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MasterMath

Voltage Drop Calculator

How much voltage is lost along the way on a circuit, with the size of cable you already have or the one you are considering.

Currency and number format for

Voltage drop

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Voltage drop—
In volts—
Size used—
Minimum size for 3 %—
Power of the circuit—
Conductor—

How this was worked out

    Indicative result. The sum is the physics of voltage drop and is for checking an installation. The permitted limits, the minimum size for each circuit and its protection are set by the wiring regulations of each country. An electrical installation is signed off by a professional.

    The formula

    ΔV = factor × ρ × length × current ÷ cross-section

    Where it comes from

    It is the same formula as the sizing one, rearranged. Here the cable is known and what you want is how much voltage arrives at the far end: at 230 volts, a 3 % drop is nearly seven volts missing at the socket. That matters because a motor starts worse on low voltage and a light fitting loses output, and above all because that lost voltage does not disappear: it turns into heat inside the cable.

    How to work it out by hand

    1. Multiply the system factor by the resistivity and the length
    2. Multiply by the current
    3. Divide by the cross-section: those are the volts lost
    4. Compare them with the supply voltage for the percentage

    What is worth knowing

    It is worth understanding where that voltage goes: it is dissipated as heat inside the conductor, and that heat is wasted power paid for on the bill, which also warms the cable. A cable running hot increases its resistance, so more voltage drops and it heats further still. That is why drop limits are not only about appliances working properly: they are a thermal limit too. If an existing installation shows a high drop, the fixes are a larger size, a shorter route, or splitting the load across several circuits.

    Frequently asked questions

    How is voltage drop calculated?

    Multiply the system factor by the resistivity, the length and the current, then divide by the cross-section of the cable.

    How much voltage drop is acceptable?

    Regulations usually allow 3 % for lighting and 5 % for the rest, measured from the board. Check the rules where you are.

    What happens if the drop is too high?

    Motors start worse, light fittings lose output and the cable warms up, because the lost voltage turns into heat inside the conductor.

    How do I reduce voltage drop?

    A larger cross-section, a shorter route, or splitting the load across several circuits. Cross-section is the only one you can change without redoing the route.

    Why does three phase drop less?

    Because the factor in the formula is √3 rather than 2: the current does not have to go and return along two conductors.