Here is a lengthy informal answer from the NFPA to a similar question on this topic.
Q] Is rigid metal conduit (RMC) allowed to be routed and connected an in-wall nonmetallic box?
A] This question, like most others involving the application of electrical products, can be answered through a two-part approach. First, it must be determined whether there is a Code rule that specifically prohibits the use of a nonmetallic box with a metal conduit, metallic tubing or metal-armored cable assembly. Second, it must be determined whether there are any restrictions in the product listing or guide card information that limits the types of wiring methods that can be use with a nonmetallic boxes. The requirement of 110.3(B) regarding the installation and use of listed electrical products is always applicable to questions concerning the proper application of electrical products.
Although Article 314 does not specifically require that all boxes be listed, the fact of the matter is the majority of boxes used in electrical installations are listed. For this question there are provisions in Section 314.3 under which nonmetallic boxes are permitted to be used with metal raceways or metallic sheathed cables thus the answer to the first part is yes, the Code does permit the application. In reviewing the guide card information for three specific product categories in the Underwriters Laboratories Inc. General Information for Electrical Equipment Directory (commonly referred to as the "White Book") there are three product categories that specifically describe various types and applications of nonmetallic boxes. Depending on the guide card information found in each of these product categories, the answer to the second part is mixed. Let's look a little more at each of the considerations.
Section 314.3 provides the requirement that covers the types of wiring methods that can be connected to nonmetallic outlet, device, pull and junction boxes. The main rule restricts nonmetallic boxes to use only with open wiring on insulators, concealed knob and tube, cables with entirely nonmetallic sheaths, nonmetallic raceways and flexible cords. However, there are two exceptions to this requirement that permit the use of metallic wiring methods with nonmetallic boxes. The conditions under which these exceptions can be used ensure that where installed with metal wiring methods, nonmetallic boxes do not interrupt the electrical continuity between all metal raceways or metal-armored cables that attach to the box. The provisions within the two exceptions to maintain electrical continuity between all metal raceways and metal-armored cables connected to the box correlate with the general requirement for electrical continuity of wiring methods contained in 310.10. This is of paramount concern since the metal raceway or metal cable-armor is either the sole equipment grounding conductor or performs this function in parallel with a wire-type equipment grounding conductor installed in the raceway or cable.
The first exception permits the use of nonmetallic boxes with metal raceways or metal-armored cables provided that there is an internal bonding means installed to ensure electrical continuity between all metal cable sheaths and metal raceways. This exception covers field installation of the bonding means. The second exception covers the required bonding between metal wiring method entries via a means that is integral to the box. This provision allows for the nonmetallic box construction to provide the required bonding.
There are some instances where Article 250 does not require the bonding of metallic wiring methods to create a path for ground-fault current. This question did not indicate the type of installation involved. There are some limited energy remote control, signaling (Article 725) and fire alarm circuits (Article 760) that are supplied from a power source that is not required to be grounded in accordance per 250.20(A) for alternating current or 250.162 for direct current. In these cases, the grounding of exposed non-current carrying metal parts is not required (except where these circuits are installed in hazardous locations) per 250.86 in conjunction with 250.112(I). In regards to bonding, 250.90 specifies it is required where necessary to ensure the electrical continuity of the ground-fault current path. In the case of the limited energy circuits and systems that are not required to be grounded, it is not required to create a path for ground-fault current via the bonding of all metal raceways and cable armors. A Class 2 circuit supplying the thermostat conductors for the control of heating and air-conditioning systems is an example of a typical circuit is generally not required to be grounded. For circuits and systems that are not required to be grounded per 250.112, bonding of exposed metal parts associated with these circuits or systems is not required. Certainly, there is no prohibition in Article 250 for bonding the metal parts if this is deemed desirable.
For communication and CATV systems, Articles 800 and 820 do not contain any requirements for grounding metal raceways that are installed beyond the system protector or point of cable shield grounding. What this means is that within a building, beyond the system protector, it is not required to ground and bond metal raceways containing communication circuit conductors per Articles 250, 800, 820 and 830.
There are three product categories in the UL White Book that provide information on nonmetallic boxes. The first category is junction and pull boxes (UL identification code BGUZ). This category covers sheet-metal, cast metal and nonmetallic boxes that have a volume greater than 100 cubic inches. The guide-card information indicates that the nonmetallic boxes covered in this category are suitable for use with PVC rigid nonmetallic conduit. The second category is for nonmetallic outlet boxes (UL identification code QCMZ). This information in this category only discusses the use of outlet boxes with rigid nonmetallic conduit, however that does not necessarily indicate that nonmetallic outlet boxes with knockouts cannot be used with a metallic wiring method. That determination will have to be ascertained through the installation instructions for a particular nonmetallic outlet box. The third category covers junction and pull boxes (UL identification code BGYM) for use in Class I, Zone 0, 1, and 2 locations in accordance with Article 505. The information indicates that all boxes covered, including nonmetallic boxes, are for use with threaded rigid metal conduit, steel intermediate metal conduit, or other approved wiring method as specified in 505.15.
