Check how much voltage is lost in a cable run. Enter the current, cable size and length. You get the drop in volts and percent, the voltage left at the load and the power wasted as heat in the cable.
Resistance only, at normal operating temperature. Reactance and power factor effects on long or large cables are ignored, so treat the answer as a close estimate.
Formulas and assumptions
- Single phase: drop = 2 times I times length times resistivity, divided by cable area
- Three phase: drop = 1.732 times I times length times resistivity, divided by cable area
- Percent drop: drop divided by supply voltage, times 100
- Cable loss: number of current-carrying conductors times I squared times the resistance of one conductor
- Resistivity: 0.0225 ohm mm2 per metre for copper, 0.036 for aluminium
Example
A 20 A load on 30 metres of 4 mm2 copper at 230 V single phase loses 6.75 V, which is 2.93 percent. That is just inside a 3 percent limit, and the cable wastes about 135 W as heat.
Frequently asked questions
What causes voltage drop in a cable?
Every cable has resistance. When current flows, some voltage is lost along the way. Longer runs, higher current and smaller cables all increase the drop.
How can I reduce voltage drop?
Use a larger cable, shorten the run, raise the supply voltage (for example three phase instead of single phase) or split the load across more circuits.
What happens if voltage drop is too high?
Motors run hot and may fail to start, lights dim and electronics can misbehave. The cable also wastes more energy as heat.
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