Use a 1/2 in two-screw clamp connector for each cable, one connector per knockout. Your 14/2 and your 12/2 both fit that clamp. The knockouts your box already has are the right size, and the three-quarter-inch knockout buys you a blank. The connector itself stays a dry-side part. The clamp ends up inside the box, behind the gasket, and the box carries the weather. Install the clamps inside and seal each connector with silicone. Pack the wall opening with duct seal, then caulk the top and sides of the box so water sheds around it.
Which connector and where it sits
A two-screw clamp connector for non-metallic cable is the standard part for this job. The listing on a typical 1/2 in clamp covers NM cable and flexible cord from 0.30 to 0.45 in outside diameter. Both a 14/2 and a 12/2 sit inside that band, so each cable threads into its own knockout.
Plan on one cable per connector. Two screws bear on one sheath; with two cables in one clamp they grip the fatter 12/2 and the 14/2 slides free. That settles your knockout question by itself. Two cables means two half-inch knockouts with a connector in each, and the larger ones stay blank.
You can stop hunting for a weather-rated connector. Through a back entry the clamp sits inside the box and the wall keeps the rest. The wet side starts at the gasketed face of the box and its cover. The connector's job is grip, and a bead of silicone does the sealing. If you feed the same box with flexible conduit instead of cable, the wet-location flex conduit connector article picks that part. The cutouts and glands guide carries the knockout size tables.
What you need
- Two half-inch two-screw clamp connectors for NM cable, listed for one cable each
- The locknuts shipped with them
- Silicone sealant for the connector joints
- Duct seal putty for the wall opening
- Exterior caulk for the box-to-wall joint
- Screwdriver, cable stripper, and a 1/8 in bit if you add a weep hole
Installing the connectors through the back
- Lock out both circuits — the 15 A and the 20 A. Confirm they are dead at the cable ends before you open anything.
- Open the siding behind the box until each connector can reach its knockout from the wall side. You need room for a locknut and your fingers.
- Slide a connector onto each cable with its threads pointing toward the box. Tighten the clamp screws evenly until the sheath is gripped and centered.
- Pass each shank through its knockout from inside so the clamp sits within the box. Run the locknuts down from the wall side. Where the opening is tight, leave the locknuts as spacers on the shanks and spin the box onto the threads instead.
- Set the box level against the wall, fasten it, and pull in the slack on both cables.
- Run silicone around each connector where it crosses the knockout, inside and outside. Pack the wall opening around both cables with duct seal.
- Caulk the top and sides of the box to the siding. Leave the bottom joint open so water that gets in can weep out.
- Strip and land your conductors, and fit weatherproof while-in-use covers over any receptacles.
Keeping the clamp inside the box is what saves the wall. The smooth shoulders face the siding and you never reach behind the mounted box. If you would rather cut a larger opening and work the locknuts from behind, rear cable entry for a weatherproof box walks that version. The clearances are drawn out with the box off the wall.
The three places water gets in
Water reaches the inside of a box like this through the connector joint, the box-to-wall joint, and the wall opening behind it. Seal the connector with silicone at the knockout, and caulk the box joint on top and sides. Pack the opening with duct seal, and leave the bottom joint open on purpose.
Anything that beats those seals needs a way out. NEC 314.15 permits drainage openings of 3 mm (1/8 in) to 6 mm (1/4 in) at the low point. That covers boxes and conduit bodies listed for damp or wet locations. A 1/8 in weep in a metallic device box is the standard fix. A gasketed enclosure keeps its rating by sealing instead, so leave the drill for device boxes and manage moisture with the gasket. Condensation is the slower leak — the enclosure vent or sealed box article explains why a box breathes at night.
On masonry the same three joints need different tools. Sealing a junction box to a brick wall covers the anchors and sealant that hold where siding caulk gives up.
Working the knockouts for one 14/2 and one 12/2
The inputs, from your case:
- Cable 1: 14/2 NM-B on the 15 A circuit; Cable 2: 12/2 NM-B on the 20 A circuit, both arriving through the wall.
- Connector: one two-screw clamp per cable, its listed range covering both sheaths.
- Box: half-inch knockouts in the back.
The rule in words: each cable gets its own connector, and the clamp's listed range has to cover the cable. Count connectors before you count knockouts.
The arithmetic:
- 14/2 sits inside that range, so it takes one connector.
- 12/2 sits inside the same range, so it takes a second one.
- Two cables: 2 connectors, 2 knockouts, both the same size.
- Nothing in this job needs the larger knockout.
Decision: two half-inch knockouts and two two-screw clamps, with silicone at each joint. Carry the rule forward: one cable per connector, one connector per knockout. Any time two different gauges arrive at one box, that is two connectors, full stop.
Where the NM-B has to stop
NM-B is dry-location cable, and the NEC keeps it out of wet and damp locations. Inside your exterior wall is the dry side of that line, so the existing run stays as it is. This box is where the cable ends. When a circuit has to continue along the outside of the wall, land the NM-B here. Splice to UF-B inside the box, and send the UF out through its own clamp. Conduit is the other route — connecting PVC conduit to an outdoor enclosure covers the adapter that replaces the clamp entirely.
If the box is growing anyway
Two GFCIs crowd a single-gang bell box: the devices sit deep, and both circuits land conductors at the back. The residential answer is a 2-gang device box with in-use covers. When the box also carries a timer, a small controller or a power supply, move up to a stainless enclosure. Our 8 x 6 x 4 in 304 stainless box holds both devices and the wiring behind a hinged, gasketed door. Nominal inside space from the CAD runs 7.91 in of height, 5.91 in of width and 3.91 in of depth. You set the two half-inch entry holes on the drawing, and they arrive cut. The box is delivered in 10 business days, counted from your approval of the drawing. It is rated NEMA 4X / IP66 by the manufacturer, not UL listed. Browse stainless wall-mount enclosures for other sizes, or start from the enclosure selector when inside dimensions drive the choice.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 8 × 6 × 4 in | 304 stainless | 8 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can both cables go through one three-quarter-inch knockout instead?
They fit physically, but a two-screw clamp is listed for one cable. Some plastic saddle clamps carry a two-cable listing only when both cables are the same gauge — never 14/2 beside 12/2. The screws bear on the fatter sheath and the thin one slides. Two half-inch knockouts with a connector each is the correct answer, and it leaves you a spare knockout for a third cable later.
How close to the box does the NM cable need to be fastened?
The NEC wants NM cable secured within 12 in of every box, measured along the cable. Staple each cable inside the wall cavity before it turns toward the entry, and let the connector's clamp take the final stretch. The clamp counts as the securing point at the box itself. One staple and one clamp per cable satisfy the run.
Would a UF connector seal the NM cable better than a two-screw clamp?
No. A die-cast UF connector is made for one round-jacketed underground cable and commonly threads into a three-quarter-inch knockout. It will grip flat NM-B, but the seal still comes from the silicone you put around it. No connector changes that NM-B is dry-location cable. Keep the two half-inch two-screw clamps and spend your effort on the sealant and the caulk.
Will the GFCI receptacles fit the single-gang box?
Plan on a 2-gang box or a deeper enclosure. A GFCI body runs about 1.5 in deep, and a shallow single-gang bell box leaves almost nothing behind it for two circuits' conductors and splices. A stainless enclosure four inches deep holds both devices and the wiring with room to spare. You can have the entry holes cut to order.
