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Viva Calculator

Watt to Ampere Calculator

Calculate current from watts and voltage

Calculate Current

Method and source check

Last checked:

Automated source and implementation review, including worked examples and documented numerical limits. This review does not provide professional advice or specialist approval.

Method

Calculate current from entered real power, voltage, circuit type, and AC power factor. AC voltage is RMS; the three-phase formula assumes a balanced load and RMS line-to-line voltage.

I_DC = P / V

I_1Ο† = P / (V Γ— PF)

I_3Ο† = P / (√3 Γ— V_LL Γ— PF)

Variables and units

P
real power in W
V
DC or RMS voltage in V; line-to-line for balanced three-phase
PF
dimensionless power factor 0.01–1; editable assumption for AC
I
estimated current in A

Worked example

1200W at 120V DC is 10A; at 120V single-phase PF 0.8 it is 12.5A.

Limitations

The current calculation retains floating-point precision and rejects a nonfinite result. The UI displays current with up to six significant digits.

  • The current result assumes the entered AC power factor, RMS voltage, and a balanced load for three-phase mode.
  • The displayed current does not determine installation ratings, wiring, protection, or code compliance.
  • 1200W/120V DC β†’ 10A
  • 1200W/120V PF .8 single phase β†’ 12.5A
  • 1e308W/.01V DC rejected as nonfinite
  • 1e308 W, 1.5e308 V and PF=1 in balanced three-phase mode yield about 0.3849 A; intermediate denominator overflow does not turn this finite current into zero.

Sources

Table of Contents

How to Use

  1. Enter at least 0.01 W of real power and at least 0.01 V of voltage.
  2. Choose DC, single-phase AC, or three-phase AC. For AC, enter the power factor specified for your load.
  3. Read the calculated current and formula used.
  4. For AC, enter the documented or measured power factor; the prefilled 0.9 is only an example.

Enter at least 0.01 W of real power and at least 0.01 V of voltage

Choose DC, single-phase AC, or three-phase AC. For AC, enter the power factor specified for your load. Converting watts to amperes requires understanding the relationship between power, voltage, and current. The formula varies depending on the type of electrical circuit: DC, AC single-phase, or AC three-phase.

Power (P) is measured in watts (W), current (I) is measured in amperes (A), and voltage (V) is measured in volts (V). The relationship is defined by the formula P = V Γ— I for DC circuits.

Method source: OpenStax, Rice University. https://openstax.org/books/university-physics-volume-2/pages/15-4-power-in-an-ac-circuit

Conversion Formulas

Circuit TypeFormulaDescription
DC (Direct Current)I = P / VSimple division of power by voltage
AC Single PhaseI = P / (V Γ— PF)Includes power factor in calculation
AC Three PhaseI = P / (√3 Γ— V Γ— PF)Uses square root of 3 for three-phase systems

Where: I = Current (Amperes), P = Power (Watts), V = Voltage (Volts), PF = Power Factor (0.01 to 1)

For AC calculations, voltage is an RMS value. The three-phase formula assumes a balanced load and line-to-line RMS voltage; its power factor must match the entered load.

Power factor in this calculation

For AC, the entered power factor changes the current result. Use the value documented for the load or measured in the relevant setup; the prefilled 0.9 is only an example input.

Worked example

A 120 W DC load at 12 V gives 10 A by I = P Γ· V. The formula result alone does not determine wire, breaker, or equipment ratings.

Calculation limits

This formula result depends on the entered power, voltage, circuit type, and AC power factor. It does not determine installation ratings, protective devices, or code compliance.

Frequently Asked Questions

How do I convert watts to amperes?
For DC circuits, divide watts by volts: I = P / V. For AC single-phase, divide watts by (voltage Γ— power factor). For AC three-phase, divide watts by (√3 Γ— voltage Γ— power factor). For example, 1200W at 120V DC equals 10A (1200W / 120V = 10A).
What power factor should I use?
Use the power factor documented for the load or measured in the relevant setup. A prefilled value is only an example input.
When do I use three-phase calculations?
For AC calculations, voltage is an RMS value. The three-phase formula assumes a balanced load and line-to-line RMS voltage; its power factor must match the entered load.
Why is voltage important in this conversion?
Current is inversely proportional to voltage. Higher voltage results in lower current for the same power. For example, 1200W at 120V draws 10A, but at 240V it only draws 5A. This is why high-power appliances often use higher voltages.

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