Grounding system sizing and selection
(1) Grounding electrode (2) Grounding electrode conductor (GEC) (3) Equipment grounding conductor (EGC) (4) DC system ground (5) AC system ground
Properly grounding all non-current carrying metallic equipment in the electrical system, as well as both the AC and DC side of a PV system, is recommended to help mitigate electrical fires, lighting and electrocution risks to system users. This is no easy task though as designing and installing a proper grounding system is one of the more complicated aspects of off-grid PV system design as there are many different components that are required in order for a grounding system to function properly. The appropriate design varies regionally based upon not only the electrical code, but also the building, soil type and available materials.
A few universal notes on grounding electrodes:
- In areas with dry rocky soil additional grounding electrodes is a good idea and will only improve the grounding of a system.
- Grounding electrodes should always be installed outside of a structure in order to guide any lightning induced surges away from people.
- If there is more than one grounding electrode, for example one near a building and another near the PV source, both of them should be connected (bonded) together to avoid any voltage difference between the two electrodes.
- Electrodes can be encased in concrete to improve their conductivity to earth. There are even special types of concrete available for this purpose. This is mmandated by the electrical code in certain areas.
Step 1: Determine the type of grounding electrode
Determining the right grounding electrode for a particular project is best done by consulting local electricians. There are some general principles with regard to choosing a grounding electrode:
| Type | Description |
|---|---|
| Ground rod | Ground rods are driven into the ground - typically with a hammer. They are either made from copper or steel coated with copper. They must penetrate 2.5-3 m of ground in order to be able to establish a solid connection with the earth, which can be extremely difficult to achieve in rocky soil. |
| Ground plate | Ground plates are thin large plates with significant surface area that are buried in the ground. They are either made from copper or steel coated with copper. They must be bured at least 75 cm below the earth's surface. A ground plate should have a surface area of at least 1850 cm². Ground plates work well in areas with rocky soil where a driven ground rod is not an option. |
| Building rebar (Ufer) | The rebar of a building, if it is all tied together properly and encased in concrete, can serve as a grounding electrode. There are special connectors that are used to tie a grounding system into the rebar of a building. If a Ufer ground is an option, it will serve as a longer lasting and superior grounding electrode than any other option. |
| Water pipes | If a metallic water pipe leaves the building and has more than 3 m in direct contact with the soil, it can serve as a grounding electrode, but another electrode must still be installed (ground rod, ground plate, or Ufer ground). The connection to a water pipe should be made outside of a home - there are special connectors made for this purpose. |
Step 2: Determine the size of grounding electrode conductor (GEC)
The grounding electrode conductor severs as the main connection between the rest of the system and the grounding electrode(s). The grounding electrode conductor must be sized to the size of the largest overcurrent protection device in the system deteremined in Overcurrent protection device sizing
| Largest OCPD size | Minimum AWG size | Minimum mm² size |
|---|---|---|
| 15 | 14 | 2.5 mm² |
| 20 | 12 | 4 mm² |
| 60 | 10 | 6 mm² |
| 100 | 8 | 10 mm² |
| 200 | 6 | 16 mm² |
| 300 | 4 | 25 mm² |
