Introduction
Photovoltaic (PV) systems that generate electricity from
sunlight are being installed in ever increasing numbers
throughout the United States and the rest of the world. Over
150 megawatts of PV modules are being produced worldwide
annually and these PV modules, when exposed to sunlight, will
be generating electricity for the next thirty years and
longer. Utility-interactive PV systems (that can feed power to
the electrical utility grid) and stand-alone PV systems are
being installed in both rural and urban locations on
residential dwellings and commercial buildings. They are
either utility-interactive or they provide power for lights,
water pumps, appliances, and communications equipment in
stand-alone applications. A few PV systems are owned and
operated by utility companies, but most are not, and they fall
under the provisions of the National Electrical Code (NEC) and are inspected by the Authority Having
Jurisdiction (AHJ).
This article will briefly describe both
stand-alone and utility-interactive PV systems and highlight
code issues that electrical inspectors should be aware of when
inspecting them.
PV Cells, Modules
and Arrays
PV modules use solar cells to convert
sunlight directly into direct current (dc) electricity. A
number of cells are connected in series or parallel to form a
PV module, which is the smallest commercially available,
listed product for power applications. PV modules range in
power output from about six watts to about 300 watts with
nominal output volt- ages from 6 to 90 volts (Figure
1, PV
module test bed at New Mexico State University). PV modules
are connected in series and parallel to increase voltage and
current output; these groups of modules form PV panels or
arrays. PV arrays may consist of any number of modules (for
instance 30 watts or less at 12 volts) up to tens of thousands
of modules with megawatt outputs at over a 1000 volts. One of
the largest systems was utility-interactive and was installed,
owned, and operated by a non-utility organization and,
therefore, fell under the NEC.
Photovoltaic cells are made of a number of
materials. The oldest commercially available technology uses a
silicon material that is processed to produce a final product.
Newer technologies are using various thin films to produce PV
modules that are expected to lower the cost of the product.
The most common PV module uses cells made of silicon wafers
that are laminated between a glass front plate and rear
insulator (sometimes glass) with an aluminum frame. PV modules
made from thin-film photovoltaic materials may be constructed
in a similar manner, but some manufacturers are exploring less
expensive manufacturing methods such as laminating the PV
material directly to metal roofs or producing roofing tiles
that also serve as PV modules. Many of the PV modules on the
market (manufactured in the US and elsewhere) have been listed
to standards established by Underwriters Laboratories (UL) and
many also have fire ratings for use on rooftops.
Stand-Alone
There are thousands of residential
stand-alone PV systems in the US plus thousands of stand-alone
communication sites, water pumping systems, emergency call
boxes, and lighting systems. In many states, any electrical
system that requires field-installed wiring should be
installed according to local codes or the NEC and should be
inspected.
As the name implies, stand-alone systems
are not connected to the utility grid and are self-contained
producers of electrical power. They may use batteries for
energy storage. These systems may also have engine-driven
backup generators. A typical residential stand-alone system
might have a PV array rated from 1 to 4 kW, a 4 kW inverter, a
1000 amp-hour battery bank, and a 3 to 6 kW backup generator (Figure
2, Residential roof showing fixed and tracking PV
modules). An engine fueled by gasoline, propane, natural gas,
or sometimes diesel fuel would drive the generator. PV systems
that include a generator or second renewable energy source
such as a wind turbine are known as hybrid systems (Figure
3,
Hybrid PV system).
Commercial stand-alone PV systems used for
power at telecommunications sites, National Parks, and in
military installations may have PV arrays sized from 10 kW to
200 kW with proportionally sized battery banks and backup
generators (Figure
4, Pinnacles National Monument PV System).
Remote (and some urban) lighting and water
pumping systems may have only a few PV modules. Water pumping
systems usually do not have batteries and operate only during
daylight hours.
With the addition of a battery bank, most
stand-alone PV systems will also have a charge controller to
control the charging and discharging of the batteries (Figure
5, Battery charge controller). The charge controller may
contain a low-voltage disconnect to protect the batteries from
excessive discharge. The low-voltage disconnect may be a
separate device. Many inverters have internal low-voltage
disconnect controls.
There are more than 1000
utility-interactive PV systems in the U.S. but the number is
expected to increase due to federal and state subsidies and
the Million Solar Roofs Initiative of the federal government.
For more information see the US Department of Energy’s web
page at www.eren.doe.gov/millionroofs .
A residential PV system in an urban
location will usually be a utility-interactive system (Figure
6, PV modules on roof). The output of the PV array (typically
500-2,000 watts) will be connected to a listed,
utility-interactive inverter (dc-to-ac power conversion) (Figure
7, Utility-interactive inverter). The interactive
inverter will produce energy only when connected to the
electrical power grid that is operating at near 60 Hz and 120
or 240 volts. If the grid is "down" for some reason,
the inverter will produce no power and, in fact, will usually
stop producing power (de-energize) even in
"brownout" conditions. This creates a safer
environment for the utility lineman by preventing exposure to
PV power on lines that have been disconnected from the grid.
Because of the NEC requirements to de-energize,
utility-interactive PV systems do not pose any of the dangers
associated with engine-driven generators that are illegally
connected during outages after a storm.
Utility-interactive PV systems must be
connected to the utility with a dedicated circuit. There
should be no receptacle or other outlets on this circuit
between the inverter output and the load center. Section
690-64(b)(2) places restraints on the size of the PV system
that can be connected to any particular load center or other
circuit. These restraints are particularly important in
commercial installations where the load centers and
distribution panels are operated at near capacity.
AC PV Module
The ac PV module is a new type of
utility-interactive PV system (Figure
8-Two ac PV modules).
These listed devices have a small (100-300 watt) inverter
attached (factory integrated) directly to the back of the PV
module or modules (Figure
9, Rear view of ac PV module showing
attached inverter). There is no field-connected or installed
dc wiring, and the system operates much like an ac appliance.
It is listed as a single unit. The only output is alternating
current, and there are no accessible dc voltages. The output
terminals of the ac PV module are not energized until they are
connected to a 120-volt 60-Hz utility grid. This type of PV
system may become increasingly popular since the unit cost of
these low-power systems is small compared to larger systems
with multiple components. The installation and code
requirements of the ac PV modules are considerably simplified.
Code Requirements
Article 690 of the NEC specifically
addresses installation requirements for PV electrical systems,
but most of the rest of the code is also applicable. Where
there are conflicts in the code requirements between Article
690 and other articles of the Code, Article 690 takes
precedence due to the unique nature of PV modules as
electrical generators.
Wiring
Aside from the exposed single-conductor
module wiring allowed by NEC Section 690-31, the rest of the
wiring, in both ac and dc circuits, should comply with the
requirements of Chapter 3 (Wiring Methods and Materials) or
Chapter 4 (Equipment for General Use) of the NEC. Exposed
single-conductor wiring is not normally allowed by the NEC inside buildings and also should not be used in the
well-designed and installed PV system. Flexible, portable
power cable (NEC Chapter 4) should only be used in PV arrays
where the movements of sun-tracking devices require the extra
flexibility.
PV modules and arrays do not have the
capability of generating high fault currents like a battery or
generator. The PV output is proportional to the intensity of
the sunlight on the module. The rated current (both
short-circuit and operating) of a PV module is measured in a
laboratory under standard test conditions. The standard
conditions used for rating may be exceeded in actual use, on a
daily basis, for three hours or more. Consequently, the
instructions provided with PV modules require that all
conductors and overcurrent devices be rated to handle 125
percent of the rated short-circuit current. This requirement
was contained in the UL Standard 1703 and the 1996 NEC. In
the 1999 NEC, the requirement is contained entirely within
the NEC.
Temperature
Derating of Conductors
Since PV modules operate at elevated
temperatures and the wiring to the modules may be in conduits
that are exposed to sunlight and the weather, temperature
derating of conductors is necessary. PV modules may operate at
temperatures of 20-40°C (68-104°F) above the ambient
temperatures. In the hot, sunny Southwest, where ambient
temperatures reach 45°C (113°F), the PV module junction box
on the back surface of the module may reach 75°C (167°F).
Conductors with 90°C-rated insulation should be used, and the
ampacities of conductors in the junction boxes need to be
derated accordingly. Conductors in conduit are considered to
be in exposed locations and should have insulation rated for
wet use. For exposed conductors, type USE-2 or type TC cable
meets the 90°C/wet requirements. In conduit, conductor types
RHW-2, THWN-2, THW-2, or XHHW-2 meet both the 90°C and wet
requirements.
Overcurrent
Protection
As in other electrical systems, each
conductor or circuit in a PV system should be protected from
overcurrent. Since a PV system may have more than one source
of energy, some circuits may have power sources at both ends
thereby requiring overcurrent protection at more than one
location. Although PV modules are current-limited power
production devices, the UL-listing and labeling may require an
overcurrent device for each module or series string of modules
to protect the modules and wiring from external sources of
power. In most systems with batteries or utility-interactive
inverters, the module wiring must be protected from high fault
currents originating from the batteries or the utility grid
back feeding through the system.
DC source-circuit combiner devices and
power centers used in PV systems are somewhat like ac load
centers used in conventional ac systems. The individual PV
source circuits connected to PV power centers resemble ac
branch circuits connected to circuit breakers in an ac load
center.
Current-limiting fuses such as the Class
RK-5 or Class T fuses are used to protect wiring and devices
connected to batteries since large fault currents are
possible. Where current-limiting fuses are not used, each
overcurrent device in the circuit (either a fuse or circuit
breaker) must have an interrupt rating capable of withstanding
any fault currents that may occur. Typically, overcurrent
devices with interrupt ratings of 20,000 amps or higher are
used with battery circuits.
In the dc circuits of a PV system, only
overcurrent devices listed for dc operation are allowed by the NEC. Underwriters (UL) Standard 1703 and the 1996 NEC require
that the voltage rating of the overcurrent devices should be
at least 125 percent of the rated, open-circuit voltage of the
PV array output since rated open-circuit voltage increases as
temperatures go below 25°C (77°F). In the 1999 NEC, a new
Table 690-7 includes the UL 1703 requirement and provides
correction factors that are less than 125 percent for
localities where the modules will experience more moderate
temperatures.
Disconnects
The requirement for disconnects for PV
systems are covered in Article 690 of the NEC. Generally, a
disconnect is needed for each source of power or energy
storage device in the system. If these disconnects are
inadequate to isolate equipment like inverters and charge
controllers for servicing, additional disconnects may be
required for these components.
PV modules and arrays are energized when
illuminated, but a disconnect at the PV array location is not
required by the NEC unless the system is so large (more than
10 kW) that subarray disconnects would facilitate maintenance
actions. Usually, only a single disconnect for the PV array is
used near the power center or other combiner box (stand-alone
systems), or near the inverter (utility-interactive systems).
This main PV disconnect disconnects the entire PV array from
all equipment, thereby removing the PV source of power from
the system. It also isolates the PV array and array wiring
from all other sources of power in the system, but it does not
make the array safe for maintenance. All switchgear used in dc
circuits are required to be appropriately listed and labeled
for use in dc circuits.
Ground-fault
Protection
Section 690-5 (Ground-fault Protection) of
the 1999 NEC requires that any PV array installed on a
dwelling be provided with ground-fault protection (changed
from the 1996 term "Ground-fault Detection and
Interruption") to minimize the possibility of fires. This NEC requirement is unique to PV systems and is not related
to the common GFCIs used for shock protection or the
ground-fault equipment required on high-current circuits used
for equipment protection. Utility-interactive inverters may
contain ground-fault protection circuits or the ground-fault
protection is available as an option. Stand-alone and
utility-interactive PV systems that are roof-mounted on
dwellings need ground-fault protection. AC PV modules
generally are meeting the requirement with equipment-type,
ground-fault-protection-circuit breakers installed as part of
the dedicated branch circuit for those modules.
Listed Equipment
Availability
Listed hardware for PV installations is now
commercially available. Listed combiner boxes and power
centers that contain the necessary overcurrent devices are
sold as components (Figure
10, Listed Power Center). Several
inverters with power outputs of less than 5 kW are listed. A
number of listed battery charge controllers are now on the
market. Components (such as inverters) for larger systems are
not listed yet and may have to be examined for safety.
Conventional junction boxes, pull boxes, conduit, and other
familiar materials are used throughout the systems.
Batteries and
Engine-generators
Batteries are covered in Article 690 of the NEC. The installation of batteries, their use in dwellings,
allowable operating voltages, current limiting, battery
interconnection, and charge control are all spelled out.
Batteries and engine-generators generally are not listed.
Batteries may either be sealed types (valve regulated
lead-acid—VRLA or gelled electrolyte) or the more common
flooded lead-acid batteries. All batteries can vent hydrogen
gas when over-charged and they contain an electrolyte. Both
types of batteries should be installed so that the exposed
terminals are not accessible to unqualified persons. The
flooded batteries should be installed in an acid resistant,
non-conducting container to contain spilled electrolyte in the
unlikely event that the battery case is damaged.
While the batteries should be installed in
a well-vented area, power venting is not required for small
systems. Hydrogen gas is very difficult to contain and normal
room ventilation is usually adequate to ensure dispersion of
the gasses. Venting manifolds common to each cell should be
avoided. Conduit entrances to battery enclosures should be
below the tops of the batteries since hydrogen gas rises, and
the conduit ends need to be sealed with an appropriate
material to prevent hydrogen gas from entering power centers
or other switchgear.
Installations
Typically, PV power systems are installed
by persons trained and experienced in one discipline or trade.
The code-familiar person such as an electrical contractor or
electrician may be unfamiliar with the unique particulars of
PV system installations. Conversely, the PV system designer or
vendor is often an expert on PV installation requirements but
is not familiar with the intricacies of the NEC. For this
reason, the most satisfactory installations are the result of
a team effort, from project commencement to completion, that
includes a PV designer, an electrical contractor/electrician,
and the Authority Having Jurisdiction (AHJ).
Summary
Photovoltaic power systems have been
installed throughout the US and increasing numbers are being
installed each year. Most of these systems fall under the
requirements of the National Electrical Code.
Equipment, knowledge, and experience is available that allows
these systems to be installed in full compliance with the NEC. Best results are obtained when a PV systems designer
works with an electrician or electrical contractor and the
Authority Having Jurisdiction.
Additional
Information
A manual entitled Photovoltaic Power
Systems and the National Electrical Code: Suggested Practices authored by John Wiles and published by Sandia National
Laboratories is available without charge from the author or on
the internet at www.sandia.gov/PV .
The author writes a bi-monthly column
called Code Corner in Home Power Magazine, which covers
the NEC requirements for PV energy installations in some
detail. The magazine is available on the Internet at
www.homepower.com and subscriptions are available by calling
800-707-0836.
Presentations by the author on PV systems
and the NEC® are available to groups of 30 or more electrical
inspectors, electrical contractors, electricians, and other
interested IAEI members. The presentations run from 6-8 hours
and consist of overhead and 35mm slides and hardcopy handouts.
John Wiles is a Program Manager and Research Engineer
at the Southwest Technology Development Institute
located at New Mexico State University in Las Cruces,
New Mexico. He has been involved in renewable energy
systems since 1976 and has lived in an off-grid,
stand-alone PV-powered residence since 1990.
He is Secretary for a NFPA Task Group on Article 690
thatsubmitted 57 proposed changes to Article 690 of the
1999 NEC. He can be reached at SWTDI/NMSU •Box 30,001/MSC
3 SOLAR • Las Cruces, NM 88003 • 505-646-6105 •
FAX 505-646-3841• jwiles@nmsu.edu.
This work was supported by the United
States Department of Energy under Contract
DE-AC04-94AL8500. Sandia is a multi-program laboratory
operated by Sandia Corporation, a Lockheed Martin
Company, for the United States Department of Energy. |
