The 80% Rule (Why Breakers Trip)
NEC Code says continuous loads (running 3+ hours) shouldn't exceed 80% of breaker capacity.
15A Breaker
Max 12A
1440W @ 120V
20A Breaker
Max 16A
1920W @ 120V
30A Breaker
Max 24A
5760W @ 240V
Which Panel Size?
100A
Small home - No central AC, gas appliances, minimal circuits.
200A
Standard modern home - Central AC, electric range, 20+ circuits. Most common.
400A
Large home / EV charging - Electric heating, pool, multiple AC units, future expansion.
Electrical Load Sizing Formula
To calculate breaker and panel sizes, convert appliance power ratings to amps by dividing watts by voltage, then sum them up. For continuous loads (running 3+ hours), multiply by 1.25 to conform to the NEC 80% rule.
ext{Load Current (Amps)}The rate of electrical current flowing through the circuit
ext{Required Breaker Size} ext{Continuous Amps} imes 1.25
ext{NEC 80% Rule}Breakers should only be loaded to 80% of capacity for continuous usage
Manual Step-by-Step Calculation
Suppose you are installing a continuous electrical load (like a space heater) that consumes 1,500 Watts at 120 Volts. Let's find the correct breaker size.
1
1. Calculate Load Current in Amps
Divide power in watts by the circuit voltage.
rac{1,500 ext{ Watts}}{120 ext{ Volts}} = 12.5 ext{ Amps}
2
2. Apply continuous load safety factor (125%)
Under NEC rules, continuous loads require a 125% multiplier for circuit sizing.
12.5 ext{ Amps} imes 1.25 = 15.625 ext{ Amps}
3
3. Choose Standard Breaker Size
Round up to the next standard circuit breaker size (which are 15A, 20A, 30A, etc.).
lceil 15.625
ceil
ightarrow 20 ext{ Amps}
4
Result
You must run this load on a 20-amp circuit.
20-Amp Breaker Required
Frequently Asked Questions
How do I calculate electrical load for my home?
To calculate electrical load, use the formula: Amps = Watts ÷ Volts. Add up the wattage of all circuits and appliances, then divide by your voltage (typically 120V or 240V). For example, a 1,500W space heater on 120V uses 12.5 amps. Sum all circuit loads to determine total panel requirements.
What size electrical panel do I need for my house?
Most modern homes need a 200-amp panel, which supports central AC, electric ranges, and 20+ circuits. Smaller homes without central AC may use 100A. Larger homes with electric heating, EV chargers, pools, or multiple AC units often require 400A service for adequate capacity and future expansion.
What is the NEC 80% rule for electrical circuits?
The National Electrical Code (NEC) requires that continuous loads (running 3+ hours) should not exceed 80% of a circuit breaker's rated capacity. For example, a 20-amp breaker should carry no more than 16 amps continuously. This safety margin prevents overheating and nuisance tripping.
How many amps does a typical house use?
Average US homes use 100-200 amps at peak load. Daily usage varies: central AC uses 15-20A, electric dryer 20-30A, electric range 40-50A, and EV charger 30-50A. Calculate your specific needs by adding all major appliances.