Electrical Panel Capacity Calculator
Determine whether your electrical panel has enough remaining capacity before adding new circuits or appliances. Critical for home additions, EV chargers, or workshop upgrades.
Last updated: September 2026
Formula below · 2 sources (nfpa.org, Wikipedia) · Updated Sep 2026
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About this calculator
This calculator follows NEC 220.87, the code method for adding load to an existing service. Take the highest demand recorded over the last 12 months (from utility demand data or a 30-day recording meter), multiply it by 125%, and add the new load; the total must not exceed the service rating. New continuous loads, such as an EV charger (NEC 625.42) or electric heat, count at 125%. If your existing figure is instead a full NEC Article 220 calculated load, it already includes demand factors and counts at 100%. Formula: Remaining Amps = Panel Rating − Existing × (1.25 measured or 1.0 calculated) − New Load × (1.25 continuous or 1.0 non-continuous). A positive result is spare capacity; a negative result means the service is over capacity and needs an upgrade, load management or load reduction first. Do not add up breaker ratings as the "existing load"; that is not a demand figure.
How to use
Scenario: 200 A service, measured 12-month peak = 120 A, new EV charger drawing 32 A (continuous). 1. Select Panel Rating = 200 A. 2. Enter Existing Peak Demand = 120 A and choose "Measured 12-month peak". 3. Enter Additional Load = 32 A and choose "Continuous". 4. Remaining = 200 − 120 × 1.25 − 32 × 1.25 = 200 − 150 − 40 = 10 A. The service can take the charger, with 10 A to spare. With the defaults (120 A + 30 A, both at 125%), 200 − 150 − 37.5 = 12.5 A remain.
Frequently asked questions
What is the 80% rule for electrical panels and why does it matter?
The 80% rule comes from NEC 210.20(A) and 215.3: a breaker or feeder must be rated at least 125% of the continuous load it carries, so a 20 A breaker should carry no more than 16 A continuously. For a service, the NEC instead compares the calculated (or measured) demand with the service rating: NEC 220.87 takes the 12-month peak demand at 125% plus the new load, and the total must not exceed the rating. This calculator applies those multipliers to the loads rather than shrinking the panel rating.
How do I calculate the existing load on my electrical panel?
The NEC 220.87 approach uses the maximum demand actually recorded over a year: ask your utility for demand data, or use a recording meter for 30 days on the main feeders. Alternatively, an electrician can do a full NEC Article 220 load calculation, which already includes demand factors (enter it at 100%). Adding up the amperage of all installed breakers does not give a demand figure and badly overstates the load. A licensed electrician should confirm the result before major additions.
When should I upgrade my electrical panel to a higher amperage?
You should consider a panel upgrade when the available capacity after your planned additions is negative, when you frequently trip the main breaker, or when you're adding high-draw loads like an EV charger, hot tub, or central air conditioning. Older homes with 60 A or 100 A panels often cannot support modern electrical demands and benefit from upgrading to 150 A or 200 A service. Panel upgrades also make sense if your current panel uses outdated technology like fuse boxes or recalled breaker brands. Always consult a licensed electrician before upgrading, as the work requires permits and utility coordination.