Where installing Class 2 or Class 3 remote control, signaling, or power limited circuits in a raceway or where installing power-limited fire alarm circuits in a raceway, is it necessary to figure conduit fill?
Will using cable rather than individual conductors change the method of calculating raceway fill?
Is de-rating of the allowable ampacity of the conductors necessary based on the number of conductors that are installed in the raceway?
Should there be concern over heating effects that these conductors and their insulation may be subjected to during the operation of the system?
These are all questions that seem to plague contractors and engineers as they start designing low-voltage and fire alarm systems.
Remembering the basics of the National Electrical Code (NEC) is probably the best place to start when attacking the problem as previously outlined. Section 90-3 states that Chapters 1-4 generally apply to all electrical wiring systems. These first four chapters provide wiring methods, grounding and bonding for systems, ampacity tables, and other general requirements necessary for any electrical installation.
Chapters 5, 6, and 7 are devoted to special occupancies, special equipment, or special conditions, and these three chapters supplement or modify the first four general chapters of the NEC since they deal with special electrical installations.
With that in mind, Section 725-3 for Class 2 or 3 circuits and Section 760-3 only require those sections of Article 300 that are specifically referenced within Articles 725 or 760 to apply to low-voltage or power-limited fire alarm systems. For example, Section 300-17 provides information on the number and size of conductors. However, this particular section is not referenced within either Article 725 or 760. Therefore, it does not apply.
Section 300-17 basically states that the number and size of conductors must not be more than will permit dissipation of heat from the conductors and ready installation or removal of the conductors without damage, either to the conductors or the their insulation. There is also a Fine Print Note (FPN) in Section 300-17 that provides specific section numbers in raceway articles for conduit fill. Fine Print Notes are not enforceable in the NEC but provide extra information.
The Class 2 or Class 3 low-voltage system and the power-limited fire alarm circuits may be installed using wiring methods suitable for nonpower-limited or power circuit wiring. For example, electrical metallic tubing (EMT) or flexible metal conduit, with individual conductors or sets of cables, would be acceptable. These same circuits may also be installed using power-limited cables without a raceway.
If cables are used without a raceway, care must be taken as to the type of cable used. If a raceway is used to enclose the conductors or cables, the Article that applies to the type of raceway selected would apply to the installation.
Remember! Sections 725-3 and 760-3 only gave us permission to disregard Article 300, unless specifically referenced. They did not give us permission to disregard all of Chapter 3. Since raceway articles are located in Chapter 3, if we use one of these raceways, we must comply with the requirements in that specific article. We must provide the proper support, number of maximum bends, proper reaming and threading of the raceway where necessary, and proper raceway fill.
Each raceway article has a specific section that references the number of conductors that can be installed in that specific type of raceway. For example, electrical metallic tubing (EMT) is covered by Article 348 and has Section 348-8 covering the number of conductors that may be installed. It states that the number of conductors installed in a single EMT cannot exceed the percentage fill specified in Table 1, Chapter 9. Since only Article 300 is exempted from applying, the EMT fill must be calculated for even low voltage or fire alarm circuits.
Table 1 of Chapter 9 allows 53 percent fill for one conductor, 31 percent fill for two conductors, and 40 percent fill for more than two conductors. The Notes to Table 1 provide some directions for using this table in any calculation and should be reviewed thoroughly before applying the table. Note 1 allows Appendix C tables to be used for raceway fill where conductors are all the same size. Note 5 allows multiconductor cables to be installed using the actual dimensions (diameter of the cable) to be inserted into the formula for the area of a circle =1/4 times p times diameter squared or 0.7854 X d2. Note 9 is an extremely important note that allows a multiconductor cable of two or more conductors to be treated as a single conductor for calculating the percentage of conduit fill. If it is an elliptical cable, the cross-sectional diameter used is based upon the major diameter of the ellipse.
Now using all of this information as a basis, the answers to the questions in the first paragraph become easy to provide. Based upon each raceway used for the installation, the fill of the raceway must be calculated. If individual conductors are being used, either Table 5, Chapter 9 conductor dimensions can be used or Appendix C tables may be used. Using Table 5, Chapter 9, if 22 #18 TFN conductors are to be installed, then either the actual diameter of the conductor could be inserted into the area of a circle formula previously given, or, since it is already provided in Table 5, the following calculation would apply:
22 X 0.0055 in.2 per conductor = 0.121 in.2 total for all conductors
Using that value in Table 4 of Chapter 9 for EMT, the 40 percent fill for a * inch EMT calculates to 0.122 in.2 of conductors (without being overfilled). Therefore, * inch EMT would be the answer. Using EMT Table C1 in Appendix C, if 22 #18 TFN conductors are to be installed, * EMT would be permitted.
If cable is employed rather than individual conductors, the cable diameter must be determined. This information is normally available from the cable manufacturer. The diameter of each cable (if it is a multiconductor cable, remember to use the major diameter of the overall cable and treat it as a single conductor) must be inserted into the area of a circle formula to determine the square inch size of each cable. Once that is determined, treat the fill calculation the same as the calculation that was done for the individual conductors.
# of cables X [0.7854 X d2] = in.2 total for all cables
Use the square inch total of all of these cables in Table 4 of Chapter9 to determine the size of EMT necessary to hold the cables.
The last two questions posed in the first paragraph are dealing with the same issue; de-rating the conductors based upon the amount of current that will be applied to each conductor and the effect overheating would have on the insulation of the conductors.
The answer is yes, there should be concern for the amount of current that is placed on these conductors since excessive heating could adversely affect the insulation of any or all of the conductors. However, the normal de-rating associated with power and lighting conductors would not apply since these are remote-control, signaling, or power-limited conductors.
However, these Class 2, Class 3, or power-limited fire alarm circuits conductors could be treated the same as Class 1 conductors in accordance with Section 725-28(a) or Section 760-28(a) for nonpower-limited fire alarm circuits, if there is sufficient concern about heating effects on the insulation. Similar to the methods used for Class 1 or nonpower-limited fire alarm circuits, the de-rating factors of Section 310-15(b)(2)(a) would apply only where the conductors carry a continuous load in excess of 10 percent of the ampacity of each conductor.
Calculations dealing with conduit fill and conductor ampacity for Class 2, Class 3, or power-limited fire alarm circuits, as you can see, are easy if you follow the few simple rules in the NEC. Keep this in mind the next time you do an installation. The more often you use the calculations, the easier they become.
ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at email@example.com.