Any time you are working with burglar alarm, access control, sound, nurse call, or intercom systems, as well as computer networks, lighting dimmer controls, and low-voltage industrial controls, you may very well be working with circuits that fall under Art. 725, which addresses three kinds of circuits: remote-control, signaling, and power-limited.
A remote-control circuit controls other circuits through a relay or solid-state device, such as a motion-activated security light.
A signaling circuit that provides an output that is a signal or indicator, such as an annunciator light.
A power-limited circuit operates at no more than 30V and 1,000VA, such as a thermostat.
If a circuit is one of these three types and isn't an integral part of a device or appliance, it must meet Art. 725 requirements.
It's important to remember that Art. 725 circuits differ from power and light circuits in both usage and power limitations. Consequently, Art. 725 provides its own requirements for minimum wire sizes, derating factors, overcurrent protection, insulation requirements, wiring methods, and materials.
As with all other Articles in Chapters 5, 6, and 7, follow all rules required in Chapters 1 through 4 [90.3] except where Art. 725 differs. One way in which it differs is it might exclude most of the requirements found in Chapter 3.
Article 725 provides a mix of both reduced requirements and additional requirements over Chapters 1 through 4. Because of the reduced energy in most Art. 725 installations, this situation doesn't compromise safety, but it normally will provide significant cost savings.
Many people make the mistake of calling Art. 725 a “low-voltage” Article. Nothing could be farther from the truth, considering the fact that Art. 725 contains rules for circuits that may actually have a voltage of up to 600V. Another error many people make is automatically thinking that all Art. 725 installations are “safe.” Once again, this is not always the case.
As stated previously, some Art. 725 circuits can operate up to 600V; others can operate at up to 100V. Circuits of this voltage are obviously not “safe” from an electrical shock perspective. In reality, the only types of circuits in Art. 725 that are truly “safe” from an electrical shock and fire perspective are Class 2 circuits, which make up only one-third of Art. 725.
Article 725 consists of four parts:
Part I, General information.
Part II, Class 1 circuits.
Part III, Class 2 and Class 3 circuits.
Part IV, Listing requirements (for manufacturers).
To understand and correctly apply Parts I, II, and III, you must know the voltage and energy levels of the circuit, as well as the purpose of the circuit. The first thing you must know is whether the circuit is Class 1, 2, or 3. The actual circuit is the wiring system between the load side of the source and the connected equipment. Classifying the circuit is as simple as knowing the power source for the circuit.
Circuit classifications. Class 1: There are two variations:
Power-limited circuit. The power source for these limits the output to 30V (AC or DC) and 1,000VA [725.21(A)]. They are necessary when the energy demands of the system exceed the 100VA energy limit of Class 2 or Class 3 circuits [Chapter 9 Table 11(A)].
Nonpower-limited circuit. These can operate at up to 600V, and the power output is not limited [725.21(B)].
You'll find Class 1 remote-control circuits in motor controllers and in shunt-trips for circuit breakers.
Class 2 and Class 3: These are the possible power supplies for Class 2 and Class 3 circuits [725.41(A)]:
Listed Class 2 or Class 3 transformer.
Listed Class 2 or Class 3 power supply.
Other listed equipment marked to identify the Class 2 or Class 3 power source.
Listed information technology equipment limited power circuits.
Dry cell batteries are inherently Class 2 power sources — if under 30V and with a capacity less than that available from series-connected Number 6 carbon-zinc cells.
Class 2 circuits operating at 30V or less are limited to 100VA. Those operating at over 30V are limited to 5mA [Chapter 9, Table 11]. It is these rules that make a Class 2 circuit safe. The voltage is limited to render the circuit safe from an electrical shock perspective, and the current (energy) is limited to render the circuit safe from a fire perspective. Remember that limiting the voltage does not automatically make a circuit safe, contrary to popular belief. Class 2 circuits typically involve low-energy, low-voltage loads such as thermostats, programmable controllers, burglar alarms, and security systems. Class 2 circuits also include twisted-pair or coaxial cable for interconnecting computers in LANs [725.41(A)(3)].
Class 3 circuits are used when the power demand is between 0.5VA and 100VA, for circuits over 30V [Chapter 9, Table 11]. Class 2 circuits are considered safe from a fire perspective due to the energy limitations, but considering the fact that they may operate at up to 100V, they are not necessarily safe for electrical shock purposes.
Confining combustion. Firestop any Art. 725 circuits, if you install them through fire-resistant rated walls, partitions, floors, or ceilings. Always follow the manufacturer's instructions for the specific type of cable and construction material, such as drywall, brick or other materials. Fire stopping openings in fire-resistant walls, floors, and ceilings ensure that the installation won't substantially increase the possible spread of fire or products of combustion [300.21].
What about abandoned cable? To limit the spread of fire or products of combustion within a building, you must remove the accessible portion of cable that isn't terminated at equipment or identified for future use with a tag [725.2] (Fig. 1 on page 56).
This rule doesn't require removing concealed cables that are abandoned in place. By definition (Art. 100), cables in raceways are concealed. Thus, you don't have to remove cables that are in raceways.
Air-handling spaces. You can install Class 2 and Class 3 cables in ducts or plenums, if:
Doing so is necessary for the direct action upon, or sensing of, the contained air, and
You install the cables in electrical metallic tubing, intermediate metal conduit, or rigid metal conduit as required by 300.22(B). Note that even though cables may be plenum rated, they cannot be installed inside of a duct or inside of a plenum.
You can install plenum-rated Class 2 and Class 3 cables [725.82(A)], and plenum signaling raceways [725.82(I)] containing plenum-rated cables above a suspended ceiling or below a raised floor used for environmental air movement [725.61(A)] (Fig. 2).
You can install nonplenum-rated Class 2 and Class 3 cables above a suspended ceiling or below a raised floor used for environmental air movement, but only if you install them in electrical metallic tubing or other raceway types identified in 300.22(C)(1).
You can install any type of Class 2 or Class 3 cable above a suspended ceiling or below a raised floor that is not used for environmental air movement.
Type CL2 and Type CL3 cables installed beneath a raised floor in an information technology equipment room (computer room) that meets the requirements of 645.4 don't have to be plenum-rated [300.22(D) and 645.5(D)(5)(c)].
Miscellaneous requirements from other Articles. Except where Art. 725 refers to specific Art. 300 requirements, Art. 300 requirements do not apply to Class 2, and 3 circuits [725.3], but Art. 300 does apply to Class 1 circuits [725.25]. Boxes or other enclosures aren't required for Class 2 or Class 3 splices or terminations, because Art. 725 doesn't reference 300.15 for Class 2 and Class 3 circuits. But you do need a box for a splice or termination in a fire-rated assembly, regardless of what Class the circuit is comprised of [300.21].
Hazardous (classified) locations. Follow Art. 500 through Art. 516 — specifically 501.150, 502.150 and 503.150.
Cable trays. Follow Art. 392. Separate Class 2 and 3 cables in cable trays from lighting and power conductors by a solid fixed barrier, or install them with Type MC cable [725.55(H)].
Miscellaneous requirements of Art. 725. Design to allow access. You can't let an accumulation of cables interfere with removing suspended-ceiling panels. Locate the cables to allow moving suspended-ceiling panels, thereby permitting access to electrical equipment [725.7].
Mechanical execution. Equipment and cabling must be installed in a neat and workmanlike manner [725.8]. Use the structural components of the building to support exposed cables, so the cables won't be damaged by normal building use. Secure the cables with straps, staples, hangers, or similar fittings designed and installed so as not to damage the cable.
Independent support wires attached to the suspended ceiling per 300.11(A) can support raceways and cables (Fig. 3).
Cables run parallel to framing members or furring strips must be protected (where they are likely to be penetrated by nails or screws) by installing the wiring so it isn't less than 1¼ inches from the nearest edge of the framing member or furring strips, or by protecting the cables with a 1/16-inch-thick steel plate or equivalent [300.4(D)] (Fig. 4).
Safety-control equipment. Class 2 and Class 3 circuits used for safety-control equipment must be reclassified as Class 1, if the failure of the remote-control circuit or equipment introduces a direct fire hazard or life hazard [725.11].
Where damage to remote-control circuits of safety-control equipment would introduce a hazard [725.11(A)], install all conductors in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing — or suitably protect them from physical damage [725.25].
Room thermostats, water-temperature regulating devices, and similar controls for electrically controlled household heating and air conditioning are not considered safety-control equipment.
Any time you are working with remote-control, signaling, and power-limited circuits (30V or less, and under 100VA), remember that Art. 725 covers them unless they are integrated into a device or appliance. This means you'll have to determine if you're working with Class 1, 2, or 3, and then apply the corresponding requirements. Doing so can reduce installation costs while still maintaining the safety and integrity of your installation.