Code Explained

Load Calculations: Does Your Panel Have Room? (NEC Article 220)

Before adding a heat pump, EV charger or shop, we check whether the service can handle it. Article 220 gives the methods, including one that uses a year of real meter data.

By IES team, Innovative Electrical Services (IES), Auburn, CA · Reviewed November 2026 · 2 min read

Key takeaways
  • A load calculation estimates the demand on a service or feeder, not just a sum of breaker sizes.
  • Article 220 has a standard method and optional methods for dwellings (220.82 and 220.83).
  • For existing buildings, 220.87 lets you use a year of actual peak demand data, plus a 25% cushion.
  • Load management can sometimes avoid a service upgrade.

“Is my panel big enough?” is one of the most common questions we get, usually when someone is planning an EV charger, heat pump, hot tub or shop. Adding up all the breaker numbers doesn’t answer it, because not everything runs at once. The NEC gives several ways to estimate the real demand, in Article 220.

Why breaker sizes don’t add up

A 200 A panel full of breakers can easily total 600 A or more on the handles. That is normal. Lights, receptacles and appliances cycle on and off, and the code’s methods apply demand factors that reflect realistic use while staying conservative.

The methods, in brief

  • Standard calculation (Parts II and III): builds up the load from general lighting (based on floor area), small-appliance and laundry circuits, fixed appliances, cooking equipment, dryers, heating and cooling, and motors, with demand factors for each.
  • Optional calculation for a dwelling (220.82): a simpler method for a single dwelling unit that groups “general loads” and applies a demand factor, then adds the larger of heating or cooling.
  • Existing dwelling (220.83): a method for adding loads to an existing home based on the existing loads plus what you are adding.
  • Existing installations using real data (220.87): see below.

Using real meter data (220.87)

For an existing building, the maximum demand can be based on actual measurements:

  • Take the highest demand recorded over one year (many utilities can provide 15-minute interval data), multiply by 125%, add the new load, and check that the total fits the service or feeder rating.
  • If a year of data isn’t available, a 30-day continuous recording of the highest 15-minute average demand can be used, but not if the building has solar, wind or a peak-shaving system that would hide the true load.

This method often shows a home has more room than a paper calculation suggests.

When the numbers are tight

  • Load management. Energy management systems and some EV chargers can limit or shed load so the service isn’t exceeded. The code allows these to be used in the calculation when set up as the rules describe.
  • Efficient equipment. A heat pump water heater draws far less than a standard electric tank.
  • A service upgrade. Sometimes it is the right answer, and it is a good time to fix older equipment and add surge protection.

We do the calculation in writing so you, the inspector and the utility can all see the same numbers. See our guide on panel upgrades for what the process looks like.

Sources and further reading

  1. Electrical Contractor Magazine, Using optional calculations for determining existing loads
  2. NFPA 70, National Electrical Code, 2023 edition (free read-only access)

Local rules decide. These articles explain general ideas and point to the code sections we use. They are not legal or engineering advice and don’t replace the code itself. California adopts the NEC with its own amendments, cities and counties can add more, and the local inspector (the AHJ, or authority having jurisdiction) has the final say on any job. Read the code free at NFPA’s free access page.