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Voltage Drop Table

wire size table for solar power projects

To achieve maximum efficiency from your power system you do not want to under-size your wire between various components. If in doubt use a larger wire size than the charts below suggest. We will generally size the wire to maintain no more than a 2 - 3 percent voltage drop regardless of the system voltage, however this factor is more critical on low voltages. For 120/240 volts a maximum of five percent loss is acceptable unless specific equipment dictates otherwise.

The difference in cost of the next larger-size wire is usually insignificant to the overall
costs where performance may be affected over the life of the components.
In addition, improperly sized wire can shorten the life of your components.


Before using these tables, some calculations must first be performed by the system designer to insure accurate current ratings and safety factors are used. For solar modules, the UL-1703 standard requires the short circuit current (lsc) rating of the PV module be multiplied by 125% before other factors are applied. This is because the PV module electrical performance can increase above the factory rating due to the combination of increased operating temperatures and enhanced irradiances over 1000W/M2 (such as edge-of-cloud effect).

NOTE:   The "edge-of-cloud effect" is also a consideration when sizing a charge/voltage controller for solar modules. Use the short-circuit current rating to determine the controller.

The NEC requires the short circuit current ratings of the PV module or array be multiplied by 125% before calculating ampacities of any cable or over current device. This is in addition to the UL required 125%! This yields a multiplier of (125%x125%=156%) or 1.56. Although this may seem redundant, good design practice dictates both factors should be used when determining wire size from the solar array to the battery.

When calculating wire size from battery to load, multiply the load current by only 125% (1.25).
Example:  A 24V solar array of four Solarex MSX-120 modules, 2 in series by 2 parallel  modules located 30 ft. from the battery; lsc=7.60 A (one module); 7.60 x 2 = 15.2A (two modules); 15.2A x 1.56 = 23.71A. Using the 24 volt table, locate a value equal to or greater than 24 amps from the left column, follow a line to the right until a distance at or above 30 ft. is found. For this example, using the 24 volt table, we find 25 amps at 36 feet to be the best selection (for not more than 5% loss) indicating a wire size of #8 AWG.

For larger batteries and  inverters (battery to inverter cable connections)
a  minimum 4/0 cable is recommended.

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Maximum Ampacity for Copper and Aluminum Wire
Wire Size Copper Aluminum
14 25 .
12 30 25
10 40 35
8 55 45
6 75 60
4 95 75
2 130 100
1 150 115
1/0 170 135
2/0 265 150
4/0 360 205

NEC allows rounding up cable ampacity to next standard fuse or breaker.
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