Does this calculator check current-carrying capacity?

No. This tool computes voltage drop only. Ampacity depends on installation method, grouping, ambient temperature and the wiring code that applies in your jurisdiction, and must be verified separately against that code.

Cable Voltage Drop Calculator

Q: What voltage drop is acceptable?

Common practice is 3 percent for lighting circuits and 5 percent for other loads, measured from the source to the farthest outlet. IEC 60364-5-52 and the NEC informational note to 210.19(A) both present these as recommendations rather than hard limits. Solar DC arrays are usually designed tighter, at 1 to 2 percent, because every volt lost is energy never delivered.

Q: Why does conductor temperature change the result?

Copper resistivity rises about 0.393 percent per degree Celsius and aluminium about 0.403 percent. A conductor calculated at the 20 degree reference but running at 70 degrees has roughly 20 percent higher resistance, so the real drop is about 20 percent worse than the cold figure.

Q: Is the three-phase result line-to-line or line-to-neutral?

Line-to-line. The calculator applies the square root of three factor to the resistance of one conductor, which gives the drop in the line-to-line voltage when the load is balanced.

Size a conductor so the load actually gets the voltage it needs. Handles DC, single-phase and three-phase circuits, copper or aluminium, with temperature correction and parallel runs. Runs entirely in your browser — nothing is uploaded.

What this calculator does

It answers one question: given this current, this distance and this conductor, how much voltage is lost on the way, and is that acceptable. It also works backwards — tell it the drop you can live with and it returns the smallest standard size that gets you there.

The calculation

ρ(T) = ρ₂₀ × [1 + α × (T − 20)]
R = ρ(T) × L ÷ (A × n)
ΔV = I × 2R (DC and single-phase)
ΔV = √3 × I × R × cos φ (three-phase, line-to-line)
P_loss = I² × 2R or 3 × I² × R

Where ρ₂₀ is 17.241 Ω·mm²/km for copper and 28.264 Ω·mm²/km for aluminium, α is 0.00393 /°C for copper and 0.00403 /°C for aluminium, L is one-way length, A is conductor area and n is the number of parallel runs per phase.

Temperature is the part people skip

Reference tables are quoted at 20 °C. A cable sized from those tables and then run at 70 °C has about 20 % more resistance than the table implies, so the drop is about 20 % worse than the cold calculation. On long runs that is the difference between passing and failing. Set the temperature field to the conductor's expected hot-running value, not the ambient.

Worked example

A 1000 V DC solar string: 13 A, 120 m of 4 mm² copper, conductor at 60 °C.

ρ(60) = 17.241 × [1 + 0.00393 × 40] = 19.95 Ω·mm²/km
R = 19.95 × 0.120 ÷ 4 = 0.599 Ω
ΔV = 13 × 2 × 0.599 = 15.6 V → 1.56 %
P_loss = 13² × 2 × 0.599 = 202 W

1.56 % passes a 3 % limit comfortably, but 202 W is not free — across a year of full-load operation that is about 1 770 kWh of energy turned into heat in the cable. Real arrays spend most of the day below peak, so treat that figure as an upper bound, but it is exactly why solar DC sides are usually designed to 1–2 % rather than the 3 % used for building wiring.

What this tool does not do. It calculates voltage drop only. It does not check current-carrying capacity, short-circuit withstand, or protective-device coordination — those depend on installation method, grouping, ambient conditions and the wiring code in force where you are. Confirm ampacity against your local code before ordering cable.

Which drop limit should you use?

3 % — the usual figure for lighting and general building circuits.
5 % — often allowed for other loads, and for feeder plus branch circuit combined.
1–2 % — solar DC arrays and battery leads, where lost volts are lost energy.
Motor starting — inrush can be 6× running current; check the drop at starting current, not just at running current, or the contactor will chatter.

Both IEC 60364-5-52 and the NEC informational note to 210.19(A) present 3 %/5 % as recommendations rather than mandatory limits, so the figure is a design choice. Underground distribution and utility work often uses different figures again.

Frequently asked

What voltage drop is acceptable?

Common practice is 3 % for lighting and 5 % for other loads, measured to the farthest outlet. Solar DC arrays are usually designed to 1–2 %, because every volt lost is energy never delivered.

Why does conductor temperature change the result?

Copper resistivity rises about 0.393 % per °C, aluminium about 0.403 %. Calculated at 20 °C but running at 70 °C, a conductor has roughly 20 % higher resistance, so the real drop is about 20 % worse than the cold figure.

Does this check current-carrying capacity?

No. This tool computes voltage drop only. Ampacity depends on installation method, grouping, ambient temperature and your local wiring code, and must be verified separately.

Is the three-phase result line-to-line or line-to-neutral?

Line-to-line. The √3 factor is applied to the resistance of one conductor, giving the drop in line-to-line voltage under balanced load.

Can I use it for aluminium?

Yes — select aluminium and the resistivity and temperature coefficient switch accordingly. Expect roughly 1.6× the drop of an equivalent copper conductor.