Voltage drop calculator
Voltage drop is the most common reason a circuit that passes on paper misbehaves in practice. It scales with length and current and inversely with area.
A
mm2
V
Voltage drop
3.598%
- Voltage lost
- 8.2752 V
- Voltage at the load
- 221.725 V
- Power lost as heat
- 165.50
- Minimum area for 3% drop
- 2.998 mm2
Show the working
- Current = 20
- One-way run length = 30
- Conductor cross-section = 2.5
- Supply voltage = 230
- System = 2
- Conductor = 1.724e-8
- Conductor temperature = 20
- r_per_m = 1.724e-8 / (2.5 * 1e-6) * (1 + 0.00393 * (20 - 20)) = 0.006 896
- Voltage lost = 2 * 20 * 0.006 896 * 30 = 8.2752
- Voltage drop = 8.2752 / 230 * 100 = 3.597 91
- Voltage at the load = 230 - 8.2752 = 221.725
- Power lost as heat = 8.2752 * 20 = 165.504
- Minimum area for 3% drop = 2 * 20 * 1.724e-8 * 30 / (230 * 0.03) * 1e6 = 2.998 26
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- drop = k I rho L / A, with k = 2 for single phase and sqrt(3) for three phase
- resistivity is temperature-corrected at 0.00393 per kelvin for copper
Most codes suggest 3% on a branch circuit and 5% overall. These are guidelines about performance, not usually hard legal limits.
Reactance is ignored, which is fine for small cables and increasingly wrong above about 50 mm2.