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Ohm's law and voltage drop calculator

Solve for volts, amps, ohms or watts, and estimate voltage drop on a run.

Updated: Oct 2026.

For learning only. Check your code book. These calculators are study aids, not design tools. Always follow the code that applies where you work, the manufacturer's instructions and your supervisor. Nothing you type is sent to us or anyone else.

Ohm's law

Enter any two values and leave the others blank. Uses V = I × R and P = V × I.

Voltage drop (simplified)

Enter the conductor resistance per kilometre from your code book's tables or the manufacturer's data. This estimate uses resistance only: it ignores reactance, power factor and temperature, which your code book's method may include.

Studying for the Construction Electrician exam? See the Construction Electrician guide and our free practice sample.

Sources

  1. NIST — SI units (opens in a new tab)
  2. Red Seal Program — Construction Electrician (formulas provided at the exam) (opens in a new tab)

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