Size the tap enclosure's equipment grounding conductor from the breaker ahead of the feeder, not from the tap conductors. A 400 A feeder calls for 3 AWG copper in the NEC's EGC sizing table, and the 150 A breaker inside the tap box changes nothing. Listed steel EMT can do the whole job instead of a wire: the NEC counts a metal raceway made up with its listed fittings as a valid equipment grounding conductor. You pull a wire in three cases only — the specification demands wire type, part of the run is PVC or liquidtight, or voltage-drop upsizing scales the conductor. Everything else, from hub choice to door bonding to the ground rod question, follows from that sizing rule. The enclosure grounding and bonding guide carries the fundamentals; the sections below give you the tap numbers.
The upstream breaker sets the EGC size
The NEC's EGC sizing table does one thing: it sizes the wire-type EGC from the rating of the overcurrent device ahead of the feeder or branch circuit. Not from the conductor size, and not from any breaker downstream. A ground fault inside the tap box must heat that conductor long enough for the device ahead of the feeder to trip, so the table is built around its rating.
Read the copper column straight across:
| Feeder overcurrent device (A) | Copper EGC (AWG) |
|---|---|
| 100 | 8 |
| 200 | 6 |
| 400 | 3 |
| 600 | 1 |
The aluminum column runs one to two sizes larger, so read it directly instead of converting. Whatever the tap looks like downstream, the EGC number comes from the row at the upstream device.
Why the tap conductors mislead
A tap exists so its conductors can be smaller than the feeder. Under the NEC's 25 ft tap rule for industrial installations, the tap carries at least one third of the upstream device's rating. That is 133 A, and 1/0 copper covers it at 150 A in the 75 °C ampacity column of the NEC's conductor table.
Two wrong paths open here. Sizing the EGC from the 1/0 conductors buys copper the rule does not ask for. Sizing it from the 150 A breaker inside the tap box returns 6 AWG, two AWG sizes undersized for a fault the upstream device must clear. The tap conductors and the downstream breaker describe the load side; the EGC has to survive the fault side.
One rule does reach back to the tap conductors: upsize them for voltage drop and the NEC scales the wire EGC up by the same area ratio. The worked example below runs that arithmetic. Keep the grounding path separate from the grounded conductor, too. Beyond the service equipment, neutrals land on an isolated neutral terminal and grounds land on the ground bus, which is how NEC 250.6 keeps neutral current off the box walls. The bond for a tap box is the EGC path, not the neutral.
When the raceway is the EGC
Listed metal raceways sit on the NEC's list of permitted equipment grounding conductors: steel EMT, IMC and rigid metal conduit. Industry ground-fault testing shows the steel raceway carrying the return current for the seconds a breaker needs to clear, which is the same job a wire does. With EMT on the whole run, you pull no bonding conductor for the tap box; the raceway is the EGC.
Three conditions make it hold:
- Listed fittings, set-screw or compression, made up wrench-tight at every coupling and at both ends of the run.
- A threaded hub at each enclosure entry, seated on bare metal.
- One metal path end to end. A PVC or liquidtight flexible segment breaks it, and a wire EGC runs through that segment.
Engineered specifications often call for a wire-type EGC anyway; pull it at the upstream device's table size even though the EMT already qualifies. The fitting logic follows the raceway: when a device lands on a box whose EGC is the conduit, the bond is the fitting. Bonding a device to a metal box when EMT is the EGC works through that detail. Rigid conduit behaves the same way; rigid conduit as grounding conductor with NEMA 4X boxes covers it. For where a metal path stops and a wire takes over, wiring methods into a NEMA 4 enclosure compares EMT, PVC and cable. A fiberglass or plastic tap box offers no metal path at all; grounding a fiberglass enclosure walks through the wire EGC and back-panel bonding it needs.
Bonding the tap enclosure itself
With the raceway or the wire as the EGC, the box is a link in the fault path. NEC 250.96(A) puts it plainly: metal parts serving as the EGC, enclosures included, get bonded to carry the fault current safely. In practice that is three contact points.
- Each EMT entry: a listed hub made up wrench-tight. A stainless wall in a bare #4 finish seats the hub directly. On powder-coated steel, NEC 250.12 has you remove paint from threads and contact surfaces, unless the fitting is designed to cut through and make its own connection.
- The door: its own bonding jumper across the hinge, sized and routed as control panel door bonding sets out.
- The interior: a ground lug or ground bus bolted to the bare wall, with the entry done as grounding conductor enclosure entry: connector and bond describes. Insulated EGCs of 6 AWG and smaller must be green or green with a yellow stripe under NEC 250.119.
A ground rod at the tap box does nothing for fault clearing. NEC 250.4(A)(5) leaves the earth out of the effective ground-fault current path, so the fault returns through the raceway or the wire to the source. A rod is at most an auxiliary electrode under NEC 250.54, connected alongside the EGC rather than in place of it. The equipment grounding conductor vs plant grid article untangles the two roles. Before you open an energized tap box: lock out the feeder and confirm absence of voltage.
Worked example: sizing the EGC for a 400 A feeder tap
The inputs, with where each one comes from:
- Feeder protected by a 400 A molded-case breaker upstream (the worked case for this topic; substitute your own feeder rating).
- Tap run of 25 ft to a stainless tap box, inside the NEC's 25 ft industrial tap limit.
- Tap conductors 1/0 copper, 150 A at 75 °C (the 75 °C ampacity values this site's guides use).
- A 150 A breaker in the tap box feeding the load.
The rules, in words: a 25 ft industrial tap carries at least one third of the upstream device's rating. The wire EGC comes from the NEC's EGC sizing table at that upstream rating and no other number.
The arithmetic:
- Tap ampacity: 400 A ÷ 3 = 133 A. The 1/0 conductors at 150 A cover 133 A, so the tap is sized right.
- EGC from the right row: the sizing table at the upstream rating gives 3 AWG copper.
- The wrong row, for contrast: the table at the 150 A breaker gives 6 AWG, two AWG sizes short of the conductor the fault path calls for.
- The voltage-drop case: voltage drop pushes the tap conductors from 1/0 (105,600 circular mils) to 250 kcmil. The NEC scales the EGC by the same area ratio, using the standard AWG areas: 250,000 ÷ 105,600 = 2.37. The 3 AWG EGC (41,740 circular mils) must reach 41,740 × 2.37 ≈ 98,800 circular mils. 1 AWG gives 83,690 circular mils and falls short; the next size up, 1/0 at 105,600 circular mils, covers it.
Result: the table row at the upstream device is the wire size, or the size scaled by the area ratio once the tap conductors grow for voltage drop. Decision: with EMT over the whole 25 ft and a silent specification, pull nothing and bond through listed hubs. With a wire specified, the table row at the upstream rating is the number; the 150 A breaker in the box stays out of the calculation.
What to put on the order
| Your case | Bonding hardware | Wire EGC |
|---|---|---|
| EMT end to end, specification silent | Threaded-hub cutouts at the EMT trade sizes, door bonding stud | None; the raceway is the EGC |
| Wire EGC specified | Hubs plus a ground bus or welded stud | The sizing table row at the upstream device |
| PVC or liquidtight in the run | Hubs on the metal ends, a gland for the flex | Wire EGC through the non-metallic segment |
| Conductors upsized for voltage drop | Ground bus sized for the larger lug | Scaled by the area ratio, 2.37x in the worked example |
A stainless tap box serves all four cases without a coating to scrape: 304 for most indoor and washdown sites, 316L where chlorides, brine or salt air sit near the box. Our custom stainless enclosures are built to your hub layout and cutout drawing, rated NEMA 4X / IP66 by the manufacturer; not UL listed. Every box is delivered in 10 business days, counted from your approval of the drawing. The 3D enclosure builder takes the cutouts, the ground-bus option and the NEMA type straight into the quote.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does the EGC for a feeder tap follow the tap conductors or the feeder?
The device ahead of the feeder sets it, straight from the copper column of the NEC's EGC sizing table. Sizing the wire from the breaker inside the tap box instead returns a conductor two AWG sizes short of what the upstream device needs to clear a fault, which is the mistake this sizing rule exists to prevent.
Can EMT serve as the equipment grounding conductor for a tap box?
Yes. Steel EMT, IMC and rigid metal conduit sit on the NEC's list of permitted equipment grounding conductors, and published ground-fault testing shows the raceway carrying return current until the breaker clears. The usual failure is a fitting left hand-tight: make every set-screw or compression joint wrench-tight, and the raceway does the whole job.
When do you have to pull a wire EGC on a feeder tap?
Three cases: the specification calls for wire type, which engineered jobs often do; the run includes PVC or liquidtight flexible conduit, which breaks the metal path; or you upsized the tap conductors for voltage drop, which scales the wire EGC by the same area ratio. Going from 1/0 to 250 kcmil, a 2.37x increase, moves the EGC up two conductor sizes to stay ahead of the ratio.
How do you bond the tap enclosure itself when EMT is the EGC?
The box joins the fault path. Each entry gets a listed hub wrench-tight to bare metal; NEC 250.12 calls for paint and coatings to come off the contact surfaces unless the fitting is designed to cut through. On a painted box, clean a full ring under each hub rather than a spot. The door gets its own bonding jumper; the ground bus lands on bare wall.
Does the tap box need its own ground rod?
It clears no fault. NEC 250.4(A)(5) keeps the earth out of the effective ground-fault current path, so the fault returns through the EMT or the wire EGC to the source. Where an electrode exists at the box anyway, NEC 250.54 treats it as an auxiliary electrode: connect it to the equipment grounding conductor and clear faults on the metal path.
