Utility
Voltage Drop Calculator
Volts lost down a cable run, single-phase, three-phase or DC, in mm² or AWG. Conductor temperature and stranding are inputs, because both move the answer — and the resistance figures reproduce the standards' own tables.
The run
- One way, not there and back. Single-phase and DC current returns along a second conductor, and the calculation already doubles the length for it; three-phase uses √3 instead.
- Applied as cos φ. Reactance is not modelled, which is fine for the smaller sizes where resistance dominates; above about 50 mm², or on a long run at a poor power factor, use the cable maker's mV/A/m figures instead.
The cable
- Resistance climbs about a fifth between 20 °C and 70 °C, so a run that passes cold can fail at working temperature. 20 °C is the reference tables are quoted at, 70 °C is PVC at its rating, 90 °C is XLPE, and 75 °C is what NEC Chapter 9 Table 8 uses.
The limit to check against
Neither figure is offered as the rule. NEC sets no mandatory limit and recommends 3% on a branch circuit with 5% overall in informational notes; IEC-derived standards publish 3% for lighting and 5% for other uses as recommended values. The binding number is whatever your local wiring rules or the specification say.
The drop
2.69%
6.19 V
Within the 3% limit.
- Voltage at the load
- 223.8 V
- Conductor resistance
- 5.157 mΩ/m
- Smallest size that passes
- 4 mm²
Smallest that passes on voltage drop alone. Whether it can carry the current is the separate question above, and this figure does not answer it. "Nothing in the range" means the run is too long for the sizes offered — that wants a different approach, not a bigger number.
How to use it
- Choose the supply, then enter the nominal voltage, the load current and the one-way run length.
- Pick the conductor material, the size ladder your drawings use, and the cross-section.
- Set the conductor temperature to what the insulation is rated for, and tick stranded if it is.
- Choose the limit your wiring rules or specification impose.
- Read the drop, and use the smallest passing size only as the voltage-drop half of a sizing decision.
How it works
Resistance per metre is resistivity divided by the nominal cross-section, corrected for temperature and, if stranded, multiplied by 1.02 for the lay of the strands.
The drop is that resistance times the current, the length, and a factor for the supply: 2 for DC and single phase because the current returns along a second conductor, √3 for three phase.
Power factor is applied as cos φ against the resistive drop. Reactance is not modelled, which is why the page points at the cable maker's figures for large sizes.
The smallest passing size walks the chosen ladder from the bottom and stops at the first that comes in under the limit — on voltage drop only.
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Common questions
Can I use this to pick a cable size?
Only for the voltage drop half of the decision. Cable size is fixed first by current-carrying capacity — what the cable can pass without overheating — which depends on the insulation, the installation method, grouping with other circuits, ambient temperature and the protective device, and comes from your standard's tables. Once a size passes on capacity, check here that it also passes on drop. Doing it the other way round is how a cable ends up undersized.
Do I enter the length one way or both?
One way — the distance from the board to the load. The return conductor is already accounted for: single-phase and DC double the length, and three-phase uses √3 rather than 2, which is where the difference between the two comes from.
Why does conductor temperature matter?
Because copper resistance rises about 0.4% per degree, so a conductor at 70 °C has roughly 20% more resistance than the same one at 20 °C. A run checked at the reference temperature can be comfortably inside the limit and outside it once the cable is loaded and warm. Pick the temperature your insulation is rated to rather than the one the table was printed at.
Where do the resistance figures come from?
From resistivity and the nominal cross-section, not from a copied table: copper at 1/58 Ω·mm²/m at 20 °C, corrected for temperature. That reproduces NEC Chapter 9 Table 8 to three figures for solid conductors, and for stranded once the 2% lay factor is applied — which is the check that the constants are right rather than merely consistent.
Is 3% or 5% the correct limit?
Neither, as a general answer. NEC has no mandatory limit and recommends 3% on a branch circuit and 5% overall in informational notes; IEC-derived standards publish 3% for lighting and 5% for other uses. What binds you is the local wiring regulation or the project specification, so both are offered and you choose.