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5% Voltage Drop Table
The following tables represent the maximum one-way distance
for various wire gauge, voltage and current combinations based upon a 5%
voltage loss for 120v and 24v. Five percent is acceptable in most low voltage
systems, except 12 volt (see above). NOTE: To obtain a 2% figure
from these tables, divide the given distance shown in the table by 2.5.
For calculating distances for 48 volt systems, multiply the 24 volt
distances by 2.
For 240 volt systems, multiply the 120 volt distances by 2.
Below stepped lines check ampacity. The (=) indicates over 1000 feet.
Top table = 120/240 volts; Lower table
= 24/48 volts;
or slide page to view voltage -------------------->
Below stepped lines check ampacity. The (=) indicates over 1000 feet.
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 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 system.
In addition, improperly sized wire can shorten the life of your components.
**IMPORTANT**
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).
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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 - see below. |
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.
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 (two
modules); 7.60 x 2 = 15.2A (four 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 the controller, a minimum 20A would be needed
- 15.2A x 125% = 19A, and the nearest available size controller is a 20A
unit.
| 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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