You can cut a hole in the block and keep the old box, or cover the old box completely and splice in a new weatherproof box. Both pass, and your choice hangs on one measurement. Vinyl siding and PVC mounting blocks are combustible, so the working box must finish flush with the block face or project past it. Behind a noncombustible surface such as brick or stucco, a box may sit up to 1/4 in [6 mm] back. Keep every splice in the accessible box, and when the house ground comes up short, extend it with a 6 in [150 mm] jumper landed on the green 10-32 screw.
What you need
- Screwdrivers, a hole saw or jigsaw for the block opening, and a file to deburr the cut.
- A straightedge and tape measure to read the setback of the old box behind the block face.
- A breaker you can lock off, plus a non-contact tester and a meter.
- Two bare-ground jumpers of the NEC free length, cut from wire the same size as the circuit ground: 14 AWG copper on a 15 A lighting circuit.
- A green wire connector that takes three grounds, and a 10-32 green ground screw if the box has no threaded ground boss.
- A weatherproof extension ring measured deeper than the setback, or a mounting block with an integrated weatherproof box.
- Exterior sealant for the block's top and side flanges.
Prove the circuit dead
Switch the lighting breaker off and lock it, then test before you touch a conductor. Check conductor to conductor and conductor to ground with the meter; a 120 V circuit reads zero on both only when it is truly off. A non-contact tester flags live cable through the insulation, but the meter is what proves the splice safe to touch. Keep the circuit off until the cover is seated and the fixture is screwed home.
Measure the setback, then pick the fix
Lay a straightedge across the block's cut opening and measure down to the old box's front edge. The NEC sets how deep that edge may sit: in a combustible surface the box must sit flush with the finished face or project from it. In a noncombustible surface it may sit up to 1/4 in [6 mm] back. Your vinyl siding and the block itself are combustible, so the box you work out of must finish flush at the block face: 0 in of setback. The NEC also covers the gap left around a box recessed in a noncombustible surface, and restoring a recessed box cover seal in stucco walks through that repair.
| Measured setback behind the block face | Surface at the opening | What passes |
|---|---|---|
| Flush to projecting | Vinyl, wood, PVC (combustible) | Reuse the box as it sits |
| Up to 6 mm back | Brick, stucco, concrete (noncombustible) | Reuse and fill the gap tight |
| Any setback | Combustible | Extension ring out to flush or projecting, or a block with an integrated box |
| More than 6 mm back | Noncombustible | Extension ring out to 6 mm setback or flush |
Worked example: the straightedge bridges the opening and the old box edge reads three-quarters of an inch [19 mm] behind the block face. Vinyl makes the surface combustible, so the deficit is three-quarters of an inch. Fit a weatherproof extension ring measured at 1 in [25 mm] deep. One inch minus the three-quarters deficit leaves the working edge projecting a quarter of an inch past the face, which the rule allows. When the cables run out of length at that depth, cover the old box empty and swap to a block with an integrated weatherproof box. A box stamped Type 3R or Type 4X is rated by the manufacturer, not UL listed; the NEMA 3R vs NEMA 4 guide spells out what each type survives outdoors.
If the wall behind the siding has no box at all, mounting an outdoor fixture with no junction box in the wall is the article for that case.
Steps from blank block to sealed fixture
- Hold the block over the old box and trace the box outline on its back. Cut just inside the line so the box edge meets the block face with no extra gap. Deburr the cut.
- Inspect the old box before you cover it: cracks, corrosion, or old splices inside. Move any splices into the new working box; corroded connections inside junction boxes shows why corrosion spreads in damp outdoor boxes.
- Fasten the block so the screws land on solid sheathing, not just vinyl. Wavy Dutch lap leaves gaps behind a flat block, and fixing a box gap at the siding with a mounting block shows the shim-and-seal build-out.
- Pull the supply and fixture cables into the working box until each shows at least 6 in [150 mm] of free conductor past the sheath end. The NEC calls for that length at every splice point.
- Join the grounds first: supply ground, fixture ground and a jumper under one green connector. On a metal box, land a second jumper on the green 10-32 screw. Scrape paint off the boss first, because NEC 250.12 requires paint, lacquer and enamel removed from threads and contact surfaces at grounding connections.
- On a nonmetallic box, land the jumper on the fixture's green screw instead; the plastic box has nothing to bond.
- Splice the hots and neutrals with connectors sized to the conductor count, fold the tails in without pinching, and seat the gasket.
- Screw the fixture to the block, seal the top and side flanges, and leave the bottom edge open as the weep path.
When the house ground is too short
The NEC wants the same 6 in of free conductor at each splice point, measured from the sheath end. A ground trimmed to half that comes up a couple of inches short, and stretching it solves nothing. Cut a jumper to the NEC length, twist the short end, the fixture ground and the jumper under one green connector, then land the jumper's far end on the screw. The short piece ends inside the connector, and the jumper carries the bond forward.
When the fixture lead is short too, cut two jumpers of that length and put both under the same connector: one runs to the 10-32 screw on a metal box, the other to the fixture's bond terminal. Every ground ends up tied together, which is the point. NEC 250.4(A)(5) requires the grounding conductors to form an effective ground-fault current path, and the earth is not considered part of it. The enclosure grounding and bonding guide covers that path in depth.
Checks when done
- Tug each conductor at its connector with the circuit still off; nothing pulls free, including both jumpers.
- Measure each conductor from the sheath end: at least 150 mm of free conductor sits inside the box.
- Lay the straightedge across the block: the working box edge is flush or projecting on a combustible face.
- Meter from the fixture's bond lead to the panel's ground bus: a beep or a few ohms proves the bond.
- Pour a cup of water over the block top, wait 10 minutes, and look inside the box for drops.
- Restore power: the fixture holds, and any GFCI upstream stays quiet.
Mistakes that cost the seal
- Splices left in the buried box: the block hides them for good, and the next person opens the fixture box expecting a full set of conductors. Move every splice into the working box.
- Skipping the extension ring on a deep box: an edge three-quarters of an inch behind vinyl leaves a combustible recess in the wall.
- Caulking the bottom flange: seal top and sides and let the bottom drain, or water sits against the gasket.
- Landing one ground on the screw and leaving another loose: join all grounds under one connector first, then bond with the jumper.
- Cutting the block opening oversized: a visible gap around the working box undoes the weatherproofing you just built.
- Cutting into the old box itself: the block is trim, but a field cut in a rated box can end its rating; what a panel keeps after you cut it and mount devices explains how that works.
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.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 6 × 6 × 4 in | 304 stainless | 6 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 8 × 6 × 4 in | 304 stainless | 8 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 10 × 8 × 4 in | 304 stainless | 10 x 8 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can I make the splices in the old box and cover it with the block?
Put the splices in the new working box instead. A box buried behind a mounting block cannot be serviced, so it passes only when it holds nothing but cables passing through. Pull both cables into the accessible weatherproof box, splice there, and the covered box becomes a harmless raceway point. If the cables cannot reach the new depth, add an extension ring or swap to a block with an integrated box.
Does a metal or plastic box change where the ground lands?
Yes: a green 10-32 screw bonds a metal box, while a nonmetallic box has nothing to bond, so the jumper lands on the fixture's bond terminal instead. Either way, join the supply ground, the fixture ground and every jumper under one wire connector before anything lands on a screw. No ground ever depends on the screw alone.
How short can the house ground be before it needs a jumper?
The NEC calls for at least six inches [150 mm] of free conductor at each splice point, measured from the end of the sheath. A ground trimmed a couple of inches falls roughly half short. Twist the short piece, the fixture ground and a fresh jumper under one connector, then let the jumper carry the bond to the screw.
Do I caulk all four edges of the mounting block?
Seal the top and both side flanges and leave the bottom flange open as the weep path. A continuous bead across the bottom traps water behind the block and holds it against the box gasket, which is how outdoor connections corrode. A dab of sealant under each fastener head on the top and sides keeps those holes dry too.
