Connecting a wiring trough to a panel without losing the rating

The short answer: put the rated fitting on the side that faces the weather. A plain threaded joint is enough on the side that does not. For a trough mounted under an enclosure, that usually means one gasketed screw-in hub, often called a Myers-type hub, in the top of the trough. The hub's threaded body passes through a close nipple into the bottom of the panel and is secured there with locknuts. One rated fitting does the sealing work. The second joint only has to be mechanically sound and tight.

Why one rated end is enough

Think about where water actually is. Rain hits the outside of the enclosure and the exposed top of the trough. Everything between the trough's top wall and the panel's bottom wall is sheltered: vertical surfaces in the shadow of both boxes, not exposed to hose-directed or wind-driven water. That is why the rain-exposed surface gets the gasketed, rated hub, and the sheltered surface gets locknuts on the same nipple.

Ratings are manufacturer ratings, not UL listed, and they describe the whole box after every opening is made. If you want a deeper look at what a rating survives once you modify the box, see what NEMA rating a panel keeps after you cut it and mount devices. The same logic applies here. Every penetration must be closed with a fitting that restores the level of protection the box was built for.

What you need

• A gasketed screw-in hub (Myers-type) sized to the nipple, with the gasket intact and the right hole size for its threads.
• A close nipple of the right length and thread type for the hub and the panel knockout.
• Two locknuts and, where the joint needs it, a locknut with a bonding or sealing ring per the fitting instructions.
• A hole saw or punch matched exactly to the hub's required hole size, plus a deburring tool.
• A torque or firm-tightening reference: the fitting maker's instructions.

If the opening will feed sealed cable instead of a nipple into another box, the sealing rules differ. The guide on how to install a cable gland without losing the rating covers that case.

Steps

1. Confirm the hole locations. Hold the trough against the panel bottom and mark the trough top so the nipple lands plumb and clear of internal hardware on both sides. Check that the trough top is solid or has a knockout matching the hub; a marked but unpunched footprint tells you where the maker intended a hub to sit.
2. Cut the trough opening with the hole size the hub specifies, not the nipple size. Deburr both edges. A ragged or oversize hole can keep the gasket from seating and cost the rating.
3. Insert the hub from inside the trough so the gasket sits flat against the inner wall. Tighten the hub nut evenly until the gasket compresses; do not crush it.
4. Thread the close nipple into the hub body hand-tight, then wrench it a fraction past hand-tight per standard practice for tapered threads. Do not use pipe dope on the gasketed side unless the hub instructions allow it.
5. Pass the nipple through the panel knockout and secure it with a locknut on each side of the panel wall. Tighten the locknuts firmly; this joint is mechanical, not weather-sealing, but a loose one lets the assembly work loose and stresses the hub gasket.
6. Bond the joint as your installation requires. Where the trough and panel are metal and the raceway serves as an equipment grounding path, follow the bonding rules your installation is built to; the guide on enclosure grounding and bonding covers the details.
7. Support the trough on its own mounting feet or brackets. The nipple and hub are not structural members. A hanging trough will eventually lever the hub and open the gasket.
8. Close any unused knockouts in both boxes with listed plugs before you call the job done.

Checks when done

• The hub gasket is evenly compressed, with no twist or pinch visible.
• The nipple is plumb and the trough hangs square on its own supports.
• No unused knockout or opening is left open on either box.
• The trough drains or seals as its rating intends; a rain-rated trough is built to shed or drain the water it takes.

Then run water over the assembly from above with the power off and check inside the trough for drips at the hub gasket. This is a functional check, not a rating test, but it finds a pinched gasket before the first storm does.

Mistakes that cost the rating

• Rated hubs on both ends but no gasket seating on either: two bad joints instead of one good one.
• A hole cut oversize and shimmed with tape or sealant. Sealant is not a fitting; see why in the guide to cutouts, glands and HMI windows.
• Using the nipple to hang or align the trough, which works the hub loose over time.
• Leaving a knockout open in the trough, which defeats the rating even if the hub joint is perfect.
• Sideways entry into the trough wall on a rain-rated box. Rain-rated boxes shed water down their sides; a horizontal penetration into a wall can admit it. Keep the entry on the protected top surface or follow the box maker's stated entry zones.

If the layout pushes you toward side entries, conduit bodies or flexible connectors, the article on wiring methods into a NEMA 4 enclosure: EMT, PVC or cable compares the options. The piece on bottom conduit entry into a NEMA 4 floor-mounted enclosure covers the same joint geometry from below.

When to order the opening instead of cutting it

If the trough top is solid, the hole is one clean hub size, and you have a punch or hole saw of that size, cutting it in the field is normal work. Once you need several openings, an oversize cutout for a bolted hub, or entries that must line up with the panel above, order the box with the cutouts factory-made. On a stainless trough or a custom body, a factory-cut opening comes from an approved drawing, the edges are finished, and the rating question is settled by the manufacturer rather than argued in the field. Our custom enclosures are built from one piece to your width, height, depth and cutout layout, delivered in 10 business days from your approval of the drawing.

For the drawing itself, send hole positions with dimensions from a datum, not just "match the panel". The cutout drawing checklist for an enclosure quote lists what a quote needs so the hub footprint lands where the nipple expects it.

What to put on the order

• The gasketed hub, sized for the nipple and the wall thickness of the trough top.
• The close nipple, correct length so it spans both walls without bottoming out.
• Locknuts for the panel side, plus any bonding ring your installation requires.
• Listed plugs for every unused knockout.
• If the trough is custom: a drawing showing the hub hole size and position on the top wall, and the stated rating for the finished box.

Keep the rated side honest and the sheltered side tight. That is the whole joint.

Enclosures for this job

Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.

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