Voltage drop in long wire runs — how does it affect sizing?

Question

Accepted Answer

Understanding Voltage Drop Voltage drop occurs when electrical current flows through a wire, causing some of the voltage to be lost as heat. This is a critical consideration in off-grid solar systems, where long wire runs between the solar array and charge controller can result in significant voltage drop. To calculate the voltage drop, use the following formula: Voltage Drop (Vd) = (I x L) / A, where I is the current in amps, L is the length of the wire in feet, and A is the wire’s cross-sectional area in circular mils (CM).
Calculating Wire Size To determine the required wire size, we need to calculate the current flowing through the wire. A typical solar array will produce 12-20 amps of current, depending on the array size and operating conditions. For example, if we have a 1000-watt solar array operating at 17.5 volts, the current would be approximately 57 amps. Using the voltage drop formula, we can calculate the required wire size based on the desired voltage drop, such as 3%. For a 100-foot wire run, the minimum wire size would be 4 AWG, assuming a 3% voltage drop. However, this calculation assumes a relatively low operating voltage, and actual wire size may need to be increased to accommodate higher operating voltages.
Sizing the Solar Charge Controller When sizing the solar charge controller, we must also consider the voltage drop in the wire run. A general rule of thumb is to size the charge controller to handle the maximum possible current, including the wire losses. In our example, if the wire run requires a 4 AWG wire, the maximum current would be approximately 45 amps. To accommodate this voltage drop, we would need to size the charge controller to handle 45 amps, rather than the 57 amps produced by the solar array. This ensures that the charge controller can handle the maximum possible current, even with the wire losses.