Voltage Drop in PV Systems

Voltage drop is a common name for the electrical potential loss that inevitably occurs when current flows through a conductor between the source and the load. Conductor resistance and reactance combine to make a small heating load that draws power from the circuit whenever a normal load is attached. The amount of voltage drop—also known as voltage rise, resistive losses, or wire loss—is dependent on circuit voltage, current and length; conductor size and material; and other complicated factors, such as the type of raceway, the geometry of the conductors, the number of phases and the temperature.

Why Worry about Voltage Drop?

There are two problems with voltage drop in PV systems. First, it represents wasted energy. Transforming electrical energy into heat in circuit conductors is lost energy production. Second, voltage drop can cause PV inverters to stop working properly under certain conditions. For example, if the dc bus voltage drops below the inverter’s minimum MPPT voltage, then the inverter will operate in a limited state. If the ac bus voltage rises above the maximum grid voltage set point, then the inverter will stop operating completely.

What can you as a PV system designer do about voltage drop? You can optimize the schematic design and layout of equipment strategically to minimize voltage drop. You can also consider upsizing certain system conductors to further reduce voltage drop.

Quantifying Voltage Drop

To understand how you can reduce voltage drop, you have to quantify it. In its simplest form, a circuit conductor can be considered a long, lowresistance electric heater obeying Ohm’s Law. The voltage across the heater is the current multiplied by the resistance, or V = I x r. The resistance will depend on the size, material, temperature and length of the conductor. Typical values for standard conductors are given in tables as ohms per 1,000 feet of conductor length. Therefore, the basic equation for dc voltage drop is:

V_drop = I x r
             = I x (2 x L x R) / 1,000
             = (2 × L × R × I) / 1,000

where I is the circuit current, L is the one-way circuit length and R is the conductor resistance per 1,000 feet. The basic equation can be modified for single-phase or 3-phase ac circuits, different temperatures and different power factors.

A good place to start for manual voltage-drop calculations is provided by the NEC. Values for dc resistance per 1,000 feet can be found in Chapter 9, Table 8. Reactance and ac resistance per 1,000 feet can be found in Chapter 9, Table 9. The NEC Handbook lists some voltage-drop equations and gives example calculations following the Tables.

To be clear, however, the NEC does not require any particular amount of voltage drop. Instead, Article 210 on branch circuits and Article 215 on feeders both have a fine print note (FPN) suggesting that “reasonable efficiency of operation” occurs if voltage drop is limited to less than 3% each on feeders and branch circuits, and less than 5% overall (210.19(A) FPN No. 4 and 215.2(A)(3) FPN No. 2.) These FPNs are not binding and certainly were not written with PV in mind. Nevertheless, they provide useful reference points. One general requirement the Code does make in Article 250.122(B) is that if the current-carrying conductors are upsized for voltage drop, the equipment grounding conductor (EGC) must also be proportionally upsized. The 2008 NEC includes an exception to this rule in Article 690.45(A), stating that it is not necessary to upsize the dc EGC in a PV system to address voltage drop. As a result, the EGC in PV source and output circuits is generally sized according to Table 250.122.

Another option for performing voltage-drop calculations is to use one of the many available electrical calculator programs. Some programs can be used online, and some can be downloaded to your computer. Handheld calculators with special buttons for electrical functions are also available. Most of these calculators are reasonably accurate and good for standard situations. However, most will not show you exactly how the calculations are being done so that you can verify the answer you are getting. Most will not adapt very well to nonstandard applications like PV. For example, many allow dc voltages up to only 48 V, and few allow adjustment for temperature or power factor.

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