Step 8 - Calculate and select BOS components

The final step in the design process is to calculate the balance of system (BOS) components needed for the PV system. These are the wiring between the different components designed in the previous steps as well as Disconnects/fuses, over current protection and lightning protection devices. Lightning arrester is mounted between the outdoor array and the indoor equipment to prevent voltage spikes and current surges from damaging equipment and possibly starting fires. Over current protection is used to prevent circuit overload conditions from creating a fire hazard in conductors and equipment. Fuses and circuit breakers are acceptable over current protection devices. Disconnects are switches used to open an electrical circuit and maybe combined in a single box with a fuse. Disconnects are used to isolate the various energy components for protection, maintenance, testing or to enable and disable the system. Disconnects/Fuses are recommended between all the major components of the PV system. For information on availability and pricing visit my partners Balance of Systems section.

Wiring: PV system wiring is substantially different from conventional AC wiring as they generally use lower voltage and often have larger wire sizes compared to AC systems. NEC article 690 - Solar Photo Voltaic Systems - has complete details on the code requirements for wiring and other electrical aspects. Some key information is also described in the Standards and Codes section on this site. Wire size selection is based on two important criterions - Ampacity and Voltage drop. Ampacity refers to the current carrying ability of a wire. The larger a wire is, the greater its capacity to carry current. Using a wire with an Ampacity less than the current flow will cause the wire to overheat leading to losses and even fire. Table 310-16 in the NEC code book lists the standard Ampacity of copper wires. Voltage drop is the loss of voltage due to a wire's resistance and length. It is important to take into consideration the voltage drop and minimize the energy loss in the wiring in designing the wiring for your PV system. Using a larger wire size, decreasing the current flow or decreasing the length of the wire are all solutions to reduce voltage drop.

The Ampacity ratings for insulation types RHW-2 and THWN-2 with copper conductors at 30 degrees C is shown below in table W-1. The Wire resistance of the same copper conductors is shown below in table W-2.

Wire Ampacity (Ratings for types RHW-2 and THWN-2 with copper at 30 C)
Table W.1

Wire Resistance ratings for the above and effect of temperature on resistance

When designing the wiring between the different components, ensure the voltage drop is less than 2% of the rated voltage.

Use the following calculations to determine system wire sizes.

PV Combiner box to Charge controller and Charge Controller to Battery:

Pick the wire size from table w-2 based on the value calculated above. An important variable in the above formula is the length of the conductor. Typically the length of the conductor from the PV Array combiner box to the charge controller is going to be much longer than the conductor from the charge controller to the battery, So it important to calculate the sizing for both sections.

Battery to Inverter:

Pick the wire size from table w-2 based on the value calculated above.

Inverter to Main Panel
Use standard AC wiring to connect the Inverter to the main panel.

Over current protection:Over current protection is used to prevent circuit overload conditions from creating a fire hazard in conductors and equipment. Fuses and circuit breakers are acceptable over current protection devices. When the current exceeds a fuse or circuit breaker's rated amperage, the circuit will open and stop all current flow. A fuse that has "blown" must be replaced while a circuit breaker may be reset. Circuit breakers must be UL listed and be DC rated.

The rating of an over current device must be less than or equal to the Ampacity of the wire used between the two systems it is connecting. When an over current device is placed at the connection between two different wire sizes, the rating must be less than or equal to the smaller wire.

Use the following calculations to determine system fuse/disconnect sizes:

Battery to Inverter: Fuse rated for Inverter Total Amps calculated in the previous section

Standard ratings for Fuses and Breakers are given below in Amps:

Grounding
Grounding is an involved topic and the following are some of the reasons to ground your system

• To limit voltages due to lightning, line surges or unintentional contact with higher voltage lines
• To stabilize voltages and provide a common reference point being earth
• To provide a path in order to facilitate the operation of over current devices.

There are two specific ways of grounding a system - equipment grounding and system grounding. Equipment grounding provides protection from shock caused by a ground fault and is required in all PV systems by the NEC code. A ground fault occurs when a current-carrying conductor comes in contact with the frame or chassis of an appliance or an electrical box. A person who comes in contact with the frame or chassis will receive a shock. System grounding is taking one conductor from a two wire system and connecting it to the ground. The NEC code required this for all systems over 50 Volts. In a DC system this means bonding the negative conductor to ground at a single point in the system. Locating this grounding connection point as close as practicable to the PV Source better protects the system from voltage surges due to lightning. If you choose not to system ground a PV system under 50 Volts, both conductors need to have over current protection which is often more cumbersome and costly. Most PV installers chose to system ground even if the system operates under 50 Volts.

This completes the design methodology for a Grid-Interactive system.
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• Step 0: Determine the Solar Hours of your location
• Step 1: Estimate the critical loads that need to be supported
• Step 2: Determine the days of Autonomy for the system
• Step 3: Select a "Nominal" DC side voltage that the design will be based on
• Step 4: Select the PV Panels to be used for the system
• Step 5: Select the Battery manufacturer and model to be used in System
• Step 6: Select an appropriate Charge controller
• Step 7: Select an appropriate Inverter
• Step 8: Calculate and select BOS components