Voltage Drop FAQ

Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. As current travels through a conductor (like a wire), the inherent resistance of the material causes a portion of the voltage to be lost as heat before reaching the load.

While specific requirements vary by region and local codes, a common engineering guideline is to maintain a voltage drop of no more than 3% for lighting circuits and 5% for other loads. These are standard recommendations to ensure equipment operates efficiently and safely.

First, find the cable resistance (R) using R = ρ × L / A, where ρ is the resistivity of the material. For Single Phase, the formula is: Vd = 2 × I × R (accounting for the return wire). For a balanced Three Phase system, the formula is: Vd = √3 × I × R.

To reduce voltage drop, you can: 1) Increase the cable size (cross-sectional area) to lower resistance, 2) Decrease the length of the cable run, 3) Reduce the load current, or 4) Use a material with lower resistivity (like copper instead of aluminium).

Copper is a better conductor (lower resistivity) than aluminium, meaning a copper wire will have less voltage drop than an aluminium wire of the exact same size. However, aluminium is lighter and often more cost-effective for large distribution lines.

In our calculator and standard formulas, you should enter the one-way distance from the source to the load. The formulas automatically account for the return path.

Yes, this calculator is completely free to use. It accurately implements standard engineering formulas based on standard resistivity values at 20°C.

No. This tool provides estimates for planning and educational purposes only. Always verify calculations against your specific local regulations (NEC, IEC, BS 7671, etc.) and consult a qualified engineer.

Equipment at the end of the cable receives less voltage than it needs. Lights may dim, motors can run hot or struggle to start, electronics may behave unreliably, and the lost voltage is wasted as heat in the cable.

Current flows to the load and back again, so it passes through the cable twice. That is why single phase uses Vd = 2 × I × R.

In a balanced three-phase system the three line currents are 120° apart in phase, so the line-to-line voltage drop works out to √3 (about 1.732) × I × R.

No. It uses conductor resistance at 20°C and ignores power factor and reactance. Treat the result as an estimate and check final designs against your local standards.