt resistance load has 24 ohms of resistance. What current does it draw?
- I = V ÷ R
- I = 240 ÷ 24
- I = 10 amperes
Its power is 240 × 10 = 2,400 watts. You can check with V² ÷ R: 240² ÷ 24 also equals 2,400 watts.
The 10-amp answer is load current. If the question asks for a conductor or protective-device rating, you still need the applicable NEC rules, including any continuous-load requirement.
Practice electrical calculations with JCR
Learn to find answers in the code book, work through calculations and practice for test day.
Live classes teach the 2020 NEC. Check your state’s exam requirements before choosing a course. NC prep includes NC business and law. For another state, also study any business-and-law material required for your license. JCR carries the 2020 NEC softbound code book.
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Choose in-person classes, live Zoom or recorded courses. Review NEC navigation, grounding and bonding, and other electrical topics. Check the course’s NEC edition, state approval and credit hours before enrolling.
See CE courses and class datesExample 2: a balanced three-phase load
A balanced load is rated 24 kVA at 208 volts, three phase. Find line current.
- Convert kVA: 24 × 1,000 = 24,000 VA.
- Use line-to-line voltage: 208 V.
- I = 24,000 ÷ (1.732 × 208).
- I ≈ 66.6 amperes.
The 24 kVA rating already gives apparent power, so do not divide by power factor again. If the problem instead gives 24 kW of electrical input and a power factor of 0.80, the current is 24,000 ÷ (1.732 × 208 × 0.80), or about 83.3 amperes. Watts and volt-amperes give different answers when power factor is below 1.
Where horsepower causes mistakes
One mechanical horsepower is about 746 watts of output. A motor needs more electrical input power than its mechanical output because it has losses. A calculation from output horsepower may require both efficiency and power factor.
For motor conductor and branch-circuit short-circuit protection calculations, use the full-load current tables and exceptions in 2020 NEC 430.6(A)(1). For overload protection, check the motor’s nameplate current and 430.32. Our 2020 NEC motor breaker example and overload example show the distinction.
Check these before choosing an answer
- Did you convert kW or kVA to watts or VA?
- Is the voltage line-to-line or line-to-neutral?
- Does the problem require power factor or efficiency?
- Did you calculate current, or the final conductor or breaker size?
- Did you round only after finishing the calculation?
For the physical relationships behind these formulas, the U.S. Department of Energy’s electrical science handbook is a useful reference.
Common questions
Can I bring this formula sheet into my exam?
Only if your exam expressly permits it. Many programs prohibit loose notes. Practice the formulas and follow the current reference rules.
Is watts divided by volts always amperes?
It works for DC and purely resistive single-phase loads. Other AC power problems can require power factor and, for balanced three phase, √3.
Examples use the 2020 NEC. References checked October 25, 2026.
