Photovoltaic (PV) power systems are being
installed by the thousands throughout the United States. In
states like California, New York, New Jersey and a few others
where financial incentives are available, the PV business is
booming. Over 560 megawatts of PV modules were produced
internationally in 2002 and annual production is increasing
each year.1The first PV cells produced nearly fifty years ago are still
producing power and modern PV modules are expected to produce
energy for the next forty years or longer. The power output
from PV systems ranges from a few hundred watts to several
megawatts. Most of the systems are not operated or owned by
any electric utility and therefore come under the requirements
of the National Electrical Code. They must be inspected
to ensure the safety of the owners, operators, service
personnel, and the public.
The Code requirements for a typical
residential PV system are at least as complex as those for
residential wiring, and the direct current (dc) portions of
the system coupled with the alternating current (ac)
interconnection to the utility grid make PV installations
somewhat unique. Because the PV industry is thriving and
growing rapidly, individuals, companies, and organizations
with varying degrees of knowledge, skill, and experience are
installing these systems. Large (and some small) PV systems
integrators and vendors working with experienced electrical
contractors who have jointly pursued additional PV-specific
training and who work closely with the local permitting and
inspecting authorities usually (but not always) perform the
best, most Code-compliant installations.
On the other hand, individuals or
organizations who have little or no experience or training in
installing electrical systems of any type are installing more
than half of new PV systems. These systems may be unsafe (not Code-compliant)
at initial installation, may develop hazardous conditions over
the life of the system, may be hazardous to operate or
service, and may fail to deliver the full performance of a
well-designed and installed PV system.2
The authority having jurisdiction (AHJ) is
the key player in ensuring that these less-than-good PV
installations do not proliferate further. Inspectors need to
demand additional training in the inspection of PV systems and
then inspect these systems very closely. Yes, it is a
relatively unfamiliar technology, but 80 percent of the Code already familiar to inspectors applies and it is relatively
easy to learn the inspection requirements that are unique to
PV systems.
The North American Board of Certified
Energy Practitioners (NABCEP at www.NABCEP.org), a voluntary
certification program for PV installers, has certified the
first installers. The training, experience, and skill
requirements of PV designers/installers obtaining this
certification will help to ensure that safer, higher quality
PV systems are installed.
PV
System Types
Two
main types of PV systems are being installed in the U.S.:
utility-interactive (grid-connected), photos
1, photo 2, photo 3, and photo
4, and stand-alone (off grid), photo
5 and photo 6.
Both types use PV modules connected in series and parallel to
form PV arrays that produce dc energy at various voltages from
about 12 volts to 600 volts. Generally, energy storage
batteries are found in the stand-alone systems, but few are
found in the utility-interactive systems. Variations of each
system are possible with some utility-interactive systems
having battery banks to provide energy when the utility power
is not available. The larger residential stand-alone systems
will usually have a back-up generator, and these systems are
known as hybrid stand-alone systems.
Utility-interactive
Systems
Utility-interactive
PV systems are by far the most numerous of the types of PV
systems being installed. A typical residential system might
have a PV array and an inverter (converts dc to ac) capable of
delivering 2500 watts of ac power to either ac loads in the
house or to the utility grid when the power output is in
excess of those local loads. In residential PV systems, single
and multiple inverter installations are common. The single
inverter may have an ac output rating of 700 to 3500 watts,
and systems are frequently seen with 2–4 inverters used to
increase the system power output (see photo
7). Residential PV systems have had ac outputs up to 30
kW! These residential-sized inverters interface with the grid
at 120 volts or 240 volts, are listed to UL Standard 1741, and
have all of the necessary safety equipment built in and
verified as part of the listing process. The inverters
inherently meet NEC 690.5 ground-fault protection
equipment requirements (fire protection) for use with PV
arrays mounted on the roofs of dwellings.
In commercial systems, the three-phase
inverters usually start at about 10 kW and go up to 250–300
kW, and interface with the grid at 208–480 volts and higher
(see photo 8).
The utility-interactive inverters have all
of the automatic ac line disconnect devices built in that
protect the utility linemen who are working on a supposedly
unenergized utility feeder. The utility-interactive PV
inverter will not energize a dead line and in fact will
disconnect from the line when the line voltage varies more
than 10 percent from nominal or when the frequency varies by
more than a few hertz from the normal 60 Hz. Many PV owners in
California were surprised when their utility-interactive PV
systems did not work during the rolling utility blackouts and
brownouts a few years ago. Utility-interactive PV systems with
battery backup were popular for months following the
blackouts.
Stand-alone
Systems
Stand-alone
systems are typically installed in remote areas where the
utility grid is not available or where the connection fees to
the grid are higher than the costs of an alternative energy
system. While stand-alone systems sales are far smaller than
the fast-growing utility-interactive PV system business, there
is and has been a steady market for off-grid systems.
The stand-alone inverter converts dc energy
stored in batteries from the PV array to ac energy to support
the loads (see photo
9). Inverter power ratings are from about 250 watts to
5000 watts for residential systems and, as before, multiple
inverters may be connected together for greater power outputs.
Battery banks usually operate at a nominal 12, 24, or 48 volts
so the current levels to the inverters can be hundreds of amps
at full load. The stand-alone inverter may not include the
Section 690.5 ground-fault equipment, so if the dwelling
installation has the PV array on the roof, an external,
field-installed ground-fault protective device must be used.
Larger stand-alone systems are found at
national parks, telecomm sites, and federal facilities. These
can be as small as the residential system with ac outputs in
the 2–10 kW range, but they can also have single inverters
up to 250 kW. A few of these larger systems have multiple
large inverters with combined outputs approaching 500 kW or
more. Battery banks for the larger systems operate in the 200–600
volt range and dc currents to the inverters can be hundreds of
amps at these higher voltages.
Starting with the PV modules on the roof,
here are some areas where inspectors need to examine the
systems closely.
Conductor
Types and Ampacity Calculations
PV
modules may be connected with any of the numerous NEC chapter 3 wiring methods suitable for fixed installations. The
conductors and wiring methods must be suitable for the outdoor
(wet) environment, be suitable for exposure to sunlight, and
able to operate in temperatures in the 65–80°C range.
Conductors should be sized based on ampacity calculations
after temperature corrections. Section 690.31 of the NEC also allows exposed, single-conductor cables (USE-2, SE, and
UF) for interconnecting the modules, and many PV modules have
the appropriate conductors permanently attached as wiring
leads with connectors.
Ampacity calculations for the circuits from
the PV modules are based on the rated short-circuit current
marked on the back of each module. The PV module is a
current-limited source, and this rated short-circuit current
must be multiplied by several factors that account for
environmental factors and Code requirements to reach a
conductor ampacity figure. Because of the unique nature of the
solar resource and the way PV modules generate current, all PV
source currents are considered to be continuous with no
non-continuous currents. These continuous output currents
under real-world outdoor conditions may exceed the rated
short-circuit current, and this requires that a multiplier of
125 percent be applied to the rated value [NEC 690.8(A)]. A
second 125 percent multiplier (for a combined multiplier of
156 percent) is then applied to ensure that conductors and
overcurrent devices are not operated at more than 80 percent
of rating [NEC 690.8(B)].
Also, because the rated open-circuit
voltage is measured in a laboratory at room temperature
(25°C/77°F) and the modules operate over a wide range of
outdoor temperatures (including solar heating) from -40°C
(-40°F) to 80°C (176°F), the rated open-circuit voltage is
multiplied by a temperature-dependent factor that can be as
large as 125 percent (NEC 690.7). Although the modules
have a rated open-circuit voltage of 22 to 44 volts, when they
are connected in series, the rated open-circuit voltage of the
string can rapidly approach 600 volts under cold weather
temperatures.
When batteries are used, many installers
use welding cable (listed and unlisted) or automotive battery
cables to avoid having to deal with stiff 4/0 AWG and larger
standard cables. Neither of these cable types meet NEC requirements, but flexible THW and RHW cables are available
that do meet NEC chapter 3 requirements for fixed
electrical installations.
Color
Codes
The
color codes for dc wiring are just like those for ac wiring.
Remember in the early days of the Code, Edison was a dc
man. The grounded conductor (usually the negative conductor)
is to be white or have three white stripes. There is no color
code specified for the ungrounded conductor (usually the
positive conductor) in dc systems and installers use red or
black. A common error is to use a red conductor for the
positive conductor (OK) and a black conductor for the grounded
negative conductor (not to Code), but this doesn’t
meet Code and is a holdover from electronic wiring
practices (see photo
10). Since USE-2, SE, and UF conductors are generally
available only with black insulation, there is an allowance in
Section 200.6(A)(2) to let PV module interconnection cables be
marked white even when they are 6 AWG and smaller. Current
outputs from typical PV modules generally result in
interconnection conductors in the 14–10 AWG range. Of
course, when the source circuits are paralleled, the currents
increase and the conductor must be larger.
Disconnects
Since
most, if not all, PV systems will have the negative dc
conductor from the PV array operating as a grounded conductor,
there should never be a switch, fuse, or circuit breaker in
this grounded conductor. This is particularly important in PV
source and output circuits where a disconnected (and energized
with respect to ground) conductor that was marked white as a
grounded conductor could represent a shock hazard whenever the
PV array was illuminated. A common Code violation on PV
systems operating from 100–600 volts dc is to use a fused or
unfused safety switch for the PV disconnect that breaks both
the ungrounded positive conductor and the grounded negative
conductor (see photo
11).
Some automated ground-fault protection
devices (required for roof-top dwelling unit PV systems by
690.5) may open the negative conductor during a ground-fault
action, and this possibility is noted with marking
requirements providing an appropriate warning (see 690.5 and
690.13).
PV
DC Disconnect Location
NEC 690.14(C)(1) requires that the position of the main PV dc
disconnect and the conductors routed from the PV array on the
roof to that disconnect be addressed in much the same manner
as ac service-entrance conductors and disconnects. The
conductors from the PV array must remain outside the structure
until they get to the PV dc disconnect that must be in a
readily accessible location at the point where the conductors
first penetrate the building. The disconnect may also be
located immediately inside the structure at the point of
penetration and that location is acceptable in many
jurisdictions.
PV installations are frequently seen where
the PV source and output circuit conductors penetrate the
roof, are routed through the interior of the structure and
finally reach the dc PV disconnect that may or may not be
mounted in a readily accessible location. An installation such
as this does not meet Code and the daylight-energized
conductors inside the building structure pose potential
hazards for emergency response personnel who are unable to
turn them off.
There is no Code requirement for a
disconnect to be located on the roof at the PV array (much
like an air-conditioning service disconnect) because the PV
array is always energized in the daytime, the PV array needs
minimal servicing, and the disconnect would not make the PV
array any safer for servicing [NEC 690.14(C)(5)].
Section 690.18 establishes the means to make a PV array safe
for servicing including covering the modules with an opaque
material.
Overcurrent
Protection
PV
modules are subject to reverse current flow (under
line-to-line and ground faults) from external sources such as
strings of other PV modules connected in parallel, backfeed
from batteries in systems that have them, and backfeed from
the utility grid through inverters. These reverse currents may
damage the module if allowed to exceed the value marked on the
back of the module as the "Maximum Series Fuse." For
this reason, most series-connected strings of modules require
an overcurrent device in the circuit to protect all modules in
the string from the reverse overcurrents. The overcurrent
device (either fuse or circuit breaker) is usually located at
the source of the overcurrents (usually at a PV combiner box)
and also provides the Code-required overcurrent
protection for the conductors in that circuit.
Some recent utility-interactive inverters
are certified by the manufacturers to be not capable of
backfeeding currents from the grid and these inverters can be
connected to one, two and possibly more strings of modules in
parallel with no overcurrent devices in the dc circuits [NEC 690.9(A)Ex.].
Grounding
Grounding
PV systems is no more or less complex than grounding any other
electrical system. The intent is to keep all exposed metal
surfaces that could be energized at the same zero potential
with respect to earth no matter whether they are associated
with the dc part of the system or the ac part. Since the
inverter is the common element between the dc portion of the
system and the ac portion, its enclosure is common to both
equipment-grounding systems and keeps the exposed metal
surfaces at the same potential. Of course, the size of the
equipment-grounding conductors will vary with the various
circuits and those associated with PV source circuits have
unique sizing requirements (690.45).
Inverters usually have internal
transformers that isolate the dc grounded conductor from the
ac grounded conductor, so essentially the dc system becomes a
separately derived system at least in fact, if not by Code definition. Internal connections in the residential-size
utility-interactive inverters will usually provide the bonding
connection between the grounded dc negative conductor and the
equipment-grounding system. Provisions are also made for the
connection of a dc grounding electrode conductor. In
stand-alone systems, the inverters generally do not have the
dc main bonding jumper, so this connection will have to be
made by the installer.
The grounding of the ac grounded conductor
is accomplished in the existing service equipment on
utility-interactive systems and in the ac distribution
equipment (ac load center—a.k.a. in Code language:
panelboard in an enclosure) in stand-alone systems.
The dc and ac grounding electrode
conductors are either connected to separate grounding
electrodes (which are then bonded together) or are connected
to a single grounding electrode or grounding system. Changes
proposed for Article 690 in the 2005 NEC should clarify
this area since Article 250 doesn’t specifically address
electrical systems where both ac and dc grounding are
required.
The
Utility Connection–Commercial
In
most residential PV installations and many commercial
installations, the output of the utility-interactive inverter
is commonly connected to a backfed circuit breaker in the load
center associated with the service entrance. NEC 690.64(B) establishes rather strict requirements on how these
interconnections must be made, and the ratings for the load
center and the language of subparagraph (2) deserves special
attention. Note that in a commercial installation the sum
of the ratings of all overcurrent devices feeding the panel must not exceed the rating of the panel. Circuit
breakers feeding the panel would be the main and any PV
breakers. In many commercial installations, the main breaker
is sized the same as the panel rating, and this leaves no
ability to install a back-fed PV breaker. One solution is to
install a larger panel with a higher rating while retaining
the original size main breaker. This will allow PV breakers to
be added up to the difference between the main breaker rating
and the rating of the panel. Another solution is to add a
second service entrance to the building if the service
entrance voltage matches the inverter output voltage.
The NEC 690.64(B)(2) requirement
applies to all load centers/distribution equipment and feeders
between the point where the PV inverter is connected and the
service entrance panel. As can be readily seen, if the 15-amp
PV system feeds a 400-amp panel that receives power from a
1000-amp service-entrance panel, the situation gets
problematic at that 1000-amp panel. If this 1000-amp panel has
a 1000-amp main breaker, then there is no allowance to connect
a backfed circuit at this point, which would be the 400-amp
breaker feeding the remote 400-amp panel. And, yes, the
backfed "PV breaker" at this point has to be counted
as 400-amps even if the output of the PV inverter at that
400-amp panel only required a 15-amp breaker. A little
inspector understanding and common sense in this area will go
a long way, but all those involved must keep in mind that
after the OK is given, the installation has been completed,
and everyone has forgotten the project, the unwary occupant
may add loads to the system that could overload one of the
panels if NEC 690.64(B)(2) has not been followed.
The
Utility Connection–Residential
Residential
PV applications are given a 120 percent allowance on the sum
of the breakers feeding a load center, because most
residential panels are more lightly loaded than commercial
panels based on Code requirements (see NEC 690.64(B)(2)Ex). The 120 percent allowance will allow moderate
sized PV systems (up to 7680 watts on a 200-amp panel) to be
installed on dwelling units without modifying the service
equipment. However, larger residential PV systems can have
outputs that will exceed this 120 percent allowance and the
existing installation must be modified.
Summary
Excellent
performing, Code-compliant PV systems are being
installed throughout the country; however, many don’t
measure up. Inspectors, being familiar with most aspects of
the NEC, can easily pick up the necessary additional
knowledge required to inspect PV systems. PV systems designers
and installers are gaining additional training and experience,
but everyone can benefit from that final once over by the AHJ.
For
Additional Information
If
this article has raised questions, do not hesitate to contact
the author by phone or e-mail. E-mail: jwiles@nmsu.edu
Phone: 505-646-6105
See the article "Photovoltaic Power
Systems and the National Electrical Code" in the
January/February 1999 issue of the IAEI News for
additional information on PV systems.
A PV Systems Inspector/Installer Checklist
will be sent via e-mail to those requesting it. A copy of the
100-page Photovoltaic Power Systems and the National
Electrical Code: Suggested Practices, published by Sandia
National Laboratories and written by the author, will be sent
at no charge to those requesting a copy by e-mail. The
Southwest Technology Development web site (http://www.nmsu.edu/-tdi)
maintains all copies of the "Code Corner Columns"
written by the author and published in Home Power Magazine over the last 10 years.
The author makes 6–8 hour presentations
on "PV Systems and the NEC" to groups of 40
or more inspectors, electricians, electrical contractors, and
PV professionals for a very nominal cost on an as-requested
basis.
This work was supported by the United
States Department of Energy under Contract DE-FC04-00AL66794
1 PV News, Vol. 22, No.
3, March 2003. Paul Maycock, Editor. 4539 Old Auburn
Road, Warrentown, VA 20187
2 Wiles, John C. Jr., William
Brooks, and Bob-O Schultze. "PV Installations, A
Progress Report" presentation at the 29th IEEE Photovoltaics Specialists Conference, New Orleans,
LA, May 2002
John Wiles works at the Southwest
Technology Development Institute (SWTDI) at New Mexico
State University. SWTDI has a contract with the US
Department of Energy to provide engineering support to
the PV industry and to provide that industry, electrical
contractors, electricians, and electrical inspectors
with a focal point for Code issues related to PV
systems. He serves as the secretary of the PV Industry
Forum that submitted 30 proposals for Article 690 in the
2005 NEC. He provides draft comments to NFPA for Article
690 in the NEC Handbook. As an old solar pioneer, he
lives in a stand-alone PV-power home in suburbia with
his wife, two dogs and two cats—permitted and
inspected, of course
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