Calculate current and evaluate circuit safety
Electrical current — measured in amperes (A) — is the flow of electric charge through a conductor. Ohm's Law defines the relationship between current, voltage, and resistance:
Current (I) = Voltage (V) ÷ Resistance (R)
For example, a 230V supply connected to a 46Ω load produces a current of 230 ÷ 46 = 5A.
The three Ohm's Law quantities are always related. If you know any two, you can find the third:
| To find | Formula | Example |
|---|---|---|
| Current (I) | V ÷ R | 12V ÷ 4Ω = 3A |
| Voltage (V) | I × R | 3A × 4Ω = 12V |
| Resistance (R) | V ÷ I | 12V ÷ 3A = 4Ω |
Current ratings matter for choosing the right wire size and circuit breaker. Exceeding the rated current causes heat buildup in the wire — sustained overheating degrades insulation and creates fire risk.
| Current range | Typical application | Wire size (copper) |
|---|---|---|
| <2A | Low-voltage electronics, signal cables | 0.5 mm² |
| 2–6A | Lighting circuits, small appliances | 1.0–1.5 mm² |
| 6–13A | Standard household outlets (UK/EU) | 1.5–2.5 mm² |
| 13–20A | High-power appliances, kitchen circuits | 2.5–4.0 mm² |
| 20–32A | Electric showers, EV chargers | 4.0–6.0 mm² |
Direct current (DC) flows in one direction — used in batteries, solar panels, and electronic circuits. Alternating current (AC) reverses direction at the grid frequency (50Hz in Europe, 60Hz in the US) — used for mains power distribution. Ohm's Law applies to both, but AC circuits also involve impedance from inductors and capacitors, which adds complexity beyond simple resistive loads.
Every conductor has a maximum current rating determined by its cross-sectional area and insulation type. Running more current than the wire is rated for causes resistive heating — the wire acts like a heating element. At moderate overloads, the insulation degrades over time. At severe overloads, it can ignite. Always use wire rated for at least 125% of the continuous load current.
Divide voltage by resistance. A 24V supply connected to a 12Ω resistor produces 24 ÷ 12 = 2A of current.
Standard household circuits in Europe are rated at 16A or 20A. In the US, most outlets are on 15A or 20A circuits. The safe current for continuous loads is 80% of the circuit rating — 12.8A on a 16A circuit, 16A on a 20A circuit.
Excessive current causes the wire to heat up. The circuit breaker should trip before damage occurs, but sustained marginal overloads can degrade insulation without tripping the breaker. Signs of overloaded wiring include warm outlets, flickering lights, and frequently tripping breakers.
Voltage is the electrical pressure that drives current through a circuit — like water pressure in a pipe. Current is the rate of charge flow — like the volume of water flowing per second. A high-voltage source can produce very little current if the resistance is high, and vice versa.
Yes. Most metals increase in resistance as temperature rises — this is called a positive temperature coefficient. This means a hot wire has higher resistance and draws less current than a cold one. At the extreme, tungsten filament bulbs start with very low cold resistance (high current surge) that increases dramatically once the filament heats up.