Load current is the amount of electrical current drawn by any device or component that consumes power in a circuit. When you plug in a kettle, turn on a motor, or charge your phone, each device pulls a specific amount of current from the power source to do its job. That current flowing through the device is the load current, measured in amperes (amps).
How Load Current Works in a Circuit
Every electrical circuit has two basic parts: a source (like a battery or wall outlet) and a load (the device consuming power). The load is whatever does useful work, whether that’s producing light, heat, motion, or processing data. Load current is simply the current that flows through that device during operation.
The amount of current a load draws depends on two things: the voltage supplied and the resistance of the load. This relationship follows Ohm’s Law, which states that current equals voltage divided by resistance. A 120-volt outlet powering a device with 12 ohms of resistance produces 10 amps of load current. Lower resistance means more current flows; higher resistance means less.
The source also plays a role. Every power source has some internal resistance, which means the total current in the circuit is determined by both the source’s internal resistance and the load’s resistance combined. In practical terms, this is why a weak battery can’t power a high-demand device: the source can’t deliver enough current without its own voltage dropping significantly.
Calculating Load Current
For DC circuits and purely resistive AC loads, the math is straightforward. You can calculate load current three ways depending on what values you know:
- Current = Voltage ÷ Resistance (when you know the supply voltage and the load’s resistance)
- Current = Power ÷ Voltage (when you know how many watts the device uses and the voltage)
- Current = √(Power ÷ Resistance) (when you know wattage and resistance but not voltage)
AC circuits with motors, compressors, or other non-resistive loads add a complication called power factor. In these circuits, the voltage and current waveforms fall out of sync, so the effective power delivered is reduced. The formula becomes: Power = Voltage × Current × cos(θ), where θ is the phase angle between voltage and current. A power factor of 1.0 means perfect alignment; anything less means the device draws more current than a simple wattage-divided-by-voltage calculation would suggest.
How Different Loads Draw Current
Not all loads behave the same way. Resistive loads like heaters, incandescent bulbs, and toasters draw current that rises and falls perfectly in sync with the voltage. The current and voltage peak at the same moment, which makes them the simplest to calculate and manage.
Inductive loads, like motors, fans, and compressors, cause the current to lag behind the voltage. The current peaks after the voltage does. This phase lag is why motors have a lower power factor and often draw more current than their wattage alone would predict. Capacitive loads, such as certain LED driver circuits and power-factor correction equipment, have the opposite effect: current peaks before voltage.
This distinction matters practically because inductive loads, which are extremely common in homes and industrial settings, require wiring and breakers sized for the actual current they draw, not just the useful power they consume.
Full Load Amps on Motors
If you’ve looked at the nameplate on a motor, you’ve likely seen “FLA” or “Full Load Amps.” This is the current the motor draws when running at its rated power output under normal conditions. A 460-volt three-phase motor, for example, might list an FLA of 4.2 amps. That number is critical for sizing the overload protection on the circuit, which is typically set to no more than 115% of FLA.
Motors also draw a much higher surge of current when starting up, sometimes five to seven times their full load current. This inrush current is brief but important to account for when selecting circuit breakers and fuses, since they need to tolerate the startup spike without tripping while still protecting against sustained overloads.
Typical Load Current for Household Devices
To put load current into everyday context, here’s what common appliances draw from a standard outlet:
- Phone charger or laptop: less than 0.5 amps
- Desktop computer: about 1.3 amps
- Microwave: around 6.5 amps
- Hair dryer: about 10 amps
- Washing machine: around 10 amps
- Tumble dryer: up to 11 amps
- Kettle, iron, or single oven: up to 13 amps
- Electric vehicle (slow charge): up to 13 amps
The gap is enormous. A phone charger draws roughly 25 times less current than a kettle. This is why high-draw appliances like ovens and dryers often need dedicated circuits with heavier wiring.
Why Wire Size Depends on Load Current
Every wire has a maximum safe current-carrying capacity, called its ampacity. Push more current through a wire than it’s rated for and it heats up, potentially melting insulation or starting a fire. The relationship between load current and wire thickness is direct: heavier loads need thicker wires.
In residential wiring, 14-gauge wire handles 15-amp circuits (lighting and general outlets). Twelve-gauge wire handles 20-amp circuits for kitchen appliances and heavy-duty receptacles. Ten-gauge wire is used for 30-amp circuits powering equipment like dryers and small HVAC units. A smaller gauge number means a physically thicker wire with lower resistance, which is why it can safely carry more current. In commercial spaces, 12-gauge is often the minimum, with 10-gauge used for higher-demand equipment.
Signs of Excessive Load Current
When a circuit carries more current than it’s designed for, the first symptom is heat. Wires, breakers, and connections warm up beyond their normal operating temperature. You might notice a warm panel surface, a distinct “hot electrical” smell, or breakers that trip repeatedly. Loose connections make the problem worse because they create points of high resistance that can glow red-hot and eventually arc.
Frequent breaker trips are the most common sign that load current exceeds the circuit’s capacity. This usually means too many devices are running on the same circuit, or a single device has developed a fault that causes it to draw abnormally high current. Overloaded circuits are one of the leading causes of electrical fires, which is why breakers exist: they cut power before the wiring reaches dangerous temperatures.
How to Measure Load Current
Measuring load current requires placing an ammeter in series with the circuit, meaning the current must flow through the meter on its way to the load. This is different from measuring voltage, where you simply touch probes across two points. To measure current, you break the circuit at one point, then connect the meter’s leads to each side of the break so all current passes through it.
If you’re using a multimeter, start with the highest current range setting and work your way down to get the most accurate reading. One important safety rule: never connect an ammeter in parallel across a voltage source. An ammeter has almost no internal resistance, so connecting it directly across a power source creates a near short circuit, producing a dangerous current surge that can destroy the meter or cause injury. Clamp-style ammeters offer a safer alternative for many situations, since they measure the magnetic field around a wire without requiring you to break the circuit open.