Electrical Load Calculator
Calculate electrical load in watts, amps, and recommended circuit size.
This electrical load calculator finds the current draw (amps) of a device or circuit from its power rating (watts) and voltage, and helps confirm whether a circuit's breaker size is adequate for the connected load.
Enter the wattage and voltage, and the calculator returns the amperage and a suggested circuit size.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| 1,500W appliance on a 120V circuit | 12.5 A | Amps = 1,500 ÷ 120 = 12.5 A - within a standard 15-amp circuit's capacity, though above the 80% continuous-load threshold of 12 A. |
| 3,600W electric dryer on a 240V circuit | 15 A | Amps = 3,600 ÷ 240 = 15 A. |
The Amperage Formula
Amps = watts / volts. A 1,500-watt appliance on a 120-volt circuit draws 12.5 amps (1500/120). Comparing this figure against a circuit's breaker rating (commonly 15 or 20 amps for standard household circuits) shows whether the circuit has enough capacity for that load.
The 80% Continuous Load Rule
For continuous loads (running 3+ hours), electrical code commonly recommends not exceeding 80% of a circuit's rated capacity - a 20-amp circuit should generally carry no more than 16 amps of continuous load, providing a safety margin against overheating and nuisance breaker trips.
How to Use the Electrical Load Calculator
- Open the Electrical Load Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the electrical load calculator works.
Frequently Asked Questions
How do you calculate amperage from watts?
Divide the wattage by the voltage: amps = watts / volts. For a 120-volt circuit, a 1,200-watt device draws 10 amps.
What is the 80% rule for circuit loading?
For continuous loads (3+ hours), electrical code commonly recommends not exceeding 80% of a circuit's rated amperage - meaning a 20-amp circuit should carry no more than 16 amps of continuous load.
What happens if a circuit is overloaded?
An overloaded circuit can trip its breaker repeatedly, or in older systems without proper protection, pose a fire risk from overheating wires - staying within the circuit's safe rated capacity avoids both issues.
How do I use this electrical load calculator?
Enter the known values, review the units or settings, and the calculator updates the result instantly. The formula and example on this page show how the answer is produced.
What does the Electrical Load Calculator calculate?
Calculate electrical load in watts, amps, and recommended circuit size. It is designed for fast browser-based calculations without sign-up, downloads, or manual spreadsheet setup.
What formula does this electrical load use?
The formula is: amps = watts / volts. This rearrangement of Georg Ohm's 1827 formula (combined with the power equation P=IV) is embedded in the National Electrical Code, first published by the National Fire Protection Association in 1897 following a wave of building fires attributed to unsafe wiring - modern code requirements, including the 80% continuous-load derating rule, trace directly back to that safety-driven regulatory history.
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Last updated: September 27, 2026.