Question

Using the sample PV system below, calculate the wire sizes needed for various portions of the PV system. The system consists of the following specification and equipment:

a. DC system voltage = 24 volts

b. Ten 100 – Watt modules, each with nominal voltage of 12V. The short circuit current (I_sc) of each panel is 7.2 amps and the maximum power (I_mp) of each panel is 6.2 amps.

c. Eight batteries, each is 6V and rated at 350 amp-hours.

d. One charge controller that is 24 volts and rated for 60 amps.

e. One 2500-watt inverter (inverter efficiency: 90%) with an input DC voltage of 24 volts (lowest operating DC voltage: 22 V) and output AC voltage of 120 volts

f. Total DC load is 500 watts at 24 volts

g. Voltage drop requirements and one-way distances between the components of the system:

• 2% and 45 feet between PV combiner box and Charge controller
• 2% and 10 feet between charge controller and battery bank
• 2% and 5 feet between battery bank and inverter
• 2% and 15 feet between battery bank and DC load center

Answer 1

Detailed Instructions for using the Wire Size Calculator

Step 1 - The first step is to decide on the voltage for your system: 12, 24, or 48 volts. The main issue is the wire size needed for the (usually) fairly long run to the Solar Panels. Simply stated, the higher the voltage, the smaller the wire size that is needed to carry the current. The formula P=E*I says that the wattage/power P is equal to the voltage E times the current I in a circuit. So, you can see that as the voltage goes up the current goes down since E*I always = P. (More details on formulas are available under Watt & Power). Less current means smaller (less expensive) wire. So, as a general rule, you would normally choose a higher system voltage. The only reason not to would be if you planned on using lots of 12 volts DC only equipment. Also, keep in mind that whatever system voltage you decide on 12, 24, or 48, all of your equipment must work on this voltage. If you choose 24 volts for example, your solar panels, charge controller, inverter, and battery bank will all need to be 24 volts. By playing with the numbers in the Wire Size Calculator you can get an idea of what voltage will be best for your system.

Step 2 - Next, enter the maximum amps/amperage that your solar panels will produce. This will be the rating of one panel times the number of panels in your array. If you put two 12 volt panels in series to increase the voltage to 24 volts, you would count the two panels as one. The same would be true if wiring two 24 volt panels to equal 48 volts. The reason for this is that in a series circuit the voltage increases, but the current or amperage stays the same. More details on this are available under Battery Wiring Diagrams which explains series and parallel wiring. For example: 10 solar panels rated at 5 amps at 12 volts. You want a 24 volt system so you wire 2 panels in series to make 24 volts. You do this 5 times. The 5 pairs will be wired in parallel where the current adds to give you 5 sets times 5 amps per set equals 25 amps. Enter the 25 as the maximum amps your wires need to carry.

Step 3 - This is the distance in feet from your solar panels to the charge controller and battery bank location. Even though you will actually be running 2 wires, one negative & one positive, do NOT double the distance. The Wire Size Calculator assumes this and does it for you in the calculation.

Step 4 - The loss you will get in the transmission of the electrical power from your solar panels to your equipment location is due to the resistance of the wire. This cannot be avoided. A common practice is to use 3, 4, & 5 percent figures for 12, 24, and 48 volt systems respectively. I like the 3 percent choice for all systems, but even 5 percent is not too bad. The Wire Size Calculators' answers are based on copper wire using the standard AWG (American Wire Gauge) sizes. Also note that 00, 000, and 0000 gauges (generally refered to as 2/0, 3/0 and 4/0 are progressively larger in size and are represented in the Wire Size Calculator as -1, -2, and -3. If you enter numbers that would result in sizes larger than -3 (pretty darn big), you will get an error message to that effect. In this case, the best response would be to increase system voltage (resulting in less current required) or/and increase the percent of loss.

By the way, a -3 size wire (4/0) is pretty large and if used in a 48 volt system with a 5% loss factor, you could move 100 amps over 250 feet. This would be a VERY large system. The calculator does not fiqure systems larger than this. Whatever gauge wire you use, you must also make sure it is capable of carrying the amount of current your system will produce.