National Electrical Code Explanations

Based on the 2020 NEC

by Mark Lamendola

National Electrical Code Tips: Article 760, Fire Alarm Systems, Part 5

All of the following tips apply to Power Limited Fire Alarm (PLFA) circuits.

  1. The NEC is pretty serious that you keep these conductors separate from electric light, power, Class I, NPLFA, and medium-power network-powered broadband communications circuit conductors. How serious? It lays out the requirements in nearly a full page, from 760.136(A) through 760.136(G). As a general principle, keep them separate unless there is no other way even if you can meet one of the requirements of 760.136(A) through 760.136(G).
  2. First, 760.136(A) it gives you a laundry list of items in which these conductors can't be placed if those other conductors are present [760.136(A)]. It would be hard to imagine something they missed, but if you are serious about committing a code violation you can probably think of something.

  3. Then the NEC makes an exception to 760.136(A) by saying you can install these conductors with Class I, NPLFA, and medium-power network-powered broadband communications circuit conductors but only if you separate them with barriers [760.136(B)]. Note that electric light and power are conspicuously absent from this exclusion. Again, keep these seperate from other conductors unless there is no other way.


  4. As a means to acheive the separation required by 760.136(B), you may use raceway [760.136(C)]. That is, you can enclose these conductors within a raceway inside an enclosure to separate them from Class I, NPLFA, and medium-power network-powered broadband communications circuit conductors. Again, electric power and light conductors are not included. Why would this be? Because those conductors can generate a much higher magnetic field, making inductance into the PLFA conductors possible. The raceway doesn't "ground" or "divert" the magnetic field; it is the physical distance created by the raceway that protects the PLFA conductors from induction from the Class I, NPLFA, and medium-power network-powered broadband communications circuit conductors (the magnetic field weakens with the square of the distance); this distance is not enough to ensure that, for example, a 480V motor lead won't induce a voltage into the NPLFA conductors.
  5. But there is still an exclusion for those electric light and power conductors. It even includes the Class I, NPLFA, and medium-power network-powered broadband communications circuit conductors. You can run these in the same compartments, enclosures, etc., but only where they are introduced solely to connect equipment connected to PLFA circuits and comply with one of two conditions [706.136(D)]. Yes, those conditions; always a catch.



     

  6. One of those conditions is that you keep a minimum of 0.25 inches of separation between the NPLFA conductors and all of the others [760.136(D)]. The other condition is that the circuit conductors operate at less than 150V to ground (there goes your 480V motor circuit!) and either the PLFA circuit conductors are installed as NPLFA conductors or they are installed using Type FPL, FPLR, or FPLP (or permitted substitutes) plus they must meet a few other requirements.

  7. And if the enclosure has only a single opening, you can run all these conductors through a single fitting but you must separate them with a continuous and firmly fixed nonconductor such as flexible tubing [760.136(E)].

  8. If you install PLFA conductors in hoistways, you must run them in rigid metal conduit or one of the other metallic raceways listed in 760.136(F).
  9. For all other applications, you must keep the NPLFA circuit conductors separate from those other conductors by at least two inches. However, you can ignore this requirement if you meet one of two conditions [760.136(G)].
  10. The first of the aforementioned conditions is that all of those other conductors or all of the NPLFA conductors are in a raceway or in metal-sheathed, metal-clad, or TypeUF cables. The second of the aforementioned conditions is that the two groups of conductors (the NPLFA is one group, all the other conductors are the other group) are separated by a continuous and firmly fixed nonconductor (e.g., porcelain tubes or flexible tubing) plus the insulation on the conductors [760.136(G)(1) and (2)]. If you can name everyone who drowned on the ill-fated voyage of the Titanic, you don't have to meet either of these conditions if you can also stand on your head for five minutes continuously while drinking a case of Coors.

OK, there's a fictional requirement added in item 10. Can you spot it?