Amp conversion is the mathematical process of translating other electrical units—like watts, volt-amps, or horsepower—into amperes to determine the actual current draw of a circuit or device. Getting this single number right dictates everything from the AWG wire gauge you pull through conduit to the trip rating of the breaker in your panel. The most common point of confusion for DIYers is mixing up real power (watts) with apparent power (volt-amps) during AC calculations, or assuming a battery's milliamp-hour (mAh) rating equals its continuous amp output.

The Core Amp Conversion Formulas (DC and AC)

To convert power to current, you need to know the system voltage and whether you are dealing with Direct Current (DC) or Alternating Current (AC). AC circuits introduce a variable called Power Factor (PF), which accounts for the phase shift between voltage and current caused by inductive or capacitive loads like motors and transformers.

Understanding Power Factor (PF): In a purely resistive DC circuit or an AC resistive load (like a space heater or incandescent bulb), PF is 1.0. For inductive loads like AC motors or compressors, PF typically ranges from 0.80 to 0.90. Ignoring PF on an inductive load will result in calculating a lower amp draw than reality, leading to undersized wires and nuisance breaker trips. For a deep dive on measuring this in the field, refer to Fluke's guide on power factor.

1. DC Circuits and AC Resistive Loads

For DC systems (solar panels, battery banks) or purely resistive AC loads, the formula is straightforward:

I (Amps) = P (Watts) / V (Volts)

2. AC Single-Phase Circuits

For standard household 120V or 240V circuits powering inductive loads (microwaves, refrigerators, power tools):

I (Amps) = P (Watts) / (V (Volts) × PF)

3. AC Three-Phase Circuits

For 208V or 480V three-phase systems common in workshops and commercial buildings, you must account for the square root of 3 (approximately 1.732):

I (Amps) = P (Watts) / (1.732 × V (Volts) × PF)

Worked Numeric Example: Sizing a Breaker for a Workshop Heater

Let’s walk through a real-world scenario. You are installing a 1,500-watt portable workshop heater on a standard 120V single-phase circuit. Because it is a resistive heating element, the Power Factor is 1.0.

  1. Base Amp Conversion: 1,500W / (120V × 1.0) = 12.5 Amps.
  2. Apply the Continuous Load Rule: Under NEC Article 210.20, any load expected to run for 3 hours or more is considered 'continuous' and must be multiplied by 125%.
  3. Adjusted Ampacity: 12.5A × 1.25 = 15.625 Amps.
Result: A standard 15-amp breaker is insufficient because 15.625A exceeds its 80% continuous capacity (12A). You must step up to a 20-amp breaker and pull 12 AWG copper wire (rated for 20A at 60°C/75°C) to safely handle the continuous thermal load without tripping or melting the terminal lugs.

Where You Meet Amp Conversion in Practice

You will rarely see a device labeled simply in 'amps' on its nameplate without also listing watts, VA, or horsepower. Here is where amp conversion becomes critical on the jobsite or at the workbench:

  • Motor Sizing (HP to Amps): You cannot simply use 746 watts per horsepower to size a motor breaker. Motors have high inrush currents and varying efficiencies. Instead, the NEC mandates using specific lookup tables (like NEC Table 430.248). For example, a 1 HP single-phase motor at 115V has a full-load amp (FLA) rating of 16A, not the theoretical 6.5A you'd get from a raw wattage conversion. Always use standard engineering tables for motor FLA.
  • Solar Charge Controllers: When sizing wires between a solar array and an MPPT charge controller, you convert the array's maximum wattage to amps at the battery's nominal voltage, not the panel's Vmp. A 400W panel charging a 12V battery bank requires wire sized for at least 33.3A (400W / 12V), plus the NEC 125% solar multiplier.
  • UPS and Inverter Sizing (VA vs Watts): Uninterruptible Power Supplies (UPS) are often rated in Volt-Amps (VA), not Watts. A 1500VA UPS with a 0.8 power factor can only support 1200 real watts. Converting the VA rating to amps requires dividing by the voltage, but sizing the load requires converting your device's watts to VA first to ensure you don't overload the inverter's internal transformer.

Common Appliance Amp Conversion Reference Table

Use this reference table for standard US household appliances. Note that breaker sizes assume standard NEC 80% continuous load derating where applicable, and 14 AWG wire is the absolute minimum for 15A circuits (12 AWG recommended for voltage drop mitigation on long runs).

Appliance / Load Rating Voltage Est. PF Calculated Amps Min Breaker / Wire
Space Heater 1500 W 120V 1.0 12.5 A 15A / 14 AWG (Non-continuous)
Window AC Unit 1200 W 120V 0.85 11.7 A 15A / 14 AWG
Electric Dryer 5500 W 240V 1.0 22.9 A 30A / 10 AWG
Well Pump (1 HP) 1 HP 240V 0.80 8.0 A (FLA) 15A / 14 AWG
Microwave Oven 1100 W 120V 0.90 10.1 A 15A / 14 AWG

Frequently Asked Questions

How do I convert watts to amps for a 240V appliance?

Divide the wattage by 240, then divide that result by the power factor. For purely resistive 240V appliances like baseboard heaters or electric water heaters, the power factor is 1.0. Therefore, a 4,500-watt water heater draws exactly 18.75 amps (4500 / 240). Because water heaters are considered continuous loads under NEC guidelines, you multiply 18.75 by 1.25 to get 23.4 amps, dictating a 25-amp or 30-amp breaker and 10 AWG wire.

What is the amp conversion for 1 horsepower?

The theoretical electrical equivalent of 1 mechanical horsepower is 746 watts. However, you should never use 746W to calculate motor amp draw because it ignores motor efficiency and power factor. A real-world 1 HP motor operating at 120V with 80% efficiency and a 0.85 power factor will draw roughly 10.8 amps under full load (746 / (120 × 0.80 × 0.85)). Always defer to the Full Load Amps (FLA) printed on the motor's physical nameplate or NEC Table 430.248 for breaker sizing.

How do I convert milliamp-hours (mAh) to amps?

Milliamp-hours measure battery capacity over time, not instantaneous current draw. To convert mAh to Amp-hours (Ah), divide by 1,000 (e.g., a 5000mAh power bank is 5Ah). To find the continuous amp draw a battery can safely supply, you must look at its 'C-rating' or maximum continuous discharge rate specified by the manufacturer, not its mAh capacity. For example, a 2000mAh (2Ah) LiPo battery with a 20C rating can safely deliver 40 continuous amps (2Ah × 20C).

Does amp conversion change if I use a step-down transformer?

Yes, current changes inversely with voltage to conserve power (minus minor efficiency losses). If you use a 240V to 120V step-down transformer to run a 1200-watt tool, the secondary (120V) side will draw 10 amps (1200W / 120V). However, the primary (240V) side will only draw 5 amps (1200W / 240V). When sizing the wire and breaker for the wall outlet feeding the transformer, you calculate based on the primary 240V current (5A), but the wire feeding the tool from the transformer must be sized for the secondary 120V current (10A).