Lessons · Electrical · the 80 percent rule
The 80 percent rule: a circuit that runs for hours is sized at 125 percent
A load that runs for three hours or more is continuous, and its circuit is sized at 125 percent of the load; said the other way, a breaker may carry only 80 percent of its rating continuously.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
A row of electric baseboard heaters runs all night. A sign, a walk-in cooler, a bank of grow lights. Size those to the bare amps and the breaker trips on a cold Tuesday at 3 a.m., or worse, does not trip and the wire cooks.
How to think about it
Ask how long the load runs at once. Three hours or more: multiply the amps by 1.25 before you pick the breaker and the wire. Checking an existing circuit: multiply the breaker by 0.8, and that is the most continuous load allowed on it.
Worked example
Baseboard heaters: 2400 W at 240 V, I = 2400 / 240 = 10 AThe load, on for hours every winter night.
Required: 10 A × 1.25 = 12.5 AContinuous, so 125 percent.
Breaker: 15 A. Wire: 14 AWG copper is rated 15 AThe next standard size up, and a wire that matches it.
A 20 A breaker may carry 20 × 0.8 = 16 A continuouslyThe same rule read backwards. Sixteen amps of heater, not twenty.
Your turn
A 24 A continuous load. Write the line that gives the required circuit rating.
24 × 1.25 = A
Solve one, graded on the server
The trap
Applying 125 percent to a load that runs for twenty minutes. A toaster, a microwave, a garage door opener are not continuous. Oversizing everything wastes copper and, on a motor circuit, can leave the wire less protected than it should be.