Article 690, Solar Photovoltaic Power Systems, has been in
the National Electrical Code (NEC) since 1984.
An NFPA-appointed Task Group for Article 690 proposed changes
to Article 690 for both the 1996 and 1999 codes. The Task
Group, supported by more than 50 professionals from throughout
the photovoltaic (PV) industry, met seven times during the
1999 code cycle to integrate the needs of the industry with
the needs of electrical inspectors, and end users to ensure
the safety of PV systems. The Task Group proposed 57 changes
to Article 690 and all the changes were accepted in the review
process. The performance and cost of PV installations was
always a consideration as these changes were formed, but
safety was the number-one priority. All of the proposals were
well substantiated and coordinated throughout the PV industry,
and with representatives of Underwriters Laboratories, Inc
(UL). The Task Group was led by Ward Bower of the Photovoltaic
System Applications Department at Sandia National
Laboratories. Ray Weber, Chair of Code-Making-Panel #3 (CMP-3)
requested the formation of the Task Group for Solar
Photovoltaic Systems. Paul Duks of UL provided valuable
background information and technical coordination with
applicable UL standards.
The most significant changes that were made in Article 690
for the 1999 NEC, along with some of the rationale, are
discussed in the remainder of this article.
Figure 690-1, often a source of confusion to many who
thought it was a design diagram for a PV system, has been
completely revised and expanded to identify the PV-unique
components in various types of PV systems and to show how they
may interrelate. A copy of the new Figure 690-1 is shown as Figure
1 and Figure 2 of this report.
Many definitions in Section 690-2 (Definitions) were
updated and clarified and five new ones were added to define
the terms used in Article 690. For example, the term
"power conditioner" was replaced with the more
commonly used term "inverter." All references to
solar hot water control systems were removed. The new and
evolving ac PV module was defined as an Alternating Current
Module to retain consistency of terms used in the NEC®.
Definitions related to stand-alone, hybrid, and
utility-interactive systems were added or revised to better
define each and to include the hybrid PV systems. The common
terminology appearing in Section 690-2 will aid the Authority
Having Jurisdiction (AHJ) and PV installers to better
understand the systems and to communicate more effectively
during the installation and inspection process.
Section 690-4 (Installation) was revised to clarify the
interconnections of modules. This change allows "daisy
chaining" modules from junction box to junction box as
long as ampacity and temperature requirements for wiring and
devices are met. New language in 690-4 also allows
interconnected modules in systems under 50 volts to be
considered as a single-source circuit.
Section 690-5 (changed from Ground-fault Detection and
Interruption to Ground-fault Protection) for the PV array on
dwellings, was revised extensively to provide clarity and to
allow alternative methods to satisfy the requirement for
ground-fault protection while still maintaining system safety.
Listed equipment that may be included in utility-interactive
inverters, power centers, and as separate components is now
available to meet this requirement. This fire-protection
requirement on dwellings (which will hopefully never be
needed) is now well defined. The hard-to-define term
"disable" was removed from this section and from
Section 690-18. Providing an indication of a fault and
labeling the hardware is required in the 1999 edition. The
fact that ground-fault protection equipment may automatically
disconnect the grounded conductor of an array in the event of
a fault is also covered with the requirement for a warning
label placed near the ground-fault indicator.
A new Section 690-6 (Alternating-Current Modules) was added
to fully define the connection requirements of ac PV modules.
Among other things, a ground-fault protection device is
required on the dedicated branch circuit used for connecting
the ac module to the load center. That protection device is
required to disable the ac module. Disabling an ac PV module
is accomplished by removing the ac grid connection. Since the
duplex outlet on a receptacle type GFCI violates the dedicated
circuit requirement, a service entrance panel or blank face
device must be used. This ground-fault protection requirement
is intended for fire protection on dwellings and not shock
protection. Ground-fault equipment protection circuit breakers
that fit in the service entrance panel or in a separate panel
and that trip at 20-30 milliamp are suitable.
The changes in the 1999 Article 690 will require changes in
the documentation for calculating maximum voltages and
currents for PV modules. Today, the UL requirements for PV
modules are found in the instruction manual of listed modules.
The old UL standard 1703 required the instruction manual to
state the requirements for multiplying module open-circuit
voltage and short-circuit current by 125% before going to the NEC.
With the 1999 changes, those UL requirements have been
included in Article 690-7 (Maximum Voltage) and 690-8 (Circuit
Sizing and Current) of the NEC. Section 690-7 includes
a new Table 690-7 that now makes the voltage multiplier a
function of the lowest expected ambient temperature. Only when
the expected temperature reaches -21°C (-5°F), does the
factor increase to 1.25 as found in the old UL1703 standard.
If the modules are to be installed where the coldest expected
temperature is a balmy 10-25°C (50-77°F), then the
correction factor on open-circuit voltage is only 1.06.
Section 690-7 also limits the maximum voltage on one- and
two-family dwellings to not more than 600 volts.
In a similar manner, Section 690-8 was revised to include
the 125% solar enhancement multiplier required for PV source
circuit and PV output circuit current calculations previously
found in the PV module instruction manual as part of the
listing documentation. Section 690-8 now includes both the
125% multiplying factor required to deal with daily variations
in PV module output and the 125% multiplier used to derate all
conductors and overcurrent devices throughout the code. The
combined factor of both 125% multipliers for PV source and
output circuits is 156%, while all other circuits in the
system are subject to only a single 125% multi-plier or the
80% conductor-derate required throughout the code.
The new NEC language for system voltage and circuit
current calculations for wire sizes requires careful
coordination with the UL Standard 1703. The new 1999 NEC requirements may conflict with the UL Standard 1703 until it
is modified to remove the solar enhancement and voltage
temperature requirements from the module instruction manuals.
In the meantime, there may be modules in the pipeline that
still have the UL requirement in the instruction manual. Those
using the 1999 NEC are now cautioned not to duplicate
the solar enhancement requirement.
Section 690-9 (Overcurrent Protection) now has exceptions
that do not require overcurrent devices on some types of
circuits. These exceptions generally apply to small,
single-module, direct-connected water pumping systems where
there is no chance of high fault currents from other sources.
It should be noted that overcurrent devices in PV source
and output circuits should be rated at 156% of the
short-circuit currents from the modules. Obviously, with this
rating, these overcurrent devices will not respond to fault
currents solely from the connected modules. They will,
however, protect the module conductors from backfeed currents
from other sources such as parallel-connected modules,
batteries, and even currents from ac sources back feeding
through inverters.
Section 690-10 (Stand-Alone Systems) is a new section that
should benefit the installer and owner of stand-alone PV
systems. The code now allows the PV system inverter ac current
output to be less than the rating of the building load center
or service entrance equipment. A 500-watt inverter may now be
connected to the input of a 120/240-volt, 200-amp load center
for stand-alone applications. The conductor that is used for
this connection has to be rated to carry only the 500-watt
output of the inverter, not the 48,000 watts that the service
entrance can carry. Also, Article 690-10 spells out that a
single 120-volt inverter may be connected to a 120/240-volt
load center when certain conditions are met. There must be no
240-volt circuits and no multi-wire branch circuits in the
building. Of course, appropriate overcurrent devices must be
installed at each end of this cable unless a tap rule as found
in Section 240-21 (Location in a Circuit) can be applied.
Section 690-13 (Disconnection Means, All Conductors) was
revised to clearly state that a switch, fuse, or circuit
breaker should not be placed in a grounded conductor except
where the grounded conductor is automatically interrupted to
comply with the ground-fault protection required in Section
690-5.
AC PV modules may be grouped together on a single circuit,
and a single disconnect-device for all modules is allowed
according to additions in Section 690-15 (Disconnection of
Photovoltaic Equipment). Ampacity calculations using the sum
of the maximum output current of the ac modules still apply.
Section 690-17(Switch or Circuit Breaker) and 690-33
(Connectors) allow the use of a connector for a
disconnect-device as long as it is listed for the use and
meets other code requirements for polarization, guarding,
personnel safety, and grounding. This applies to conventional
systems and to ac PV modules.
The markings required on ac PV modules are listed in a new
Section 690-52 (Marking, Alternating-Current Photovoltaic
Modules). These markings are similar to those required for
conventional PV systems required in Section 690-51.
Utility-interactive systems received considerable attention
in the 1999 NEC because of the expected proliferation
of these systems. Marking the points-of-connection of these
systems is required by Section 690-54 (Interactive System
Point of Connection). Most of Part G (Connection to Other
Sources) was revised to allow easier connection of
utility-interactive systems while still maintaining high
levels of safety. The changes included a revised requirement
for using listed equipment in interactive systems, a new
requirement for inverters to de-energize upon loss-of-utility
in interactive systems, allowable unbalanced grid connections,
and a clarification of the allowable point-of-connection for a
PV system.
Section 690-72 (Storage Batteries, Charge Control) was
revised to require control of the charging process except the
1999 changes require no battery charge controls on systems
where the maximum charging currents are very low (less than 3%
of battery capacity expressed in amp-hours).
A new Part I (Systems Over 600 Volts) was added to Article
690 to specifically address PV systems operating over 600
volts. Some of the larger utility-interactive systems may
operate above 600 volts. The new section directs that systems
greater than 600 volts meet the requirements of the new
Article 490 (Equipment, Over 600 Volts, Nominal) that has been
added to collect all parts of the code for over 600 volts into
one Article. The new Section I defines the maximum battery
voltage as the highest voltage experienced under charging
conditions. Maximum system voltage is used for the PV source-
and output-circuits.
The 1999 NEC Handbook (available from NFPA) includes
significantly more detail, substantiation, and explanations of
Article 690 and the changes that were made for 1999. It is
also an excellent reference for other articles of the NEC.
If you have questions about the implementation of PV
systems following the requirements of the NEC, feel
free to call, fax, email, or write John Wiles at the location
below. Sandia National Laboratories sponsors the activities in
this area as a support function to the PV Industry. 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.
Authors: John C. Wiles of Southwest Technology
Development Institute New Mexico State University , Box
30,001/ MSC 3 SOLAR , Las Cruces, NM 88003 , Phone
505-646-6105 , Fax 505-646-3841 and email: jwiles@nmsu.edu
Ward Bower, Sandia National Laboratories, Division 6218
MS 0753 , Albuquerque, NM 87185 , 505-844-5206, Fax
505-844-6542 , email: wibower@sandia.gov |
