EMT feeder bonding bushing requirement: when to fit one

Short answer: a 2 in EMT feeder that ends in a plain hole you cut needs no bonding bushing at 250 volts to ground or less, provided it carries its own EGC. Every 120/208 V and 120/240 V panel qualifies. Above 250 volts to ground, as on a 480Y/277 V system at 277 volts to ground, the NEC requires the metal raceway bonded at that wall. A bonding bushing is the way to make it. On any painted wall the locknut still has to bite through to bare metal. The voltage test, the termination steps and the proof-of-bond checks follow.

When does an EMT feeder into a panel need a bonding bushing?

At 250 volts to ground or less, an EMT feeder that carries its own equipment grounding conductor lands in a plain, fully cut hole. Use a standard connector, a wrench-tight locknut and an insulating bushing. The NEC asks for no bonding bushing there.

Two conditions change that answer. The first is voltage: the NEC's raceway bonding rules bite only above 250 volts to ground. The second is the hole itself: above that threshold, a concentric or eccentric knockout must take a bonding bushing or bonding wedge. Standard locknuts are out in that case (when a bonding bushing is required at a concentric knockout). A field-cut plain hole sidesteps that rule, because a full round hole leaves the wall metal continuous under the fitting.

Because the run starts on the load side of the outdoor main disconnect, it is a feeder. The tighter make-up rules that apply at the service point are not in play (standard locknut bonding at services). Pick the termination from the voltage to ground, not from the breaker size or the conduit size.

What counts as over 250 volts to ground?

The test is the voltage from each phase to ground. A 480Y/277 V system puts 277 volts on every phase to ground and sits over the 250-volt line. A 120/208 V panel reads 120 volts to ground and stays under it.

Read the system off the disconnect label or the panel schedule before you buy a single fitting. An ungrounded 480 V delta runs 480 volts phase to phase and can reach 480 volts to ground after one ground fault, so treat it as over the line. A 600Y/347 V system puts 347 volts to ground. The high leg of a 120/240 V delta reads 208 volts to ground, still under the threshold.

Above the line, the code asks one thing of the entry: bond the metal raceway to the grounded wall so its continuity there is assured. Where the entry is a concentric or eccentric knockout, the plain locknut-and-bushing make-up is ruled out entirely. The listing side of that rule is covered in boxes vs enclosures for bonding over 250 volts.

What does the feeder's own EGC change?

The EGC moves fault clearing off the raceway. NEC 250.4(A)(5) defines the effective ground-fault current path as the conductor route back to the source and rules the earth out of it. The feeder's own ground clears the fault, not the tube wall.

That decides what the bond at the wall is for. The NEC still requires raceways, cables, equipment and enclosures to be bonded together so they can safely conduct any fault current likely to be imposed on them (NEC 250.96(A)); the locknut, or the bonding bushing above 250 volts, is that bond. The EGC clears the fault, and the bonded raceway stays at the enclosure's potential instead of drifting between fittings.

Size the EGC from the overcurrent device ahead of the feeder, not from the phase conductors' ampacity. Table 250.122 covers the feeder sizes you will actually meet:

An insulated EGC of 6 AWG and smaller must be green or green with one or more yellow stripes (NEC 250.119). Land it on the subpanel's ground bar, not on a painted-in ground screw.

How do you make up a 2 in EMT termination step by step?

For a 2 in EMT feeder ending in a hole you cut yourself, plan on a 2.47 in [62.7 mm] hole. That is the standard knockout diameter for 2 in trade size under UL 50, Table 71-1A. Deburr both faces and keep the hole clear of concentric stampings.

The kit is short: a chassis punch sized for 2 in trade or a hole saw finished to size, a deburring tool, a steel EMT connector, a locknut, an insulating bushing, a bonding bushing when the voltage test calls for one, and a star-toothed locknut if the wall is coated. The cutouts guide pairs conduit and gland sizes with the punches that cut them (the cutouts, glands and HMI window guide).

De-energize the feeder and prove absence of voltage at the lugs before the wall comes off, then work the list:

1. Read the voltage to ground off the disconnect label or the panel schedule: 120 volts needs no bonding bushing, 277 volts or more needs one.
2. Center-punch the wall away from any concentric stampings, cut the entry at about 2.47 in [62.7 mm], and deburr both faces.
3. Pass the steel connector through from the inside, set-screw or compression type; compression holds its bond better on runs that see vibration or washdown.
4. Wrench the locknut tight against bare metal. Above 250 volts to ground, make up a bonding bushing instead and land its bonding conductor on the ground bar; the 480 V case is walked through on the bonding bushing requirements for 480 V EMT raceways page.
5. Where the wall is thick or the opening runs oversize, compare a bonding bushing or bonding hub at an enclosure before you drill.
6. Seat the insulating bushing against the cut edge; the NEC expects conductor insulation protected where 4 AWG and larger pass a metal wall.
7. Land the EGC on the ground bar at the torque marked on the lug.

Worked example, 100 A at 480Y/277 V: three 3 AWG copper phase conductors carry 100 A at 75 °C terminations under the NEC ampacity rules, and the 8 AWG copper EGC matches the 100 A breaker per Table 250.122. The 277 V-to-ground reading puts the entry over the 250 V line, so the bonding bushing goes on. Fill check: 3 × 0.2679 in² = 0.8037 in², plus 0.0848 in² for the 8 AWG EGC, gives 0.8885 in² of THHN. The 2 in EMT allowance is 1.342 in² at the 40% fill used with three or more conductors, so the run sits at about 66% of the allowance. On a powder-coated subpanel the star locknut bites to bare metal, and the ohms check below should read under 1 Ω.

How do you bond through paint on a coated enclosure?

NEC 250.12 requires paint, lacquer and enamel removed from threads and contact surfaces at grounding and bonding connections. On a powder-coated wall you either bite through the coat with a star-toothed locknut or strip a bare-metal window under the fitting first.

Powder coat is an insulating film about 2-4 mils [50-100 µm] thick. A plain locknut flattened onto it can leave the joint reading ohms instead of a solid bond. Three fixes, in the order most crews reach for them:

• Drive a star-toothed locknut wrench-tight so its teeth cut through to bare metal all round.
• Strip a paint window the width of the locknut rim to bare metal, then make up with a plain locknut.
• Where the coating has to stay unbroken, bolt a bonding jumper lug from the raceway to the ground bar.

Any of the three ends with metal on metal; the tell at inspection is a bright ring where the rim sat. The grounding and bonding guide carries the rest of the enclosure-side practice (the grounding and bonding guide).

Which checks prove the bond at the end?

Put a four-wire low-ohms meter between the EMT at the entry and the subpanel's ground bar. A made bond reads well under 1 Ω, and a reading of a few ohms usually means the locknut is riding on paint.

• Re-read after final torquing; with a sound joint, lead resistance is most of what the meter still shows.
• Look for the toothed rim's bright ring all the way round a coated wall.
• Tug the EGC lug and torque it to the marking on the bar.
• Check the insulating bushing sits flush and no conductor bears on the cut edge.
• On 480 V work, confirm the bonding bushing's pigtail is landed and tight.

Re-run the ohms check after any work that moves the conduit, not on a calendar. Write the reading into the panel's records so the next crew has the number.

When does a stainless enclosure make this easier?

A bare 304 or 316L stainless wall has no paint to strip, so the locknut lands on brushed metal and the bond is made on the first wrench pull, year after year.

That is worth asking for on the next box. The custom stainless route cuts plain round entries at the fitting sizes on your drawing, takes the burrs off both faces, and delivers the body in 10 business days from drawing approval (custom stainless enclosures with factory cutouts). For washdown and chloride areas the 316L grade holds up better, and 304 covers most indoor washdown work. The termination steps above stay the same, minus the paint fixes.

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.

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