Junction box splice on feeder: what NEC allows

Short answer: yes, the NEC lets you splice a feeder in a junction box. Three conditions decide it: the splice stays accessible, the box is sized for the pull, and the conductors still match the breaker. On a 200 A, 208/120 V feeder that means 3/0 copper at 75 °C terminations, a copper equipment ground, a 2 in raceway and a pull box at least 16 in long for a straight-through pull. A 1 AWG conductor (130 A) would only serve a 125 A breaker. The rumor that feeders cannot be spliced is just that. The NEC asks for an accessible, properly sized enclosure, and this article walks you through each rule with the numbers to order against.

Does the NEC allow a junction box with splices on a feeder?

Yes: the NEC places no ban on splicing feeder conductors. A junction box with a removable cover is the standard place to do it, as long as the splices stay accessible and the box is sized for the conductors and raceways you run into it.

The rules come from the NEC's box articles, the same ones that govern every splice. The splice sits in an enclosure or a listed splice device. The box stays reachable. The box is sized for what enters it. Branch-circuit splices follow the same logic in smaller wire; the splicing branch circuits in a junction box under the NEC article covers that case.

A feeder splice continues the same three conductors and ground; it is not a tap. A tap pulls a new circuit off the feeder and picks up protection rules of its own. Keep the difference straight on the one-line diagram before you count anything.

When a splice box is refused on a job, the reason is one of those three conditions, not a ban. Ask which condition failed, fix it on the drawing, and the splice goes back in.

What does accessible mean for a feeder splice box?

Accessible means you can open the cover by hand or with simple tools. A box behind finished wall board or buried under soil of any depth fails inspection.

The NEC puts it this way: a box containing splices stays reachable without removing parts of the building, the equipment or any finish. A lay-in ceiling counts when the tile lifts by hand and the cover face stays clear of conduit and duct. A fixed ceiling does not count.

Choose the location the way the next tech will meet it: on the wall beside the disconnect, on the strut rack with the machine, or at the machine base with the cover facing an aisle. Label the box 'feeder splice' and keep the cover screws in place, so the next person opens it without hunting.

What conductors does this feeder need?

At the breaker's 75 °C terminations the minimum copper for the example is 3/0 AWG, rated 200 A. 1 AWG (130 A) and 2/0 (175 A) fall short. The NEC equipment grounding conductor table calls for a 6 AWG copper ground ahead of a 200 A breaker.

The values come from the NEC's 75 °C copper ampacity table, used with lugs and breakers marked 75 °C, the standard case for industrial gear. Take the smallest conductor that covers the breaker; the breaker covers the load.

The ground is its own pick. NEC Table 250.122 sizes it from the breaker ahead of it, so the device above sets 6 AWG copper, and the same size lands on both sides of the splice.

A wrong pair shows up at the splice first. A 1 AWG conductor under a breaker that delivers 200 A runs 54 % over its 130 A rating: (200 - 130) / 130 = 54 %. The insulation runs hot, and the burned splice in an outdoor junction box article describes exactly that failure. If the machine draws less than the breaker rating, fit the breaker to the load instead. A 125 A device takes 1 AWG (130 A) and shrinks the raceway and box in the next two sections.

What raceway size carries three 3/0 conductors and a ground?

Three 3/0 THHN conductors and one ground of that size occupy 0.855 in² of cross-section. That passes the 40 % fill limit in a 2 in raceway (25 % full) and fails it in 1½ in (42 % full), so 2 in is the trade size to order.

The arithmetic, for the feeder over a 40 ft [12 m] run with two 90° bends:

• A 3/0 THHN conductor takes 0.268 in²; the ground in THHN takes 0.051 in².
• Total: 3 × 0.268 + 0.051 = 0.855 in².
• 1½ in rigid conduit: internal diameter 1.610 in, area π/4 × 1.610² = 2.04 in²; 0.855 ÷ 2.04 = 42 %, over the limit.
• 2 in rigid conduit: internal diameter 2.067 in, area 3.36 in²; 0.855 ÷ 3.36 = 25 %, passing with margin.

The NEC caps raceway fill at 40 % for three or more conductors. EMT and IMC internal diameters differ from rigid by hundredths of an inch, and the verdict holds: 1½ in fails, 2 in passes. The margin is not cosmetic either, since a 40 ft pull with bends goes easier at 25 % than at 40 %.

If the load shrinks to a 125 A breaker, three 1 AWG conductors plus an 8 AWG ground total about 0.50 in² and fit 1¼ in rigid at 33 % full. Where the raceway meets the box, the hub or a gland plate carries the entry and the seal; the gland plate requirement for conduit-fed junction boxes article explains when the plate is needed.

How do you size the box for large feeder conductors?

Boxes with 4 AWG and larger conductors are sized by dimensions, not cubic inches. A straight pull needs a box at least 8 times the largest raceway's trade size long. An angle pull needs 6 times that size plus the sum of the other raceways on the same wall.

Volume fill ends at 6 AWG. The NEC box-fill table that allots 2.25 in³ to each 12 AWG conductor covers conductors of that gauge and smaller; above it, pull-box dimensions decide.

For the example, 2 in raceways entering opposite ends give a straight pull of 8 × 2 in = 16 in of box length. Add splice hardware room: compression splices for 3/0 copper stand several inches tall, so an 8 in deep box lets the conductors lie out instead of folding. An angle pull into one wall pair starts at 6 × 2 in = 12 in to the opposite wall, plus 2 in for every additional 2 in raceway in that group.

The outdoor service conductor pull box article works the pull-box spec in detail. The how to size an electrical enclosure guide covers the cabinet rules that take over once the box holds devices instead of bare splices.

When does a splice box need working clearance in front of it?

A splice-only box has no clearance rule of its own. The NEC working-space rule applies to equipment likely to be serviced while energized, so a box holding a disconnect or breaker needs at least 3 ft [900 mm] of clear depth in front for 0-150 V to ground, Condition 1.

A 208/120 V system is 120 V to ground, so the Condition 1 depth is 3 ft [900 mm]. NEC 110.26(A) also asks for at least 30 in of width or the equipment width, and room for doors to swing 90°. Height gets two numbers: 6.5 ft [2.0 m] of headroom or the equipment height, and the highest breaker handle no more than 2.0 m [6 ft 7 in] above the floor under NEC 240.24(A).

So the bare splice box can sit close to the machine, while the relocated safety switch needs its 3 ft [900 mm] of standing room. The junction box working clearance above a ceiling article works through the case where the box lands over a lay-in ceiling.

How do you make and bond the splice?

Make each splice with connectors listed for the conductor size and metal. Torque bolted types to the marked value, crimp compression types with the matched die, and insulate to at least the conductor's insulation rating. On the example that means three 3/0 copper splices plus the ground.

De-energize, lock out and prove absence of voltage on all three phases and the neutral before the cover comes off. A 208 V three-phase arc burns badly.

Bond the box into the fault path while you are in there. Made-up rigid metal conduit threads can carry the fault current, with bonding bushings where the joint breaks; otherwise land a bonding jumper from the ground conductor to the box. The earth is never the fault path under NEC 250.4(A)(5). The bonding a metal junction box with spliced feeder conductors article shows the terminations, rigid conduit as grounding conductor with NEMA 4X boxes covers the conduit path, and the enclosure grounding and bonding guide collects the rules.

On a wet site the connection is the part that fails first; the page on how 600 V conductor splices stay sealed outdoors shows the sealed-splice hardware that keeps water out of a splice for its whole life.

What goes on the order for a feeder splice box?

Specify a gasketed stainless pull box with a hinged or removable cover, at least 16 in long for the straight pull, 2 in hubs or cutouts for the raceways, and a bonding point for the ground. Order the raceway, conductors and splice connectors to match.

Then the decision by site:

• Dry indoor panel room: a Type 12 painted-steel box does the job; save the stainless for the washdown areas.
• Washdown, food and beverage, outdoor: 304 stainless at Type 4 or 4X.
• Salt air, brine and chemical splash: 316L stainless at Type 4X.
• Buried section: the box stays above grade, since burial fails accessibility.

Send the drawing with the raceway sizes, hub positions, cover swing and mounting points. We cut every entry from one piece: the custom enclosures builder takes your width, height, depth and cutouts. The stainless size finder filters catalog lines from 6 x 4 x 4 in to 60 x 36 x 16 in outside, stainless cutout drawings runs the drawing checklist before we cut, and the cutouts and glands guide lists what the drawing needs. Custom work is delivered in 10 business days from your approval of the drawing.

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