Short answer: run it. LFMC from a PVC junction box to a metal enclosure is a normal NEC combination when a full-size EGC rides in the conduit. Bond the metal enclosure end with a listed liquidtight fitting, jumper the jacket to the EGC inside the PVC box, and size that EGC from the breaker ahead of it. The 20 ft of steel jacket is continuity backup, and the fault current rides the wire. The sections below take the bond end by end, then the trench, the tests and the box itself.
Does LFMC from a PVC junction box to a metal enclosure need bonding at both ends?
One bond decides the run: the metal enclosure end, made through a listed liquidtight fitting. At the PVC box the jacket takes one short jumper to the EGC inside, the bond NEC 314.3 calls for at a nonmetallic box. The enclosure is where a fault has to leave the raceway and reach the source, so the fitting there does the real work. The PVC wall carries no current; it is an insulator. The jumper at the PVC end exists to keep the jacket from floating as an unbonded metal segment. The fundamentals sit in the grounding and bonding guide; this page applies them to the liquidtight run.
What carries the fault current when an EGC runs inside the LFMC?
The wire EGC carries every fault — 10 AWG copper for a 60 A breaker, 8 AWG for 100 A and 6 AWG for 200 A under NEC Table 250.122 — while the steel jacket is backup. The earth counts for nothing under NEC 250.4(A)(5). The code lists metal raceways, liquidtight included, among the things that may serve as an EGC when terminated in listed fittings. With a wire inside, you keep that as margin rather than as the path.
| Breaker ahead of the run | Copper EGC per NEC Table 250.122 |
|---|---|
| 20 A | 12 AWG |
| 60 A | 10 AWG |
| 100 A | 8 AWG |
| 200 A | 6 AWG |
Worked example, the same feed over 20 ft of LFMC with two hots, a neutral and a ground: the feeders are 6 AWG THHN copper, because the NEC's 75 °C column carries 6 AWG at 65 A, and 65 A covers 60 A. The EGC is 10 AWG green, the size Table 250.122 asks behind a device this rating. That wire adds 1.2 Ω per 1000 ft × 20 ft ÷ 1000 = 0.024 Ω to the fault path — milliohms against the breaker's job. Pull the EGC with the feeders and land it on the ground bar, not only on the fitting.
Which fitting bonds LFMC at the metal enclosure end?
Use a listed liquidtight fitting with a positive grounding path at the enclosure wall. Land the EGC on the ground bar beside it, and strip paint and brushed residue from the contact area, as NEC 250.12 requires at bonding connections. A listed fitting compresses a sealing ring onto the jacket and carries ground through its own body. The wall and the jacket become one metal exactly where a fault wants out. On larger raceway, or where the job plan wants a visible lug, add a bonding bushing with a short jumper to the bar — the heavier version of the same single bond. The listed conduit hubs article shows how bonding fittings carry fault current at equipment. Order the hole from the factory: the cutouts, glands and windows guide shows what a round, deburred cut keeps that a field drill costs.
What bonds the LFMC jacket at the PVC junction box end?
NEC 314.3 lets metal raceway enter a nonmetallic box only where the metal parts are bonded together inside it. So the PVC end takes one short green jumper from the fitting's grounding lug to the EGC landed in the box. PVC insulates, so the box wall never carries current; the jumper is for the jacket. Skip it and the 20 ft of steel between the boxes floats as an unbonded segment, exactly the loose metal the code wants tied down. A fitting with an integral ground lug makes the work one wire nut and one ring terminal. The same logic — nonmetallic walls bond their contents, not their walls — runs through grounding a fiberglass enclosure. Where the entry lands on a knockout, the bonding jumper at an eccentric knockout gives the jumper pattern at the hole.
What changes when the transition box is metal instead of PVC?
A metal transition box joins the fault path, so it takes the same fitting plus an EGC pigtail to the box itself. That bond meets NEC 250.96(A)'s demand that metal parts conduct the fault current likely to hit them. The bond work sits close to the PVC case; the metal box simply becomes a conductor that must carry fault current back to the source. Strip the coating under the ground screw, land the pigtail with the splices, and the box is in the path on purpose. Bonding a metal junction box with spliced feeder conductors walks that pattern end to end. Many builders pick the metal box here for toughness alone; the grounding difference is one extra pigtail. Where the choice matters for the machine around it, use a metal enclosure, not a plastic box sets out the case for metal walls.
Can LFMC go in the ground between two buildings?
Only LFMC marked for direct burial goes in the trench. The NEC's minimum-burial table puts it at 24 in [600 mm] of cover; switching the buried leg to rigid or intermediate metal conduit cuts that cover to 6 in [150 mm]. Read the direct-burial mark on the coil before it goes in the ground; unmarked flexible metal conduit stays above grade. Where the route can be struck by a shovel or a wheel, the NEC calls for rigid metal, intermediate metal, Schedule 80 PVC or an equally tough wall. Sleeve the rise into each building and seal the low entry where water will sit. At the far building, a driven rod is an auxiliary electrode under NEC 250.54: tie it to the EGC if you like, and keep the conductor back to the source as the return. The rod steadies voltage; the EGC clears faults.
What should you test before you close the enclosure?
Ohm the jacket to the ground bar and expect under 1 Ω: the 20 ft of EGC wire is about 0.024 Ω by the arithmetic above, so a reading near an ohm points at a loose fitting or a missing jumper. Lock the feeder out and prove it dead before you clip a meter to anything. Then work the short list:
- Torque each fitting to the maker's stated value; the sealing ring holds the bond and the seal together.
- Read the insulated EGC green or green-yellow end to end, as NEC 250.119 sets for insulated conductors 6 AWG and smaller.
- Open the PVC box and confirm the jumper lands on the ground bundle, not on a spare conductor.
- Re-test continuity after every splice: jacket to bar, and jacket to jacket across the run, each under 1 Ω.
What goes on the order for the metal enclosure end?
Order the enclosure with a factory cutout sized to the fitting and a ground stud or terminal bar. A 16 x 12 x 6 in (H x W x D) 304 stainless wall-mount box takes a feed of this size with room for the splice, the bar and the bend at the terminal. The cutout arrives round and sealed-ready, so the fitting threads in without a field drill. That size leaves room inside for duct you add later, and the 16 x 12 x 6 in 304 stainless wall-mount enclosure is the standard pick.
Near salt spray or brine, order the same box in 316L. Send your own layout to custom enclosures, or pick from stainless wall-mount enclosures, set material and cutouts. Delivery is in 10 business days anywhere in the world, counted from your order; for custom work, from your approval of the drawing. Our boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Decide it like this: listed fitting at the metal end, one green jumper inside the PVC box, wire EGC sized by the breaker per Table 250.122, and direct-burial jacket at 24 in [600 mm] or rigid metal at 6 in [150 mm].
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 16 × 12 × 6 in | 304 stainless | 16 x 12 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 12 × 6 in | 316L stainless | 16 x 12 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Is a grounding bushing needed at the PVC end of an LFMC run?
A bushing is one way; a fitting with an integral ground lug plus a short jumper is the other. NEC 314.3 wants the metal raceway parts bonded together inside a nonmetallic box either way, so the 20 ft of steel jacket does not float between the boxes. The PVC wall itself is an insulator and carries no bond.
What EGC size pairs with a 60 A subpanel feed in LFMC?
NEC Table 250.122 reads 10 AWG copper behind that breaker, 8 AWG behind 100 A and 6 AWG behind 200 A. The feeders can be 6 AWG copper, which the NEC's 75 °C column carries at 65 A. Keep the insulated ground green or green-yellow through 6 AWG, per NEC 250.119.
How deep does buried LFMC have to be?
The NEC's minimum-burial table sets 24 in [600 mm] of cover for liquidtight flexible metal conduit, against 6 in [150 mm] for rigid or intermediate metal conduit. Use only jackets marked for direct burial, sleeve the rise into each building, and step up to a tougher raceway where the trench can be struck.
Does the LFMC jacket have to act as the equipment grounding conductor?
It may, when terminated in listed fittings, but a wire EGC inside lets the jacket stand as continuity backup. Size the wire from the breaker ahead of the run using NEC Table 250.122. NEC 250.4(A)(5) counts only metal in the fault path, so the earth between buildings plays no part.
Should the transition junction box be metal or PVC?
Either works with the grounding done right. PVC takes one jumper inside the box; metal takes the same jumper plus a pigtail to the box itself. Metal buys impact toughness on a feeder this size where knocks happen, and it joins the fault path deliberately rather than by jumper alone.
