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, which is why both are widely used. You simply need a larger aluminium cable to match the performance of copper.

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 (by multiplying by 2 for single-phase systems).

Yes, this calculator is completely free to use. It accurately implements standard engineering formulas based on standard resistivity values at 20°C. However, actual voltage drop in the field may vary due to factors like temperature, installation method, and load characteristics.

No. This tool provides estimates for planning and educational purposes only. It does not replace local electrical codes or standards (such as the NEC, IEC, or BS 7671). Always verify calculations and limits against your specific local regulations and consult a qualified electrical 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. If your result is over the limit, use a larger cable or shorten the run.

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, where R is the resistance of one conductor, and why you enter only the one-way length.

In a balanced three-phase system the three line currents are 120° apart in phase, so the voltage drop between two lines works out to √3 (about 1.732) × I × R instead of 2 × I × R. For three phase, use the line-to-line supply voltage (for example 400 V), not the phase-to-neutral voltage.

Small differences are normal. Calculators may use different resistivity values, a different conductor temperature, or include extra factors such as power factor and cable reactance. This calculator uses resistance at 20°C only, so treat it as an estimate and check final designs against your local standard.

No. It uses conductor resistance at 20°C and ignores power factor and reactance. A cable that is carrying load runs warmer than 20°C, and its resistance rises with temperature (roughly 20% higher at 70°C for copper), so the real drop is usually somewhat higher. Reactance can matter for large cables, so this calculator is best for quick checks and small to medium sizes.

Enter the line-to-line voltage, such as 400 V (or 415 V, 480 V, depending on your supply). The percentage drop is calculated against the value you enter.

Single phase: I = P ÷ V (for a resistive load; for motors and similar loads, I = P ÷ (V × power factor)). Three phase: I = P ÷ (√3 × V × power factor). Use the rated or maximum expected load, not the average.

Yes, for DC the drop in a two-wire circuit is also 2 × I × R, so choose “Single phase”, enter the DC voltage, the current and the one-way cable length. For low-voltage DC (12 V or 24 V) even a small drop is a large percentage, so keep the limit tight.

Yes. Switch the unit to feet and the calculator converts to metres before calculating. Cable sizes are in mm²; if you have AWG cable, convert it to the nearest mm² size using a chart before using the calculator.

It is the smallest standard size in the list that keeps the voltage drop within your chosen limit, for the inputs you entered. It only looks at voltage drop.

Not necessarily. Voltage drop is only one check. A real design must also consider current-carrying capacity (ampacity), installation method, grouping and ambient temperature, protective device rating, short-circuit withstand and local code requirements. Always use the larger of all the requirements, and confirm with a qualified electrician or engineer.

Resistance is inversely proportional to the cross-section, so doubling the cross-section roughly halves the voltage drop. For example, going from 4 mm² to 8 mm² halves the drop, though 8 mm² is not a standard size, so you would pick 10 mm².

Aluminium’s resistivity is about 60% higher than copper (0.0282 vs 0.0175 Ω·mm²/m), so you generally need a cross-section roughly 1.6 times larger for the same drop. Aluminium is lighter and cheaper, which is why it is often used for large feeders.

Yes. A motor can draw several times its running current when it starts, which causes a temporary, larger voltage drop. This calculator uses the current you enter, so for motors check the running case and, separately, the starting case against your motor supplier’s or local code’s limits.