Cable voltage drop calculator
How much of your supply voltage is lost in the cable before it reaches the spindle, and how much of it turns into heat on the way. Everything here comes from the resistivity of the metal — no tables, no maker data.
The calculation
Voltage drop in a run
Length is the one-way distance, not the total copper you buy. Cross-section is per conductor.
What the number means
Three numbers come out, and they answer three different questions:
- Drop in volts is what the load actually sees missing. Subtract it from the supply and you have the voltage at the spindle terminals under load.
- Drop as a percentage is the one that gets compared against a limit. Whether the limit is 3 %, 5 % or anything else is a decision made by your installation rules and your drive's tolerance of a low supply — this site does not publish a number for it, for the reason given on every page: it is maker- and installation-specific.
- Power lost in the cable is heat you are paying for twice: once as electricity, and once as whatever it takes to get the heat out of the enclosure. On a long run at high current it is not a rounding error.
The table below holds the current and the length fixed and walks through standard conductor sizes, which is usually the faster way to answer "is my cable too thin" — you are rarely choosing the length, and you are often choosing the cable.
On the output side of a VFD
The calculation above is a DC-resistance calculation. It is sound for the supply side of the drive, where the cable carries 50 or 60 Hz. The output side is not 50 Hz — it is a PWM waveform at the drive's carrier frequency, and two things stop being negligible there:
- Skin effect. Current crowds towards the surface of the conductor as frequency rises, so the effective resistance is higher than the DC value. The higher the carrier frequency, the more this matters.
- Cable impedance, not just resistance. Over a long run the cable's inductance and capacitance combine with fast voltage edges at the drive to produce reflected waves, which can put voltage peaks at the motor terminals well above what the drive is outputting.
Both make the real situation worse than the number printed here, in the same direction — so the result is a floor, not a ceiling, for the motor side. This is also why this page does not publish a maximum cable length. The figure depends on the drive's carrier frequency, the insulation system of your particular spindle, and whether you have fitted an output reactor or a dV/dt filter. Those are all maker-specific, and a single number borrowed from another machine is worse than no number. Read cables, reactors and filters for what actually mitigates it.
When it matters and when it does not
- Long supply runs to a workshop sub-panel — this is where drop is usually worst, because the current is the whole machine's and the length is the building's.
- Single-phase supplies — the same current and length produce a larger percentage drop than on three-phase, because the return conductor is also carrying it and the voltage is lower to begin with. See single-phase input.
- Thin cable on a big spindle — a 2.2 kW spindle on a long 1.5 mm² run is the classic case. The drive trips on undervoltage at acceleration and nothing looks wrong at idle.
- Short runs inside one enclosure — usually negligible. If your number came out under about half a per cent, this is not your problem.
Related
- Cables, reactors and filters — what to do about the high-frequency half of the problem
- VFD ↔ spindle wiring diagram — including how the protective conductor is sized
- VFD sizing calculator — the current that goes into the box above
- Unit converter — mm² to AWG is not here; use the cross-section from the cable jacket
Parts
Shielded cable & connectors
Questions
Is this the same as the maximum cable length a maker publishes?
No, and this site deliberately does not publish that number. It depends on your drive's carrier frequency, the insulation system of your spindle and whether you have fitted an output reactor or filter — all maker-specific.
Why does the real drop on the motor side come out higher than this?
Because the drive output is PWM at the carrier frequency, not 50 Hz. Skin effect raises the effective resistance and the cable's own inductance and capacitance start to matter, so the figure here is a floor for the motor side.
What do I do if the drop is too large?
Go up a conductor size — that is the only fix that reduces resistive drop. If the drop is small and a long motor run still misbehaves, it is a high-frequency problem, and reactors and filters are the answer rather than thicker copper.
Sources and method
Three kinds of statement appear on this page, and each is labelled where it occurs:
- Derived — resistance from
R = ρ L / A, three-phase drop fromΔV = √3 · I · R, single-phase drop fromΔV = 2 · I · R. Resistivity at 20 °C is 1.7241×10-8 Ω·m for copper and 2.8264×10-8 Ω·m for aluminium, corrected for temperature with α = 0.00393 and 0.00403 per kelvin respectively. All of it is printed with the result. - Rule of thumb — workshop practice, not a standard. Given with the range it is meant to hold over, and it loses to your manual wherever the two disagree.
- Maker-specific — parameter numbers, connector pin numbers, minimum brake resistance, maximum cable length, bearing speed limits. Not published here. They move between firmware revisions of the same model, and a wrong one is worse than none.
Standards and documents
- IEC 60228 — conductors of insulated cables: nominal cross-sections and resistance classes
- IEC 60204-1 — electrical equipment of machines: general requirements
- IEC 61800 series — adjustable speed electrical power drive systems
- The manual for your own drive and spindle — the only valid source for maker-specific values