Short answer: keep the mains and the coax apart. One undivided box with live 120 V terminals is no place for a coax shield. So you use two separate weatherproof boxes, or a listed two-compartment box whose rigid barrier holds coax at least 1/4 in [6 mm] from every power terminal. The coax itself needs no box at all: a 3/8 in [9.5 mm] grommeted hole, a 2 in [50 mm] drip loop, an O-ring-sealed outdoor connector and a 14 AWG [2.1 mm²] copper bond at the entry do the whole job. Inside, a 2-gang box of at least 18 cu in with an outlet-and-coax faceplate finishes it.
Can coax and power share one junction box?
Not in one undivided box: live 120 V screws beside a coax shield break the NEC's separation rules for low-voltage wiring. Use two boxes, or a listed two-compartment box whose rigid barrier holds coax at least 1/4 in [6 mm] from every power terminal and splice.
Inside a box the picture changes from the wall. The cable sheath comes off, so terminals, splices and bare grounds sit exposed at 120 V. The coax braid is bare metal you handle every time you swap a patch cable. One conductor burning off a screw, or one nicked insulation, puts line voltage on that shield and on everything plugged into it. The barrier exists for exactly that failure.
Two arrangements pass. Mount the coax box beside the power box, so the existing weatherproof box stays untouched. Or buy a two-compartment frame: one enclosure, a rigid metal or plastic barrier between the compartments, so even a loose live wire cannot reach the coax. Hold Class 2 wiring 1/4 in [6 mm] clear of power terminations; that is the clearance the barrier guarantees. Some 2-gang weatherproof covers accept two separate 1-gang boxes and give you the same separation under one lid.
Pairing a PoE camera with a 120 V floodlight in the same box raises the identical question. The same separation logic answers it there too.
Does coax need a weatherproof box at all?
It doesn't: a coax pass-through is a low-voltage signal cable that seals at the connector. The waterproofing belongs to an O-ring outdoor connector, a grommeted hole and a drip loop. A box earns its keep only where you splice or extend the coax inside a wall or ceiling.
A straight run through the wall seals itself. Outdoor F-connectors carry an O-ring under the barrel, the jacket fills the grommet, and water tracking the cable drips off the loop. A box starts to earn its place when you splice, split or amplify — a splitter, an amplifier, a powered tap. Low-voltage wiring wants a box wherever its conductors are spliced inside a wall or ceiling. Leave the existing power box exactly as it stands. The coax crosses the wall through its own penetration about 12 in [300 mm] away. The connector-level sealing details live in weatherproofing coax connections in an outdoor junction box.
What parts do you need to run coax and power to one wall?
For one inside wall you need a 2-gang box of at least 18 cu in, a faceplate with one receptacle and one F-81 coax jack, a 3/8 in [9.5 mm] grommet, a ground block with 14 AWG copper, and one weatherproof connector.
| Part | Spec to buy | Why the number |
|---|---|---|
| Inside box | 2-gang, at least 18 cu in | the fill count for a receptacle plus one 12/2 run lands near 15.75 cu in |
| Faceplate | 2-gang, one receptacle plus one F-81 jack | both services leave through one plate |
| Coax | RG6 with outdoor compression connectors | the O-ring in the connector is the waterproof part |
| Wall grommet | fits a 3/8 in [9.5 mm] hole, grips a 0.27 in [6.9 mm] jacket | seals the penetration without pinching the cable |
| Ground block plus bond | 14 AWG [2.1 mm²] copper minimum | the NEC minimum for a coax entrance bond |
| Sealant | exterior polyurethane or silicone bead | top and sides only; the bottom stays open |
| Outdoor option | two 1-gang weatherproof boxes under one 2-gang cover | keeps the compartments separate outside |
The same discipline scales up to industrial walls. Keep the 120 V cabinet and the coax terminations in two stainless boxes mounted side by side instead of mixing them. Let the custom line cut the entries exactly where the cables cross the wall. An 8 x 8 x 4 in 304 stainless wall-mount box, rated NEMA 4X / IP66 by the manufacturer; not UL listed, gives you about 8 x 8 x 4 in [203 x 203 x 102 mm] of outside envelope — room for a ground block, a coax surge protector and two cable coils. It is delivered in 10 business days. Any F-81 bulkhead you fit into a cutout needs a rating to match the box: a bulkhead rated lower drags the whole assembly down to its own figure. Browse 304 and 316L wall-mount enclosures or the custom enclosure builder to size it.
How do you seal a coax hole through an outside wall?
Drill a 3/8 in [9.5 mm] hole and angle it a few degrees downward. Push in an RG6-sized grommet, and let the coax sag 2 in [50 mm] below the entry before it climbs to its fitting. Seal top and sides; leave the bottom open.
Worked example — hole, jacket and connector. RG6's jacket measures 0.27 in [6.9 mm] across. A 3/8 in [9.5 mm] bit leaves 0.375 − 0.270 = 0.105 in [2.7 mm] of total clearance. That is enough for the grommet to grip the jacket without pinching it. The compression connector runs about 0.44 in [11 mm] across the nut — wider than the 9.5 mm hole — so the connector cannot follow the cable through: feed bare coax first, crimp the connector on the inside end. Keep every bend wider than 10 × 0.27 = 2.7 in [69 mm] of radius, the 10-jacket-diameter sweep installers use for RG6.
Angle the hole a few degrees downward from the outside face so water running down the wall passes the hole instead of entering it. Push the grommet in dry and feed the cable. Then run a bead of exterior sealant across the top and both sides; the bottom edge stays open so anything that gets past the seal drains out. The drip loop does the rest: 2 in [50 mm] of sag below the entry means water tracking the jacket drips off before it reaches a box or the plate. The glands, drip loops and drains article works the same logic at enclosure scale. The cable-entry cover article shows the boxed variant when one penetration serves several cables.
How do you wire the boxes, step by step?
Nine steps take you from a dead breaker to a tested plate: mount the 2-gang box, pull the 12/2 and the bare RG6, set the grommet and drip loop, crimp the connector, bond the ground block with 14 AWG copper, then close everything up and test.
- Switch the breaker off and read 0 V at the outlet with a meter before you open anything.
- Pull the 1-gang inside box and set a 2-gang old-work box of at least 18 cu in in its place.
- Bring the 12/2 feed to it and leave 6 in [150 mm] of free conductor, the length the NEC asks for at every box.
- From outside, drill the 3/8 in [9.5 mm] hole a few degrees downward, about 12 in [300 mm] to one side of the existing power box.
- Push the RG6 grommet into the hole and feed the bare coax from the outside face toward the inside box.
- Form the drip loop on the outdoor drop: 2 in [50 mm] of sag below the entry before the cable climbs.
- Mount the ground block at the entry and bond it to the grounding electrode system with at least 14 AWG [2.1 mm²] copper.
- Crimp the outdoor compression connector on the indoor end, land the receptacle on the 12/2, and seat the F-81 jack on its own side of the plate.
- Bead sealant across the top and both sides of the grommet, leave the bottom edge open, close both boxes, and test.
Two finish details. The fill count for that inside box comes to about 15.75 cu in: three 12 AWG conductors at 2.25 cu in each, 4.5 cu in for the receptacle, 2.25 cu in for the bundled grounds and 2.25 cu in for internal clamps. An 18 cu in box leaves 2.25 cu in spare, and the coax jack adds none, because fill counts power conductors only. Terminating the 12/2 follows the same practice as connecting Romex to a weatherproof outdoor box. The grounding and bonding guide shows which parts of the house count as the electrode system your 14 AWG bond must reach.
How do you check the job when it's done?
Five checks settle it: 120 V at the receptacle, near-zero ohms from the ground block to the electrode conductor, a drip loop 2 in [50 mm] below the grommet, no bend tighter than a 2.7 in [69 mm] radius, and a dry inside after a 30 s hose rinse.
- Receptacle tester: correct wiring at 120 V, the house nominal.
- Bond: a meter from the ground block's screw to the electrode conductor reads near 0 Ω.
- Drip loop: the low point sits 2 in [50 mm] below the grommet on the outdoor face.
- Bends: nothing tighter than the 2.7 in [69 mm] radius; no white stress marks on the jacket.
- Water: a gentle 30 s hose rinse over the entry leaves the inside of both boxes dry.
- Sealant: an unbroken bead on top and both sides, the bottom edge open to drain.
Run the same inspection order a mixed-voltage junction box checklist uses: separation first, terminations second, water third.
Which mistakes let water in or fail inspection?
The failures repeat themselves: a shared undivided box, a connector forced through the grommet, a bottom edge sealed shut, a bond to a distant pipe, and a box too small for its conductors. Each has a one-line fix you can apply before the cover goes on.
| Mistake | What it costs you | The fix |
|---|---|---|
| One undivided box for coax and power | line voltage on a shield you touch | two boxes, or a divider holding 1/4 in [6 mm] of clearance |
| Connector pushed through the grommet | torn grommet, slow leak | the 0.44 in [11 mm] nut cannot pass a 9.5 mm hole; crimp after the wall |
| Bottom edge sealed shut | trapped water pools in the box | bead the top and sides; the bottom drains |
| Bond left on the crawl-space pipe | the entry stays unprotected; surges ride the shield in | ground block at the entry; a driven rod may assist under NEC 250.54 |
| Hole far larger than the grommet | a sealant-only plug that sags open | 3/8 in [9.5 mm] for a 0.27 in [6.9 mm] jacket |
| Kink under the 2.7 in [69 mm] radius | reflected signal, dropouts on the line | re-route with a full sweep and check the picture |
| Gland or bulkhead rated below the box | the assembly drops to that part's rating | buy parts rated to the box's Type and follow their stated installation torque |
So the decision, case by case. Coax only passing through the wall: no outdoor box at all — grommet, drip loop, ground block, 14 AWG [2.1 mm²] bond. Coax that splits or amplifies outside: a second weatherproof box beside the power box, in its own compartment. Both services forced to one spot: a listed two-compartment box with the rigid barrier and its 1/4 in [6 mm] clearance. Indoors, the 2-gang box with the combo faceplate is the whole answer.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 8 × 8 × 4 in | 304 stainless | 8 x 8 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
What is the divider box for coax and power called?
Suppliers list it as a two-compartment or divider box: one weatherproof frame, two isolated compartments, a rigid barrier between the power side and the low-voltage side. Outdoors, two 1-gang weatherproof boxes under one 2-gang cover achieve the same separation. Indoors, a plain 2-gang box of at least 18 cu in with an outlet-and-coax faceplate is the standard answer.
How far from Romex should coax run inside the wall?
Inside a wall, both jackets are insulation, so a parallel run is fine; the separation rule bites inside boxes, where the power sheath is stripped and terminals sit live. For signal quality, keep parallel coax and power runs about 12 in [300 mm] apart and cross at 90 degrees where they must meet. The 1/4 in [6 mm] and divider rule applies at the terminations.
Can the coax stay bonded to the crawl-space copper pipe?
Move it to the entry. A ground block on the outside wall, bonded with at least 14 AWG [2.1 mm²] copper to the grounding electrode system, protects the run where lightning actually hits; a distant crawl-space bond leaves the whole exterior leg unprotected. NEC 250.54 allows a driven rod as an auxiliary electrode — it joins the system, and the bond to the electrode system does the work.
Would running fiber instead avoid the separation problem?
Mostly, yes. Nonconductive fiber removes the shield bond and the induced-hum concerns entirely, so the entry needs only a grommeted hole sized to the jacket, a drip loop and a dry spot for the media converter or ONT. Feed the converter's power from the receptacle side, and the separation question disappears with the copper.
