Mounting a ground bar in an outdoor NEMA 3R enclosure

Short answer: tap the back wall for a ground bar only when the steel catches two full threads of the screw, and through-bolt it with a nut when it does not. A 10-32 screw needs 0.0625 in [1.6 mm] of wall; 16 gauge sheet measures 0.0598 in and misses by about 0.003 in. That is why most 16 x 16 in outdoor pull boxes get a nut instead of a tap. Type 3R is a drainage rating, so a bolted, sealed penetration is normal practice and threaded mounting holes break no rule. Put the bar on the back wall above the drain line, keep the holes out of the drip path onto terminals, and strip the paint off every contact point.

What a Type 3R box is built to do

NEMA 250 defines Type 3R as an enclosure for outdoor use against rain, sleet and snow, and the design leans on drainage rather than sealing. UL 50E keeps the drain openings in the bottom of a Type 3R enclosure between 1/8 in [3.2 mm] and 1/4 in [6.4 mm]. Water that blows past the door runs out the floor instead of pooling. A damp floor after a storm is the box working; the NEMA 3R vs NEMA 4 guide maps where 3R stops and Type 4 begins.

Read the mounting question that way and it gets simpler. You are not protecting a watertight box; you are adding penetrations that shed water the way the factory ones do. The practice rule: seal every hole you drill, and keep each one out of the drip path above energized lugs. Threaded mounting holes themselves raise no objection in a wet location. The wall thickness behind them is the whole argument, and that is arithmetic.

The two-thread rule the tap has to meet

The NEC's list of permitted connections for grounding and bonding conductors accepts machine screws and bolts with the threads fully engaged: at least two full threads. Listed terminal bars, lugs, pressure connectors and welds round out the same list. Two threads is the floor because a partial thread strips once vibration and fault-current heating work on the joint; two full turns keep the clamp.

Contact is the other half of the joint. NEC 250.12 requires paint, lacquer and enamel to come off the threads and contact surfaces of grounding connections, so a star washer biting through powder coat or a filed bright ring under the bar is part of the job, not a nicety. And the bar is only half the bonding: NEC 250.96(A) calls for metal raceways, enclosures and other parts serving as grounding conductors to be bonded together so they can carry the fault current likely to hit them. Every metallic conduit lands in a threaded hub or a listed bonding locknut, never a plain snap-in. For a full cabinet rather than a pull box, the backplane bonding vs ground bar comparison sorts the two arrangements out, and the grounding and bonding guide carries the fundamentals.

Counting the threads: a 10-32 screw in a thin back wall

Take the common case: a 16 in x 16 in steel pull box mounted outdoors, fed by six conduit runs. The equipment grounding conductors from all six runs land on one bar.

Inputs, and where each comes from:

• Screw: a 10-32 machine screw, cut at 32 threads per inch — the bar hardware of standard indoor practice.
• Rule: two full threads of engagement, from the NEC connection methods above.
• Wall: 0.0598 in [1.52 mm] of standard steel at 16 gauge from the Manufacturers' Standard Gauge for sheet steel; the galvanized column lists the same gauge at 0.0635 in [1.61 mm].

The rule in words: a screw catches one full thread for every 1/32 in of wall, so two threads need 2 ÷ 32 threads/in = 0.0625 in [1.59 mm] of engagement. The wall offers 0.0598 in x 32 threads/in = 1.91 threads. The shortfall is 0.0625 − 0.0598 = 0.0027 in [0.07 mm], call it 0.003 in.

Result: a standard thin wall holds 1.9 threads, and the galvanized sheet holds 2.03 on paper — a margin that mill tolerance of about ±0.005 in erases from panel to panel. The decision falls out by wall thickness:

Threshold: tap at 0.0747 in (14 gauge) or thicker, through-bolt anything thinner. The nut makes thread count irrelevant because it clamps the joint instead of relying on the wall's threads. The same arithmetic is why tapped ground-bar studs fall short in plenty of thin-gauge panelboard cabinets.

What you need

• Copper ground bar with lugs sized for the largest conductor in the box; for six conduit runs, count one lug per equipment grounding conductor plus two spares.
• 10-32 x 1/2 in machine screws and external-tooth star washers for a tapped wall of 14 gauge or better.
• 10-32 x 1 in screws, nuts and washers for a through-bolted thin wall; 1 in spans the bar, the 0.0598 in wall and the nut with room to spare.
• No. 21 drill bit (0.159 in) for the 10-32 tap and a 13/64 in [5.2 mm] bit for the #10 clearance hole.
• 10-32 NC tap, tap wrench and a deburring tool.
• Outdoor-rated sealant for the outer face; the machine-enclosure sealant selection guide covers which chemistry takes years of sun and rain.
• A file and star washers for paint removal at the contact points (NEC 250.12).
• A torque driver for the lug screws, set to the marking printed on the bar.

Mounting the bar, step by step

1. Lock out the feed and test for absence of voltage inside the box before any drilling or tapping.
2. Mark the spot on the back wall: above the drain line, beside the conduit entries, with a screwdriver swinging 90 degrees clear. Keep the holes out of the drip path above energized lugs.
3. Read the wall with calipers on a knockout plug or bare edge: tap at 0.0747 in (14 gauge) or more; through-bolt at 0.0598 in and anything thinner.
4. Tapping route: drill the No. 21 hole square to the wall, tap the 10-32 thread fully, clear the chips, and confirm the screw fights you for two complete turns. Assemble with a star washer under the screw head.
5. Through-bolt route: drill the 13/64 in clearance hole, deburr both faces, assemble screw head inside, bar in the middle, nut and external-tooth washer outside, then snug it down evenly. Lay a sealant bead over the nut and washer on the outer face; the outdoor mounting-screw sealing walkthrough details the bead and the bedding.
6. Bring every contact point to bare metal: file or star-washer the bar-to-wall and nut-to-wall spots. Paint left underneath is resistance sitting in the fault path.
7. Land the conductors: each equipment grounding conductor under its own lug at the bar's marked torque, and the grounding electrode conductor onto the same bar or through it uninterrupted. Current-carrying conductors pass straight through with no splice, the way a pull box is meant to work.

Checks that prove the connection

• Tug each conductor at its lug with a firm hand pull; nothing creeps out of the lug.
• Meter from bar to enclosure wall: the reading matches the leads touching each other, essentially 0 Ω. Repeat from bar to each conduit hub.
• Walk the outside face: a sealant bead over every added nut and washer, no bare drilled edge left to rust.
• Open the door after the first real rain. Water on the floor that drains away is Type 3R doing its job; a wet streak down the back wall from your holes means the penetration gets resealed.

Mistakes that cost the box later

• Tapping the thin sheet anyway. The 1.9 threads strip under vibration and the bar works loose within a season of storms; what a wall screw does to a sealed rating gets sharper still in the screws-through-the-wall article.
• Bottom-mounting the bar. The floor is where drained water and dirt gather, so floor-mounted bars corrode and vanish under grime; the ground bar placement article works through wall versus mounting plate.
• Paint left under a lug or nut. That is the exact joint NEC 250.12 tells you to strip to bare metal.
• Bonding through a plain locknut. Use the threaded hub or a listed bonding locknut so the raceway truly bonds to the box wall.
• Treating a driven ground rod beside the pole as the fault path. NEC 250.54 makes it an auxiliary electrode tied to the equipment grounding conductor, and NEC 250.4(A)(5) keeps the earth out of the effective ground-fault current path.

Order the holes instead of drilling them

If you are specifying the enclosure rather than retrofitting a pull box, put the ground-bar mounting on the drawing. We build custom enclosures from one piece: your material, your size, every cutout. That includes tapped holes or welded studs for the bar, in Type 3R, Type 4, Type 4X and the other type numbers, delivered in 10 business days from your approval of the drawing.

For salt air, brine or chemical washdown, take the 316L line; the stainless steel guide covers gauge, welds and care. Our wall-mount stainless boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed.

Decision, then: retrofitting an existing painted 3R pull box, through-bolt the bar with a nut and a sealant bead. Specifying a new box, order the mounting holes or studs in the drawing and keep the drill in the van.

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