Splice all three equipment grounding conductors together in one joint, twist a pigtail into that joint, and land the pigtail on the box with its green grounding screw. NEC Table 250.122 sizes each ground from the breaker ahead of its circuit: 14 AWG copper for 15 A, 12 AWG for 20 A. The pigtail takes the largest ground in the box, which in the worked example below is the microwave circuit's ground. Metal conduit can carry the fault path, but only when every fitting in the run is listed for the raceway and tightened the way its listing assumes. Nobody can inspect a joint hidden inside a fitting, so pull green THHN through the conduit on any box you plan to service and let the wire carry the path.
Why every ground joins the same joint
The code rule is blunt: where circuit conductors are spliced in a metal box, every equipment grounding conductor that enters that box is spliced to the others and bonded to the box. NEC 250.96(A) says the same thing from the metal side: parts serving as grounding conductors are bonded together so the joint can carry any fault current the circuits can impose. A fault needs a low-resistance metal loop back to its breaker, and it needs that loop to survive long enough to trip in cycles. NEC 250.4(A)(5) rules the earth out as the loop.
One pigtail off one circuit fails the other two. Bond the box from the microwave circuit alone and a hot-to-box fault on the dishwasher circuit dead-ends. The breaker sees no fault current, the box sits at 120 V, and the loop closes only when something conductive arrives. Each ground keeps its own size under NEC Table 250.122, so the joint mixes the two conductor sizes and hands the box the largest. The grounding and bonding guide holds the sizing fundamentals; a feeder spliced in a metal box follows the same joint logic.
What you need
- Green 10-32 grounding screw: a metal box built for grounding carries a tapped 10-32 hole beside the knockouts.
- A 6 in [150 mm] pigtail of green THHN sized to the largest ground in the joint — in the worked example, the ground for the 20 A circuit.
- One more pigtail per device if a receptacle or switch shares the box.
- Green THHN to pull through the conduit when the run has no wire EGC; insulated EGCs 6 AWG and smaller are green or green with yellow stripes (NEC 250.119).
- A listed wire connector sized for the joint — six circuit grounds plus the pigtail in the worked example below.
- Listed cable connectors for each NM cable entry.
- Drivers for the screw and fittings, and a wrench for threaded hubs.
- A meter that reads low ohms, for the dead test and the continuity check.
The joint, step by step
- Prove the box dead. Open all three breakers and read 0 V between every pair of circuit conductors and from each conductor to ground before the first wire moves.
- Collect the grounds. Straighten the green THHN from each conduit and the bare ground from each NM cable; the bare conductor is a full EGC, not spare copper.
- Twist one joint. Every ground in the box goes into it — six circuit grounds plus the pigtail in the worked example. Seat them in a connector listed for the count, or split the joint and bridge the halves with a short jumper of the largest ground size, then tug each conductor hard enough to trust it.
- Land the pigtail on the box. Loop it under the green 10-32 screw in the tapped hole beside the knockouts and tighten onto bare metal.
- Tighten every fitting in the raceway run. Both set screws on each EMT connector go in until they dimple the conduit wall, compression nuts pull to a firm stop, threaded hubs go wrench-tight through full thread depth.
- Close and test. Fit the NM connectors, fold the joint in without pinching, fit the cover, re-energize, and run the continuity check below.
The splice rules that govern the conductors themselves — connector counts, box fill, which wires may join — belong to the branch-circuit splicing side of the job; grounding adds only the pigtail. For sizing the box around the joint, see how to size an electrical enclosure.
Three circuits, worked end to end
Take a remodel box fed by three 30 ft runs: a 20 A microwave circuit, a 15 A disposal circuit and a 15 A dishwasher circuit, each THHN in EMT from the panel and NM cable out to the appliance. NEC Table 250.122 sizes the conduit grounds at 12 AWG for the microwave and 14 AWG for the other two; each NM cable carries a matching bare ground. The joint therefore holds seven conductors: green and bare for the microwave at the larger size, green and bare twice more at the smaller, and the pigtail at the larger size again.
The continuity budget is the number to remember. The larger copper ground runs about 1.6 Ω per 1,000 ft, so the 30 ft from box to panel adds 30 × 1.6 ÷ 1,000 ≈ 48 mΩ. The 14 AWG legs add about 76 mΩ each at 2.5 Ω per 1,000 ft. A meter from box to ground bar reads tens of milliohms when the joint is sound. The cost of skipping the joint: on a hot-to-box fault the breaker never trips, and the enclosure holds 120 V until a hand closes the loop.
When the conduit can carry the ground
The NEC accepts metal raceways as the equipment grounding conductor when the raceway and its fittings make one electrically continuous run from panel to box. That makes the fitting, not the conduit, the weak link: the run is only as continuous as its loosest screw. A fitting earns its place two ways — it is listed for that raceway type, and it is installed the way the listing assumes.
| Fitting at the box | Carries fault current when | The tell you can see |
|---|---|---|
| EMT set-screw connector | Both screws are tightened until they bite the conduit wall | Screw heads recessed, wall dimpled under each screw |
| EMT compression connector | The nut is pulled to its full stop | Nut at its stop, conduit fully bottomed |
| Threaded hub on rigid or IMC conduit | Threads engage full depth, wrench-tight | Thread visible at the hub face |
| Raceway with unknown fittings | Nothing — treat the raceway as no path | Pull green THHN and land it at both ends |
So the set-screw suspicion is half right: the fitting is legitimate on paper, and the risk is the installation nobody watched. On a run you tightened screw by screw, let the raceway carry the path. On an existing run, or any box you plan to service, pull the wire — the larger ground pulled green through 30 ft of EMT is minutes of fishing. The path then survives a loose screw, a corroded hub or a fitting swapped years from now. For hub placement on an outdoor or sealed box, see the mounting guide and the cutout, gland and window guide.
Checks that prove the path
With the breakers open, read ohms from the box to the ground bar in the panel. Tens of milliohms passes on a kitchen-length run — 48 mΩ of conductor in the example — and a full ohm means a connector that never gripped or a fitting that never bit. Reseat the suspect joint and read again. After the power is back, the design proves itself the only way that matters: the breaker trips instantly on the first fault. Note the milliohms and the date in the panel logbook so the next technician has a baseline.
Mistakes that break the ground path
- Grounding from one circuit. The pigtail-off-one-circuit shortcut leaves the other circuits without a fault path through the box; the first hot-to-box fault energizes the enclosure and waits.
- A green screw on paint. NEC 250.12 requires nonconductive coatings to come off threads and contact surfaces at grounding connections, so take the contact point down to bare metal first.
- A sheet screw in an untapped hole. The tapped 10-32 hole gives the screw a full seat; a self-tapper into thin sheet gives it a couple of threads and a joint that works loose.
- Screws leaned on instead of driven. A set screw that only flattens against the wall carries no fault current; it has to bite.
- A bare NM ground trimmed for tidiness. That conductor is the appliance's fault path; it goes into the joint at full length.
The same job in a stainless enclosure
Scale the joint to industrial hardware and the logic survives intact. Three motor or control circuits land in a 10 x 8 x 6 in 304 stainless wall-mount enclosure (manufacturer-rated NEMA 4X / IP66; not UL listed) through hub cutouts you place in the drawing — the custom builder takes each hub by position and size. The bond stays the same: every EGC in one joint, the largest-ground pigtail to a boss or grounding screw on bare metal, and a green bonding jumper across the hinge so the door is bonded open or closed, the way UL 508A panel shops build it. In brine, sanitizers or salt air, the 316L version of the same 10 x 8 x 6 in box takes the corrosion while the grounding job stays identical. The wall-mount range runs from 6 x 4 x 4 in upward when the joint count outgrows the box.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 10 × 8 × 6 in | 304 stainless | 10 x 8 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 10 × 8 × 6 in | 316L stainless | 10 x 8 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Do all three circuit grounds really go in one joint?
Yes — every equipment grounding conductor that enters a box with splices joins the others and bonds to a metal box. The kitchen box in the worked example holds six grounds plus one pigtail: green and bare conductors from the 20 A microwave circuit, green and bare from the 15 A disposal and dishwasher circuits, and the pigtail for the green 10-32 screw. Drop one circuit from the joint and that circuit loses its fault path through the box.
Are set-screw fittings an acceptable ground path?
By the listing, yes: the NEC accepts a metal raceway with listed fittings as the equipment grounding conductor, and an EMT set-screw connector carries fault current once both screws bite the conduit wall. The connector also has to match the raceway type. What nobody can do is inspect the joint after the cover goes on, which is why a box you service gets a wire EGC instead.
How big should the pigtail to the box be?
The largest equipment grounding conductor in the joint: 12 AWG copper when the biggest breaker feeding the box is 20 A, 14 AWG at 15 A, per NEC Table 250.122. Feeders scale the same way — 10 AWG at 60 A, 8 AWG at 100 A, 6 AWG at 200 A — so a 200 A service joint gets a 6 AWG pigtail.
Can a ground rod at the box do the job instead?
The wire back to the panel is the path; the earth is not. NEC 250.4(A)(5) rules soil out of the effective ground-fault current path, because tripping a 20 A breaker instantly takes milliohms of metal while a rod through soil reads tens to hundreds of ohms. A rod beside the box is at most an auxiliary electrode under NEC 250.54, connected alongside the EGC, never instead of it.
