ASK THE EXPERTS: Voltage-Drop Calculations

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There have been numerous discussions in Home Power about voltage drop in DC or AC circuits. The calculations have all been very useful for me. However, I have a concern that has not been addressed, or perhaps it has and I have just missed it.

It seems as though the data used in these equations is always “best-case-scenario” information. For instance, in calculating voltage drop on a DC circuit from array to inverter, STC Vmp, Imp, wire distance, and desired voltage drop is used. It seems that using worst-case-scenario data—Vmp on the hottest day for your location; projected voltage and amperage of the string at, say, 20 years—would provide more accurate values that reflect real-world conditions.

Steven Johnson • via email

You are wise to design the system for at least a 20-year life, and to estimate cumulative losses and compensate for them. And you are correct—wire-sizing calculations at the full rated power of the modules for a new system will result in “oversizing” as the array ages and its output decreases.

Every string inverter has a minimum and maximum input voltage range. The maximum may be hit on a record-low-temperature, sunny winter morning when the sun first strikes the array. During this short period at dawn, the current is very low. As soon as the sun’s intensity and array current increase, the modules warm up just enough to drop the voltage below that maximum. Calculating maximum low-temperature VOC is necessary to size the equipment to handle that maximum voltage. The array’s DC voltage will gradually decrease over time due to normal PV module degradation, so this calculation covers the best-case situation when the PV modules are new.

Calculating minimum DC voltage is of greater concern, as all inverters have a minimum acceptable voltage for establishing full maximum power-point tracking (MPPT). Good design practice is to maximize the number of modules in each series string to reduce the potential for the inverter’s DC input voltage to ever drop below the minimum necessary for efficient operation.

With inverter AC voltage, the concern is voltage rise, not drop, because (due to Ohm’s law: V = I × R) the AC output wire’s resistance causes the inverter to see an elevated AC grid voltage at the inverter’s AC output terminals. The same calculations for voltage drop are used when considering voltage rise, though, and typically designing for 1.5% as a maximum voltage change will minimize the possibility of the inverter shutting off due to combined high grid voltage and undersized conductors. Here too, degradation of output power would only improve this problem, since again looking at Ohm’s law, the voltage rise will decrease further with less current flowing.

Amperage is less of a concern. When the system is new, an array capacity that is oversized relative to inverter wattage may experience power clipping during midday periods of high insolation. Normal degradation over time will decrease this power clipping and voltage drop.

With battery-based systems, the old rules about sizing conductors to minimize voltage drop do not apply to most modern controllers. Past systems required large-gauge wires to keep the voltage from a 12 V array high enough to equalize a flooded battery in the summer heat. Most modern MPPT charge controllers allow maximum array VOC to reach 150 V to 250 V (with some allowing up to 600 V) to charge a 24 V or 48 V battery bank. The fact that these charge controllers can use higher-voltage arrays to charge lower-voltage batteries has changed the equation.

As with inverter PV string sizing, as long as the input is within the charge controller’s range, using worst-case data would have little effect. A reduction in array current would actually reduce voltage drop, and a reduction in array voltage would have no consequence on wires and equipment sized to handle the initially higher voltage.

Finally, the National Electrical Code “Conductor Properties” table used in voltage-drop calculations assumes a wire temperature of 167°F. When annual production is considered, wire temperatures are, on average, much lower, and therefore actual voltage drop year-round is less.

Allan Sindelar sindelarsolar.com

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