The 2005 NEC has been published and
Article 690 has some changes that will benefit the
Photovoltaic (PV) Power Industry and electrical inspectors by
making the Code easier to understand and by allowing
modified installation procedures. As jurisdictions adopt the Code (some as early as January 1, 2005—others possibly not for
years), the new requirements may be applied. These
requirements and other significant changes will be covered in
this article.
Optional Inverter
Locations
The intent of the 2002 NEC was to have
utility-interactive inverters mounted in readily accessible
locations. However, these devices are relatively robust,
require little maintenance, and generally are constructed with
outdoor enclosures. Section 690.14(D), new to the 2005 NEC,
allows utility-interactive inverters to be mounted in areas
that are not readily accessible. A readily accessible area is
one that can be approached without opening a locked door,
removing building materials, or using a ladder or other device
to reach the location. In the 2005 Code,
utility-interactive inverters may now be mounted on the roof
of a building near the PV array. However, dc and ac
disconnects must be located at the inverter and an additional
ac disconnect must be located in a readily accessible location
as required by 690.14(A)–(C), usually at ground level. These
disconnects are Code requirements and may not satisfy
any utility requirements for a readily accessible,
visible-blade, lockable ac disconnect for the PV system. These
disconnect requirements were covered in the article on PV
systems in the March/April 2004 issue of the IAEI News.
PV Source and
Output Conductors Allowed Inside the Building
Section 690.14 generally requires that PV source and output
conductors remain outside a building until they reach a
readily accessible disconnect at the point of first
penetration. Section 690.31(E) now permits conductors from the
PV array on the roof of a building to be run inside the
building before reaching the first readily accessible
disconnect if those conductors are installed in metallic
raceways. Metallic raceways would include the various types of
rigid metal conduit and flexible metal conduits. Non-metallic
raceways (PVC) are not allowed by this provision because they
do not provide the physical protection, fire containment or
ground-fault detection afforded by metallic raceways. Now, the
PV installer can legally hide the conductors from the roof
inside the building without running unsightly conduits down
the outside of the structure as was required in the 2002 Code.
While Section 690.14(A and B) read the same in the 2005 NEC as they did in the 2002 Code, an exception addressing
690.31(E) has been added to 690.14(C)(1) to address the
allowance for metallic raceways inside the building. If
metallic raceways are not used, then the PV source and output
circuits must remain outside the building until they reach the
readily accessible disconnect at the point of first
penetration.
Ungrounded PV
Systems Now Permitted
Section 690.35 was added to permit the use of ungrounded PV
arrays where neither of the circuit conductors is grounded as
is currently required for systems operating over 12 volts
nominal. This permissive (not mandatory) requirement was added
to the Code to allow utility-interactive inverters to
be used that have no internal or external isolation
transformer. Without a transformer, the inverter efficiency
can be increased while the weight and cost can be reduced. The
equipment grounding system still must be present and there are
several other requirements that will help to ensure that these
ungrounded systems are as safe as the grounded systems. These
additional requirements for ungrounded systems are loosely
based on PV design and installation practices used in Europe
where the Europeans have had far more experience with
ungrounded power systems than we have had in the United
States.
1. Disconnects and overcurrent protection
will be required in both of the now-ungrounded conductors.
2. A ground-fault protection device will be
required on all ungrounded PV systems even when the PV array
is not mounted on the roof of dwellings where such a device is
currently required (see 690.5).
3. The conductors from the PV array will be
installed in raceways (conduit) or be part of a
multi-conductor sheathed cable. This requirement is to
duplicate the protection provided by a double-insulated cable
that is not presently available in the US. Underwriters
Laboratories (UL) is developing a new standard for
double-insulated cables, and such cables are being designed
for use with PV modules. Until such cables are available, the
current use of modules with single-conductor pigtail wiring
and MultiContact® connectors will not be allowed
on ungrounded PV arrays.
4. Because many people think that
ungrounded PV systems are inherently safer than grounded
systems, a warning label will be required at all points where
the ungrounded conductors are terminated. Labels with the
following warning will have to be attached by the installer at
points like junction boxes and disconnects where the
conductors are attached to terminals that may require service.
Warning
Electric
shock hazard. The direct current circuit conductors of
this photovoltaic power system are ungrounded but may be
energized with respect to ground due to leakage paths
and/or ground faults.
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5. Inverters or charge controllers used in
ungrounded systems must be specifically listed for that
purpose by Underwriters Laboratories or other acceptable
testing and listing agencies like ETL or CSA.
Installers and inspectors should note that
most of the currently-available PV equipment intended for use
on 12 to 48-volt PV systems is designed to be used only on
grounded PV systems and would generally not meet the
requirements listed above for ungrounded PV systems. This
equipment frequently has overcurrent devices and disconnects
installed in only one of the current-carrying conductors and
the other current-carrying conductors are frequently connected
to a common bus without overcurrent protection. Also, most 12
to 48-volt PV systems will continue to use inverters that have
transformers to obtain the necessary 120-volt ac output
voltage from the lower dc input voltage.
Grounding System
Clarifications
Section 690.47(C) clarifies the requirements for grounding
systems that have both ac and dc grounding requirements.
Typically, all PV systems with inverters must have both the ac
and the dc side of the system grounded since the internal
transformer in the inverter isolates the dc grounded conductor
from the ac grounded conductor. The inverter essentially
creates a separately derived dc system when this isolation is
considered. Normally the ac part of the PV system is grounded
at the ac service disconnect (utility-interactive systems) or
the ac load center (stand-alone systems) and is accomplished
by the existing ac system. The Code allows the dc
grounding electrode conductor to be routed to one or two
locations: (1) to a dc grounding electrode which then must be
bonded to the ac grounding electrode, or (2) directly to the
ac grounding electrode where it is connected to that electrode
with a separate clamp. The size of the grounding electrode
conductor is determined by 250.66 (ac) and 250.166 (dc), and a
bonding conductor, when used, must be sized the larger of the
two. See the "Perspectives on PV" in the
September/October issue of the IAEI News for additional
details on grounding.
Backfed Breakers
May Not Need To Be Clamped
The addition of Section 690.64(B)(5) takes precedence over the
code requirement [in Section 408.16(F)] that all backfed
circuit breakers must be clamped to the internal busbar. This
revision does not require that backfed circuit breakers be
clamped to the internal load center busbar where they are
connected to a listed utility-interactive inverter and where
all circuit breakers in the panel are secured with a front
panel. Installers (and inspectors) were having a great deal of
difficulty in finding load centers that had provisions for
clamping backfed breakers that were not in the main breaker
position. Since a backfed breaker connected to a
utility-interactive inverter immediately goes dead when
unplugged, the dangers associated with such breakers connected
to a rotating generator (which may stay energized) do not
exist. Furthermore, if an unqualified person uses a
"tool" to remove the cover from a load center
(thereby allowing any breaker to be removed), the main lug or
main breaker terminals and the exposed bus bars may present
greater hazards than an unplugged backfed breaker.
Section 690.72(B)(2)(2) clarified the
requirements of diversion loads in relation to diversion
charge controllers in systems with batteries. The current
rating of the load must be equal to or less than the current
rating of the controller (a technical requirement), the
voltage rating of the diversion load must be greater than the
maximum battery voltage, and the diversion load must have a
power rating of 150 percent of the power rating of the PV
array. These modified requirements allow the PV system
designer to properly specify a diversion load that is
consistent with the requirements of the diversion load
controller while maintaining the required safety margins for
the system.
Summary
These are the major changes for the 2005 NEC. It is
unfortunate that some large PV markets, like California, will
not immediately adopt the 2005 NEC. Inspectors in those
regions are encouraged to review the changes in the Article
690 for 2005, and apply them judiciously where appropriate. I
encourage all PV systems designers and installers to get a
copy of the 2005 NEC and better yet the 2005 NEC Handbook that has significantly expanded comments on
the intent of the Code requirements.
The PV Industry Forum has already started
formulating proposals for the 2008 NEC and they must be
finalized before the end of November 2005. Send me your
comments and suggestions on PV safety for the 2008 NEC and I will ensure they get the thorough review they deserve.
Inspector comments and suggestions for
changes to Article 690 are particularly welcome. The
"best" PV systems (safest, most durable, most
reliable, highest performing) have usually resulted from a
close collaboration between the PV designer, the code–familiar
installer, and the electrical inspector.
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
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 with their
address 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 ten
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.
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 |
