EMT as EGC in parallel feeders: upsize or let the raceway carry it

Short answer: make up every EMT coupling and connector properly and let the raceway carry the ground fault — the upsized wire EGCs in all ten sets are redundant copper. Ten parallel raceways are ten parallel ground-fault paths, and bonded steel raceway tests at or below the impedance of the wire that NEC Table 250.122 sizes. If you pull wire EGCs anyway, the rule bites harder, not softer: each raceway gets the full size for the feeder's overcurrent device, grown in proportion to its phase conductors. In the 1200 A feeder worked through below, that turns a 3/0 copper EGC into 250 kcmil in every one of ten raceways.

How a bonded steel raceway returns the fault current

The NEC lists metal raceways, EMT among them, as equipment allowed to serve as the equipment grounding conductor. That allowance rests on measured performance: decades of published impedance tests put bonded steel raceway at or below the table-sized wire on feeders of this size. Two properties earn it. The raceway surrounds its phase conductors, so the fault loop is concentric and its reactance is tiny — a wire pulled alongside the phases cannot match that geometry. The steel wall also carries more cross-section than the table's wire, which cuts resistance.

Your instinct about surface area is half the answer. Ten sets give ten parallel paths, and parallel impedances divide: two raceways roughly halve the impedance, ten cut it toward a tenth. Each path is a good one on its own, which is the other half. Steel wins on geometry, not on material — steel resists current more than copper, so the concentric loop and the extra cross-section do the work. Those same ten paths also share normal stray current between panels; the grounding and bonding guide shows what that looks like.

The code holds you to the joints, not the steel. NEC 250.96(A) requires metal parts meant to serve 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. Made up wrench-tight, EMT couplings and connectors are part of that path. The grounding and bonding guide walks the full route from fault to source.

Why the wire EGC still grows with the phases

NEC Table 250.122 sizes a wire EGC from the rating of the overcurrent device ahead of it: 3 AWG copper for a 400 A device, 3/0 copper at 1200 A. The table pairs the wire with the fault current its circuit can deliver until that device clears. Upsize the phases for voltage drop and the same breaker now clears more current through a lower-impedance loop, so the NEC's proportional rule grows the wire EGC by the same percentage increase in circular mils as the phases.

The physics is heating, not impedance. Fault heating rises with current squared times time, and the wire EGC has to survive the full clearing interval. A 3/0 wire left at its old size behind 25% more phase copper runs hotter for longer before the breaker trips. The proportional rule keeps the wire's thermal margin matched to what the circuit can now deliver.

In parallel raceways the code puts a wire EGC in every raceway and sizes each one from the feeder's overcurrent device — you never split one EGC across the sets. And the wire rule never looks at the conduit: it sizes the wire as if the raceway were plastic, because the raceway is an alternative path, not a partner. Run both and each wire still has to do the whole job alone, which is why the wires come out redundant when the EMT is bonded properly. The same full-size-per-raceway logic shapes how you divide conductors inside the box; the backplate and DIN rail layout guide works the fill side.

A 1200 A feeder upsized from eight to ten sets of 1/0

The inputs, with their sources:

• Overcurrent device: a 1200 A breaker, the feeder being built.
• Base phases: 8 sets of 1/0 AWG copper at 150 A each (75 C column of the NEC ampacity table) — 8 x 150 A = 1200 A.
• Voltage-drop fix: the calculation adds 2 sets, so the run becomes 10 sets of 1/0 AWG. The NEC's parallel-conductor rule starts paralleled sets at 1/0 AWG copper, so more sets is the fix.
• Phase copper: 1/0 AWG is 105,600 circular mils (cmil), so 8 sets give 844,800 cmil and 10 sets give 1,056,000 cmil.

The rule in words: size the wire EGC from the overcurrent device with NEC Table 250.122, then grow it by the same percentage increase in circular mils as the phases.

1. Base EGC: Table 250.122 at the breaker rating calls for 3/0 AWG copper, 167,800 cmil, in each raceway.
2. Phase increase: 1,056,000 cmil / 844,800 cmil = 1.25, a 25% upsize.
3. Upsized EGC: 167,800 cmil x 1.25 = 209,750 cmil.
4. Round up to the next standard size: 250 kcmil, or 250,000 cmil.

The result: ten raceways would each carry a 250 kcmil copper wire EGC — 250,000 cmil against the 105,600 cmil of the 1/0 phases beside it, 2.4 times the copper of the conductor it travels with.

That is the threshold to watch: whenever the wire EGC outsizes the phases it rides with, price the bonded raceway-only design first. Bond the EMT per NEC 250.96(A) and omit the wires; the raceway is the EGC in all ten sets. If the specification demands wire EGCs, all ten go in at 250 kcmil, full size, no sharing.

Where the raceway path breaks

Steel rarely fails; joints do. Each of these puts a series element in the path:

• Couplings and connectors left finger-tight: the whole run is only as good as its worst joint.
• Concentric knockouts: rings left in the wall do not guarantee metal-to-metal contact, and the code requires bonding around them — a bonding bushing or jumper finishes the path. The cutouts and glands guide covers knockout dimensions and fitting seating.
• Paint and coatings under fitting seats: NEC 250.12 requires nonconductive coatings such as paint removed from threads and contact surfaces at grounding and bonding connections.
• A section replaced with PVC or liquidtight: the metal path stops there, and everything downstream loses its EGC.
• Corrosion at wet-location joints: oxide between set screw and wall adds impedance one joint at a time.

De-energize, lock out and verify absence of voltage before re-torquing fittings or punching new knockouts. Then prove the run: a continuity reading from the far-end box back to the source should sit well under 1 Ω for a feeder of a few hundred feet, and a bad joint shows up as a jump.

What it changes at the enclosure

The raceway hands the fault to the last enclosure, and the box has to accept it. Bond the enclosure through its fitting on bare metal; the grounding and bonding guide covers the bonding points and conductor sizes.

Mounting height matters when you work on those connections later: keep the highest breaker handle no more than 2.0 m (6 ft 7 in) above the floor and leave room for the door to swing 90 degrees, per the enclosure mounting guide. And when the design still pulls a wire out of a sealed outdoor box, keep the exit tight — the sizing rules in the enclosure sizing guide leave the bending space you need at the terminals.

What to specify, case by case

Decide by the path, not by habit. A feeder in bonded steel EMT needs no wire EGC whatever its phase count; a feeder with any nonmetallic break in the raceway, or a spec that names conductors, gets a full-size wire EGC in every raceway, grown with its phases. Size the box that terminates the run with the enclosure sizing guide before the conductors go in.

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.

Questions