Electrical Load Calculator
Add your appliances and circuits to calculate total electrical load, amperage draw, and recommended breaker sizing. Uses the NEC 80% safety margin rule.
Circuits / Appliances
Common Appliance Wattage Reference
Use this table to look up typical wattage ratings for common household and commercial appliances. Actual wattage varies by model and usage.
| Appliance | Typical Watts | Typical Circuit |
|---|---|---|
| LED Light Bulb (60W equiv.) | 10 | 120V / 15A |
| Incandescent Light Bulb | 60 | 120V / 15A |
| Ceiling Fan | 75 | 120V / 15A |
| Laptop Computer | 65 | 120V / 15A |
| Desktop Computer | 200 | 120V / 15A |
| Television (LED/LCD) | 100 | 120V / 15A |
| Refrigerator | 150 -- 200 | 120V / 20A |
| Microwave Oven | 1,000 -- 1,500 | 120V / 20A |
| Toaster | 900 | 120V / 20A |
| Coffee Maker | 1,000 | 120V / 20A |
| Dishwasher | 1,200 -- 1,800 | 120V / 20A |
| Garbage Disposal | 500 | 120V / 20A |
| Washing Machine | 500 | 120V / 20A |
| Electric Dryer | 4,000 -- 5,500 | 240V / 30A |
| Electric Oven / Range | 8,000 -- 12,000 | 240V / 40-50A |
| Water Heater (electric) | 4,000 -- 5,500 | 240V / 30A |
| Space Heater | 1,500 | 120V / 20A |
| Window Air Conditioner | 900 -- 1,400 | 120V / 20A |
| Central Air Conditioner | 3,000 -- 5,000 | 240V / 30-50A |
| Heat Pump | 3,000 -- 6,000 | 240V / 30-60A |
| Hair Dryer | 1,500 | 120V / 20A |
| Vacuum Cleaner | 1,000 | 120V / 15A |
| Garage Door Opener | 600 | 120V / 15A |
| Sump Pump | 800 | 120V / 20A |
| EV Charger (Level 2) | 3,800 -- 11,500 | 240V / 30-50A |
| Hot Tub / Spa | 6,000 -- 7,500 | 240V / 50-60A |
| Well Pump | 750 -- 1,500 | 240V / 20A |
| Electric Furnace | 10,000 -- 20,000 | 240V / 60-100A |
Wattage values are typical ranges. Always check the nameplate rating on your specific appliance for accurate load calculations.
How Electrical Load Calculation Works
The Basic Formula
Electrical load calculation uses Ohm's Law and the power formula to determine how much current (amps) your devices draw from the electrical supply:
Amps = Watts / Volts (single phase)
Amps = Watts / (Volts x 1.732) (three phase)
For example, a 1,500-watt space heater on a 120V circuit draws 12.5 amps (1,500 / 120 = 12.5).
The NEC 80% Rule
The National Electrical Code (NEC) Article 210.20 requires that the total load on a circuit breaker does not exceed 80% of its rating for continuous loads (loads running 3 hours or more). This means:
- A 15A breaker should carry no more than 12A of continuous load
- A 20A breaker should carry no more than 16A of continuous load
- A 30A breaker should carry no more than 24A of continuous load
- A 50A breaker should carry no more than 40A of continuous load
This safety margin protects wiring from overheating and reduces the risk of fire.
Breaker Sizing
To select the right breaker, take the total continuous amperage and multiply by 1.25 (the inverse of the 80% rule). Then round up to the nearest standard breaker size. Standard residential breaker sizes in the US are: 15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 60A, 70A, 80A, 90A, 100A, 125A, 150A, and 200A.
Panel Load Calculation
For whole-house panel calculations, add up all circuit loads. The NEC allows demand factors for certain load types to account for the fact that not all circuits run at full load simultaneously. This calculator shows the raw calculated load without demand factors -- for a permitted load calculation per NEC Article 220, consult a licensed electrician.
Standard US Breaker Sizes
| Breaker Size | 80% Capacity | Common Uses |
|---|---|---|
| 15A | 12A | Lighting circuits, general outlets |
| 20A | 16A | Kitchen outlets, bathroom outlets, garage |
| 30A | 24A | Electric dryer, water heater |
| 40A | 32A | Electric range, small cooktop |
| 50A | 40A | Electric range, EV charger, hot tub |
| 60A | 48A | Sub-panel, large appliances |
| 100A | 80A | Sub-panel, small home main panel |
| 200A | 160A | Typical residential main panel |
Frequently Asked Questions
How do I calculate electrical load in amps?
Divide the total wattage by the voltage. For a single-phase circuit: Amps = Watts / Volts. For example, a 1,800W load on a 120V circuit draws 15 amps. For three-phase systems, divide by voltage times 1.732 (the square root of 3).
What is the NEC 80% rule for breakers?
The National Electrical Code requires that continuous loads (running 3+ hours) not exceed 80% of the breaker's rated ampacity. A 20-amp breaker should carry no more than 16 amps of continuous load. This provides a safety margin against overheating.
How do I determine what size breaker I need?
Calculate total amperage for the circuit, then multiply by 1.25 for continuous loads. Select the next standard breaker size at or above that number. For a circuit drawing 18 amps continuously, you would need at least 22.5A (18 x 1.25), so a 25A or 30A breaker.
What is the difference between 120V and 240V circuits?
In US residential wiring, 120V circuits serve most outlets and lighting through a single hot wire and neutral. 240V circuits use two hot wires and are reserved for heavy-duty appliances like dryers, ranges, water heaters, and central AC. At the same wattage, a 240V circuit draws half the amperage of a 120V circuit, allowing for smaller wire gauges over long runs.
How do I calculate total panel load?
Add up the wattage of all circuits connected to the panel. Divide by the service voltage (typically 240V for whole-panel calculations in US homes) to get total amperage. Compare this to your main breaker rating. The NEC allows demand factors for certain load types, which can reduce the calculated total.
Can I add a new circuit to my existing panel?
You can add circuits as long as the total load does not exceed the panel's capacity. Use this calculator to add your existing loads plus the new circuit. If the total exceeds 80% of panel capacity, you may need a panel upgrade. Always consult a licensed electrician before modifying your electrical panel.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.
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Privacy & Disclaimer
Privacy: This calculator runs entirely in your browser. No data is transmitted or stored.
Disclaimer: This calculator is for estimation and educational purposes only. It does not replace professional electrical engineering judgment. Always consult a licensed electrician for actual electrical work and ensure compliance with your local electrical code. Improper electrical work can cause fire, injury, or death.
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Electrical Load Calculator FAQ
How do I calculate electrical load in amps?
Divide the total wattage by the voltage. For a single-phase circuit: Amps = Watts / Volts. For example, a 1,800W load on a 120V circuit draws 15 amps.
What is the NEC 80% rule for breakers?
The National Electrical Code (NEC) requires that continuous loads not exceed 80% of the breaker's rated ampacity. This means a 20-amp breaker should carry no more than 16 amps of continuous load, providing a safety margin for heat and transient surges.
How do I determine what size breaker I need?
Calculate total amperage for the circuit, then select a standard breaker size that is at least 125% of the continuous load (the inverse of the 80% rule). Common residential breaker sizes are 15A, 20A, 30A, 40A, 50A, 60A, and 100A.
What is the difference between 120V and 240V circuits?
In US residential wiring, 120V circuits serve most outlets and lighting. 240V circuits are used for heavy appliances like dryers, ovens, water heaters, and central AC units. At the same wattage, a 240V circuit draws half the amperage of a 120V circuit.
How do I calculate total panel load?
Add up the wattage of all circuits and appliances connected to the panel. Divide by the service voltage (typically 240V for whole-panel calculations in US homes) to get total amperage. Compare this to your main breaker rating (commonly 100A, 150A, or 200A).
What is a demand factor?
A demand factor accounts for the fact that not all circuits are at full load simultaneously. The NEC allows applying demand factors to certain load types (e.g., general lighting and small appliance circuits) to reduce the calculated total load to a more realistic value.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.