A linear load is an electrical device where the current flowing through it stays perfectly proportional to the voltage supplying it. In practical terms, this means the device draws power in a smooth, predictable wave pattern rather than in choppy bursts. If you plot the voltage and current on a graph, both form clean sine waves, the classic S-shaped curves of alternating current (AC) power.
How a Linear Load Works
In an AC power system, voltage rises and falls in a smooth, repeating wave pattern called a sine wave. A linear load responds to this wave obediently: as voltage increases, current increases in direct proportion, and as voltage decreases, current follows right along. This relationship follows Ohm’s law, the foundational rule of electrical circuits that ties voltage, current, and resistance together.
The key characteristic is that a linear load doesn’t alter the shape of the current waveform. What comes out looks like what went in. The current wave may shift slightly in time relative to the voltage wave (more on that below), but it remains a smooth sine wave throughout. This predictable behavior makes linear loads the simplest type of load for a power system to handle.
Common Examples
The most familiar linear loads are devices built around simple electrical components: resistors, inductors, and capacitors. In everyday terms, that translates to:
- Incandescent light bulbs, which are essentially resistors that glow when heated
- Electric heaters and baseboard heating elements
- Simple electric motors that connect directly to the power supply without electronic speed controllers
- Toasters, kettles, and stovetop heating coils
Heating loads and direct motor loads are classic examples because they always draw sinusoidal current from the power supply. They consume electricity without distorting the waveform or creating electrical “pollution” on the grid. If every device in your home were a linear load, the power company’s job would be much simpler.
Power Factor in Linear Loads
Even though linear loads keep the current waveform smooth, they don’t always use power with perfect efficiency. Power factor measures how effectively a device converts the electricity it receives into useful work. It’s expressed as a number between 0 and 1.
A purely resistive linear load, like a space heater, has a power factor of 1. All the electricity delivered to it gets converted into heat. But when a linear load contains a coil (inductor) or capacitor, the current wave shifts in time relative to the voltage wave. This shift means some energy sloshes back and forth between the power source and the device without doing useful work. An electric motor running without any electronics, for example, might have a power factor around 0.89, meaning roughly 89% of the power delivery is doing productive work.
For linear loads specifically, the power factor depends entirely on this time shift between voltage and current. Engineers call this the “displacement power factor” because it reflects only the displacement (time offset) between the two waveforms, not any distortion in their shape.
How Linear Loads Differ From Non-Linear Loads
The distinction matters because most modern electronics are non-linear loads. Computers, LED drivers, phone chargers, variable-speed motors, and fluorescent lighting all contain circuitry that converts AC power internally. These devices don’t draw current in a smooth sine wave. Instead, they pull current in abrupt short pulses, creating a choppy, distorted waveform.
Those distorted waveforms generate harmonics, which are extra frequencies layered on top of the normal 50 or 60 Hz power signal. Harmonics cause real problems: overheating in wiring and transformers, interference with sensitive equipment, and reduced efficiency across the electrical distribution system. Linear loads produce essentially no harmonic distortion. Under normal operating conditions, the voltage distortion at any point where linear loads connect to the grid is negligible, effectively zero.
This is why the linear vs. non-linear distinction comes up so often in electrical engineering and power system design. A building full of computers and LED lighting presents a fundamentally different challenge to the electrical system than a building full of heaters and incandescent bulbs, even if both buildings consume the same total wattage.
Why It Matters for Power Systems
If you’re sizing a generator, selecting an uninterruptible power supply (UPS), or designing an electrical panel, knowing whether your loads are linear or non-linear changes the equipment you need. Linear loads are straightforward: you calculate the total wattage, account for power factor if motors are involved, and select equipment accordingly.
Non-linear loads require oversized neutral conductors, harmonic filters, and power conditioning equipment to handle the distorted current waveforms they create. A UPS rated for a certain wattage of linear loads may not handle the same wattage of non-linear loads without derating, meaning you’d need a larger unit.
In practice, purely linear loads are becoming less common as more devices incorporate electronic controls. Even modern motors often use variable-frequency drives that make them behave as non-linear loads. Understanding the concept of a linear load gives you a baseline for how electricity is supposed to flow in a clean system, making it easier to recognize and address the complications that non-linear devices introduce.