Class 1 Circuits

By Jun 15, 2007
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According to the National Electrical Code, Part 2

In 725.23, The National Electrical Code (NEC) covers overcurrent protection (OCP) for Class 1 circuits. Plainly stated, this section allows the ampacity of the conductors to be used, and by complying with 725.28 and 310.15, whichever is appropriate, the derating factors can be disregarded. However, such a rule applies only to 14 AWG conductors and greater sizes. OCP and ampacities for smaller conductors (16 and 18 AWG) also are covered. Don’t forget overloading in control-circuits is not likely to occur due to the fact that they are never heavily loaded when in use. Conductors used for remote-control often are used as intermittent duty circuits and do not operate continuously as other circuits.

Class 1 derating factors

The derating requirements covered in 725.28 deal with the adjustment factors that are clearly outlined in Table 310.15(B) (2)(a), but when applicable, electricians must be very careful not to forget to apply the correction factors listed below Table 310.16. As pointed out in the example to 310.15 (A) (2), the 10 foot or 10 percent rule must be applied when exposing the circuit conductors in a metal cable or raceway to a temperature and distance greater than the NEC permits. I would like to repeat that many times electricians forget about these requirements and expose such circuits to higher temperatures than permitted.

However, problems usually don’t occur because the circuit conductors are never loaded to their permitted ampacities. But if loaded to their limited ampacities, overloading can occur, and overheating of insulation and components starts to set in. Electricians can spot these heating problems at terminations by visually inspecting and verifying if any of the following conditions are present:

1. Baked-out insulation

2. Slow-melting insulation

3. Very hot insulation

4. Burned terminals or lugs

5. Loose connections

Remember, it is very important to observe and adhere to 310.10 when dealing with Class 1 circuits mixed and associated with other conductors as permitted by the NEC. It is also important to comply with the section to prevent overloading and heating problems, as previously mentioned.

Selecting wiring methods

Basically, there are no special wiring methods required for Class 1 circuits. Installers of Class 1 circuits can choose from any of the ordinary wiring methods listed in Chapter 3 of the NEC. Section 725.25, in my opinion, treats such circuits pretty much the same as branch-circuit wiring. However, Article 300 imposes some restrictions on installations of Class 1 circuits when they are mixed with other circuit conductors in raceways and enclosures.

Users can locate these rules in 725.26, which modifies the general requirements found in Article 300. For example, 300.3(C)(1) permits conductors rated at 600 volts or less to occupy a common raceway or enclosure, but 725.26(A) permits such mixing of Class 1 circuits with other circuit conductors only when such circuits are functionally associated. Note that the Class 1 and power wiring circuits are limited to 600 volts.

Section 725.26(B) clearly permits Class 1 circuits to share a raceway, cable or enclosure with other conductors where they are functionally associated. In manhole installations, separation of Class 1 circuits from other conductors can be provided by using one of the appropriate wiring methods of Chapter 3, using a fixed separation, fixed nonconductors or a fastening to supports method. In cable trays, a fixed barrier can be used as a separator, or a separation can be provided by installing either the power conductors or the Class 1 conductors in a metal-enclosed cable or with an approved manner acceptable to the authority having jurisdiction.

Raceways and enclosures housing remote-control circuits are required to be grounded only where the power system is required to be grounded. A grounded system consists of a grounded circuit conductor that has been intentionally connected to earth ground or a body that serves in place of the earth.

When grounding equipment associated with remote-control circuits, Section 250.112(I) of the NEC must be consulted, and grounding is required if Part II or Part VII of Article 250 requires. Sections 250.20 and 250.162 cover AC and DC systems, which are required to be grounded, and these requirements should be reviewed very carefully before choosing a power source for Class 1 remote control circuits. Note that metallic raceways, boxes, fittings and enclosures associated with these circuits are required to be grounded when the AC power source supplying such circuit conductors is required to be grounded per 250.20.

In the next issue, I will discuss Class 2 and 3 circuits. EC

STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.

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