Is a steel MC connector allowed in a plastic box?

Only when the connector is listed for use with nonmetallic enclosures. NEC 110.3(B) requires listed equipment to be installed in accordance with its listing instructions.

How is MC cable grounded in a nonmetallic box?

Through the equipment grounding conductors, which NEC 250.148 requires to be spliced or joined together inside the box, since a nonmetallic box provides no bonding path for the sheath.

What governs boxes in wet locations?

NEC 314.15 requires boxes in damp or wet locations to be listed for the location and installed so water cannot enter or accumulate, with approved drainage where needed.

Can you use a metal MC connector on a PVC junction box?

It depends entirely on the connector listing and on the box. A standard steel MC connector with a locknut is listed for use with metal boxes and enclosures. Landing it in a nonmetallic box is not automatically permitted, and Section 110.3(B) of the 2023 NEC makes that listing enforceable, not advisory.

The 110.3(B) rule

Section 110.3(B) states that listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling. The instructions for a typical MC cable connector describe the enclosures and knockout thicknesses the connector was evaluated with. If the sheet says for use with metal enclosures only, that is the end of the discussion. Many manufacturers do list MC and AC connectors for use with both metal and nonmetallic boxes, and the packaging or the catalog page will say so.

What the box article requires

Section 314.17(A) requires openings through which conductors enter to be adequately closed. Section 314.17(C) covers nonmetallic boxes and conduit bodies. Where nonmetallic boxes are used with messenger supported wiring, open wiring on insulators, or concealed knob and tube wiring, conductors must enter through individual holes. Where flexible tubing is used to enclose the conductors, the tubing must extend from the last insulating support to not less than 6 mm (1/4 in.) inside the box. Where nonmetallic sheathed cable or multiconductor Type UF cable is used, the sheath must extend not less than 6 mm (1/4 in.) inside the box and beyond any cable clamp.

The key structural point is that a PVC box has no continuous metal path, and a bare metal knockout entry in a plastic wall does not grip the connector the way a steel knockout does. Overtightening a locknut in a plastic wall cracks the box.

The grounding problem

The bigger issue is the equipment grounding path. Type MC cable under Section 330.108 uses the combination of the metal sheath and an equipment grounding conductor, or an equipment grounding conductor alone depending on the cable type. When you land that cable in a metal box, the connector bonds the sheath to the box. In a PVC box there is nothing to bond to, so the sheath must be bonded through the equipment grounding conductors under Section 250.148, which requires equipment grounding conductors entering a box to be spliced or joined together within the box. Section 250.118(10) lists the conditions under which the Type MC sheath itself qualifies as an equipment grounding conductor.

Better options

If the design wants a nonmetallic box because of corrosion or wet location concerns, look at three alternatives. Use a metal box with a corrosion resistant finish where 300.6 permits it. Use a PVC box with a listed transition from MC to liquidtight flexible nonmetallic conduit. Or use a nonmetallic box that the manufacturer lists with a specific MC connector, which some do for exactly this situation. Section 314.15 governs boxes in damp or wet locations and requires listed weatherproof boxes with drainage where water can enter.

Field takeaway

Read the connector instruction sheet. If it is not listed for nonmetallic enclosures, do not use it there. And if you do land MC in a PVC box with a listed fitting, make sure every equipment grounding conductor is spliced together in the box per 250.148, because the box will not do it for you.