What is a Breaker Panel Load Calculation?
Your home's electrical panel (often called a breaker box) is the heart of your electrical system. It receives power from the utility company and distributes it safely to all the circuits in your house.
Every panel is rated for a maximum "ampacity" (usually 100, 150, or 200 Amps in modern residential homes). If you attempt to draw more power than the panel is rated to handle, the main breaker will trip, shutting off power to the entire house to prevent a catastrophic fire.
Before adding a large new appliance—like a hot tub, a Level 2 EV charger, an electric furnace, or a central AC unit—the National Electrical Code (NEC) requires a formal "Load Calculation" to ensure your current panel has enough capacity to support the extra power draw.
Understanding VA (Volt-Amperes)
To calculate panel load, electricians measure power in Volt-Amperes (VA). For standard residential, purely resistive loads, VA is essentially the same thing as Watts.
You can find the VA rating of any appliance by looking at its manufacturing sticker. If the sticker only lists Volts and Amps, you simply multiply them together:
Volts × Amps = VA
(Example: A 120-volt microwave drawing 10 amps uses 1,200 VA).
The 80% Continuous Load Rule
You cannot legally or safely load a breaker panel to 100% of its rated capacity if those loads run continuously. The NEC dictates that for continuous loads (anything running for 3 hours or more, like a car charger or heating system), the circuit and the panel can only be loaded to 80% of their maximum rating.
Conversely, you must multiply your continuous loads by 125% (1.25) when sizing the panel to ensure you have a 20% safety buffer built in.
How to Calculate Required Panel Ampacity
To determine the minimum size panel you need, you must total up all the VA requirements of your home and divide by your system's voltage (which is 240 Volts in standard North American residential split-phase systems).
The Formula
- Add up the Total VA of all lighting, general receptacles, and dedicated appliances (accounting for continuous load multipliers and NEC demand factors).
- Divide the Total VA by the System Voltage (usually 240V).
- The result is the minimum required panel Ampacity.
Required Amps = Total VA ÷ System Voltage
Example Calculation
After performing a detailed NEC Article 220 load calculation for a medium-sized home, you determine the total demanded load is 28,800 VA.
- Total VA:
28,800 - System Voltage:
240 - Required Amps:
28,800 ÷ 240 = 120 Amps
Because panels are not sold in 120-Amp sizes, you would be required to install a 150-Amp or 200-Amp panel to safely power this home.