Why Adding Up Your Breakers Gives the Wrong Answer

Adding up your breakers tells you almost nothing about panel capacity. Breakers protect wires; they don't describe what your home draws at the same moment. That's why the breakers in a 125 A panel can add up to several hundred amps while the home still has room — and why the National Electrical Code® uses a load calculation instead.

What a breaker number really means

A 20 A breaker means the wire behind it can safely carry 20 A.¹ It doesn't mean anything plugged into that circuit uses 20 A, or that it runs all the time. A bedroom circuit with a phone charger and a lamp might draw half an amp for most of the day.

What the load calculation does instead

For an existing home, NEC® 220.83 starts from what you actually have:

• 3 VA per square foot for lights and outlets.
• 1,500 VA per kitchen and laundry circuit.
• Nameplate ratings of fixed appliances: range, dryer, water heater, EV charger.²

Then it applies a demand factor: the first 8,000 VA counts in full and the rest at 40%, because not everything runs at once. New heating or cooling counts at 100%. The full method is in our plain-English guide to the load calculation.

A worked example

Take a 1,500 ft² home with an electric range, dryer and water heater. With typical breaker sizes, those three appliances alone sit on 110 A of breakers — before a single light or outlet. The calculation below lands at about 73 A of the 100 A main.

Spaces are a different question

Running out of breaker spaces is real, but it's separate from running out of power. A full panel may still have capacity, and an empty one may not. Check both — the free Panel Capacity Worksheet has room for each.

Ready to check your own home? Start the free check.

Technical notes

1. Branch-circuit breakers are chosen to protect the conductors (NEC® 240.4); the load is calculated separately under Article 220. A 20 A rating sets the ceiling for that wire, not how much or how often the circuit is used. Sources: NFPA 70 (NEC®), free access; LBNL technical brief, 2024. ↩

2. For the optional method, the code uses nameplate ratings for fixed appliances and fixed VA values for lighting and small-appliance circuits — never breaker sizes. A 30 A two-pole breaker feeding a water heater adds the heater's nameplate — typically 4,500 VA — not 30 A × 240 V = 7,200 VA. Municipal worksheets list typical values, e.g. City of Morgan Hill. ↩