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I = V / R
Current equals voltage divided by resistance (Ohm's Law)
I = P / V
Current equals power divided by voltage
Electrical current (I) is the rate at which electric charge flows through a conductor, measured in amperes (A). One ampere equals one coulomb of charge passing a given point per second. Current is driven by voltage (the electrical pressure) and opposed by resistance (the friction). Using Ohm's Law, current equals voltage divided by resistance (I = V / R), or equivalently, power divided by voltage (I = P / V). Understanding current is essential for sizing wires, selecting fuses and circuit breakers, and ensuring that components operate within their safe limits.
Determine whether you know the voltage and resistance (to use I = V / R) or the power and voltage (to use I = P / V). Check the device nameplate, datasheet, or measure with a multimeter.
Use volts for voltage, ohms for resistance, and watts for power. If resistance is given in kilohms, convert to ohms by multiplying by 1,000. Mixing units is the most common source of errors.
For Ohm's Law: divide voltage by resistance. For example, 12V across a 240 ohm resistor gives 12 / 240 = 0.05A (50 mA). For power: divide watts by volts. A 60W bulb at 120V draws 60 / 120 = 0.5A.
Use a clamp meter around a single conductor, or wire an ammeter in series with the load, to confirm the calculated value. Ensure your meter is rated for the expected current range before measuring.
Exceeding a wire's current rating causes overheating, insulation degradation, and potentially fire. Calculating current ensures you select properly rated wires, fuses, and circuit breakers.
Knowing the current draw of a device lets you estimate runtime from a battery. A 2,000 mAh battery powering a 200 mA device lasts approximately 10 hours under ideal conditions.
LEDs, transistors, and ICs all have maximum current ratings. Exceeding these limits even briefly can cause permanent damage. Accurate current calculations protect your components.
Approximate current consumption at rated voltage for common household and electronic devices.
| Device | Voltage | Power | Current Draw |
|---|---|---|---|
| LED Indicator | 3.3 V | 0.066 W | 20 mA |
| USB Device | 5 V | 2.5 W | 500 mA |
| Smartphone Charger | 5 V | 10 W | 2 A |
| LED Light Bulb (10W) | 120 V | 10 W | 83 mA |
| Laptop Charger | 120 V | 65 W | 0.54 A |
| Microwave Oven | 120 V | 1,200 W | 10 A |
| Hair Dryer | 120 V | 1,800 W | 15 A |
| Electric Oven | 240 V | 5,000 W | 20.8 A |
| EV Charger (Level 2) | 240 V | 7,200 W | 30 A |
One ampere (A) equals 1,000 milliamps (mA). Small electronics like LEDs and sensors typically operate in the milliamp range, while household appliances draw current in full amps. Always pay attention to the unit prefix when reading datasheets.
Current through the human body above approximately 10 mA can cause muscle contraction, and above 100 mA through the heart can be fatal. However, high current at low voltage (like a car battery) is less dangerous through skin because dry skin resistance limits the current that can enter the body.
In AC circuits with capacitors or inductors, you must use impedance (Z) instead of simple resistance (R). Impedance accounts for the phase-shifting effects of reactive components. For purely resistive AC loads, I = V / R still applies correctly using RMS values.
Use a clamp meter, which measures the magnetic field around a conductor to determine current without requiring physical contact with the wire. This is the safest and most convenient method for measuring current in live circuits, especially at high current levels.
The supply voltage will drop, the supply may overheat, or its protection circuit will shut it down. Well-designed power supplies have overcurrent protection that limits output or disconnects the load. Always ensure your supply is rated for at least 20% more than your maximum expected current draw.