Volts to Amps Calculator

Example Data Table

Numbers are illustrative for planning and quick checks.

Formula Used

• Ohm’s law: I = V / R
• DC with real power: I = P / (V × η)
• Single-phase AC with real power: I = P / (V × PF × η)
• Three-phase AC with real power: I = P / (√3 × VLL × PF × η)
• Single-phase apparent power: I = S / V
• Three-phase apparent power: I = S / (√3 × VLL)

Where η is efficiency as a decimal, and PF is power factor.

How to Use This Calculator

1. Select the electrical system: DC, single-phase, or three-phase.
2. Choose a method based on available site data.
3. Enter voltage and your method inputs: watts, kVA, or ohms.
4. For AC loads, enter power factor and efficiency if known.
5. Press Calculate Amps to view results above the form.
6. Use CSV/PDF buttons to save the latest calculation.

Professional Notes for Construction Work

Current estimates help you size conductors, select protective devices, and validate generator or inverter capacity. For temporary power boards, plan for inrush where motors, compressors, or welders are present. When nameplate values list kVA, the apparent-power option mirrors how transformers and generators are rated. If you only have watts, include power factor to avoid underestimating current on inductive loads. Efficiency lets you account for losses between input power and delivered work, which matters when selecting supplies for tools and equipment.

Always check local code requirements, ambient derating, cable grouping, and duty cycle before final sizing. Use these numbers to size breakers and conductors properly.

Technical Article

1) Why amps matter on active sites

Current drives conductor heating and protection limits. For temporary distribution boards, a small change in load can push a circuit beyond breaker ratings. For example, a 2.0 kW single-phase tool at 230 V draws about 10 A at PF 0.90 and 95% efficiency, while the same watts at PF 0.80 rises to about 11.4 A. Those extra amps affect cable temperature, voltage drop, and nuisance tripping.

2) Typical construction voltages and loads

Common site supplies include 12–24 V DC for controls and lighting, 120–240 V single-phase for hand tools, and 400–480 V three-phase for hoists, pumps, welders, and compressors. A 10 kVA three-phase supply at 400 V (VLL) is about 14.4 A per line, which helps you sanity-check generator and feeder sizing.

3) Picking the correct method

Use the real power method when you have watts or kW (motors, heaters, and measured loads). Use the apparent power method when equipment is rated in VA or kVA (transformers, generators, UPS units). Use Ohm’s law when resistance is known (heating elements and test loads). Selecting the correct method prevents underestimating current.

4) Power factor data you should know

Power factor represents how effectively current produces real work in AC systems. Pure resistive heaters are near PF 1.00. Many induction motors operate around PF 0.80–0.95 depending on loading. If PF is not included, current can be underestimated by 10–25% on inductive equipment, increasing cable and breaker requirements.

5) Efficiency and real-world losses

Efficiency captures losses in motors, drives, and power supplies. Portable tools may operate around 85–95% depending on load and design. Using 100% can look cleaner, but it can understate current. For instance, 1.5 kW at 230 V, PF 0.90 draws about 7.25 A at 100% efficiency but about 7.63 A at 95%.

6) Three-phase voltage type selection

Three-phase nameplates may quote line-to-line (VLL) or line-to-neutral (VLN). In wye systems, VLN ≈ VLL/√3. For a 400 V VLL system, VLN is about 231 V. Choosing the wrong type can shift calculated current by √3, which is significant for feeders and protective devices.

7) Interpreting results for circuit planning

Use calculated amps to estimate breaker selection, conductor size, and derating headroom. For continuous or long-duty loads, keep operating current comfortably below protective limits. Where extension runs are long, consider voltage drop: higher current increases drop and reduces tool performance. Treat the output as a planning figure before final design checks.

8) Practical workflow on site

Start with the equipment nameplate: voltage, watts or kVA, and any PF or efficiency data. Enter the values, compare results across methods if labels are incomplete, and log the final number using the CSV/PDF exports for job records. When motors are present, plan for inrush and consider measured clamp readings during commissioning for verification.

Use these calculations to support safe planning; final selection should follow applicable electrical codes and project specifications.

FAQs

1) Can I convert volts to amps without power or resistance?

No. Voltage alone cannot determine current. You need power (W/kW), apparent power (VA/kVA), or resistance (Ω) to compute amps for a specific load.

2) What should I enter for power factor if it is unknown?

Use a conservative estimate. Many motors fall between 0.80 and 0.95. If you are unsure, choose 0.85 to avoid underestimating current for inductive loads.

3) Does efficiency apply to heaters and resistive loads?

Resistive heaters are typically close to 100% electrical-to-heat conversion, but wiring and control losses still exist. Use 98–100% if you need a realistic planning value.

4) Why does three-phase use √3 in the formula?

In balanced three-phase systems, total power relates to line-to-line voltage and line current by a √3 factor. This comes from the phase relationships between voltages and currents.

5) Should I use kW or kVA for generators?

Generators are commonly rated in kVA. Use the apparent-power method for current. If you also know PF, you can estimate real power as kW ≈ kVA × PF.

6) How accurate are these results for motor starting current?

The calculator estimates running current. Motor starting current can be several times higher for a short duration. Use manufacturer data or measured inrush values for protective device coordination.

7) Can I use this for conductor sizing directly?

Use it as an input to sizing, not the final answer. Conductor selection also depends on code tables, insulation rating, ambient temperature, grouping, installation method, and allowable voltage drop.