Short answer: a listed metal raceway made up wrench-tight is an equipment grounding conductor the NEC accepts, so a feeder with no green wire can be correct as built. Before you reuse that run, add a wire EGC wherever the raceway could be broken. NEC Table 250.122 sizes the wire from the overcurrent device ahead. The worked example below is a plant feeder — a 175 A breaker, three 3/0 THHN copper conductors in 2 in rigid metal conduit, feeding a boiler panel — with the conduit-fill arithmetic shown.
When can a metal raceway be the only equipment grounding conductor?
A metal raceway that is listed for the purpose and installed complete is one of the equipment grounding conductors the NEC accepts. Threaded rigid steel couplings made up wrench-tight, or listed fittings on EMT, both qualify, so no separate green wire is demanded on a feeder of this size.
The NEC lists metal raceways among the wiring methods it accepts as equipment grounding conductors. What it demands in exchange is continuity at every joint. NEC 250.96(A) requires metal parts serving as equipment grounding conductors — raceways, enclosures, fittings — to be bonded together so they can safely conduct any fault current likely to be imposed on them. NEC 250.4(A)(5) calls that an effective ground-fault current path and rules the earth out as part of it. A feeder that hops from a breaker enclosure to a disconnect switch and on to a panel is one chain. Each coupling, hub and cabinet wall in it either carries fault current hard enough to trip the breaker or leaves the fault sitting in the box. The fundamentals live in the grounding and bonding guide.
Is set-screw EMT a ground path you can trust?
EMT with set-screw or compression fittings is a permitted equipment grounding conductor, and dry, still buildings have run that way since the 1960s. In vibrating, washed-down or knocked-around corners, threaded rigid steel is the raceway that keeps its continuity.
The code accepts both; plant experience separates them. A set screw grips the smooth EMT wall by friction. Vibration, temperature cycling or one hard twist on a die-cast fitting can open the path right there. Compression fittings bite a corrugated sleeve, and threaded rigid steel makes up metal to metal along the whole joint. That is why factory floors stay with threaded work. Pick by environment: dry ceilings and office walls take EMT with every fitting tight and each box bonded. Machine aisles, washdown corners and fork-traffic routes take threaded rigid steel.
What size wire EGC does the breaker call for?
Size the wire EGC from the rating of the overcurrent device ahead of the feeder using NEC Table 250.122, not from the feeder copper. A 175 A breaker lands on the 200 A row, which sets the copper and aluminum sizes in the table below.
The row follows the breaker or fuse, not the wire: 3/0 copper carries 200 A at 75 C under the NEC ampacity tables and still takes that row. The rows you meet most:
| Overcurrent device ahead | Copper EGC | Aluminum EGC |
|---|---|---|
| 100 A | 8 AWG | 6 AWG |
| 200 A | 6 AWG | 4 AWG |
| 300 A | 4 AWG | 2 AWG |
| 400 A | 3 AWG | 1 AWG |
| 600 A | 1 AWG | 2/0 AWG |
The wire is a second path, not a rival. Fault current splits between raceway and wire, and the wire still clears the breaker after a coupling gets knocked loose.
Will the wire fit in the pipe you already own?
It fits: three 3/0 THHN feeder conductors plus one 6 AWG THHN EGC occupy 0.8544 in² of the 1.363 in² that a 2 in rigid metal conduit may hold at its 40% fill limit. The wire goes into the pipe you already own.
- Allowance: the NEC conduit-fill tables give 2 in rigid metal conduit 3.407 in² of internal area, and the limit for three or more conductors is 0.40 x 3.407 = 1.363 in².
- Load already in the pipe: three 3/0 THHN conductors at 0.2679 in² each come to 3 x 0.2679 = 0.8037 in².
- Add the EGC wire at 0.0507 in²: the bundle reaches 0.8037 + 0.0507 = 0.8544 in², which is 62.7% of the 1.363 in² allowance.
- Spare room: even a 4 AWG THHN EGC at 0.0824 in² fits, taking the bundle to 0.8861 in².
What moves the numbers: 2 in EMT holds 3.356 in² total and 1.342 in² at 40%, so the bundle sits at 63.7% there. Thicker XHHW-2 insulation, or a fourth 3/0 already in the pipe (1.363 - 1.0716 = 0.291 in² left), eats the margin while the 6 AWG still clears it.
Which fittings keep a raceway a continuous conductor?
Threaded couplings and listed bonding hubs, made up wrench-tight and kept free of paint at every contact surface, keep the raceway continuous from breaker enclosure to disconnect. A conduit through a concentric or eccentric knockout needs a bonding jumper or a listed bonding fitting around that ring.
NEC 250.12 requires paint, lacquer and enamel to be removed from threads and other contact surfaces at grounding and bonding connections, unless the fitting is designed to make removal unnecessary. A lug landed on powder coat is decoration. On a service or a run above 250 V to ground, the NEC wants the bond made around a concentric or eccentric knockout ring, and the eccentric-knockout bonding jumper article shows the make-up. Through-penetration entries take a threaded hub instead, and the conduit hubs at service equipment article sorts out which ones pass inspection. For the box end of the run, the rigid conduit with NEMA 4X boxes article covers bonding steel raceway to a stainless enclosure.
How do you add the wire EGC to a live plant feeder?
Adding the wire to an existing run is a day's work for one electrician, and the steps below keep the conductors and the raceway both doing their jobs. Work dead: lock out the feeder and verify absence of voltage at every enclosure before the first cover comes off.
- Read the breaker rating and take the wire size from NEC Table 250.122: 175 A lands on the 200 A row in the table above.
- Re-run the fill arithmetic with the real conductor areas before the pull, as in the example above.
- Pull the wire insulated green, or green with one or more yellow stripes; NEC 250.119 sets that color rule for insulated EGCs of the size from step 1 and smaller.
- Land the wire on the ground bus or a bonding lug in each enclosure, made up on bare metal.
- Remake every coupling and hub wrench-tight and fit the bonding jumper at any knockout ring.
- Mark both paths on the panel schedule so the next hand knows the feeder carries raceway and wire together.
Which checks prove the fault path when you are done?
Three checks close the job: a raceway reading from enclosure to enclosure well under 1 Ω, every thread and set screw remade wrench-tight, and the wire EGC landed and torqued at both ends. A run that fails any of them is not yet a fault path.
Measure the raceway first, with the feeders disconnected. A sound 40 ft run of 2 in steel reads a fraction of an ohm on a four-wire meter, and a value near 1 Ω marks a joint to remake. Then read the wire path, which comes in lower still. In corners that get hosed, repeat the raceway reading after washdown season, and once a year where vibration lives, because set screws relax with time and heat.
What should the enclosure side of the bond look like?
Give the wire EGC a landed, inspectable termination in every enclosure on the run: a lug or ground bar bonded to bare enclosure metal per NEC 250.12, in a box that keeps the water out where the raceway gets washed.
Grounds land on the ground bar or bonding lugs while neutrals stay on their isolated terminal; NEC 250.6 exists to keep neutral current off metal parts. When the switch enclosure itself is the corroding part, a stainless wall-mount box is the swap: hinged gasketed door, removable mounting panel, wall-mount feet, 304 or 316L, from 6 x 4 x 4 in to 60 x 36 x 16 in (H x W x D), rated NEMA 4X / IP66 by the manufacturer; not UL listed. The UL 50E and NEMA 250 listing guide covers when an inspector wants the UL mark.
A 24 x 20 x 8 in 304 box has room for a switch, its lugs and a ground bar, with the #4 brushed panel taking lugs on bare metal. The 316L twin handles brine and salt air, with about 60 C (140 F) the chloride stress-corrosion threshold for both grades. NEC 312.6 sets wire-bending space at terminals, and it grows with conductor size — the enclosure sizing guide walks the arithmetic. Send the hub and window cutouts on the drawing (custom enclosures, stainless wall-mount enclosures) and the box arrives cut and gasketed, delivered in 10 business days from your approval of the drawing.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 316L stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does a wire EGC in parallel with the raceway cause circulating current?
No. In normal operation both paths sit at ground potential and the load current runs in the circuit conductors, so nothing circulates. During a fault the two paths work in parallel and each helps trip the breaker. The wire keeps its own rating too: a 6 AWG copper EGC carries 65 A continuous at 75 C, far above any share it sees on a healthy feeder of that rating.
What happens when the breaker rating falls between two rows of NEC Table 250.122?
Take the next row up. A 175 A breaker lands on the 200 A row, so the wire EGC is 6 AWG copper or 4 AWG aluminum. An adjustable-trip breaker sizes from the trip setting actually installed, so a device set at 175 A uses the same row.
Can the equipment grounding conductor be bare copper instead of green insulation?
Yes, where the wiring method allows it. NEC 250.119 sets the color rule for insulated EGCs: they must be green or green with one or more yellow stripes. A bare copper EGC skips that rule and saves insulation diameter, which matters most when the fill calculation is tight, as in the conduit example above.
Which stainless grade suits a washdown boiler room corner?
304 stainless covers most washdown, food and outdoor splash work, and 316L earns its keep where brine, salt air or chemical splash lives; molybdenum raises pitting resistance while about 60 C (140 F) is the chloride stress-corrosion threshold for both grades. Our wall-mount boxes in either grade are rated NEMA 4X / IP66 by the manufacturer; not UL listed.
