Centaur Round Plastic SB Nail-On Box - 18/20 cu in (per 75)

Size and depth, mostly. The PSBR18 is a 3 inch diameter box, 3.5 x 3.5 x 2.75 inches, with 18 cubic inches of volume. The PSBR20 is a 4 inch diameter box, 4 x 4 x 2.25 inches, with 20 cubic inches. So the 20 gives you more fill volume in a shallower box, which matters if you're working in a thin ceiling cavity or fighting a joist. The 18 is the deeper of the two if your cutout needs to stay small.

No. These are standard round plastic switch boxes for electrical wiring and devices, not fan-rated support boxes. A paddle fan needs a box specifically listed for fan support, typically metal or a braced fan box. Use the Centaur round box for ceiling-mounted luminaires and general junction work, and order a fan-rated box where a fan is going in.

New construction. Each box ships with pre-mounted nails and fastens directly to a joist or framing member before the drywall goes up. There's no bracket or wing arrangement for cutting into a finished ceiling, so for a retrofit into existing drywall you want an old-work box with retention ears instead.

Box fill is figured from the cubic inch rating, and you get 18 cubic inches on the PSBR18 and 20 on the PSBR20. Under NEC box fill rules a 14 AWG conductor counts 2.00 cu in and a 12 AWG counts 2.25 cu in, with additional allowances for the grounds (one total), the internal clamps (one, at the largest conductor size), and any device (two). The 20 cu in box gives you meaningful headroom for a multi-cable ceiling junction; the 18 is fine for a straightforward fixture drop.

Clamping is built in. The Centaur round boxes come with integral cable clamps, four on one and six on the other, so you're landing NM cable straight into the box without buying connectors. That also means you don't lose usable volume to a clamp assembly the way you would with an add-on.

They go out in cases of 75 boxes, which is how a contractor or an MRO stockroom buys them for a rough-in package. Each capacity, 18 cu in and 20 cu in, is a separate case, so a mixed ceiling layout means ordering both.

The Centaur round box is molded thermoplastic. Plastic is the common choice for NM cable in residential and light commercial wood framing: it's non-conductive, so there's no bonding requirement for the box itself, and the integral clamps take the cable without connectors. Go metal instead where you're running EMT or MC, where the raceway has to bond to the box, or where you need fixture support the plastic box isn't listed for.

It is rated 1-gang, so it will take a single device as well as serve as a plain junction point. Being round it's mainly used at the ceiling for fixture outlets and splices rather than for a wall switch or receptacle, where a rectangular box gives you a better plaster-ring fit.