More than 90 percent of the new PV systems
being installed throughout the United States are connected to
the local utility with utility-interactive inverters (figure
1). These inverters range in size from about 250 watts
(rated ac output) to about 250 kW. Multiple inverters may be
used at a single location to provide even higher outputs. The
connection requirements to the utility are established in
various sections of the Code. Unfortunately, in many
cases, these requirements are not fully understood or complied
with. This article will concentrate on the requirements of the 2005 National Electrical Code Section 690.64, Point of
Connection.
This section of the Code allows the
output of the inverter to be connected either on the supply
(utility) side of the service disconnect or on the load
(inverter) side of the service disconnect. Supply-side and
load-side connections will be addressed for non-dwelling
(commercial) installations first, followed by the requirements
for dwellings.
Supply-Side
Connections—690.64(A)
Connecting to the supply side of the service disconnect
usually implies that the output of the PV inverter is
connected to the conductors between the service disconnect and
the meter socket. This connection is made to allow the meter
to sense utility-generated power flowing to the load
(facility) and PV-generated power flowing back to the utility
when local power production exceeds local loads. Using a
single meter allows relatively easy implementation of net
metering where the meter runs forward and backward (depending
on power flow) and the customer eventually pays for only the
net energy used or produced. Figure
2 shows a diagram of such a connection, and figure
3 shows the picture. In the picture, the disconnect shown
is an existing feeder disconnect (1980s vintage) for the
building and the connections for the PV conductors are at the
bottom, which are on the supply side of the service disconnect
for the building. The conductors leading into the building
connect to the building load center that has a main circuit
breaker serving as the service-entrance disconnect.
The inverter will normally be connected
through a disconnect/overcurrent protection device before
being connected to the service-entrance conductors between the
meter and the service disconnect. This is equivalent to
connecting a second service entrance to the building and the
disconnect/overcurrent device (circuit breaker or fused
disconnect) should be rated as service-entrance equipment.
Elsewhere, Article 690 requires that the output circuit from
the inverter be sized and protected at 125 percent of the
rated continuous ac output of the inverter. Obviously, the
existing service-entrance conductors must be at least this
size in case they have to handle the full rated output of the
PV system. Like other service conductors, the conductors
between the disconnect/overcurrent device and the existing
service-entrance conductors are not protected, and it is
suggested that they be as large as the disconnect/overcurrent
device terminals will accept. It is also suggested that these
conductors be kept as short as possible and that they follow
the general requirements for service-entrance conductors.
Since the inverter output circuit is not a load or feeder
circuit, I do not believe that the general tap rules are
applicable.
Of course, the connection could be made
with the addition of a new meter, and this would be a complete
second service entrance to the facility. Usually, this
complicates the measuring and billing for energy used or
produced where net metering is in effect and the system is
associated with a building or structure. However, this
complete separate service entrance is frequently used on the
larger (100 kW and up) systems.
Since many utilities require a visible
blade, lockable (open) disconnect between the output of the
inverter and the utility point of connection, the disconnect
described above and required by the NEC, may also serve
as the utility-required disconnect. In some cases, the utility
will not allow a fused disconnect, so a second, non-fused
disconnect must be added.
Load-Side
Connections—690.64(B)
Load-side connection requirements are more numerous than
supply-side connection requirements. Section 690.64(B)(1)
requires that a dedicated circuit breaker or fused disconnect
be used for the interconnection. This essentially means that
the output of each single inverter be connected to a
disconnect/overcurrent device before that circuit is connected
to any other sources or loads. See figure
4 for a circuit showing two inverters connected to a load
center (panelboard) on dedicated circuits. Figure
5 shows a picture of a load center being used to connect
two utility-interactive inverters to the grid. And, yes, those
circuits are "dead."
The requirements of 690.64(B)(2) are
complex. Here is what the section (without the exception) says
with emphasis added by the author. "The sum of the ampere
ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed the rating of
the busbar or conductor."
The key word that many readers miss is the
word "supplying." In a load center or panelboard,
the main circuit breaker supplies power to the internal
busbars, as do any backfed circuit breakers supplying power from the PV inverters. The potential problem can be seen
in figure 4. The
load center is rated at 100 amps, the main circuit breaker can
supply 100 amps to the busbars, and at the same time, the
inverters may add another 30 amps to the busbars. If the loads
were increased to 130 amps (for example, increased plug
loads), no circuit breakers would trip, but the busbars in the
center of the panel rated at 100 amps would be overloaded
carrying 130 amps.
In the deliberations for the 2002 NEC,
the determination was made that while placing the backfed PV
circuit breakers at the bottom of the panel (as far away from
the main circuit breaker as possible) would prevent
overloading the panel busbars, it was not an acceptable
long-term solution (even with placards). Placards get lost or
damaged and people who may not be familiar with PV
installations and interconnections move around circuit
breakers in load centers after the initial installation.
In designing PV systems for commercial
(non-dwelling) installations, an existing load center is
usually considered. In many commercial installations, the size
of the main circuit breaker in the load center has the same
rating as the load center itself. Therefore no additional current may be supplied to the load center from
backfed PV circuit breakers. In this case, one alternative is
to go to a supply-side connection as outlined above. Another
option is to remove the existing load center and replace it
with a new, larger load center that has a main circuit breaker
rated the same as the original main circuit breaker. The
amount of PV current that can be backfed is the difference
between the panel rating and the main circuit breaker.
In all cases the main circuit breaker, the
load center, and any conductors (including feeders) carrying
the output of a PV system must be sized for at least 1.25 x
the rated output of the inverter (see 690.8 and 690.9). As
will be seen below, the load center will usually be
significantly larger than just the size required by the PV
circuits.
In some installations, an oversized load
center is being used with an adjustable main circuit breaker.
Assuming that the main circuit breaker is set at a trip point
below the rating of the panel, then the difference between the
two ratings is the allowable current that can be backfed from
the PV array.
It is usually not a good idea to
replace an existing main circuit breaker with one that has a
lower rating or to adjust an adjustable main to a lower trip
point in an attempt to accommodate a PV system. The original
installer of the system sized that main circuit breaker based
on code-required load calculations, and if the circuit breaker
rating were changed, it could result in nuisance trips or an
overloaded circuit breaker, not to mention a Code violation.
Connecting PV
Systems to a Commercial Feeder Panel or Subpanel
In many commercial installations, the PV system is installed
on the roof of a multi-story building. The building usually
has a feeder panel or subpanel on each floor of the building,
and those panels are connected to a main panel on the ground
floor. To minimize the PV installation cost, an attempt is
made to connect the PV output to the feeder panel on the top
floor. However, figure
6 reveals a problem. While the requirements of
690.64(B)(2) are easily met at the top floor feeder panel,
they become increasingly more difficult to meet at
intermediate feeder panels and at the main panel.
For example, the backfed PV current at the
top floor 100-amp feeder panel could require only a 15-amp
circuit breaker. Section 690.64(B)(2) would normally require
that the feeder panel be increased to 125 amps (next standard
size) to accommodate the 15-amp backfed PV circuit breaker.
However, when we get to the first 400-amp intermediate panel,
the rating of the backfed circuit breaker carrying the PV
currents is now 100 amps, not the 15-amp rating of the circuit
breaker in the top floor panel. Meeting 690.64(B)(2) is more
difficult with the larger backfed circuit breaker. Since this
100-amp circuit breaker is the only circuit breaker limiting
backfed currents, its full 100-amp rating must be considered,
not just the 15-amps that it is carrying at the present time.
If only the 15 amps were considered, then at some future date
the PV array might be expanded and the intermediate feeder
panels could be overloaded since any backfed currents could
reach 100 amps before a circuit breaker tripped in the
intermediate 400-amp panel. At this point, the 400-amp panel
would have to be increased to at least a 500-amp panel to
accommodate the 100-amp backfed circuit breaker to meet
690.64(B)(2) requirements.
The same analysis applies to the main
1000-amp panel. The backfed circuit breaker is now rated at
400 amps and to meet Code, the main panel would have to
be upgraded to at least a 1400-amp panel to keep the 1000-amp
main circuit breaker. All of these difficulties could be
avoided by doing a supply-side connection (at 15 amps). Of
course, those 15-amp PV output circuit conductors would have
to be routed from the roof to the main service panel, and the
output voltage of the inverter would have to match the voltage
of the service entrance. In some cases a transformer might be
required to match the inverter output voltage to the
service-entrance voltage.
In all cases, connecting a second
service-entrance disconnect with a 15-amp rating (probably
using a higher-rated disconnect) to an existing 1000-amp
service must, of course, be accomplished in a safe,
code-compliant manner using appropriate equipment.
Applying 690.64(B)(2) to the feeder
conductors carrying backfed PV currents between the various
panels indicates that they usually will not have to be
enlarged in size when a PV system is added. There is no place
on these circuits where the feeder can be overloaded (unless
the PV output current exceeds the feeder rating) because there
are no places between circuit breakers where loads can be
connected that could be inadvertently increased as they could
be inside a panel board as shown in figure
4.
Supply-Side
Connections—690.64(B)(2) Dwelling Units
Now, let us examine the installation requirements for dwelling
units. The exception for 690.64(B)(2) reads: "For a
dwelling unit, the sum of the ampere ratings of the
overcurrent devices shall not exceed 120 percent of the rating
of the busbar or conductor."
Now we can add PV backfed circuit breakers
to the dwelling (residential) load center with some leeway
before we have to start changing equipment. Normally, the main
circuit breaker in a residential load center is rated the same
as the residential load center. This exception allows the sum
of the main circuit breaker plus the sum of any backfed PV
circuit breakers to be 120 percent of the rating of the load
center. This additional 20 percent allowance is made because,
generally, residential circuits are more lightly loaded (due
to demand factor calculations) than circuits in commercial
buildings. Where the main circuit breakers and panels have the
same rating, the exception to 690.64(B)(2) allows 20 amps of
backfed PV circuit breakers to be added to a 100-amp panel and
40 amps to be added to a 200-amp panel. Although these numbers
translate to a 3840-watt (ac inverter output) PV system on a
100-amp panel and a 7680-watt PV system on a 200-amp panel,
some people want to install bigger PV systems and that means
creative thinking must be used. These limits include the
normal 80 percent maximum continuous operating-current
limitations on the circuit breakers.
Many common PV inverters are rated at 2500
watts, 240 volts. The rated output current is 2500/240 = 10.4
amps. Using the code-required 1.25 multiplier (690.8) yields a
circuit breaker requirement of 13 amps, which rounds up to 15
amps as the rating of the backfed circuit breaker. On a
100-amp panel, with a 100-amp main circuit breaker, only one
of these inverters can be accommodated. On a 200-amp panel,
only two of these inverters may be connected limiting the PV
system to 5000 watts and not the maximum potential of 7680
watts.
However, figure
7 shows a code-compliant way to add three of these
2500-watt inverters to a 200-amp panel by using a subpanel. A
subpanel is selected to accommodate the three 15-amp backfed
circuit breakers, one from each of the 2500-watt inverters.
The main circuit breaker on this dedicated (PV-only) subpanel
has to have a minimum rating of the 3 x 10.4 x 1.25 = 39 amps
(round up to a 40-amp circuit breaker). This would also be the
rating of the backfed circuit breaker in the main panel and,
at 40 amps, would meet the Code requirements for a
200-amp main panel. Of course, two 40-amp circuit breakers
would not be needed, and only one at the main panel would
suffice.
What should the size of the subpanel be?
Using a formula derived from the Code requirements, we
see that the minimum size of the panel would be about 75 amps,
which would round up to a 100-amp, standard-sized panel.
3 * 15 + 40 <= 1.2 X, where X is the
panel size required
Solving for X gives us
X >=(45 +40)/1.2 = 71 amps
For those desiring to install larger PV
systems on residential services, the use of a supply-side
connection as outlined above can meet the Code requirements.
Line Side of Ground Fault Equipment—690.64(B)(3)
The Code generally requires that all PV inverters be
connected on the line side of any ground-fault protection
equipment with an exception that allows backfed GFP equipment
when the protected circuits have ground-fault protection from
all sources.
However, tests (by SWTDI and Sandia
National Laboratories) on the typical 5 milliamp GFCIs, 5 and
30 milliamp GFP circuit breakers have revealed that the
internal sensing and trip circuits are destroyed when they are
tripped while being backfed by a PV inverter. Conversations
with manufacturers of the larger 100–800-amp ground-fault
protection devices also indicate that these devices will be
damaged when tripped while being backfed. Therefore, it is
recommended that ground-fault protection equipment never be
backfed. A proposal deleting the exception to 690.64(B)(3) is
being developed for the 2008 NEC.
Markings Required—690.64(B)(4)
This section requires that all panelboards and fused
disconnects supplying power to a busbar or conductor be marked
showing all sources of power. This requirement is generally
met by the installation of placards containing the required
information installed by the system installer on all backfed
panelboards and fused disconnects. The placard should show the
rated output current of the inverter feeding the circuit and
the nominal line voltage of the inverter.
Backfed Circuit Breakers—690.64(B)(5)
Although another section of the Code [408.36(F)]
requires that backfed circuit breakers be clamped, changes to
690.64(B)(5) in the 2005 NEC no longer require them to
be clamped when connected to the output of utility-interactive
inverters. Section 690.3 indicates that the 690 requirements
override the 408 requirement. A fine print note explains that
circuit breakers suitable for backfeeding are not marked with "Line" and "Load"
designations.
Battery-Backed-Up,
Utility-Interactive Systems—More Complexity
The specifications in Underwriters Laboratories Standard 1741
require all utility-interactive inverters cease exporting
power to the utility grid when the utility grid voltage and
frequency deviate from very narrowly defined values. In
blackout situations, the PV system and the standard
utility-interactive inverter cease to operate and will not
even supply power to local loads. In areas where utility
blackouts are common or are anticipated to be common, some
systems are being installed that have a battery-based energy
storage system installed to provide local power during utility
outages. The batteries are connected to a specially designed
and listed utility-interactive inverter that, in the event of
a utility outage, will disconnect from the utility system and
provide a set of designated circuits with power from the PV
system and the battery. All of these actions are done
automatically with transfer devices built into the inverter. Figure
8 shows a simplified block diagram of a typical system.
Several variations are possible.
In normal operation, the utility is present
and the inverter acts as any other utility-interactive
inverter. Any power from the PV system in excess of local load
requirements is fed into the utility grid. When insufficient
power is available from the PV system, the system buys power
from the utility. The batteries are kept at full charge (float
charged) by the utility power and are generally not used.
However, when there is a utility outage, the inverter
automatically senses this outage, ceases to export power to
the utility, and feeds the backup load subpanel with ac power
derived from the PV array and the batteries. The backup loads
will receive ac power from the batteries and PV array to the
extent that the energy draw does not exceed the capacity of
the supply and storage systems.
Interfacing these systems with the utility
grid and meeting 690.64(B)(2) requirements presents challenges
for the system designer, the installer, and the inspector.
Many of these inverters have internal transfer relays that are
rated for 60-amps continuous duty, and that information is
presented in the specifications. This specification leads
designers and installers to size the backup load subpanel for
60 amps and to use a 60-amp backfed circuit breaker to connect
the inverter to the main load center where the utility
connection is made. The use of 60-amp circuit breakers in both
positions provides for best use of the internal 60-amp relay
and appears to allow maximum loads to be connected to the
backup subpanel. Unfortunately, the use of 60-amp circuit
breakers poses two problems and Code violations.
First, even though the inverter may be
rated (and can be adjusted) to carry 60 amps, the external
wiring and circuit breakers require the normal 80 percent
continuous current derating. For a 60-amp continuous current,
an 80-amp circuit breaker and conductors rated for at least 75
amps would be required. Another option, that will allow the
60-amp circuit breakers to be retained, would be to adjust the
inverter to not allow more than 48 amps of continuous current
to be handled by these circuits. That adjustment is commonly
available on most of these inverters, although there is some
question about who has access to the adjustment (qualified or
unqualified people).
A second issue is the 690.64(B)(2)
requirements discussed above. In a residential installation, a
60-amp backfed PV circuit breaker would dictate that at least
a 300-amp main panel be used (60 amp PV circuit breaker + 300
amp main circuit breaker = 360 amps = 1.2 x 300 = 360).
Residential load centers rated at 300-amps and above, while
not impossible, are not common. In a commercial installation,
the existing load center would have to be replaced with one
having at least a 60 amp greater rating. In either case, a
supply-side interconnection [690.64(A)] might be the more
practical alternative. If the full 60 amps of the inverter are
to be used, then, of course, 80-amp circuit breakers and
75-amp conductors should be used.
To further complicate the system design,
many of these systems have an external inverter-bypass switch
that is used if the inverter fails. This bypass switch,
usually consisting of a pair of interlocked circuit breakers,
is used to connect the back up subpanel directly to the main
panel when the inverter fails. These circuit breakers are
typically also rated at 60 amps and installed in a small
60-amp, three-position (three-phase) load center. Obviously
neither the circuit breakers nor the load center are rated to
carry 60-amps continuously. The use of a larger load center
and interlocked 80-amp circuit breakers would allow a full
60-amp rating for the inverter-bypass switch.
Summary
The requirements of NEC Section 690.64 can be met in
nearly all installations. While the requirements, at first
glance, are somewhat complex and sometimes overlooked,
attention to these details in the design, installation, and
inspection of these systems should help to ensure a safe,
durable, and code-compliant installation.
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 color 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 .)
A black and white printed copy will be mailed to those
requesting a copy via e-mail if a shipping address is
included. The Southwest Technology Development web site http://www.nmsu.edu/~tdi maintains all copies of the previous
"Perspectives on PV" articles. Copies of "Code
Corner" written by the author and published in Home
Power Magazine over the last 10 years are also available
on this web site.
Draft proposals for the 2008 NEC being developed by the PV Industry Forum may be downloaded
from this web site: http://www.nmsu.edu/~tdi/pdf-resources/2008NECproposals2.pdf
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. A schedule of future presentations can be found on the
SWTDI web site.
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 |
