Inspectors are more and more frequently
faced with permitting or inspecting PV systems as these
systems proliferate throughout the country due to increasing
regional financial incentive programs. Photovoltaic power is a
relatively young technology and industry. While well-qualified
people are installing many excellent, code-compliant PV
systems, others are designing and installing these systems
with little or no prior experience with electrical systems.
Unfortunately, as financial incentives continue and even
increase, more unqualified people are installing these
systems. The electrical inspector, through the permitting and
inspection process, can help the PV industry focus on the
design and installation of safe, code-compliant PV systems.
Inspector involvement early in the process often proves
beneficial to all.
The information below will help the
inspector determine if the basic design of a PV system meets
the specific requirements of the National Electrical Code as outlined in Article 690 and in other sections of the Code.
Additional information will be provided to highlight areas
that should receive special attention during the inspection.
Inspectors need to be at least as well informed, if not better
informed, than the designers and installers of these systems.
The
Permit
As a part-time inspector and plan reviewer (for utility
companies and municipalities who require code-compliant PV
systems), I always require that the vendor/installer provide a
neat, legible system diagram, a list of the conductors and
parts used (with model numbers), and the calculations used for
conductor and conduit sizing and overcurrent device rating.
Although I don’t require engineering drawings, or even CAD
drawings, unreadable, messy scribbles on the backs of
envelopes are rejected. Since both inspectors and installers
have not done thousands of PV systems, the inspector should
accept nothing less.
Conductor
Types
Module junction box temperatures may be 30–35°C higher than
ambient temperatures, and 75–80°C temperatures are not
uncommon. The conductor types selected for connection to the
PV modules should be rated for wet, 90°C conditions. In
conduit, these are normally THHN/THWN-2 or RHW-2 types. If in
free air, as allowed by the Code, the conductors most
commonly used are USE-2, and if these are to be also run in
conduit, they should be USE-2/RHW-2—particularly if the
conduit is inside the building. [See "Perspectives on
PV" in the July/August 2004 issue of the IAEI News for more information.]
Conductors in outside conduits or in PV
combiners or junction boxes exposed to the sun may be
operating at 17°C or higher than the ambient temperature [see
the fine print note No. 2 in Section 310.10 in the 2005 NEC]. In PV systems we suggest adding 20°C to the
ambient temperature to accommodate the temperature rise in
aging, dull-colored conduits. Again, this usually dictates the
use of wet rated conductors with temperature ratings of 90°C,
although some installations in cold climates might squeeze by
with 75°C insulated conductors.
Currents,
Cables, and Overcurrent Devices
The process for calculating cable sizes for PV systems in the Code is somewhat complex, particularly when conditions of use are
applied that include temperature deratings and conduit fill as
well as the temperature limitations of the terminals of
overcurrent devices. See Appendix I of the author’s PV
Power Systems and the National Electrical Code: Suggested
Practices (available free as a download—see endnote1).
A slightly abbreviated version is presented here.
Due to the unique characteristics of solar
energy and PV modules, worst-case currents are always used and
are considered continuous [see "Perspectives on PV"
in the July/August 2004 IAEI News]. In any PV source
circuit (one module or a series connected string of modules)
the individual module short-circuit current (Isc)
is multiplied by 1.56 to get the basic conductor ampacity
rating (at 30°C) and the overcurrent device (where required)
to protect this conductor and the internal module conductors.
Temperature correction factors, for the conductors connected
to the modules, of either 65°C (cooling air to the back of
the modules—4 inches or more of space) or 75°C (no cooling
air—less than 4 inches of space) are applied to the 30°C
ampacity.
Overcurrent devices (where required) are
installed electrically and physically away from the modules in
combiner circuit boxes where the PV source circuits are
combined in parallel. If the combiner boxes are exposed to
sunlight and ambient temperatures over 40°C (104°F), then it
is likely that the overcurrent devices will be exposed to
temperatures in excess of their normal 40°C maximum. In
practice, a 10–15 percent derating should be applied to the
overcurrent device rating and then it should be verified that
it would still protect the conductor.
When dealing with temperature deratings on
90°C conductors connected to overcurrent devices with
terminals rated for conductors operating at no more that 75°C
or possibly even 60°C, that 1.56 x Isc calculated
current must be below the 75°C (or 60°C) ampacity values for
the conductor size being used [see 110.14(C)].
When PV module source circuits are
paralleled in PV combiners, then the short-circuit currents of
the paralleled circuits sum together, and new conductors and
overcurrent devices must be selected to handle the increased
currents.
The voltage rating of conductors,
overcurrent devices, and disconnects must be based on the
maximum system voltage, which is the sum of the open-circuit
voltage (Voc) of all modules connected in series times a
temperature dependent factor found in NEC Table 690.7.
A factor of 1.25 can be used for any system that is installed
in locations where the record low temperature is no lower than
-40°C (-40°F).
Disconnects
On utility-interactive PV systems, disconnects are generally
required for the main PV circuit input to the inverter and the
inverter ac output (which may be a backfed breaker in a load
center). The addition of batteries in some systems will
necessitate additional disconnects. Most utilities require an
outside, visible blade, lockable disconnect between the ac
output of a PV system and the point where that output connects
to the utility. While not a Code requirement, it must
be installed in a code-compliant manner.
The disconnect must have a rating of 1.56 Isc at that point, and must have a voltage rating and be connected
in a manner consistent with the maximum system voltage.
Inverter
AC Outputs
The ac output circuits from an inverter should be sized and
protected at 125 percent of the rated steady-state output
currents even when the connected PV array will never produce
currents at or near that level. One never knows how many
additional PV modules may be connected in the future.
The connection to the utility must meet the
requirements of NEC 690.64(B)(2). In residential
systems, this section of the Code will allow a
relatively small PV system to backfeed the residential load
center. In commercial systems, either the size of the load
center must be adjusted or a second service entrance must be
added to accommodate the PV system.
The
Inspection
Good workmanship
For some reason, even experienced electricians frequently
forget to use good workmanship when installing PV systems. In
all cases, conduit should be fastened to structures for
protection against wind and ice loading. Modules and mounting
racks as well as other equipment should be firmly mounted to
structures in a manner that will resist environmental stresses
of sunlight, wind, and rain at the very least. Areas of the
country subject to earthquakes or hurricanes will require
specialized, more rugged installations.
Double lugging and worse are common in PV
installations (see photo
1 for an example).
Grounding
Grounded conductors, both ac (neutral) and dc (negative),
should be white or marked white and should never be
interrupted by a switch pole, fuse, or circuit breaker —particularly
on dc source circuits from the PV modules (see photos 2 and 3).
Grounding of module
frames, combiner enclosures and disconnects in the dc circuits
is important because they may operate up to 600 volts in
commonly installed systems. No sheet metal or "tech"
screws should be used to ground disconnect enclosures with
tin-plated aluminum lugs; proper grounding/ground bar kits
should be used (see photo
4). [See "Perspectives
on PV" in the September/October 2004 issue of the IAEI
News for more details on grounding PV modules.]
When metal conduit has been used, proper
bonding of the conduit to the enclosures should be verified,
particularly when the dc PV voltages are above 250 volts.
The ac portion of most PV systems should
have only one neutral-to-ground bond, and that bond will
frequently be in the ac load center for the system. Since the
inverter uses a transformer that isolates the dc grounded
conductor from the ac grounded conductor, the dc negative
should also have a single bond to ground. Many
utility-interactive inverters make this dc bond internally and
there should be a separate dc grounding electrode conductor
routed to either a dc grounding system or to the ac grounding
system. Any roof top PV system on a dwelling should have a
Section 690.5 ground-fault protection system and these may be
either external to the inverter or built in. The grounding
electrode conductor will be connected to this device when it
is external to the inverter.
Overcurrent Protection
Overcurrent devices in disconnect enclosures and PV
combiners located in readily accessible locations that have
exposed internal circuits should be accessible only by
qualified persons. If these devices have exposed internal
terminals and/or bus bars that could be energized when opened,
the covers should require at least a tool for access. Although
not required (yet) by the Code or UL Standards, these
devices would benefit from a warning label—on the outside:
"Warning: Electric Shock—No User Serviceable Parts
Inside" (see photo
2).
Disconnects
The location of the main PV disconnect must comply with
690.14, and unless the PV source circuit conductors are
installed in metallic raceways, they must remain outside the
structure until that first, readily accessible disconnect is
reached (see 690.31(E) in the 2005 Code).
Although the NEC allows this disconnect to be either
outside the structure or immediately inside the structure at
the point of first penetration, the PV disconnect is normally
mounted in the same manner as the ac service disconnect for
the particular jurisdiction.
On the system with batteries and larger
systems that use larger conductors (e.g., 2/0 AWG and above),
the inspector should verify that fine stranded cables (where
used) are properly terminated with connectors and terminals
listed for use with such cables (see photo
5).
Summary
Photovoltaic power systems have the potential to produce
significant amounts of energy for many years. The
well-informed inspector can make a significant contribution to
the safety, quality, durability, and even performance of these
systems. Compliance with the requirements of the NEC and the recognition that the Code gives minimum
requirements should result in a safe, durable system,
particularly if these minimums are exceeded. A well-qualified
team that includes the designer, installer and the inspector
will help ensure that these systems remain safe for their
entire life.
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.
1 A PV Systems Inspector/Installer
Checklist will be sent via e-mail to those requesting it. A
draft copy of the 143-page, 2005 edition of the Photovoltaic
Power Systems and the National Electrical Code: Suggested
Practices, published by Sandia National Laboratories and
written by the author, may be downloaded from this web site
(http://www.nmsu.edu/~tdi/roswell-8opt.pdf.) 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 ten years. Copies of previous "Perspectives
on PV" are also available on this web site.
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.
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 will be submitting 30+ proposals for Article
690 in the 2008 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 a cat—permitted
and inspected, of course.
This work was supported by the United
States Department of Energy under Contract
DE-FC04-00AL66794 |
