Grounding Explained
The following list contains the NEC® definitions (NEC® 2011,
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.
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® 2011, Article 690.41). In a DC system, this means bonding
the negative conductor to ground at one single point in the system (NEC®
2011,
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® 2011, 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® 2011, 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.
3. Ground-fault Protection
Roof-mounted, DC PV arrays located on dwellings must be provided with DC ground-fault protection (NEC® 2011, 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® 2011, 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® 2011, 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® 2011, 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® 2011, 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® 2011, 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® 2011,
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.