Splicing 350 kcmil in a pull box: sizing it right

Splicing in the box shrinks the NEC minimum from 8 x to 6 x the raceway trade size: 6 x 3 in [76 mm] = 18 in [457 mm] of entry-to-wall distance, against 24 in [610 mm] for a straight pull with no splice. An 18 x 18 x 8 in box meets that number exactly, and it still fights you on 350 kcmil, whose minimum bend radius alone is 5.6 in [142 mm]. One cable in, one out is a splice, so the right part is a listed insulated splice connector, not a power distribution block. Near the ocean, order 316L. The cable, connector and resistance numbers used below are planning values for the worked example; replace each with the dimensions and torque printed on the parts you buy.

Does the 24 in straight-pull rule still apply when you splice in the box?

No. With splices in the box the NEC swaps the straight-pull rule for the splice rule: 8 x 3 in [76 mm] = 24 in [610 mm] for a straight pull, 6 x 3 in = 18 in [457 mm] once conductors stop and turn in the box. A straight pull means the conductors enter one wall and leave the wall opposite, with no turn and no splice. The code sizes that box at 8 times the trade size of the largest raceway, which is where the 24 in figure comes from.

Once you splice, or the conductors leave by a different wall, the splice dimension governs: at least 6 times the largest raceway's trade size, plus the trade sizes of any other raceways entering the same wall in the same row. The straight length exists to keep a long pull from jamming at the entries. The splice dimension gives each conductor room to stop, turn and land.

For everything else that stacks into a box size, bending space at terminals and fill limits, the enclosure sizing guide walks it with the numbers.

So is an 18 x 18 x 8 in box big enough for 350 kcmil splices?

It meets the minimum exactly, 18 in [457 mm] being what 6 x 3 in [76 mm] demands, with zero margin left over. The code is not what breaks the box; bends, connector length and depth are. The 18 in is measured inside, from the raceway opening to the far wall, so read the inside figure on the box you have: gasket seats and flanges can shave a tenth of an inch.

Three things push past 18 in. A second 3 in raceway in the same wall row makes it 6 x 3 + 3 = 21 in [533 mm]. A 2 in raceway for the ground makes it 20 in [508 mm]. Both conduits entering one wall is a U pull, and there the 6 x rule measures between the two entries instead. The outdoor service conductor pull box article runs this arithmetic with weather and orientation in play.

Why does a power distribution block need a bigger box?

Because a distribution block goes in by its listing and the maker's instructions, and one 350 kcmil block line prints a 24 in [610 mm] minimum enclosure size that governs that block. One cable in, one out is a splice, not distribution, so the block is the wrong part here anyway. A listed block may sit in a pull box when its instructions are followed, including the spacing the sheet calls out. Blocks used ahead of the main must also carry a marking for that line-side duty.

Listed insulated splice connectors are the compliant path for a through-splice, and they are smaller. If a tap to a second load appears later, that is distribution: plan the block and its 24 in box then. For tapping loads off at the machine end, motor lead splices outdoors covers the landing hardware.

Which splice connector should you use for 350 kcmil?

Use a listed insulated splice connector with 350 kcmil printed inside its wire range: the set-screw type runs about 4.5 in [114 mm] long by 1.6 in [41 mm] wide, and a compression sleeve with heat shrink is the slimmer choice. Read four things off the body before you buy: the wire range includes 350 kcmil on both ports, the voltage rating is 600 V, the CU/AL mark matches your conductor, and a torque value is printed on it.

These are planning dimensions from generic 350 kcmil lines; the sheet that ships with the connector you buy wins. On small-conductor jobs a volume rule takes over instead, and the 8 AWG three-phase splice box article shows that math.

How much room is left inside an 18 x 18 x 8 in box?

About 17.9 x 17.9 x 7.9 in [455 x 455 x 201 mm] inside, and a single inline splice claims 15.7 in [399 mm] of the length: 5.6 in [142 mm] of bend, 4.5 in [114 mm] of connector, 5.6 in of bend. That leaves 2.2 in [56 mm], which is all the stagger you get. The NEC minimum bend for nonshielded conductors of 4 AWG and larger is 8 times the overall diameter, and 350 kcmil THHN runs about 0.7 in [17.8 mm] OD, so 8 x 0.7 = 5.6 in [142 mm] per 90-degree bend.

Now the length arithmetic: 5.6 + 4.5 + 5.6 = 15.7 in, and 17.9 - 15.7 = 2.2 in. Three phase splices want 4.5 in each to stagger along the run, and 2.2 in offers none. All three then land at one station across the width, where 3 x 1.6 in = 4.8 in [122 mm] fits easily, and stacking happens in depth instead.

Depth, with planning values: hub, locknut and bushing project about 0.9 in [23 mm] inside; two staggered rows of connectors take 1.8 in [46 mm] each with 1.0 in [25 mm] between rows; the door side eats 0.3 in [8 mm]. That sums to 5.8 in [147 mm] against 7.9 in [201 mm] of inside depth, so 2.1 in [53 mm] of clear working room remains. Call it two knuckles and a socket: the steel you saved on the box comes back as labor on the wall.

What box should you order for a coastal 350 kcmil splice?

A 24 x 24 x 10 in [610 x 610 x 254 mm] 316L wall-mount box rated NEMA 4X / IP66 by the manufacturer, 23.88 x 23.88 x 9.88 in inside, with both 3 in [80 mm] trade-size hub holes cut at the factory. Redo the sums: 23.88 - 15.7 = 8.2 in [208 mm] of stagger along the run, and 9.88 - 5.8 = 4.1 in [104 mm] of depth in front of the barrels. The torque wrench finally has swing.

The stock 316L sizes around it are the 24 x 24 x 10 in, the 20 x 20 x 10 in (19.88 in inside each way, about 4.2 in [107 mm] spare on the run) and the 24 x 24 x 12 in at 11.88 in [302 mm] deep. Each ships with a hinged gasketed door, a removable mounting panel that comes out for a pure splice can, and a door configured for a panel PC; when you want a blank door and the two 3 in entries exactly on the run, custom enclosures build it from one piece, delivered in 10 business days counted from your approval of the drawing.

Material for ocean air is 316L, full stop: salt deposits and gasket crevices pit 304 even at room temperature, and why wet-dry cycling rusts 316L explains the mechanism. Browse 316L wall-mount enclosures for the size range, and keep the EPDM gasket, good from -50 to 150 C [-58 to 302 F], for marine wetting. A UL listed box is evaluated to UL 50E; ours are rated NEMA 4X / IP66 by the manufacturer and not UL listed, and NEMA does not test or certify enclosures. The NEC in US installations is explicit that an IP code does not replace a type rating (NEC 110.28). Order hub holes cut to the UL 50 fitting-diameter table (Table 71-1A); the cutouts and glands guide covers hole sizing. Field-cutting a hole in a finished stainless box is how the 4X seal and the corrosion resistance both die. If the box will be serviced energized, leave the working space the code wants in front of it, as the junction box working clearance article shows.

How do you lay out and land the splices?

Kill the feeder, lock it and prove absence of voltage first, because a 310 A, 350 kcmil circuit does not give second chances; then run the eight steps below in order.

1. Lock out, tag, and prove absence of voltage with a tested meter. This is the one step with no shortcut.
2. Measure entry-to-far-wall inside the box; you need at least 18 in [457 mm]. With a second 3 in raceway in the same row, recompute 6 x 3 + 3 = 21 in [533 mm] and treat the box as too small.
3. Dry-fit everything, and slide the heat-shrink tubes over both cable ends before any crimp or bolt goes together. Forgetting the tube is the classic miss.
4. Form the approach with bends no tighter than 5.6 in [142 mm] radius, and let the 3 in RMC carry the conductor weight rather than the connector barrel.
5. Strip to the length printed on the connector and seat both 350 kcmil ends fully, with the conductor inside the printed wire range on both ports.
6. Torque each fastener to the value printed on the body with a calibrated wrench, then stripe every fastener.
7. Insulate, then bond: shrink full-length onto the insulation or close the shell, engage the RMC threads fully, set grounding locknuts on the hubs, and remove paint at every contact surface that carries fault current (NEC 250.12). The rigid conduit grounding article covers the conduit path in full.
8. Seat the gasket, dog the latches, and label the splice with the circuit ID and the torque date.

What checks prove the job when the power goes back on?

Read millivolts and heat. At 310 A a healthy 50 micro-ohm splice drops 15.5 mV and dissipates 4.8 W, so three barrels within a couple of millivolts of each other, and a box skin about 3 C [5 F] over ambient, say the job is sound. The 310 A figure is the 75 C copper column for 350 kcmil; the 50 micro-ohms is the worked example's assumption, so replace it with your measured values.

• Millivolts across each barrel under load: about 15.5 mV each at 310 A on the example's 50 micro-ohm joints, and the three readings close to each other.
• Thermal scan: three splices make about 14.4 W total, and 14.4 W over 1.4 m2 [15 sq ft] of stainless at 3.7 W/m2K is roughly 3 C [5 F] over ambient. One barrel running hot above that points at a bad crimp or a light torque.
• Torque stripes present on every fastener, bends still no tighter than 5.6 in [142 mm], and nothing hanging on a connector.
• Seal intact: gasket seated all around, latches dogged, and no drain or vent added, because a part rated below 4X pulls the whole assembly down to that rating.
• The label on the door matches the one-line diagram, so the next person opens the right can.

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