Wire and Cable for Wind and Solar Electric Systems
This table lists the American Wire Gauge (AWG) sizes for copper conductors. In addition to wire size, the table provides values for load (current) carrying capacity, resistance, and maximum frequency. The resistance and skin depth noted are for copper conductors only. Detailed descriptions of each item are provided below the table.
Note: These values are approximate and are not meant to be used in engineering calculations.
[Ohms / 1000 ft.]
[Ohms / km]
for 100% skin depth
AWG Notes: American Wire Gauge (AWG) is a standardized wire gauge system used predominantly in the United States to note the diameter of electrical wire. The general rule of thumb is for every 6 gauge decrease, the wire diameter doubles and every 3 gauge decrease doubles the cross sectional area. For example, two strands of #14 in parallel would be approximately equal to a single strand of #11 for current capacity.
Diameter Notes: A mil is 1/1000 of an inch.
Resistance Notes: The resistance noted in the table above is for copper conductors. For a given current, you can use the noted resistance and apply Ohms Law to calculate the voltage drop across the conductor.
Current (ampacity) Notes: The current ratings shown in the table are for power transmission and have been determined using the rule of 1 amp per 700 circular mils, which is a very conservative rating. For reference, the National Electrical Code (NEC) notes the following ampacity for copper wire at 30 Celsius:
14 AWG - maximum of 20 Amps in free air, maximum of 15 Amps as part of a 3 conductor cable;
12 AWG - maximum of 25 Amps in free air, maximum of 20 Amps as part of a 3 conductor cable;
10 AWG - maximum of 40 Amps in free air, maximum of 30 Amps as part of a 3 conductor cable.
Check your local electrical code for the correct current capacity (ampacity) for mains and in-wall wiring.
Skin Effect and Skin Depth Notes: Skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor. The skin effect causes the effective resistance of the conductor to increase with the frequency of the current. The maximum frequency show is for 100% skin depth (ie. no skin effects).
Wire and Cable Factoids
The single most important component of a wire or cable is its insulation. The selection of an insulation is governed by a number of factors such as stability and long life, resistance to sunlight (ultraviolet), dielectric properties, resistance to ionization and corona, resistance to high temperature, resistance to moisture, mechanical strength, and flexibility. There is no single insulation that is ideal in every one of these properties. It is necessary, therefore, to select a cable with the type of insulation which most fully meets the requirements of the particular installation involved.
These are some general rules and common practices when wiring solar systems. They are not meant to be all-inclusive, only general guidelines.
1. Almost all wiring is done with stranded wire or cable. Solid wire is occasionally used for long runs, but in most cases, it is not suitable for wiring panels, controls, pumps, batteries, or other components. If used, you run the risk of breaking the terminals and/or screws if the cable is flexed. It is also difficult to get a good connection with some types of terminals.
2. All outdoor wiring should be of type XLP/XHHW, TC (Tray cable), USE-2 or similar outdoor UV (sunlight) resistant insulation. Other types, such as THHN, may be used, but should only be run in conduit if it is used. Metallic or NMC (non-metallic conduit) may be used in most cases.
3. When wiring batteries, inverters, or other high current devices, crimp/solder type lugs, or cable clamps designed for good connections with large cable should be used. Do not attempt to wire the stranded wire directly to battery terminals. For most purposes, welding cable is a better choice than the more common PVC battery cable, due to it's much tougher insulation and higher temperature qualities. Welding cable is more expensive than PVC cable, but PVC will melt at fairly low temperatures.
4. Do NOT use the commonly available Romex® type solid house wire for anything BUT house AC wiring. It is not suitable for outdoor, direct burial, or water pump wiring. The insulation will fall off within a year or two if used in direct sunlight. It is also difficult to get good solid connections with solid wire on most components used in solar systems.
5. Size the wire gauge properly - the best components will not work properly if undersized wire is used. For panel and general wiring, see the wire loss table. We sell quite a bit overseas, and most of the world uses metric wire sizes. For conversion, see the metric to AWG wire size conversion table on the same page as the wire loss table.