Panel Capacity and Headroom Calculator

Every home has a main panel that caps total draw. Panels are rated in amps, commonly 100A, 150A, 200A, or 400A on a larger house. Multiply by the service voltage (240 V for US residential) to get capacity in watts.

200 A × 240 V = 48,000 W = 48 kW of capacity.

The 80% continuous rule

A continuous load is one expected to run three hours or more, and the code requires the overcurrent device to be rated at 125% of it (NEC 210.20(A) for branch circuits, 215.2 and 230.42 for feeders and services). Sizing at 125% of the load is the same thing as loading the device to no more than 80% of its rating, which is where the familiar 80% figure comes from. Intermittent loads such as an oven, a microwave or a power tool can use the full rating.

So when you are planning an addition, whether that is an EV charger, a hot tub or a new wing, the working rule is to keep the continuous portion under 80% of the panel. If a 50 A EV circuit would push you past that, the answer is a service upgrade rather than optimism.

Calculating total load

Adding up every installed breaker badly overstates reality, because nothing draws maximum from all circuits at once. Article 220 handles that with demand factors, and there are two accepted methods for a dwelling:

• Standard (220.42): the first 3,000 VA of general lighting at 100%, the next 117,000 VA at 35%.
• Optional (220.82): the first 10 kVA of general load at 100%, everything beyond at 40%.

Those are different methods with different numbers, and mixing them (10 kVA at 35%, say) gives an answer belonging to neither. Large appliances then get their own defined percentages on top.

This page uses the simpler comparison of connected load against panel rating, which is the right shape when you already have a load figure and want to know what is left. For a permit or a service upgrade you need a formal calculation, and the home panel load calculator runs the 220.82 optional method from square footage and appliance wattages to produce one. A licensed electrician still has to sign it.

The two pages measure against different lines, and that is deliberate. This one reports your headroom against the 80% figure, because you are asking whether something else will fit and the thing you are adding is usually continuous. The 220.82 page reports against the full panel rating, because 220.82 has already applied its demand factors and its result is compared to the service size directly. Applying 80% on top of 220.82 would be discounting the same load twice.

One reassurance: a 200 A panel with 160 A of breakers in it is completely normal. Breaker space is not load. Only simultaneous draw counts.