Electrical Exam · Free practical tool

Voltage drop calculator

Follow conductor resistance and the complete electrical path in a licensing calculation. State whether the supply is DC, single-phase AC or balanced three phase before choosing the multiplier.

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The method

DC: ΔV = 2LIr. AC: ΔV ≈ kLI(r cosφ + x sinφ), where k = 2 single-phase or √3 three-phase.

Before using the result

  • Supply conductor resistance at its operating temperature; use one-way route length. AC formula is the usual small-drop approximation for a lagging load. Three-phase voltage is line-to-line. No ampacity or code check is made.

Worked examples

These original training examples illustrate the method for Journeyman Electrician Core Knowledge Review (JE). Use the matching “Load example” button above to explore a variation; example values are not equipment settings or prescribed criteria.

1. Worked example

Follow conductor resistance and the complete electrical path in a licensing calculation. State whether the supply is DC, single-phase AC or balanced three phase before choosing the multiplier.

Circuit
DC two wire
One-way conductor length
30 m
Current (A)
10
Voltage (V)
230
Resistance per conductor (Ω/m)
0.007
  1. Path factor k = 2; route length = 30 m.
  2. Effective resistance term = 0.007 Ω/m.
  3. ΔV = 2 × 30 × 10 × 0.007 = 4.2 V.
  • Voltage drop: 4.2 V
  • Percentage drop: 1.826087 %
  • Estimated receiving voltage: 225.8 V

2. Comparison example

Compare a different supplied condition using the same method. For Electrical Exam, the important check is that the input meaning, units and method assumptions still match the problem.

Circuit
DC two wire
One-way conductor length
30 m
Current (A)
15
Voltage (V)
230
Resistance per conductor (Ω/m)
0.007
  1. Path factor k = 2; route length = 30 m.
  2. Effective resistance term = 0.007 Ω/m.
  3. ΔV = 2 × 30 × 15 × 0.007 = 6.3 V.
  • Voltage drop: 6.3 V
  • Percentage drop: 2.73913 %
  • Estimated receiving voltage: 223.7 V

Check your understanding

1. Which relationship is used?

DC: ΔV = 2LIr. AC: ΔV ≈ kLI(r cosφ + x sinφ), where k = 2 single-phase or √3 three-phase.

2. Which assumption should you check first?

Supply conductor resistance at its operating temperature; use one-way route length. AC formula is the usual small-drop approximation for a lagging load. Three-phase voltage is line-to-line. No ampacity or code check is made.

3. How should you verify a different unit system?

Convert every input to the displayed calculation units first. Equivalent physical inputs must give equivalent results; a unit label alone does not convert a number.

Continue with the matching exam topic

This tool connects to Journeyman Electrician Core Knowledge Review (JE) free practice. Work through a question independently, then use the calculation steps to identify where your method differs. The tool stays free whether or not you choose paid preparation.

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