Foundations
Ohm’s law: voltage
- where
- voltage (V)
- current (A)
- resistance (Ω)
Explain this equation
Works out the voltage across a resistance carrying a known current. Used for voltage drop along a conductor or across one part of a series circuit.
V is the voltage across the resistance. I is the current through it and R is its resistance. The product tells you how much of the supply is used up pushing current through that part.
Voltage drop is proportional to both current and resistance, because each ampere needs R volts to force it through R ohms.
Sense check: 2 A through 6 Ω drops 12 V. A cable loop of 0.1 Ω carrying 30 A drops 3 V.
Watch out: in a series circuit every part carries the same current, but each has its own drop. Do not apply the full supply voltage to each part.
The voltage drop across a resistor is current multiplied by resistance. This is also useful when assessing the drop along a known conductor resistance.
A series circuit has one current, but each resistance can have a different voltage drop. Calculate each drop using that same current.
Enter your values to see the result.
Common mistake
Applying the whole supply voltage to every series resistor.
Limits
DC resistance or a purely resistive AC element using consistent RMS quantities.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.