Poor-quality and improperly installed battery and inverter cables can cause problems in the function and safety of a battery-based system. Here’s how to select the right cables and install them correctly, for optimal system performance.
There is a perception that battery and inverter cables are expensive—and it is a tempting place to cut costs—but buying cheap cables can result in significantly reduced performance of the battery bank and inverter(s). It’s a lot like putting cheap tires on a high-performance car—you save some money, but you don’t get the performance and safety you might need. The common problems seen with cabling in battery-based renewable energy (RE) systems are typically due to low-quality cables and hardware, in combination with poorly made crimps and connections.
You can purchase preassembled cables or have them made to order, but you can also build them yourself. The details are important—battery cables and their ring terminal connectors (also called “lugs”) carry high current and are used in harsh environments where they can be exposed to sulfuric acid, hydrogen gas, high temperatures, and dissimilar metals.
For battery/inverter RE systems, the largest conductors in the system are usually the ones connecting all of the batteries together and then exiting the battery box to connect to the inverter. Since nearly all battery-based inverters operate at 48 VDC or lower, the cables need be large to handle high currents without significant losses. Sizing of these cables is based on the battery voltage, the inverter’s continuous amperage rating, and the length of the cable. Commonly, these cables are either 2/0 AWG (acceptable for use with a maximum of 175 A breaker or fuse) or 4/0 AWG (acceptable for use with a maximum of 250 amp breaker or fuse), but will need to be individually calculated. For example, the installation manual for OutBack Power Systems’ VFX3524 (3,500 watts; 24 VDC) inverter recommends 4/0 AWG for a battery-to-inverter cable length of 10 feet or less. This size cable would result in a voltage drop of less than 1% at full rated output of the inverter, resulting in 34 watts of losses in the 10-foot-long positive and negative conductors. Shorter cables would reduce the losses proportionally.
High-quality battery/inverter cables are made of fine-strand copper conductors with a flexible insulation covering and are available from manufacturers such as Polar Wire Products or Cobra Wire & Cable. Although finely stranded cables are not required, they make installing and servicing the system easier and reduce stress on the battery and inverter terminals. All high-quality battery cables are made with UL-listed wire and include a National Electrical Code (NEC)-required designation, such as RHW, THW, or THHW.
Lower-quality battery cables are often made from automotive or welding conductor cable. This type of cable is cheaper and easier to obtain—but is not acceptable by the NEC since it is not UL-listed or marked with the NEC wire type. While some types of welding cable have a UL listing, they have been approved using a different set of UL standards and tests, and are not marked with the required NEC wire-type designation.