Voltage Drop Calculator

Supply system
Conductor material

Resistivity is a nominal value at 20°C. A fully loaded cable typically runs hotter, so the real drop in service will usually be somewhat higher than reported here.

Diagram
DVoltage drop5.60V
Drop (% of nominal)2.43%
Voltage at load224.4V

About this calculator

This calculator estimates the voltage drop over a single cable run — enter the supply system (single-phase 230V or three-phase 400V), the load current, the cable's one-way length, its conductor cross-section, and whether the conductor is copper or aluminium. It reports the drop in volts, the drop as a percentage of the nominal voltage, and the voltage actually arriving at the load.

Formula

single-phase:  DeltaU = 2 x L x I x rho / A
three-phase:   DeltaU = sqrt(3) x L x I x rho / A

L is the cable's one-way length in metres, I is the load current in amps, A is the conductor cross-section in mm², and rho is the conductor's resistivity in Ohm x mm² / m. A single-phase circuit counts the run's resistance twice — once out on the live, once back on the neutral — which is where the factor of 2 comes from. A balanced three-phase load's line-to-line drop uses a factor of sqrt(3) instead.

rho is a nominal value at 20°C:

copper:     0.0175 Ohm x mm2 / m
aluminium:  0.0286 Ohm x mm2 / m

These are standard reference figures for conductor resistivity at 20°C, not this calculator's own invention — but they are still nominal. A real conductor's resistance rises with temperature, and a fully loaded cable commonly runs well above 20°C, so the actual drop in service will typically be somewhat higher than the figure this calculator reports. An info note is shown alongside every result as a reminder of this.

drop percent      = DeltaU / nominal voltage x 100
voltage at load   = nominal voltage - DeltaU

Nominal voltage is 230V for single-phase, 400V for three-phase.

Worked example

32A, 25m one-way, 4mm² copper, single-phase:

DeltaU = 2 x 25 x 32 x 0.0175 / 4
       = 50 x 32 x 0.0175 / 4
       = 1600 x 0.0175 / 4
       = 28 / 4
       = 7.0 V

drop percent    = 7.0 / 230 x 100 = 3.04%
voltage at load = 230 - 7.0 = 223.0 V

3.04% is over the 3% guidance threshold this calculator uses for a lighting circuit, so this exact example is shown with a warning, not a clean pass.

FAQ

Why 3% and 5%, specifically? These are widely repeated design-guidance figures — commonly around 3% for a lighting circuit and 5% for a general power circuit, measured from the origin of the installation to the load — not a single clause of one specific EU or US standard. Different guides and different jurisdictions state slightly different numbers. Treat the warning/error here as a useful rule of thumb, not a code citation, and check your own jurisdiction's actual requirement if this circuit needs to be signed off against one.

Does this calculator size the cable or select a breaker for me? No. It only reports the drop for the cross-section you already chose. Cable sizing (matching a conductor to a current-carrying capacity, ambient temperature, and installation method) and breaker/protective-device selection are both jurisdiction-dependent and are deliberately out of scope here.

Why does the drop change so much between copper and aluminium at the same cross-section? Aluminium's resistivity (0.0286 Ohm·mm²/m) is roughly 1.6x copper's (0.0175 Ohm·mm²/m), so the same run in aluminium drops proportionally more voltage — this is exactly why an aluminium conductor is typically specified one or two sizes larger than the copper conductor it replaces.

Why is the reported drop likely to be a slight underestimate in a hot cable? Because rho is fixed at its nominal 20°C value. A real conductor's resistivity — and therefore the real voltage drop — rises with temperature, and a cable carrying its rated current for any length of time typically runs well above 20°C. This calculator does not model that temperature rise.

What standard does this calculator cite? None. The drop formula itself is plain conductor-resistance physics, and the 3%/5% guidance thresholds are widely repeated design practice rather than one citable clause — see the FAQ entry above. meta.standards is deliberately left empty.

Assumptions and limits

No standard is cited for the 3%/5% thresholds used here — see the FAQ above. rho is fixed at its nominal 20°C value for both conductor materials; real in-service resistance is somewhat higher whenever the cable is meaningfully loaded. This calculator does not perform cable sizing (ampacity/derating) or breaker selection — both are explicitly out of scope, since they are jurisdiction-dependent in a way this calculator's arithmetic deliberately is not.

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