Voltage drop

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 isues, 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 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 wire sizing 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

The table below has recommended maximum voltage drop values for vaious circuits. It is important to note that other components in a circuit - terminals, fuses, breakers - add resistance and will increase voltage drop as well, therefore it is important to be conservative.

Note: Not all PV systems are wired the same. These voltage drop values are for total circuit length between one component and another. It is common to have DC busbars that have one single set of wires that runs to the energy storage system and connect to a positive and negative DC busbar that serve as a point of connection for the inverter and charge controller. In this case it is necesary to calculate the total voltage drop between the charge controller and the energy storage system, rather than just to the busbar.

Circuit	Maximum recommended voltage drop
PV source to charge controller	2%
Charge controller to energy storage	1.5%
Energy storage to inverter	1.5%
DC lighting circuits	5%
DC load circuits	3%
AC load and lighting circuits	2%

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