Lesson 6

1) During the initial site visit to check a single story building’s acceptability for a PV system, you note that the asphalt-shingled roof has a 4/12 slope and is oriented 10 degrees to the west of true south. The south-facing roof is a rectangle that is 30 feet wide and 20 feet from the eaves to the roof top. Is this building a good candidate for a PV installation? If it is and given that the roof can support the PV system and a 3-person installation crew, what would you suggest to the building owner as the largest, safe array (peak output) to install?

Answer: The roof would be a good candidate if it was shade-free from about 9 a.m. to 3 p.m. I would allow at least a foot of free roof around the array: 28 ft X 18 ft = 504 ft2 of area for the array. 504 X 12 = 6048 peak Watts max.

2) For the same building described in question 1, what conditions might you encounter that would make you reject the site for a system installation?

Answer: Tall trees, a chimney, or other shade sources, air vent pipes in the way, the shingles need to be replaced, local covenants prohibiting solar devices, or electrical system up to code.

3) What estimated cost would you tell the building owner for an installed PV system with a peak output of 3000 Watts?

Answer: $8 - $10/Watt = $24,000 to $30,000 to install the system.

4) Why is an inverter needed in a grid-connected PV installation?

PV modules produce dc electricity and the building’s electrical appliances and equipment are designed to operate on AC electricity.

5) Why is an inverter needed in a remote or stand-alone PV system?

Answer: PV modules produce DC electricity which is stored in batteries, but most of the building’s electrical appliances and equipment are designed to operate on ac electricity.

6) Given that the building owner wants to install a grid-connected PV system, how would you size the inverter?

Answer: I would determine the area available to install the PV array and work within the building owner’s present budget and the future potential array size increase to determine the PV array’s output. I’d use the array’s peak output as the minimum size for the inverter.

7) What is the color of the grounded conductor in a PV installation and how is it sized?

Answer: The grounded conductor in a PV system is white and sized to safely carry the overcurrent device protecting the circuit.

8) What is the color of the equipment/frame ground wire in a PV installation and how is it sized?

Answer: The equipment and frame grounding wire can be green or bare copper and is sized to safely carry the largest load that could occur in the circuit.

9) What function does the equipment/frame ground perform?

Answer: The equipment ground prevents current flows that could injure/kill people or harm equipment.

10) Given that a PV system uses modules outputting a nominal 12 volts at 5 amperes, the modules are 30 feet from a combiner box, and you can only tolerate a 2% voltage drop, what gauge of wire should be used to connect the modules with the combiner box? What gauge of wire if the modules strings were 24 volt at 5 amps?

Answer: [From the 3% voltage drop table -12 volt, 5 amp, 12 gauge wire for 24 feet or 38 feet in 10 gauge wire] for a 2% voltage drop, modifiy the table distances by 2/3rds (24 X .67 = 16 feet for the 12 gauge wire; 39 X .67 = 25.5 feet of 10 gauge wire; and 60 X .67 = 40 feet of 8 gauge wire). Therefore, use 8- gauge wire in the 12-volt system. For the 24-volt system, you can safely use the 12-gauge wire to connect the modules and the combiner box.

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