Article 725: Around the Circuit Part 2

Oct 1, 2006 12:00 PM, By Mike Holt, NEC Consultant

Avoiding confusion with Class 2 and 3 remote-control, signaling, and power-limited circuits

In last month's “Code Basics” article, starting on page 56 of the September issue, we spent most of our time discussing Class 1 circuits. Now, it's time to switch gears and focus on Class 2 and 3. Remember, these circuits are defined by their power sources, which must be one of the following:

A listed Class 2 or Class 3 transformer.
A listed Class 2 or Class 3 power supply.
Equipment listed as a Class 2 or Class 3 power source.
Listed information technology equipment.
A dry cell battery rated 30V or less for a Class 2 circuit.

These power sources are listed in 725.41(A)(1) through (5). Note that item No. 3 has two exceptions:

Exception 1: Thermocouples.
Exception 2: Limited power circuits.

These have an energy level at or below the limits in Chapter 9, Table 11(A) and Table 11(B). This Exception is intended to apply to programmable logic control I/O circuits.

Equipment supplying Class 2 or 3 circuits must be marked to indicate each circuit that is a Class 2 or 3 circuit [725.42]. Remember that the Class 2 or 3 circuit begins at the power supply. That means conductors and equipment on the supply side of the Class 2 or 3 power source must be installed in accordance with Chapters 1 through 4 [725.51].

Reclassification You can reclassify Class 2 or 3 circuits as Class 1 circuits simply by eliminating the Class 2 and Class 3 equipment markings required by 725.42, but you must install the entire circuit in a Chapter 3 wiring method [725.55(D)(2)(b)].

You might not want to do that, however. If you reclassify these circuits, then you can't install them with Class 2 or 3 circuits that have not been reclassified as Class 1 [725.55]. Reclassifying the circuit allows the Class 1 circuit to be installed with functionally associated power circuits per 725.26(B)(1).

Fig. 1. Class 2 and 3 cables are not permitted in the same raceway or enclosure with power or Class 1 conductors.

Separation,. Separation helps prevent a fire or shock hazard that could occur from a fault between the Class 2 or 3 circuit and the higher-voltage circuits. You can install Class 2 and 3 cables in the same raceway or enclosure with:

Communications wires and cables [800.133(A)]
Power-limited fire alarm cables [760.56]
Optical fiber cables [770.133(B)]
CATV cables [820.133(A)], and
Low-power network broadband [See 725.56(E)(5)].

You cannot place Class 2 or 3 conductors in any enclosure, raceway, or cable with conductors of electric light, power, Class 1, or nonpower-limited fire alarm circuits, except as permitted in 725.55(B) through (J), as shown in Fig. 1.

For example, you can install Class 2 and 3 circuits in the same enclosure as electric light, power, Class 1, and nonpower-limited fire alarm circuits if they are separated by a barrier (Fig. 2).

Fig. 2. Class 2 and 3 circuits can be installed in the same enclosure as electric light, power, Class 1, and nonpower-limited fire alarm circuits if they are separated by a barrier.

Combinations Listed Class 2 cables have 150V insulation, whereas listed Class 3 cables are rated 300V [725.82(G)]. Spelled out in 725.56, this has implications for running Class 2 and 3 together in the same cable, enclosure, or raceway.

You can run Class 2 and 3 conductors within the same cable with communications conductors, if the cables are listed as communications cables or multipurpose cables and they are installed per Art. 800 [800.113 and 800.133(A)(1)(b)]. A common application of this requirement is when a single communications cable is used for both voice communications and data [725.56(D)(1)], as shown in Fig. 3.

Fig. 3. Class 2 circuits within a communications cable are considered communications circuits and must meet the requirements of Art. 800 [800.133(A)(1)(b)].

Class 2 or 3 cables can be in the same raceway or enclosure as jacketed cables of any of the following [725.56(E)]:

Power-limited fire alarm circuits in compliance with Art. 760.
Nonconductive and conductive optical fiber cables in compliance with Art. 770.
Communications circuits in compliance with Art. 800.
Coaxial cables in compliance with Art. 820.

Support Don't use a raceway to support Class 2 or 3 cables. This is a common Code violation, and is not known by many installers and inspectors. Support these cables by the structural components of the building to prevent damage from normal building use. Secure cables by the appropriate straps, staples, hangers, or fittings — and install them so as not to damage the cable [725.8].

Class 2 control cables can, however, be supported by the raceway that supplies power to the equipment controlled by the Class 2 cable [725.58, 300.11(B)(2)].

Fit the application Make sure the cable or raceway being used as a wiring method within a building is listed and marked for the application. You'll find the application rules in 725.61 and the related listing requirements in 725.82. Here are some highlights.

You can't install Class 2 or 3 cables in constructed ducts or plenums [725.3(C)], unless they act directly on the contained air, are installed using a wiring method described in 300.22(B), and are plenum-rated.

Fig. 4. Listed plenum-rated raceways can be installed above a suspended ceiling or below a raised floor used for environmental air [300.22(C)(2)], but only if the cables contained in these raceways are also plenum-rated, such as Types CL2P or CL3P [725.61(A)].

You can install plenum-rated cables above a suspended ceiling or below a raised floor used for environmental air [725.61(A)]. You can also install listed plenum-rated raceways above a suspended ceiling or below a raised floor used for environmental air [300.22(C)(2)], but only if the cables contained in these raceways are also plenum-rated, such as Types CL2P or CL3P [725.61(A)], as shown in Fig. 4.

To install Class 2 and 3 cables that are not plenum-rated above a suspended ceiling or below a raised floor used for environmental air, you have to put them in a metal raceway [300.22(C)(1)]. Metal raceways containing circuit conductors from a power-supply system that operates at 50V or less aren't required to be grounded (bonded) to an effective ground-fault current path [250.86 and 250.112(I)].

You can install types CL2, CL2X, CL3, or CL3X cables in locations other than ducts, plenums, or other environmental air spaces in any occupancy. Are you installing cable between more than one floor? Then you use one of the three methods described in 725.61(B).

The requirements for Class 2 and 3 circuits can quickly become mind-boggling, if you don't approach them methodically. Follow these steps to avoid confusion and Code violations:

Understand the power level on the load side, so you can determine if a circuit is Class 1, 2, or 3.
Determine the route of the cabling and raceway. Can you simplify things by avoiding risers and environmental air space?
Understand the application. Review each of the eight application types in 725.61 to determine which application you have. This will tell you what the rules are for your application.
Follow the listings. Review your planned bill of materials against the requirements of 725.82 to ensure you are using the correct cables and raceways. Note any special installation requirements, such as those noted in 725.82(E).
Review the general requirements for remote control, signaling, and power-limited circuits, before finalizing your design, bill of materials, and work plans.

With this approach, you will find it fairly easy to meet the requirements for Class 2 or 3 circuits. Although it may be tempting to gloss over these and just get going with the installation, giving in to that temptation can mean you'll be ripping out a Class 2 or 3 installation and starting over. Save time by doing the job right the first time.

Article 725: Around the Circuit Part 1

Any time you are working with burglar alarm, access control, sound, nurse call, or intercom systems, as well as computer networks, lighting dimmer controls,...