Electrical Panel Capacity: 100A, 200A, and Load Calculations

Somewhere on the side of your house is a gray metal box that may control your ADU budget more than any design choice. Let's learn to read it.

What the numbers mean. Your main electrical panel has a capacity in amps — how much electricity the property can draw at once. Older homes commonly have 100-amp service; modern homes have 200 amps. Adding an ADU means adding a second home's worth of demand: kitchen appliances, water heating, HVAC, laundry.

The load calculation decides. An electrician (or your designer) runs a load calc — a standardized worksheet adding up both dwellings' demands with code-defined factors. The output is a yes/no: your existing service can or can't carry the combined load.

The rules of thumb:

• 200A service: usually fine. A typical house plus a modest all-electric ADU generally fits, with a subpanel feeding the unit.
• 100A service: usually not. A second full dwelling on 100 amps rarely passes the calc — plan on a service upgrade.
• Wildcards that tip the math: EV chargers (existing or planned), electric heat, pools/spas, and how electrified the ADU's appliances are.

What an upgrade involves. Replacing the panel and often the service equipment: commonly $5,000–$15,000+, plus coordination with your utility — whose approval and scheduling can take weeks to months. This is why panel capacity gets checked in feasibility, not framing: the cost is manageable when planned and painful when discovered.

Two budget-saving notes. First, going all-electric with efficient heat-pump equipment often lowers the ADU's calculated load versus resistance heating — sometimes the difference between passing and upgrading. Second, load-management devices (smart panels that shed loads momentarily) can sometimes squeeze an ADU onto existing service legally — worth asking your electrician about before writing the upgrade check.