Practice: three-phase 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 |
|---|---|---|
| I | Line current | A |
| VA | Apparent power (kVA × 1,000) | VA |
| E | Line-to-line voltage | V |
| 1.732 | Square root of 3 |
Worked example
A balanced three-phase load of 57 kVA is supplied at 480 V, three-phase. What is the line current?
- Three-phase current: I = VA ÷ (E × 1.732).
- Convert to VA: 57 kVA × 1,000 = 57,000 VA.
- E × 1.732 = 480 × 1.732 = 831.36.
- I = 57,000 ÷ 831.36 = 68.6 A.
Answer: 68.6 A
Three-phase power, explained
A three-phase system carries power on three conductors whose voltages peak one third of a cycle apart. Because of that offset, the line-to-line voltage is 1.732 times the line-to-neutral voltage: 120 × 1.732 = 208 V and 277 × 1.732 = 480 V. The same factor shows up in the power formula, so a balanced three-phase load draws VA ÷ (E × 1.732) amps on each line.
On the exam, use the nominal voltages the NEC lists for calculations unless the question gives another value. For a three-phase load, use the line-to-line voltage (208, 240 or 480 V), never the line-to-neutral voltage.
The most common trap is a single-phase load fed from a three-phase panel, such as a 208 V water heater connected to two phases of a 208Y/120 V system. It is still a single-phase load, so its current is VA ÷ 208 with no 1.732. Use 1.732 only for loads connected to all three phases.
How to solve it on the exam
- Decide whether the load is three-phase (connected to all three phase conductors) or single-phase (two wires, even if they are two phases of a three-phase system).
- Convert kVA to VA or kW to W by multiplying by 1,000.
- For three-phase, multiply the line-to-line voltage by 1.732 first, then divide the VA by that number.
- For single-phase, divide the VA by the voltage the load is connected to.
- To go the other way, kVA = E × I × 1.732 ÷ 1,000 for three-phase, or E × I ÷ 1,000 for single-phase.
Common mistakes
- Leaving out 1.732 on a three-phase load.
- Using 1.732 on a single-phase load just because it is fed from a three-phase panel.
- Using 3 instead of 1.732, or multiplying by 1.732 when you should divide.
- Using the line-to-neutral voltage (120 or 277 V) in the three-phase formula.
- Forgetting to convert kVA to VA before dividing.
Exam tips
- Learn three products: 208 × 1.732 = 360.3, 240 × 1.732 = 415.7 and 480 × 1.732 = 831.4. Dividing the VA by one of them saves a step on most three-phase questions.
- If two answer choices differ by a factor of about 1.73, the question is testing whether you know when to use the square root of 3.
Three-phase power calculator
Questions people ask
How do you calculate three-phase amps from kVA?
Multiply kVA by 1,000 to get VA, then divide by the line-to-line voltage times 1.732. For example, 45 kVA at 208 V is 45,000 ÷ (208 × 1.732) = 124.9 A.
Why do you use 1.732 in three-phase calculations?
1.732 is the square root of 3. In a three-phase system the line-to-line voltage is 1.732 times the line-to-neutral voltage, so the power formula includes this factor.
Do you use 1.732 for a 208 V single-phase load?
No. A load connected between two phases of a 208Y/120 V system is single-phase, so its current is VA ÷ 208. Use 1.732 only for loads connected to all three phases.
What voltages should I use in exam calculations?
Use the nominal voltages the NEC lists for calculations, such as 120, 208, 240, 277 and 480 V, unless the question gives another value. In the 2026 NEC they are in 120.5; in the 2023 NEC, 220.5(A).
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.