Four Box Styles, and the One That Fits Your Job
This category looks like a wall of similar gray and bronze boxes, and it is really four distinct products with different jobs. Depth separates the two cast device boxes. Shape and hub count separate the Bell box from both. Material decides which conduit system the box can join. Sorting those out first turns a confusing grid into a short list. For interior work where none of this applies, Lumera's device and outlet boxes collection is the cheaper starting point.
FS or FD: the Same Box at Two Depths
Both are cast device boxes that mount on a surface with threaded hubs, and the difference is internal depth.
An FS box is the shallow one, roughly two inches deep. It suits a straightforward drop where the wall or surface is tight and the box holds one device and a modest number of conductors.
An FD box is the deep version, closer to two and three quarter inches. That extra depth is not cosmetic: it is what gives conductors room to fold behind a device without forcing the yoke out of the box. Choose FD whenever wire fill is heavy, when conduit passes through rather than terminating, or when a bulkier device such as a GFCI receptacle is going in.
The practical rule on site is that FD costs a little more and almost never causes a problem, while FS is the one people regret on a crowded box.
Bell Boxes for Surface-Mounted Outlets and Fixtures
The Bell box is the round or rectangular outlet box seen on the outside of almost every house. It comes nonmetallic or die-cast, and its defining feature is that it arrives with three or more threaded holes already in it, sealed with closure plugs.
That design is what makes it flexible. You remove plugs only where conduit actually enters, so one part number covers a bottom entry, a back entry, or a pass-through, and the rest of the openings stay sealed. It is the standard choice for a surface-mounted exterior receptacle or a fixture on siding, brick, or a post.
Count Your Conduit Runs Before Choosing Hubs
Hub count is the specification most often decided by accident, and it is worth ten seconds of thought.
A box with one or two hubs handles a simple drop where the run ends at the device. A three or four hub box lets the conduit pass straight through or branch to another point, which saves adding a separate junction later. Hubs are threaded for half inch, three quarter inch, or one inch conduit, so match them to the raceway you are actually running.
Every hub you do not use has to stay sealed with its closure plug. An open hub means the box is no longer weatherproof no matter what the casting says, and it is one of the first things an inspector looks at.
Match the Box Material to the Conduit System
Both cast and PVC boxes work outdoors, so the deciding factor is not weather resistance. It is what the conduit is made of.
A cast aluminum box belongs on a metal raceway system, rigid, IMC, or EMT. The threaded hubs mate correctly and the metal path stays continuous for bonding, which matters because on many installations the raceway is carrying the equipment ground. Cast also takes physical impact better, which counts on a loading dock or a low wall where something will eventually hit it.
A PVC box belongs on nonmetallic conduit. It will not corrode at all, which wins outright in coastal air, agricultural buildings, and anywhere fertilizer or cleaning chemicals are present. The tradeoff is that a nonmetallic box carries no equipment ground of its own, so the grounding path has to be maintained through the conductors.
Mixing the two is where trouble starts. Threaded hubs of different systems do not always mate cleanly, and the bonding arrangement stops being straightforward. Keep the box and the conduit in the same family, and use the connectors intended for it, which Lumera's electrical fittings collection covers.
What NEMA 3R Actually Promises
NEMA 3R means the enclosure sheds falling rain, sleet, and external ice formation. That covers the great majority of exterior work: a receptacle on a wall, a switch by a door, a junction on a post.
Its limits matter just as much. A 3R box is not rated for direct hose-down, for submersion, or for the corrosive conditions a washdown area or a chemical plant produces. Where the enclosure will be pressure cleaned or sits in an aggressive atmosphere, the correct product is a higher rated enclosure, and Lumera's NEMA enclosures collection covers those.
The Cover Is a Separate Order, and the Type Matters
These boxes ship without a cover, and the cover is what actually closes the assembly. Two types are not interchangeable and the distinction catches people out constantly.
A standard weatherproof cover, often a flip lid, seals the opening when nothing is plugged in. Lift the lid to insert a cord and the seal is broken while the cord is in place. That is acceptable for an outlet used occasionally.
An in-use, or while-in-use, cover is a raised bubble that stays closed around a cord passing through it. Any outdoor receptacle that has something plugged into it for extended periods needs this type rather than a flip lid. Lumera's weatherproof covers collection carries both.
Ground Fault Protection Comes With the Location
Nearly every outdoor receptacle requires ground fault protection under the NEC, and that travels with the location rather than being an optional upgrade. Specifying a weatherproof box for an exterior outlet without planning the protection leaves the job half specified.
Weather-resistant device versions exist for exactly this position, built with corrosion-resistant contacts for the temperature swings an outdoor box sees. Lumera's GFCI collection carries the weather-resistant versions built for this position.
Mounting, Drainage, and Installation
Orient the box so water runs off the cover rather than collecting on it, and bring conduit in from the underside or the sides wherever the run allows. A top entry gives water a direct path inside the moment a fitting loosens.
Condensation is the second half of the problem, since a sealed box in a location with wide temperature swings collects moisture from the air alone. Some listed boxes permit a small drain at the low point for that reason. Follow the manufacturer's instructions rather than drilling your own, because an unlisted opening ends the rating.
Fasten the box to structure rather than to siding alone, and seal the wall penetration behind it so water tracking down the cavity is not the failure nobody looks for. Everything here connects to building wiring, so a licensed electrician should handle the circuit and confirm that the box, cover, fittings, and device all carry ratings suited to the position.
Weatherproof Box FAQs
What is the difference between an FS and an FD box?
Depth. An FS box is shallow, around two inches, and suits a simple drop where the surface is tight. An FD box is deep, closer to two and three quarter inches, giving conductors room to fold behind a device. Choose FD when wire fill is heavy, when conduit passes through, or when a bulky device like a GFCI is going in.
What is a Bell box and when do I use it?
It is the round or rectangular outlet box found on house exteriors, nonmetallic or die-cast, with three or more threaded holes sealed by closure plugs. You open only the holes conduit enters. It is the standard choice for a surface-mounted outdoor receptacle or fixture on siding, brick, or a post.
How many hubs do I need?
Count the conduit runs first. One or two hubs suit a simple drop that ends at the device. Three or four let conduit pass through or branch without a separate junction. Match the hub thread to your raceway size, and keep every unused hub sealed with its closure plug.
Should I use a cast aluminum or a PVC box?
Match the box to the conduit system, not to the weather. Cast aluminum goes with rigid, IMC, or EMT and keeps the metal bonding path continuous. PVC goes with nonmetallic conduit and will not corrode in coastal or chemical settings, though it carries no equipment ground, so grounding runs through the conductors.
Is a flip-lid cover enough for an outdoor receptacle?
Only where nothing stays plugged in. A flip lid seals an empty outlet and opens when a cord goes in. Any receptacle with something connected for extended periods needs an in-use cover that stays closed around the cord.
What does NEMA 3R cover, and what does it not?
It means the box sheds rain, sleet, and external ice, which covers most exterior wiring. It does not cover direct hose-down, submersion, or corrosive atmospheres. Washdown areas and chemical environments need a higher rated enclosure instead.
