A Class 2 cable assembly can run on a wall with no separation from a power cable in most cases; can this be done in a cable tray?
Cable trays are the only support system for wiring methods with their own article in the NEC, and they can run long distances. This creates the appearance of a raceway wiring method, often in unintended ways. Often individual conductors far smaller than the 1/0 threshold [in Sec. 318-3(b)(1)] show up in cable tray systems. Even where the conductors are equal to or larger than the minimum size, we see cable trays used for single conductors in nonindustrial occupancies, violating Sec. 318-3(b).
The fact is, cable trays generally are not recognized as raceways, and they aren't included in the general definition of race-ways in Art. 100. The definition in Sec. 318-2 underscores this point:
Cable Tray System. A unit or an assembly of units or sections, and associated fittings, forming a rigid structural system used to support cables and raceways [emphasis supplied].
Note that cables and raceways are self-contained Chapter 3 wiring methods without additional protection or separations. This is evident in Sec. 318-3(a)(1), which lists many permitted wiring methods eligible for cable tray support, all of which have their own articles in the NEC. All of these wiring methods are self-contained and can be run on various building surfaces. None of them need to run in a cable tray.
With this background in mind, suppose a cable tray were used as a support system for a group of Type MC cables. Could a cable assembly comprised of Class 2 control circuit conductors, but with an overall outer shield, be placed in the same cable tray? In this specific case, Type MC cable also was used for the Class 2 circuits.
The EC&M panel's response
We think the Class 2 cable assembly could be placed in the cable tray; however, in this case there isn't a magic bullet, some nugget of Code text that conclusively answers the question. In order to reach this conclusion, we have to look at the Code in its entirety, because there is language in Sec. 725-54(a)(1) that can be taken out of context to suggest otherwise:
(1) In Cables, Compartments, Cable Trays, Enclosures, Manholes, Outlet Boxes, Device Boxes, and Raceways. Conductors of Class 2 and Class 3 circuits shall not be placed in any cable, cable tray, compartment, enclosure, manhole, outlet box, device box, raceway, or similar fitting with conductors of electric light, power, Class 1, and nonpower-limited fire alarm circuits.
In this case, there are certainly Class 2 "conductors" involved; however, these are not individual conductors, but conductors that are part of a metal-clad cable assembly. We think with respect to cable trays, the word "conductors" must be read as individual conductors that aren't part of a Chapter 3 wiring method.
We think this is the only way to approach this because the first exception really doesn't apply:
Exception No. 1: Where the conductors of the electric light, power, Class 1, and nonpower-limited fire alarm circuits are separated by a barrier from the Class 2 and Class 3 circuits. In enclosures, Class 2 or Class 3 circuits shall be permitted to be installed in a raceway within the enclosure to separate them from Class 1, electric light, power, and nonpower-limited fire alarm circuits.
We think the term "barrier" in the first sentence should be interpreted as something that actually divides the cable tray and not the cable armor. The second sentence refers to raceways within enclosures, such as taking a length of EMT and running it inside a control panel to segregate a class of wiring. The cable armor isn't a raceway, and the cable tray isn't an enclosure. So on both counts, that part of the exception won't work either.
That leaves an interpretation of the main rule, as mentioned. The Chapter 3 wiring method could be used to enclose either the power circuit, or the control circuit, or both (but not within the same cable). In this case, both are part of a Chapter 3 wiring method, namely, Type MC cable. Given the long history of cane trays as a mechanical support for cable assemblies, this interpretation avoids discriminating against this support method in comparison to direct support to the building surface.
System separation on building surfaces
The rules for running power circuits on a building surface in conjunction with Class 2 control circuits are covered in Sec. 725-54(a)(3):
(3) Other Applications. Conductors of Class 2 and Class 3 circuits shall be separated by at least 2 in. (50.8 mm) from conductors of any electric light, power, Class 1, or nonpower-limited fire alarm circuits.
Exception No. 1: Where either (1) all of the electric light, power, Class 1, and nonpower-limited fire alarm circuit conductors, or (2) all of the Class 2 and Class 3 circuit conductors are in raceway or in metal-sheathed, metal-clad, nonmetallic-sheathed, or Type UF cables.
Exception No. 2: Where all of the electric light, power, Class 1, and nonpower-limited fire alarm circuit conductors are permanently separated from all of the Class 2 and Class 3 circuit conductors by a continuous and firmly fixed nonconductor, such as porcelain tubes or flexible tubing, in addition to the insulation on the conductors.
The parent rule calls for a 2-in. separation between systems. The relevant wording in this case is within the exceptions. Note that, particularly in Ex. 1, there is a clear distinction between conductors by themselves and similar conductors running in a raceway or in a cable assembly. We think, based on the language chosen to describe the permitted nonraceway wiring methods, this exception is also limited to Chapter 3 wiring.
For example, we don't think you can use Type PLTC cable to run Class 2 control circuits inside a panelboard, because the power conductors have only their own insulation. On the other hand, we think the same circuits could run through a panel if run in Type NM (or MC) Cable and arranged so the outer cable sheath were never breached within the panel. On a building surface, clearly Type PLTC cable for Class 2 conductors is suitable to be run next to power circuits, provided those power circuits run in a Chapter 3 wiring method that is either a raceway or a cable assembly.
The basic principle behind all of this is that conductor insulation alone is generally unacceptable to establish system separations between power and Class 2 (and Class 3) control and signaling circuits. Some further separation must be provided. In Ex. 2, porcelain tubes or other "continuous and firmly fixed nonconductor, such as...flexible tubing" can be used. Note that this exception does not use the word "barrier" to describe such flexible separators. This is further substantiation for our argument that that word shouldn't be used to describe these arrangements. Again, Sec. 725-54(a)(1) Ex. 1 can't be used, and the only way out is a careful interpretation of the main rule.
We are quite certain the practice described in the question is safe, and the Code as a whole supports it. We are aware this subject will be the subject of a proposal to clarify this, and we think that should be carefully considered.
EDITOR'S NOTE:
These answers are given by our panel of experts. I am chairing this panel, and the other panel members include Bill Summers, James Stallcup, and Dan Leaf. The opinion expressed is that of the panel. If a panelist disagrees with the majority opinion, his explanation is printed following the answer. Although authoritative, the answers printed here are not, and cannot be relied on as formal interpretations of the National Electrical Code.