Article 725 in the NEC covers
signaling systems and remote control systems. These types of
systems are not always easily distinguished from each other
because applications of signaling and remote control circuits
tend to overlap. In fact, some systems or circuits often
perform both functions. For this reason, the rules in Article
725 and the application of those rules are based on circuit
classifications rather than precise functions. (The terms remote
control and signaling will be treated as
interchangeable in this article.)
Remote control and signaling circuit
classifications are based on usage and power limitations.
Because of the power limitations and the limited uses of the
circuits covered by Article 725, special wiring methods and
installation requirements apply. Article 725 modifies the
first four chapters of the NEC with regard to some
alternative wiring methods, and provides somewhat relaxed
requirements in other respects. However, in order to maintain
the power limitations upon which the special rules are based,
other requirements are somewhat more restrictive, particularly
those that relate to separations from higher-powered circuits
and systems.
The wiring covered by Article 725 is often
called low voltage wiring. However, the scope of
Article 725 encompasses more than is implied by this term and
the meaning of low voltage is entirely dependent on context
when used in the NEC. For example, Article 490 defines high
voltage as more than 600 volts, which indicates that low
voltage is anything 600 volts or less. In fact, the title of
Section 110.34(B) includes the term low voltage and it
means 600 volts or less in that context. Yet in another part
of the same article, specifically Section 110.26(A)(1)(b), low
voltage means not greater than 30 volts rms or 42 volts peak
in ac systems or 60 volts in dc systems. Other levels of
voltage are used to determine the applicability of special
rules for lighting, such as 30 volts or less in Article 411
and 15 volts or less in Sections 680.24(A)(2) and 680.33(A).
Low voltage is defined in 551.2 as 24 volts nominal or less
for applications in recreational vehicles. The point is that low
voltage is simply not a good description or definition for
signaling circuits nor does it accurately describe the scope
of Article 725.
Limited energy or power limited are more meaningful terms in describing how the NEC defines circuits used for signaling and remote control, but
even these terms have specific meanings only in certain
contexts. According to Section 725.21(A), Class 1
power-limited circuits are allowed to have higher power
ratings than Class 2 or Class 3 circuits [see Tables 11(A) and
11(B) in chapter 9] and power supplies for power-limited fire
alarm circuits are treated as equivalent to Class 3 sources in
Section 760.41. The only accurate way to refer to signaling
circuits for use in applying the NEC and Article 725 is
through the identified circuit classes defined and used in
Article 725. However, some circuits that may appear to perform
signaling functions may actually be classified as fire alarm
circuits covered by Article 760 or may perhaps be classified
as communications circuits covered in chapter 8.
Article 725 defines three classes of
signaling circuits: Class 1, Class 2, and Class 3 circuits.
Class 1 circuits are further subdivided into Class 1
power-limited circuits and Class 1 remote-control and
signaling circuits. (Although this distinction is made to
differentiate power sources for Class 1 circuits, this
distinction is not used with regard to wiring methods or other
installation rules that apply to Class 1 circuits in Article
725.) The circuits themselves are defined by their power
supplies in accordance with Section 725.21 for Class 1
circuits and 725.41 for Class 2 and Class 3 circuits. In other
words, a user or inspector need only locate the power supply
and examine its rating and/or listing in order to determine
the class of the circuit in question.
Class 2 circuit applications are much more
common than Class 3 circuits. But circuits are neither Class 2
nor Class 3 just because they are at some "low
voltage" such as 24 volts. Most of the time a circuit
must be supplied by a source that is listed as a Class 2 or
Class 3 source in order for it to be a Class 2 or Class 3
circuit. However, some equipment that is listed other ways may
supply Class 2 circuits. For example, the limited-power
circuits of listed information technology equipment (ITE) are
recognized by 725.41(A)(4) as Class 2 circuits. An example of
this listing mark is shown in photo
1. Photo 2 is an another example of typical markings that identify a
Class 2 source on listed equipment, in this case, a garage
door opener. Other sources that are not labeled as Class 2 but
are newly recognized in the 2005 NEC are the limited
power supplies of other listed equipment such as power-limited
input/output or communications circuits of listed industrial
control equipment. These circuits are widely used in
programmed logic control systems and industrial machinery.
This new rule is found in 725.41(A)(3), Exception No. 2 and
explained in the related fine print note.
In some cases, Class 2 or Class 3 circuits
are permitted or required to be reclassified as Class 1
circuits [see 725.11 and 725.52(A), Exception No.2] or
permitted or required to be reclassified as communications
circuits [see 725.55(D)(2)(b) and 725.56(D)(1)]. As noted in
725.52(A), Exception No. 2, FPN, where circuits are
reclassified, they are reclassified for their entire length
and must comply with all of the requirements for the new
classification.
The fine print note that follows Section
725.1 explains why special classifications are applied to
signaling circuits. The difference between power and lighting
circuits and Class 1, Class 2, and Class 3 circuits is
twofold: one, usage, and two, power limitations. Class 1
circuits are primarily distinguished by usage as they are
permitted to be supplied by sources that could initiate a fire
and could be a shock hazard, and in some cases may even have
less restrictive overcurrent protection requirements than
branch circuits. However, as described in 725.2, Class 2 and
Class 3 circuits have significant power limitations that keep
either from initiating a fire under normal conditions and keep
Class 2 circuits from presenting a shock hazard under normal
conditions. A lower level of voltage may be a shock hazard in
a wet location than would be a hazard in a dry location. For
this reason, some power supplies are listed as Class 2 power
supplies in dry locations but become Class 3 power supplies if
the circuits are used in wet locations as shown in photo
3. Typically, 30 volts rms is considered to be the
threshold of shock hazard in dry locations and 15 volts rms is
the threshold for wet locations. This information may be found
in the notes to Table 11(A) in chapter 9. Photo
4
The fine print note following 725.1 also
explains how Article 725 modifies the general rules of
chapters 1 through 4 of the NEC. Article 725 allows the
use of smaller wire sizes than would be allowed for branch
circuits and permits additional uses of fixture wires
(insulation types) that are not mentioned in Article 402. It
changes the application of derating factors, specifically the
adjustment factors of 310.15(B)(2)(a), for most signaling
circuits. It modifies the requirements for overcurrent
protection, primarily because the loads are relatively small
and conductors are not subject to overloading, or because the
power sources are inherently limited. And, especially for
Class 2 and Class 3 circuits, Article 725 allows the use of
wiring methods — primarily special cable types — that are
not permitted for higher-powered circuits. (Article 725 does
not provide or permit any special wiring methods for Class 1
circuits.) As noted previously, in exchange for these
"loosened" rules, Article 725 modifies Article 300
in requiring that, in most cases, signaling circuits be
separated from higher powered circuits so that inadvertent
contact with higher powered systems will not compromise the
power limitations of the power supplies that define the
classes of signaling circuits. Although Article 725 does not
specifically mention it, the rules for grounding many
signaling circuits and related equipment are also different
from the rules for power and lighting circuits. These
modifications are found within Article 250 itself. We will
briefly examine each of these types of modifications in the
remainder of this article.
Perhaps the most evident difference between
signaling circuits and power or lighting circuits is the
permitted use of special cable types for wiring. These special
cable types are permitted because of the inherent energy
limitations of Class 2 and Class 3 circuits and their power
supplies. The special wiring methods that may be used for
Class 2 and Class 3 circuits are outlined in Section
725.52(B). In addition, Section 725.52(A) allows the use of
Class 1 wiring methods and also allows Class 2 or Class 3
circuits to be reclassified as Class 1 circuits.
Reclassification of a circuit is
significantly different from using the wiring method of
another class of circuit. The use of Class 1 methods is
permitted for a portion of a Class 2 circuit, but a
reclassified circuit becomes a Class 1 circuit for its entire
length as noted in the fine print note that follows 725.52(A),
Exception No. 1. A similar distinction must be drawn between
the permitted substitution of communications cables for Class
2 or Class 3 cables and the reclassification of the Class 2 or
Class 3 circuits as communications circuits. Nevertheless,
either way, a different cable type or wiring method is
permitted or required. For example, the typical intended use
of 4 pair Category 5 Type CM cable like that shown in photo
5 is
for a combination of Class 2 and communications circuits, but
since both types of circuits are within the same cable in this
case, they must all be classified as communications circuits
in accordance with 725.56(D)(1) and 800.133(A)(1)(b). If the
cable were used for only Class 2 circuits, the cable could be
used as a substitute cable in accordance with 725.61(G) and
the circuits would remain classified as Class 2.
There are no special wiring methods for
Class 1 circuits according to 725.25, and methods from chapter
3 must be used. However, 725.25, Exception No. 1 recognizes
the smaller conductor sizes and insulation types permitted by
725.27. The alternative insulation types are certain types of
fixture wires covered by Article 402, but in this case fixture
wires are allowed to be used in applications that are beyond
the scope of Article 402 and that are not mentioned in Article
402. All Class 1 circuits, even those that are less than 30
volts, must use insulation rated for 600 volts, but wire sizes
are permitted to be as small as 18 AWG. Wire sizes for Class 2
and Class 3 circuits are not specified by the NEC.
Where the cables listed for Class 2 and Class 3 are used, the
listing standard for the cables includes a minimum conductor
size for construction of such cables.
The uses and applications of the special
cable types described in 725.61 and 725.82 are too numerous to
review in detail in this article. However one common problem
relates to the use of cables in ducts, plenums, and other
spaces for environmental air as described in Section 300.22.
An important point to note is that since 725.3(C) requires
compliance with 300.22, all permissions for uses of cables in
ducts, plenums, and other spaces for environmental air must be
interpreted and applied to also comply with 300.22. For
example, although Section 725.61(A) seems to permit
installations of "plenum-type" cables (with the
suffix "P" in their markings) to be installed in
ducts, Section 300.22 permits such "equipment" only
where necessary for direct action or sensing of the contained
air. Wiring is equipment as defined in Article 100. Other
Class 2 and Class 3 cables may be used as outlined in 725.61
within the limits of their listings as given in 725.82.
Section 725.28(B) says that the adjustment
factors from Table 310.15(B)(2)(a) apply only to Class 1
conductors that are loaded to more than 10 percent of their
ampacity. Where Class 1 methods are used for Class 2 or Class
3 circuits, the adjustment factors do not apply. It follows,
although it is not specifically stated, that adjustment
factors would not apply to Class 2 or Class 3 conductors
installed using Class 2 or Class 3 wiring methods, but where
Class 2 circuits are reclassified as Class 1 circuits, all
requirements for Class 1 circuits do apply. [See 310.15(B)(2),
Exception No. 1 which explains that adjustment factors are
intended to apply only to power and lighting conductors.]
Whether adjustment factors apply or not, Section 725.3
requires compliance with raceway fill limits for all circuits
covered by Article 725. Note, however, that if Class 2 or
Class 3 circuits are reclassified and installed as
communications circuits as covered in 725.56(D), neither
adjustment factors nor raceway fill limitations apply [See
90.3 and 800.110, Exception]. Photo
6 shows an example of a raceway used for cable management
and physical protection where raceway fill applies to the
Class 2 circuits, but where adjustment factors do not apply.
Section 725.23 provides overcurrent
protection requirements for Class 1 circuits.
"Derating" factors of 310.15, that is, the
adjustment factors of 315.15(B)(2)(a), are not applied in
determining ampacities and the small conductor rules do not
apply in accordance with 240.4(D) and (G) for conductors 14
and larger. Overcurrent protection may not exceed 7 amperes
and 10 amperes for sizes 18 AWG and 16 AWG respectively.
[These maximum values of overcurrent protection are not the
ampacities of 18 and 16 AWG. Loads are limited to the
ampacities given in Table 402.5 according to 725.27(A) where
the ampacities are given as 6 amperes for 18 AWG and 8 amperes
for 16 AWG.] The overcurrent protection is located as required
by 725.24. These rules are similar in many ways to the rules
of 240.21, and in some cases, larger overcurrent devices or
overcurrent protection on the supply side of a power supply
may be adequate. No additional overcurrent protection is
required for Class 2 and Class 3 circuits because the power
supplies are inherently limited.
In order to ensure the integrity of the
power limiting means for signaling circuits, physical
separations from higher powered circuits are required,
especially in raceways and cables. For the most part, these
requirements are more restrictive than the requirements of
300.3(C)(1) which, according to 725.3, do not apply to the
circuits covered by Article 725. Although Class 1 circuits are
permitted to be up to 600 volts, they are generally required
to be kept separate from power and lighting circuits unless
the circuits are functionally associated. But even functional
association does not relieve the requirement for maintaining
separations between Class 2 or Class 3 circuits and power,
lighting, or Class 1 circuits. Class 2 or Class 3 circuits
that are reclassified and wired as Class 1 circuits are
subject only to the functional association restrictions of
Class 1 circuits.
Separation requirements are found in 725.26
for Class 1 circuits and in 725.55 for Class 2 and Class 3
circuits. These sections recognize that signaling circuits
will sometimes have to occupy the same enclosure to connect to
common equipment, such as to relays that control a lighting
circuit and that are, in turn, controlled by a Class 2
circuit. Alternative methods, including barriers, raceways
within enclosures, or in some cases, physical distance or
cable jackets, must be used to maintain separations within
enclosures as specified in Section 725.55. Photo
7 shows an example of separations maintained by barriers
within an enclosure. Photo
8 shows power wiring on the inside of an enclosure
separated from Class 2 wiring on the outside where the
terminals are allowed to be exposed on the outside because
they do not present a shock hazard. Class 2 transformer like
the one shown in photo
8 are often found where a single box is used to contain
supply conductors, the power supply, and the Class 2 wiring
for installations of doorbells, and these installations
violate separation rules in most cases.
Grounding requirements of equipment and
circuits covered by Article 725 are not specified in Article
725. As mentioned previously, the "special rules" in
this case are actually found in Article 250. Where equipment
such as boxes and raceways are required to be grounded or
bonded, the requirements for signaling circuits are no
different than for other circuits and systems. However, many
signaling circuits and related enclosures and metal parts are
not required to be grounded.
To determine whether raceways and other
metal enclosures for signaling use are required to be
grounded, we must first determine whether the system is
required to be grounded, that is, whether the voltage system
is required to include a grounded conductor. This can be
determined from 250.20 in most cases (see 250.162 for dc
systems). On one hand, according to 250.20(A), most Class 2
systems used for such purposes as doorbells, garage door
openers, and thermostats are not required to be grounded
because they are less than 50 volts, are derived from grounded
120-volt systems, and are not installed as overhead conductors
outside of buildings. On the other hand, a 120-volt, Class 1
circuit would usually have to be grounded in accordance with
250.20(B)(1). Second, according to 250.112(I), the equipment
supplied by these circuits is only required to be grounded
where system grounding is required by Part II (250.20) or Part
VII (250.162). Thus the equipment supplied by or containing
the Class 2 circuits in this example is not required to be
grounded but the equipment associated with the Class 1
circuits mentioned is required to be grounded. These
requirements are reiterated by 250.86, which refers
specifically to 250.112(I). The raceway shown in photo
9 is not required to be grounded because it contains only
24-volt Class 2 circuits that are derived from a 120-volt
system.
A circuit or equipment that is not required
to be grounded by the rules discussed so far in Article 250
may be required to be grounded or bonded in other cases and by
other rules. These requirements may be found in Article 725 or
in other articles. For example, Section 725.57 imposes
additional requirements on Class 2 and Class 3 circuits that
extend beyond one building and are subject to accidental
contact with higher voltages. In such cases, the Class 2 or
Class 3 circuits are required to be treated as communications
conductors in certain ways which may include connections to
grounded primary protectors and grounding of metallic cable
sheaths or shields. In addition, non-current-carrying metal
parts of all electrical systems must be bonded in hazardous
(classified) locations. Raceways or enclosures may not always
be required for signaling circuits in such areas if the
circuits are also nonincendive or intrinsically safe, but
where metal raceways or enclosures are used in hazardous
areas, they must be bonded to comply with Section 250.100.
This article has briefly examined why and
how signaling circuits are given special or different
treatment in the NEC. The classifications of the
circuits covered by Article 725 are determined and
differentiated by use and power limitations. Class 2 and Class
3 circuits are most commonly defined by listed power supplies.
The inherent power limitations of these supplies justifies the
use of alternative wiring methods and different schemes for
protecting equipment and persons from the possible hazards of
these systems. In fact, in many cases the hazards are already
significantly reduced by the nature and sources of the
circuits themselves. Article 725 provides somewhat relaxed
requirements for installations of signaling circuits. However,
in order to maintain the power limitations upon which the
relaxed rules are based, requirements for separations from
higher-powered circuits and systems must be significantly more
restrictive, and these restrictions must be carefully
observed.
Noel Williams is a consultant
specializing in electrical code requirements. He managed
two electrical contracting firms and supervised
electrical construction projects in the western states
for over 25 years. He has taught electrical classes and
seminars for over 20 years throughout the United States
and is an approved instructor for continuing education
for electricians in numerous states. He is a licensed
electrical inspector and master electrician. Noel is
author of NFPA’s Limited Energy Systems, and
coauthor of the NFPA/IAEI Electrical Inspection
Manual, and NFPA’s NEC Q and A books. He has
been an active member of the IAEI Utah Chapter for over
25 years, and is a member of their executive board. |
