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Voltage Drop Calculator and Wire Size Finder

How the voltage drop calculator works: ΔV = 2 · L · I · ρ / A (three-phase: √3 instead of 2, times the power factor). 25 m of 2.5 mm² copper carrying 16 A at 230 V drops 5.5 V = 2.4 %. It can also size the wire for a maximum voltage drop.

Source: IEC 60364-5-52, IEC 60364-5-52:2009 – Low-voltage electrical installations, wiring systems; Annex G (informative): voltage drop 3 % lighting / 5 % other uses. Updated: .

V
A
1 for resistive loads (heaters), motors often 0.8–0.9
m
distance from source to load (feet × 0.3048), not the round trip
mm²
1.5 mm² ≈ 16 AWG, 2.5 mm² ≈ 13–14 AWG, 4 mm² ≈ 12 AWG
°C
hotter in operation: at 70 °C resistance is about 20 % higher
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Result

Result
ΔV = 5.517 V (4.6 %)
Values
QuantityValue
Voltage drop5.517 V (4.6 %)
Voltage at the load114.5 V
Current16 A
Power lost in the cable88.28 W
Resistivity ρ0.01724 Ω·mm²/m
Step by step
  • Resistivity of copper at 20 °C: ρ = 0.01724 Ω·mm²/m
  • Voltage drop: ΔV = 2 · L · I · ρ · 1 / A = 2 · 25 m · 16 A · 0.01724 · 1 / 2.5 mm² = 5.517 V (4.6 %)
Above 3 %: the NEC informational notes and IEC 60364-5-52 (Annex G) recommend about 3 % for a branch circuit or for lighting. The wire must also carry the current continuously (ampacity per installation method, e.g. NEC Table 310.16) and match the overcurrent protection – this calculator does not check that.

How it is calculated

Voltage drop formulas

Part of the voltage is lost in the resistance of the cable. With one-way length L, current I, cross-section A and resistivity ρ:

The factor 2 accounts for the outgoing and return conductor. Rearranged, the required wire size is A = 2 · L · I · ρ · PF / ΔV. Inductive reactance is neglected, which is accurate enough for typical building wiring up to about 35 mm² (2 AWG).

Resistivity values

Copper: ρ = 1/58 Ω·mm²/m = 0.0172 at 20 °C (IEC 60028); aluminium: 0.0283 Ω·mm²/m (IEC 60889). Resistance rises by 0.39 % (copper) or 0.40 % (aluminium) per kelvin – set the conductor temperature for a more realistic result.

Examples

ApplicationCalculationResult
120 V circuit, 15 A, 20 m, 2.5 mm² (≈ 13 AWG)2 · 20 · 15 / (58 · 2.5)4.1 V = 3.4 %
230 V circuit, 16 A, 25 m, 2.5 mm²2 · 25 · 16 / (58 · 2.5)5.5 V = 2.4 %
12 V solar, 10 A, 5 m, 2.5 mm²2 · 5 · 10 / (58 · 2.5)0.69 V = 5.7 %

At low voltage the same drop is a much larger percentage – 12 V runs need thick wire. For AWG sizes see the AWG to mm² converter.

How much voltage drop is acceptable?

The US National Electrical Code recommends in informational notes a maximum of 3 % for a branch circuit and 5 % for feeder plus branch circuit combined. The international wiring standard IEC 60364-5-52 (informative Annex G) recommends 3 % for lighting and 5 % for other uses when supplied from a public low-voltage network.

Limitations

This calculator checks voltage drop only. Ampacity (heating by installation method), short-circuit protection and breaker sizing must also be satisfied. Mains wiring should be done by a qualified electrician.

Frequently asked questions

How do you calculate voltage drop?

For single-phase: ΔV = 2 · L · I · ρ / A with ρ = 0.0172 Ω·mm²/m for copper. Example: 30 m, 10 A, 1.5 mm² → 2 · 30 · 10 · 0.0172 / 1.5 = 6.9 V.

How do you calculate three-phase voltage drop?

ΔV = √3 · L · I · ρ · PF / A, relative to the line-to-line voltage. 50 m, 32 A, 6 mm², PF 0.9 gives 7.2 V = 1.8 % of 400 V.

How do I size a wire for voltage drop?

A = 2 · L · I · ρ / ΔV_max. For 40 m, 16 A and at most 3 % of 230 V (6.9 V): 2 · 40 · 16 / (58 · 6.9) = 3.2 mm² → 4 mm².

What wire size for 12 V DC?

At 12 V, 3 % is just 0.36 V. For 10 A over 5 m you need 2 · 5 · 10 / (58 · 0.36) ≈ 4.8 mm², so 6 mm² (about 10 AWG).

What is the maximum allowed voltage drop?

Common recommendations are 3 % for a branch circuit or lighting and 5 % in total (NEC informational notes, IEC 60364-5-52 Annex G).

Sources and legal basis

As of:

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