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All wires have a certain amount of resistance that corresponds to their diameter which will cause a certain amount of voltage in the circuit to be lost when current is flowing. High voltage drop in a circuit will cause a lower-than-expected voltage which can create performances issues but does not represent a safety hazard in itself. If a wire is improperly sized it can lead to significant loses in voltage that can cause loads to stop functioning or improper battery charging. Voltage drop must be examined for all circuits in an off-grid PV system.

The calculation can be performed easily by hand, but there are many different free calculators available online that are much quicker - see Special:MyLanguage/Resources. The factors that determine voltage drop in an off-grid PV system are:

• The maximum circuit current.
• The resistance of the conductor (wire) based upon its size.
• The length of the circuit.
• The operating voltage of the circuit.

The formula for calculating voltage drop is:

Voltage drop	= 2 x Circuit current x One-way circuit length (m) x Resistance (Ω/Km) ÷ 1000

The most important value is the percentage voltage drop for the circuit. This is calculated using the formula:

Percentage voltage drop	= Voltage drop ÷ Circuit operating voltage x 100

See Wire, overcurrent protection, and disconnect sizing and selection - Phase 4: Voltage drop for recommended voltage drop values for circuits in an off-grid PV system.

Conductor resistance values

A simplified chart with resistances for copper wires.

Copper resistance at 75°C
Size AWG	Area	Metric equivalent	Ω/km
18	0.823 mm²	1 mm²	26.5 Ω
16	1.31 mm²	1.5 mm²	17.3 Ω
14	2.08 mm²	2.5 mm²	10.7 Ω
12	3.31 mm²	4 mm²	6.73 Ω
10	5.261 mm²	6 mm²	4.226 Ω
8	8.367 mm²	10 mm²	2.653 Ω
6	13.30 mm²	16 mm²	1.671 Ω
4	21.15 mm²	25 mm²	1.053 Ω
3	26.67 mm²	—	0.833 Ω
2	33.62 mm²	35 mm²	0.661 Ω
1	42.41 mm²	50 mm²	0.524 Ω
1/0	53.49 mm²	—	0.415 Ω
2/0	67.43 mm²	70 mm²	0.329 Ω
3/0	85.01 mm²	95 mm²	0.2610 Ω
4/0	107.2 mm²	120 mm²	0.2050 Ω