What the result means
Current depends on power, voltage, power factor and whether the circuit is single- or three-phase. Higher voltage means less current for the same power, which is why large equipment runs at 240, 208 or 480 V.
For motors, horsepower is output. The calculator converts to input watts using efficiency. Real motor current varies with load, so for circuit sizing use the nameplate FLA and the tables in your electrical code.
How it's calculated
- Single-phase: I = W ÷ (V × PF)
- Three-phase: I = W ÷ (1.732 × V × PF)
- From HP: W = HP × 746 ÷ efficiency
Worked example
A 5,000 W water heater element on 240 V single-phase.
- I = 5,000 ÷ (240 × 1.0)
- I = 20.83 A
The element draws about 20.83 A. As a continuous load, check the circuit against your code's continuous-load rules.
Reference table
The values below are computed from the same calculator functions used above. They let you compare common sizes and operating points without replacing the interactive calculation or its assumptions.
| Watts | 120 V | 240 V |
|---|---|---|
| 500 | 4.17 | 2.08 |
| 1,000 | 8.33 | 4.17 |
| 1,500 | 12.50 | 6.25 |
| 2,000 | 16.67 | 8.33 |
| 2,500 | 20.83 | 10.42 |
| 3,000 | 25.00 | 12.50 |
| 3,500 | 29.17 | 14.58 |
| 4,000 | 33.33 | 16.67 |
| 4,500 | 37.50 | 18.75 |
| 5,000 | 41.67 | 20.83 |
| 5,500 | 45.83 | 22.92 |
| 6,000 | 50.00 | 25.00 |
| 6,500 | 54.17 | 27.08 |
| 7,000 | 58.33 | 29.17 |
| 7,500 | 62.50 | 31.25 |
| 8,000 | 66.67 | 33.33 |
| 8,500 | 70.83 | 35.42 |
| 9,000 | 75.00 | 37.50 |
| 9,500 | 79.17 | 39.58 |
| 10,000 | 83.33 | 41.67 |
Reference values are examples based on the stated assumptions. Verify manufacturer data, conductor ampacity and local requirements before using a result.
Sources and assumptions
The listed formulas use the assumptions shown in the worked example. Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box. Where a code table, manufacturer specification or field measurement is required, use the applicable local source.
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FAQ
What power factor should I use?
1.0 for heaters and incandescent lighting. Motors and electronics are lower; use nameplate or measured values.
Can I size a motor circuit with this?
No. Motor circuits are sized from nameplate or code-table FLA, not from horsepower math.
Why does the HP result depend on efficiency?
746 W per HP is output power. The motor draws more than that from the line because of losses.
Is a 240 V load single- or three-phase?
Residential 240 V is single-phase. Three-phase is typical in commercial buildings at 208 or 480 V.
Using a reference result in the field
A calculator is most useful when the inputs match the job being checked. Where a field asks for a length, confirm whether it is one-way or total; where it asks for current, confirm whether the load is continuous or intermittent. The unit label beside the result is part of the result; do not carry a number into another calculation without checking its units.
Reference rows are provided for comparison, not as universal design limits. They use the values and conditions stated with each table. A different conductor material, temperature, power factor or load type can change a field result. Where the page calls out a code check, equipment rating or manufacturer table, that check remains part of the job.
Use the interactive inputs for the exact conditions at hand, then review the displayed formula and assumptions. If a value looks unexpected, change one input at a time and compare it with a known point in the reference table. Keep the measured readings and job-specific notes with the work record so another technician can understand how the result was obtained.
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