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.
In beta. This lesson was written for Hone and has not yet been checked by a licensed electrician. Practice material, not professional advice. What that means.
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.