A continuous load in commercial wiring is an electrical load where the maximum current is expected to continue for three hours or more, and because of the sustained heat it produces, code requires circuits and equipment serving continuous loads to be sized at 125 percent of the continuous load. This rule ensures circuits are not run at their full limit for long periods, which would cause overheating. Understanding continuous loads helps explain why commercial circuits are often sized larger than the load alone would suggest.

Here is what a continuous load in commercial wiring is.

The continuous-load definition and the 125 percent rule are defined by code, so their application is determined by qualified electricians or engineers following the applicable code.

The definition: three hours or more

Code defines a continuous load as one where the maximum current is expected to continue for three hours or more. The three-hour threshold is the key: a load that runs at its maximum for at least that long is continuous, while loads that cycle or run for shorter periods are not. Examples of loads that are often continuous in commercial settings include much of the lighting (on all day), and other loads that run steadily for long stretches.

Why continuous loads are treated specially

Continuous loads are treated specially because sustained current produces sustained heat. A circuit carrying current continuously for hours heats up more than one carrying the same current briefly. Running a circuit at its full rating continuously would push it to its thermal limit for a long time, which is unsafe. To prevent this, code requires extra margin for continuous loads, so the circuit is not maxed out during long operation. This is the reasoning behind the 125 percent rule.

The 125 percent rule

The code rule is that circuits and overcurrent devices serving continuous loads must be sized at 125 percent of the continuous load. In practice, this means a circuit is sized so the continuous load is no more than 80 percent of the circuit’s rating (since 100 divided by 125 is 80). For example, a continuous load is multiplied by 1.25 to determine the minimum circuit and breaker size. This built-in margin keeps the circuit from running at its full limit continuously.

Continuous vs non-continuous

In a load calculation, continuous and non-continuous loads are treated differently:

• Continuous loads (three hours or more at maximum) are counted at 125 percent for sizing
• Non-continuous loads are counted at their actual value
• Where both are present, the sizing accounts for 125 percent of the continuous portion plus the non-continuous portion

Distinguishing which loads are continuous is important for sizing circuits correctly, which is part of a proper load calculation.

Why it matters

The continuous-load rule matters because it keeps circuits safe under sustained operation, which is common in commercial buildings where lighting and equipment run all day. Without the margin, circuits serving continuous loads could overheat over long operating hours. Sizing to 125 percent of continuous loads is a fundamental part of safe commercial circuit and service design, which is why qualified professionals apply it in every load calculation.

A continuous load in commercial wiring is a load where the maximum current is expected to continue for three hours or more, and because sustained current produces sustained heat, code requires circuits and equipment serving continuous loads to be sized at 125 percent of the continuous load (so the load is no more than 80 percent of the circuit). This rule keeps circuits safe under long operation, and qualified professionals apply it in load calculations.

Stella Electric sizes commercial circuits and services correctly for continuous loads for businesses across the Baltimore area and throughout Maryland, with flat-rate pricing and no overtime charges. If you need your commercial wiring sized right, we can help. Financing is available on qualifying projects.