Set load and voltage limits to estimate safe amperage ranges fast today. Pick breaker and wire targets, then export records for crews easily daily.
Single-phase current: I = (P × 1000) / (V × PF × η)
Three-phase current: I = (P × 1000) / (√3 × V × PF × η)
Where P is power in kW, V is volts, PF is power factor, and η is efficiency as a decimal.
The calculator estimates a minimum current using low load + high voltage, and a maximum current using high load + low voltage. The sizing current applies the continuous factor to the maximum.
| Power | Voltage | Phase | PF | Eff | Load min | Load max | Tol | Range (A) |
|---|---|---|---|---|---|---|---|---|
| 15 kW | 400 V | Three | 0.90 | 92% | 60% | 100% | ±5% | 16.6 – 32.1 |
| 7.5 kW | 230 V | Single | 0.85 | 90% | 50% | 110% | ±6% | 19.6 – 46.0 |
| 20 hp | 415 V | Three | 0.88 | 91% | 70% | 100% | ±4% | 21.6 – 33.7 |
Construction power demand is rarely steady. Saws cycle, pumps surge, compressors start under load, and temporary lighting expands during night work. A single “nameplate amps” figure can miss what crews experience in the field. This calculator provides a minimum-to-maximum band so supervisors can plan feeders, protection, and distribution boards with fewer surprises.
Current depends on real electrical power and how efficiently equipment converts it into work. A 15 kW motor at 400 V and 0.90 power factor does not draw the same amps as a resistive heater of equal kW. By including power factor and efficiency, the estimate aligns better with typical motors, welders, and mixed loads used on site.
Three-phase distribution often reduces current for the same power because the power is shared across phases. That can reduce conductor size and voltage drop on long runs to hoists or batching areas. The phase selector applies the correct relationship (including √3 for three-phase) so the range remains realistic for the selected supply.
Load minimum and maximum percentages model partial operation and peak work periods. For example, a pump may run at 60% during normal drainage yet approach 100% during storm events. Setting a band such as 60% to 110% helps capture both steady operation and temporary overloads from difficult starts or short-term duty.
Temporary feeders, generator sets, and long extension runs can create voltage variation. The calculator treats tolerance as a worst-case envelope: higher voltage reduces current, while lower voltage increases it. Using ±5% to ±8% is common for planning, but measure on site if equipment trips or runs hot.
For loads expected to run for extended periods, planners often size conductors and protection above the calculated maximum. The continuous sizing factor (for example 125%) converts the maximum current into a conservative “design current.” The recommended breaker is then chosen as the next standard rating above that design current.
The copper wire recommendation is a planning target, not a final design. Ampacity depends on insulation rating, ambient temperature, conduit fill, bundling, termination limits, and local requirements. Treat the displayed gauge as a quick check: if the project constraints are harsh, choose the next larger size and verify with your standard tables.
Exporting CSV or PDF helps coordination between electrical, mechanical, and site management teams. Keep a record of assumed voltage, power factor, and efficiency so later changes are traceable. When equipment is swapped, re-run the calculator and attach the updated export to the temporary power log.
Use nameplate kW if available. If only horsepower is known, select hp and enter that value. For variable loads, enter the rated value and use the load range to reflect typical operation.
Use manufacturer data when possible. If unknown, motors are often 0.80–0.95 power factor with 85–95% efficiency. Conservative inputs produce safer sizing, especially for remote or temporary supplies.
It models expected voltage variation from long feeders, generator regulation, and changing site loads. A higher tolerance increases the estimated maximum current when voltage dips, helping you plan for worst-case operation.
The adjusted value multiplies the calculated maximum by your sizing factor (often 125%). This creates a planning current used to select a standard breaker size and a reasonable wire target.
Yes, but enter realistic power factor and efficiency. Resistive heaters typically have power factor near 1. Welding and electronic loads can vary widely, so use measured data if available.
No. Motor starting current can be several times higher than running amps. For frequent starts, verify starter type, protection curves, and generator capacity, then adjust your site design accordingly.
They are planning recommendations. Final selection must consider installation method, temperature, bundling, and local electrical rules. Always validate with your standard tables and site inspection data.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.