Grounding

The following list contains the NEC® definitions (NEC® 2005, Article 100) for the grounding terms you should be familiar with.

  • Grounded: Connected to the earth or to some conducting body that serves as earth.
  • Grounded conductor: Current carrying conductor that is grounded at one point. Conventionally the white wire.
  • Grounding conductor: A conductor not normally carrying current used to connect the exposed metal portions of equipment or the grounded circuit to the grounding electrode system. Normally bare copper or green wire.
  • Grounding electrode conductor: Bare copper wire connecting grounded conductor and/or equipment grounding conductor to the grounding electrode.
  • Grounding electrode: Usually a ground rod or bare metal well casing.
  • Ungrounded conductor: Current carrying conductor not bonded with ground. Conventionally the red, positive wire on DC; conventionally black, any color besides white, gray, green, or bare copper on the AC side.

Why Ground?

The following is a list of the reasons to ground:

  • 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 the earth.
  • To provide a path in order to facilitate the operation of overcurrent devices.

There are two specific ways to group a system: equipment grounding and system grounding. It is important to know the difference between the two.

1. Equipment Grounding

Equipment grounding provides protection from shock caused by a ground fault and is required in all PV systems by the NEC®. A ground fault occurs when a current-carrying conductor comes into contact with the frame or chassis of an appliance or an electrical box. A person who touches the frame or chassis of the faulty appliance will complete the circuit and receive a shock. The frame or chassis of an appliance is deliberately wired to a grounding electrode by an equipment grounding wire through the grounding electrode conductor. The wire does not normally carry a current except in the event of a ground fault. The grounding wire must be continuous, connecting every non-current carrying metal part of the installation to ground. It must bond or connect to every metal electrical box, receptacle, equipment chassis, appliance frame, and photovoltaic panel mounting. The grounding wire is never fused, switched, or interrupted in any way. When metal conduit or armored cable is used, a separate equipment ground is not usually necessary since the conduit itself acts as the continuous conductor in lieu of the grounding wire. Grounding wires are still needed to connect appliance frames to the conduit.

2. System Grounding

System grounding is taking one conductor from a two wire system and connecting it to ground. The NEC® requires this for all systems over 50 volts (NEC® 2005, Article 690.41). In a DC system, this means bonding the negative conductor to ground at one single point in the system (NEC® 2005, Article 690.42). Locating this grounding connection point as close as practical to the photovoltaic source better protects the system from voltage surges due to lightning (NEC® 2005, Article 690.42, FPN). In grounded systems, the negative becomes our grounded conductor and our positive becomes the ungrounded conductor. If you choose not to system ground a PV system under 50 volts, both conductors need to have overcurrent protection (NEC® 2005, Article 240.21), which is often more cumbersome and costly. Most PV installers simply choose to system ground even if the system operates under 50 volts.

Grounding

3. Ground-fault Protection

Roof-mounted, DC PV arrays located on dwellings must be provided with DC ground-fault protection (NEC® 2005, Article 690.5). Many grid-tied inverters offer built-in ground fault protection. If a system is to be roof-mounted on a dwelling and the system is not using an inverter package with built-in ground-fault protection, ground fault protection must be wired in separately. Ground-fault protection isolates the grounded conductor (in DC, this is the negative wire) from ground under ground-fault conditions, as well as disconnecting the ungrounded conductor (the positive wire).

Size of Equipment Grounding Conductor

The size of the equipment grounding wire for the PV source circuits, such as the PV to battery wire run; or for grid-tied systems with no battery back up, the PV to inverter wire run, depends on whether or not the system has ground-fault protection.

If the system has ground-fault protection, the equipment grounding conductors can be as large as the current carrying conductors, the positive and negative wires, but not smaller than specified in NEC® 2005, Table 250.122. This table is based on the amperage rating of the overcurrent device protecting that circuit. For example, if the circuit breaker protecting the circuit is rated at or between 30 amps and 60 amps, you can use a #10 AWG copper equipment grounding wire. If the positive and negative conductors have been oversized for voltage drop, the equipment grounding wire also must be oversized proportionally (NEC® 2005, Article Proper ground-fault protection 250.122(b)). From the example in the Wire Sizing Exercise, you increase the necessary wire size from #6 AWG to #1/0 AWG to satisfy a 2% voltage drop requirement. Here you would have to increase your equipment grounding wire from #10 AWG to #4 AWG.

If the system does not have ground-fault protection, the equipment grounding wire must be sized to carry no less than 125% of the PV array short circuit current. For example, if your PV array has a short circuit current of 30 amps, the equipment grounding wire would have to be sized to handle at least 37.5 amps (30 amps X 1.25). Similar to the PV systems with ground-fault protection, if the positive and negative conductors have been oversized for voltage drop, the equipment grounding wire also must be oversized proportionally (NEC® 2005. Article 250.122(b)). From the example in the Wire Sizing Exercise, you increase the necessary wire size from #6 AWG to #1/0 AWG to satisfy a 2% voltage drop requirement. Here you would have to also increase the equipment grounding wire from #10 AWG to #4 AWG .

Size of Grounding Electrode Conductor

The DC system grounding electrode conductor, which is the bare copper wire connecting grounded conductor (the negative wire) and/or equipment grounding conductor to the grounding electrode (the ground rod), cannot be smaller than #6 AWG aluminum or #8 AWG copper or the largest conductor supplied by the system (NEC® 2005, Article 250.166). Even though many PV systems have larger conductors in the system (for example, #4/0 inverter cables), they can use #6 AWG copper wire for the grounding electrode conductor if that is the only connection to the grounding electrode (NEC® 2005, Article 250.166(C)).

Grounding Electrodes

Because all PV systems must have equipment grounding, regardless of operating voltage, PV systems must be connected to a grounding electrode. This is usually done by attaching the equipment grounding wire to a ground rod, via a grounding electrode conductor. PV systems often have AC and DC circuits where both sides of the system can use the same grounding electrode. Some PV systems may have 2 grounding electrodes, which is often the case for pole mounted PV arrays. One electrode is for the AC system and one electrode is for the DC system at the array. If this is the case, these 2 grounding electrodes must be bonded together (NEC® 2005, Article 690.47) with a barrier separating the AC conductors from the DC conductors.

Miscellaneous Code Issues

Stand-alone systems must have a plaque or directory permanently installed in a visible area on the exterior of the building or structure used. This sign must indicate that the structure contains a stand-alone electrical power system, and the location of the system’s means of disconnection (NEC® 2005, Article 690.56). Alternating current and direct current wiring may be used within the same system, although they may never be installed within the same conduit, or electrical enclosures without some type of physical barrier separating the AC conductors from the DC conductors.

Source: “PHOTOVOLTAICS - Design and Installation Manual” by Solar Energy International.

Solar Certification Training from Professional Solar Installers

Solar Energy International

With 18 IREC-ISPQ Certified Solar Photovoltaic Trainers and 24 NABCEP Certified Solar PV Installers — more than any other solar training organization — Solar Energy International's experienced team is on the forefront of renewable energy education. If you are seeking online solar training or in-person lab training for the NABCEP Entry Level Exam or NABCEP Installer Certification, why not receive your education from a team of the most experienced solar installer professionals in the industry? Many SEI trainers have participated in the most notable solar installations within their communities stateside, and in the developing world.

To start your solar training path today with Solar Energy International, click here.