# Wire Size Considerations

Wind/Solar Hookup Basics and Beyond.

Wire size calculator for solar panels, wind turbines, inverters, batteries and more.

Wire Size. What size wire do I need from here to there?

It is very important that the wire you choose is large enough to carry the amount of current from your charge sources, batteries and loads or you WILL experience a substantial loss of energy. Very undersized wire can lead to high heat build up in the wire which could lead to failure of the wire and surrounding devices, as well as the possibility of fire and/or injury. The cost of wire can be a substantial investment and may dictate where items are placed (how far they are from each other.) The closer you can get your charge sources to the batteries, the better as far as power transmission is concerned. Inverters need to be very close to the batteries. Often it is simply not possible to get your wind turbine or solar panels anywhere close to where the batteries will be located, thus requiring longer cable runs. The longer the cable run, the larger the wire must be in order to prevent large voltage drops (loss of power) due to the resistance of the wire. The most amount of power you would ever want to loose in a cable run of an alternate energy system is about 5 percent, yet 2 to 4 percent is by far a better target. We will first analyze wiring size for solar panels and D/C turbines. Three phase A/C turbines are more efficient in transmission of power to the batteries when the three phase rectifier is located near the batteries, so we will address 3 phase in a separate section. Lastly we will address battery inter-ties, and connections to inverters.

A common mistake in sizing wire is to turn to the NEC wire size charts to determine how much amperage a wire can safely handle. This method of sizing wire will lead to a very inefficient system since these tables are generally based on higher voltage (110/220 volts A/C), and the main concern is safely transmitting the power without heat build up. In a low voltage system, we are of course concerned with safety, yet what dictates wire size even more, is loss of power. A 10% loss of power in a 12 volt line, is 1.2 volts! This may completely prevent some devices from operating.

It is very important that certain circuits, like lighting circuits having florescent bulbs have low losses. 12 volt florescent bulbs do not operate well with even a 10% loss. This is because the internal operation of the bulb requires nearly 100% of the design voltage or the bulb will not be allowed to fully excite the gasses, giving off much less light and much more heat. Circuits having larger motors simply should not be undersized or you may find the motor does not start up properly. Always insure your inverter wire is well within this 2% drop. For battery interconnects, a 1% or 1/2% drop is even better. A 5% (and perhaps 10%) drop might be acceptable in solar panels since the cost of the wire may exceed the cost of the solar panels when there is a large distance involved. If at all possible, try to keep your power loss to less than 5% in every area of your system. An exception to this rule will be wire used for diversion or dummy loads. In this case a 10% loss is probably acceptable.

Here is a great little tool we have put together for you to calculate wire size. Simply fill in the fields and click the calculate button.

If you prefer, here is a 12 volt chart you can print out.

This table represents the maximum distance (in feet, one way) that can be achieved with a given wire size and amperage to maintain less than a 2% loss of power.

 2% Voltage Drop Guidelines — 12 volts Amps #14 #12 #10 #8 #6 #4 #2 #1/0 #2/0 #4/0 1 45 70 115 180 290 456 720 . . . 2 22.5 35 57.5 90 145 228 360 580 720 1060 4 10 17.5 27.5 45 72.5 114 180 290 360 580 6 7.5 12 17.5 30 47.5 75 120 193 243 380 8 5.5 8.5 11.5 22.5 35.5 57 90 145 180 290 10 4.5 7 11.5 18 28.5 45.5 72.5 115 145 230 15 3 4.5 7 12 19 30 48 76.5 96 150 20 2 3.5 5.5 9 14.5 22.5 36 57.5 72.5 116 25 1.8 2.8 4.5 7 11.5 18 29 46 58 92 30 1.5 2.4 3.5 6 9.5 15 24 38.5 48.5 77 40 . . 2.8 4.5 7 11.5 18 29 36 56 50 . . 2.3 3.6 5.5 9 14.5 23 29 46 100 . . . . 2.9 4.6 7.2 11.5 14.5 23 150 . . . . . . 4.8 7.7 9.7 15 200 . . . . . . 3.6 5.8 7.3 11

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