No: the cable that runs outside the building needs a wet-location rating, even when both ends terminate inside sealed boxes. Indoor NM cable is not listed for damp or wet locations, and condensation inside a conduit or box will find its insulation sooner or later. The practical choices are wet-rated UF cable, or individual THWN-2 or XHHW wires pulled through conduit; both are routine for an exterior light run.
The junction box on the outside wall is the second half of the decision. It must hold the splice, accept sealed glands, survive weather and sun, and drain or shed whatever gets past the gaskets. This article covers the cable, the box, and what to put on the order.
Why the wet rating matters
Cable ratings follow the location, not the box. A run is a wet location if it can be exposed to rain, or if it sits in conduit or a channel where water can collect. An outdoor wall run qualifies even when it looks dry: temperature swings push moist air through the conduit, and condensation forms on the cooler surfaces. That is why a sealed box alone does not rescue indoor cable.
Wet-rated insulation handles this. UF cable is rated for direct burial and wet locations. THWN-2 and XHHW-2 individual wires are dual rated for dry and wet locations, which is why suppliers usually stock one version. If you pull individual wires, use conduit for the whole run and keep the glands sealed at both ends.
Choosing between UF cable and individual wires
Both work; the trade is convenience against flexibility.
- UF cable: one jacket, three cores, no conduit needed on the exposed run where the listing allows it. Faster on a short wall run, but the jacket is stiff and the bend radius is large, which matters in a small box.
- THWN-2 or XHHW-2 in conduit: easier to pull through an existing wall conduit, easier to re-pull later, and each wire is wet rated. This is the usual choice when a conduit already runs through the wall.
- Flexible outdoor cord (wet-rated, "W" marked): sometimes used for luminaires, but it is a cord, not fixed wiring, so only where the installation rules allow it.
Match the conductor size to the load and the breaker. A typical LED luminaire draws little current, but size the wire for the breaker protecting the circuit, not the lamp.
What the junction box must do
The box is the splice point between the indoor circuit and the luminaire tail. It has four jobs:
- Hold the splice: terminals or connectors with room to work.
- Keep water out: a gasketed door and cable glands, not loose holes or gum.
- Survive the site: sun, rain, freeze-thaw, and whatever the wall faces.
- Drain: condensation or splash water needs a way out of the lowest point.
For boxes and conduit bodies in damp or wet locations, NEC 314.15 permits approved drainage openings of 3 mm (1/8 in) to 6 mm (1/4 in); check the edition your authority having jurisdiction enforces. Plan that opening, or a weep path at the gland, before you order.
Rating and material for the box
For an exterior light splice, a manufacturer rating of NEMA 4X / IP66 is the working target: hose-directed water and dust tight, with corrosion resistance. Our enclosures are rated by the manufacturer for the specified design; they are not UL listed. If your project specification requires a listed enclosure, check that with the inspector before ordering, because in US installations NEC 110.28 states that IP ratings are not a substitute for enclosure type ratings.
Material follows the wall and the climate:
- 304 stainless steel: fine for most outdoor walls, rain and sun.
- 316L stainless steel: for salt air, coastal sites, or a wall that gets washed down.
- Polycarbonate or fiberglass: non-metallic and light, but check UV rating and impact exposure.
A sealed metal box also gives you a grounding path for metal conduit and the luminaire's bond; with a non-metallic box, bond the metal parts inside per the wiring rules that apply to the installation.
Sizing the box for the splice
Work through what actually goes inside:
- The splice: three cores joined, plus the luminaire tail if it lands here. A small terminal block or three connectors.
- Service loops: a loop of spare cable on each side so you can re-terminate without re-pulling. 6 in (150 mm) per side is a practical minimum.
- Bending space: conductors need room at their terminals; larger conductors need more, and NEC 312.6 sets the minimum wire bending space at terminals for cabinets and cutout boxes.
- Gland depth: each gland takes wall thickness plus a nut, and the cable needs a straight entry length.
For a single 3-core lighting run, a box around 6 x 4 x 4 in (150 x 100 x 100 mm) usually fits all of this. Two circuits or larger glands push you to 8 x 6 x 4 in (200 x 150 x 100 mm) or more. Our stainless enclosure size finder filters wall-mount boxes by outside and inside size so you can check the splice fits before ordering.
Entry, sealing and drainage details
The rating lives or dies at the penetrations:
- One gland per cable, sized to the actual cable diameter. An oversized gland leaks even with the seal torqued down. The gland selection in the cutouts, cable glands and HMI windows guide covers matching gland to cable and cutout.
- Glands facing down, or at least sideways, on a vertical wall. A gland facing up collects water and funnels it to the seal. The outdoor junction box sealing guide covers drip loops and drain placement in detail.
- Drip loops outside the box, so water running down the cable drops off before it reaches the gland.
- Sealed conduit entry where the run passes through the wall, so the wall cavity does not breathe into the box. The waterproofing with glands guide walks through the entry sequence.
- Screws into the box, not through it: mounting feet keep the enclosure shell intact. Where screws must pass through a wall, the method in the outdoor mounting screws guide keeps the seal.
If the box sits where day and night temperatures swing hard, remember that a sealed box still condenses inside. The condensation in a sealed enclosure guide explains why, and the breather and drain sizing guide helps you decide between a vent, a drain, or both for the box volume.
Mounting height and access
Mount the box where the splice stays serviceable: above snow depth and splash height, below arm's-strain, and with the door accessible. Keep it out of direct driven rain where practical; orientation matters more than rating once water is hitting a gasketed door at pressure. On rough stone, shim or bed the box so the gasket compresses evenly, and use stainless fasteners matched to the box material so the wall of the box does not become the anode.
What to put on the order
| Item | Specification |
|---|---|
| Cable | Wet-rated: UF, or THWN-2 / XHHW-2 in conduit, sized to the breaker |
| Conduit | Full run through the wall and to the luminaire where individual wires are used |
| Junction box | 304 or 316L stainless wall-mount, manufacturer-rated NEMA 4X / IP66; not UL listed |
| Size | Roughly 6 x 4 x 4 in (150 x 100 x 100 mm) for one lighting splice; larger for more circuits |
| Glands | One per cable, sized to the cable OD, entry facing down or sideways |
| Drainage | Drain hole 3-6 mm (1/8-1/4 in) at the lowest point, or an approved drain fitting |
| Fasteners | Stainless, into mounting feet; gasket under the feet |
| Terminals | Connector block sized to the cores, with service loops both sides |
We build wall-mount stainless enclosures to your size and cutout layout from one piece, delivered in 10 business days anywhere in the world; for custom work that count starts when you approve the drawing. Send the cable diameters, the gland positions and the drain orientation with the order and the cutouts arrive in the right walls. If the run continues to a camera or another sealed device on the same wall, the outdoor camera wiring guide covers the multi-drop version of the same problem.
Questions
Can I use standard indoor NM cable from an outdoor junction box to a light?
No. Indoor NM cable is not rated for wet locations, even if both ends sit in sealed boxes. If moisture condenses in the conduit or box, the insulation is not designed for it. Use wet-rated UF cable or individual THWN-2 or XHHW wires in conduit for the outdoor run.
Does the junction box itself need to be waterproof if the cable is wet rated?
Yes. A wet-rated cable handles moisture along its run, but the splice inside the box still needs protection. An enclosure rated Type 4X / IP66 by the manufacturer, with sealed glands and a gasketed door, keeps water out of the terminals. Wet-rated wire and a sealed box solve two different parts of the same problem.
How big should the junction box be for a light splice?
Allow room for the wire nuts or terminal block, a loop of spare cable on each side, and the bending space the conductors need at their terminals. For a typical 3-core lighting run, a box roughly 6 x 4 x 4 in (150 x 100 x 100 mm) works; more circuits or larger glands need more depth.
Do I need a drain hole in an outdoor junction box?
If the box can collect condensation or the glands face upward, a small drain at the lowest point helps. For boxes and conduit bodies in damp or wet locations, NEC 314.15 permits approved drainage openings of 3 mm (1/8 in) to 6 mm (1/4 in); check the edition your authority having jurisdiction enforces.
Can I mount the junction box directly on the stone wall?
Yes, with stainless fasteners and a gasket under the mounting feet so water cannot wick behind the box. On uneven stone, a thin bed of exterior sealant or a spacer plate keeps the back flat and the gasket compressed evenly.
