How do you calculate voltage drop in a cable?

Voltage drop is calculated using V_drop = I × R_cable, where R_cable = ρ × 2 × L / A. ρ is the resistivity of the conductor material (1.68×10⁻⁸ Ω·m for copper, 2.65×10⁻⁸ Ω·m for aluminum), L is the one-way cable length, and A is the cross-sectional area. The factor of 2 accounts for the round-trip (supply and return conductors).

What is the maximum acceptable voltage drop?

The IEC 60364 standard recommends a maximum voltage drop of 3% for lighting circuits and 5% for other circuits. The NEC (National Electrical Code) also recommends a maximum of 3% for branch circuits and 5% total for feeder plus branch circuits. Exceeding these limits can cause equipment malfunction and energy waste.

Is copper or aluminum better for reducing voltage drop?

Copper has lower resistivity (1.68×10⁻⁸ Ω·m) compared to aluminum (2.65×10⁻⁸ Ω·m), so copper cables produce about 40% less voltage drop for the same cross-section. However, aluminum is lighter and cheaper, making it common for long-distance power transmission where larger cross-sections compensate for the higher resistivity.

How does cable length affect voltage drop?

Voltage drop is directly proportional to cable length. Doubling the cable length doubles the voltage drop. This is why long cable runs in buildings, solar installations, and industrial plants require careful wire sizing to keep voltage drop within acceptable limits.

Voltage Drop Calculator

Calculate voltage drop across cables and wires for copper and aluminum conductors.

Component Values

Source Voltage (V)

Current

Cable Length (one-way)

Conductor Material

Cross-Section

Results

Cable Resistance (round trip)0.1344 Ω

Voltage Drop1.3440 V

AWG Wire Size Reference

AWG	mm²	Copper (Ω/km)	Aluminum (Ω/km)
10	5.261	3.19	5.04
12	3.309	5.08	8.01
14	2.081	8.07	12.73
16	1.309	12.83	20.24
18	0.823	20.41	32.20
20	0.518	32.43	51.16
22	0.326	51.53	81.29
24	0.205	81.95	129.27
26	0.129	130.23	205.43
28	0.081	207.41	327.16
30	0.0509	330.06	520.63

Understanding voltage drop in cables

Voltage drop is the reduction in voltage across a conductor due to its resistance. Every wire has resistance proportional to its length and inversely proportional to its cross-sectional area. The total cable resistance for a round-trip circuit is R = p x 2L / A, where p is the material resistivity, L is the one-way length, and A is the cross-section.

The IEC 60364 standard recommends that voltage drop should not exceed 3% for lighting circuits and 5% for other loads. The NEC similarly recommends 3% for branch circuits and 5% total. Excessive voltage drop wastes energy as heat in the cable and can cause equipment to malfunction, especially motors and sensitive electronics.

Copper (p = 1.68e-8 ohm.m) is the most common conductor material due to its low resistivity. Aluminum (p = 2.65e-8 ohm.m) has about 60% higher resistivity but is lighter and cheaper, making it popular for overhead power lines and large installations. To compensate, aluminum cables use a larger cross-section than copper for the same current capacity.

For long cable runs -- solar panel arrays, industrial plants, EV charging stations -- voltage drop calculation is critical. Increasing wire gauge (larger cross-section) reduces resistance and voltage drop. The trade-off is cost and weight. Always verify that your cable sizing satisfies both current capacity (ampacity) and voltage drop requirements.

Cable Resistance

R_cable = ρ × 2L / A

Voltage Drop

V_drop = I × R_cable

Drop Percentage

Drop% = (V_drop / V_source) × 100

Key Points

Voltage drop is proportional to current and cable length
IEC 60364: max 3% for lighting, 5% for other circuits
Copper has ~40% less resistivity than aluminum
Doubling the cross-section halves the voltage drop
Round-trip length = 2 x one-way cable length

Applications

Residential and commercial wiring design
Solar panel array cable sizing
EV charging station installation
Industrial plant power distribution

AWG quick-reference table (copper wire)

AWG	Diameter (mm)	Area (mm²)	Max Current (A)	Resistance (mΩ/m)
10	2.59	5.26	30	3.28
12	2.05	3.31	20	5.21
14	1.63	2.08	15	8.28
16	1.29	1.31	13	13.2
18	1.02	0.82	10	20.9
20	0.81	0.52	7	33.2
22	0.64	0.33	5	52.9
24	0.51	0.20	3	84.2

NEC / IEC voltage drop guideline

NEC/IEC guideline: voltage drop should not exceed 3% for branch circuits or 5% total (feeder + branch combined). For sensitive electronics — microcontrollers, sensors, audio — keep drop below 2% to avoid measurement errors and logic instability.

Practical example

12V LED strip — 3A, 5m run (18 AWG copper)

Calculate round-trip voltage drop for a 5-metre LED strip cable run using 18 AWG copper wire.

R = 2 × 5m × 20.9 mΩ/m = 209 mΩ
Drop = 3A × 0.209Ω = 0.63V (5.2%) — exceeds 5% limit
Solution: upgrade to 16 AWG (13.2 mΩ/m) → drop = 0.40V (3.3%)

The factor of 2 everyone forgets

The mistake everyone makes is forgetting the factor of 2. Current leaves the source, runs down the wire to the load, and comes all the way back — so the copper it travels through is twice the one-way length. Drop the 2 and your calculated drop is half of reality, which is exactly the error that leaves an LED strip dim at the far end.

Voltage drop (round trip)

Vdrop = 2 × ρ × L × I / A

L is the one-way run; the 2 covers the return leg. ρ_copper = 1.68×10⁻⁸ Ω·m at 20 °C, aluminium = 2.82×10⁻⁸ Ω·m — aluminium needs about 60% more cross-section to match copper.

AWG to mm² reference (copper resistance)

European installers spec cable in mm², American ones in AWG, and a voltage-drop number means nothing until you've pinned the cross-section. The copper resistance column is the one-way mΩ per metre at 20 °C — double it for the round trip in the formula above.

AWG	mm²	Copper (mΩ/m)
10	5.26	3.28
12	3.31	5.21
14	2.08	8.29
16	1.31	13.2
18	0.823	21.0
20	0.518	33.3
22	0.326	52.9
24	0.205	84.2

Need the full AWG list with ampacity and aluminium figures? The wire gauge calculator has it, and the resistor power calculator tells you how much heat that I²R drop actually dumps into the cable. The numbers above come straight from the American wire gauge standard.

The 3% rule and temperature derating

Electrical codes draw the line at 3%: keep the drop on any branch circuit at or below 3%, and the whole run — feeder plus branch — under 5%. For a 12 V system that 3% is only 0.36 V, which is why low-voltage runs eat cable so fast.

And the table values are cold values. Copper resistivity rises about 0.39% per °C, so a cable sitting at 70 °C — bundled in conduit on a hot day, or carrying near-rated current — has roughly 20% more resistance than the 20 °C number suggests. Size for the temperature the cable will actually run at, not the bench.

Worked examples

12 V LED strip — 5 m run at 2 A

A 5 m feed in 18 AWG (21.0 mΩ/m). Watch the round trip bite into a tight 12 V budget.

Vdrop = 2 × 5 m × 0.021 Ω/m × 2 A = 0.42 V (3.5%)
Over the 3% line. Step up to 16 AWG (13.2 mΩ/m) → 0.26 V (2.2%).

12 V RV accessory — 6 m run at 8 A

A fridge or fan wired from the battery to the back of an RV. Long automotive 12 V runs are where this hurts.

16 AWG: 2 × 6 m × 0.0132 Ω/m × 8 A = 1.27 V (10.6%) — far too much.
Use 12 AWG (5.21 mΩ/m) → 0.50 V (4.2%).

Solar panel to charge controller — 10 m at 6 A

A 12 V panel feeding a PWM controller 10 m away. Long, low-voltage runs are exactly where drop steals your harvest.

10 AWG (3.28 mΩ/m): 2 × 10 m × 0.00328 Ω/m × 6 A = 0.39 V (3.3%)
Acceptable, but an MPPT controller at higher array voltage would cut the percentage hard.

Did you know? Voltage drop in long cable runs is a critical issue in solar installations and marine/automotive wiring. A 10 m run of AWG 18 wire at 5 A can drop nearly 1 V — enough to cause a 5 V microcontroller to brown-out or an LED strip to visibly dim at the far end.