To choose a cluster box unit (CBU), start from how many homes the site serves. Then pick the USPS CBU types (I to VI) whose door and parcel locker counts add up to that number. Add outdoor parcel lockers until you have about one parcel locker for every five mailboxes, the minimum USPS sets for apartment buildings. Get the site and equipment approved by your local USPS growth manager or postmaster before you order. Our cluster mailboxes are all USPS-licensed Salsbury units, so the main decisions are the configuration, the finish and the site.
This guide is for HOA boards, property managers, builders and developers. It covers the six standard CBU types, parcel lockers, pedestals and concrete pads, decorative options, the approval process, and the mistakes we see most often on new projects.
What is a cluster box unit?
A cluster box unit is a freestanding, pedestal-mounted mailbox with individually locked customer compartments, one or more parcel lockers and an outgoing mail slot. The carrier opens one master door to deliver to every compartment at once. Each resident opens only their own door with their own key.
USPS calls centralized delivery its preferred mode for new developments. In the 2018 revision of the Postal Operations Manual it says curbside, sidewalk and door delivery are generally not available for new delivery points. So if you are building a new subdivision, plan on CBUs, outdoor parcel lockers or wall-mounted 4C mailboxes unless USPS tells you otherwise.
CBUs must be built to the USPS-B-1118 CBU specification, and USPS licenses the manufacturers that make them for sale to developers and property owners. The Salsbury cluster box units we sell are made under USPS license #1CDSEQ-08-B-0026.
CBU types I to VI: door and parcel locker counts
Every licensed CBU comes in the same six configurations. The type number tells the postmaster exactly what you are installing, so use it in your plans and your approval request.
| Type | Customer doors | Door size | Parcel lockers | Mailboxes per parcel locker | Salsbury model |
|---|---|---|---|---|---|
| Type I | 8 | A (12-3/4" W x 3-1/8" H) | 2 (1 small, 1 medium) | 4 | 3308 |
| Type II | 12 | A | 1 (small) | 12 | 3312 |
| Type III | 16 | A | 2 (1 small, 1 medium) | 8 | 3316 |
| Type IV | 13 | B (12-3/4" W x 4-7/8" H) | 1 (small) | 13 | 3313 |
| Type V | 4 | C (12-3/4" W x 6-3/4" H) | 2 (1 small, 1 medium) | 2 | 3305 |
| Type VI | 8 | A | 4 (3 medium, 1 small) | 2 | 3306 |
Small parcel lockers measure 12" W x 10" H and medium lockers 12" W x 13-1/2" H. All six standard Salsbury CBUs stand 62" tall, 30-1/2" wide and 18" deep including the pedestal, and weigh 145 to 175 lb.
Which type should you choose?
- Type III (16 doors) gives the most mailboxes per pad and is the usual workhorse for subdivisions.
- Type IV (13 doors) has taller B doors, which hold magazines and flats better. It suits office parks and communities where residents get a lot of catalog mail.
- Type I and Type VI (8 doors) carry more parcel lockers per household. Use them to bring the parcel ratio up without adding a separate locker.
- Type V (4 doors) has double-height C doors. It is the right size for a small cul-de-sac, a fourplex or the leftover homes in a larger layout.
- Type II (12 doors) is a compact option when you need 12 boxes and plan to add outdoor parcel lockers separately.
How many parcel lockers do you need?
USPS's planning guide for builders, Handbook PO-632, and the Postal Operations Manual (632.622) require at least one parcel locker for every five mailbox compartments in new or remodeled apartment buildings, with no maximum. PO-632 sets no fixed ratio for CBU sites in single-family communities; it only says the centralized boxes should include secure parcel lockers. We recommend planning to the same 1:5 ratio and confirming the number with your growth manager before you finalize the layout.
Look at the table above: a Type III alone gives one locker per eight mailboxes, and a Type IV one per thirteen. That is why most projects pair CBUs with outdoor parcel lockers (OPLs). An OPL is a pedestal unit with only parcel compartments, each 12" W x 19-3/4" H x 15" D, so they also take larger boxes than the lockers inside a CBU.
- OPL Type I: 2 compartments, 16" W x 62" H x 18" D (Salsbury 3302).
- OPL Type II: 4 compartments, 30-1/2" W x 62" H x 18" D (Salsbury 3304), such as the Salsbury Outdoor Parcel Locker with 4 Compartments.
Example: a 48-home subdivision
- Three Type III units give 48 mailboxes and 6 parcel lockers.
- Planning to 1:5, 48 mailboxes need 10 parcel lockers (48 ÷ 5 = 9.6, round up).
- Add one 4-compartment OPL Type II: 6 + 4 = 10 parcel lockers.
- Result: four pedestals on the pad, 48 mailboxes, 10 parcel lockers. Leave a few spare compartments if the community will keep growing.
For more on how residents use the lockers, see what is a parcel locker and how does it work.
CBU vs 4C mailboxes: which fits your property?
Both are USPS centralized delivery equipment, but they suit different sites.
| Cluster box unit (CBU) | USPS-STD-4C mailbox | |
|---|---|---|
| USPS standard | USPS-B-1118 (licensed design) | USPS-STD-4C |
| Where it goes | Outdoors, on a pedestal and concrete pad | Recessed in a wall (surface-mount and free-standing enclosures are also sold), indoors or on an exterior building wall |
| Typical property | Single-family subdivisions, townhomes, office parks | Apartments, condos, mailrooms, mixed-use buildings |
| Configurations | Six fixed types (I to VI) | Built to order by columns and rows |
| Weather | Built to stand outside | Rated for indoor or outdoor use; USPS requires a canopy and night lighting for exterior installations |
USPS requires all new or remodeled apartment buildings to install approved 4C equipment, per PO-632. USPS-STD-4B+ boxes, horizontal or vertical, are approved only for existing installations, as replacements for old 4B boxes. For a side-by-side look, read 4C vs 4B+ vs cluster mailboxes.
Standard vs Regency decorative CBUs
Salsbury makes every CBU type in two looks. The internals, door sizes and USPS license are the same. The difference is the trim.
- Standard CBUs have the plain aluminum cabinet and pedestal. Finishes are sandstone, bronze, green, black and white. Postal gray is sold for replacement units only.
- Regency decorative cluster mailboxes add a die-cast aluminum decorative cap and pedestal cover. The Regency Type III measures 31" W x 71-3/4" H x 18-1/2" D. The extra height comes from the cap, which is worth checking against any canopy or kiosk roof you plan.
HOAs with architectural guidelines usually choose Regency to match street lights and signage. Builders working to a budget usually choose standard units. Both pass the same USPS approval.
Pedestals, concrete pads and mounting
Every Salsbury CBU and OPL ships with its pedestal. The pedestal height varies by type so that the finished unit is 62" tall:
- Types I, II and V use the tall pedestal (28-1/2", Salsbury 3395).
- Types III, IV, VI and both OPLs use the short pedestal (14-1/2", Salsbury 3385).
Pedestals bolt to a concrete pad. CBUs are not set in the ground like a residential post. Salsbury offers two anchor kits: an expansion wedge anchor kit (3393) for existing concrete and a J-bolt kit (3394, four 1/2" x 8" bolts) cast into new concrete.
Salsbury's installation instructions call for reinforced concrete with a compressive strength of 3,000 psi at 28 days, with pad thickness from 4" to 8" depending on the pad size in their table. An existing pad you reuse must be at least 48" wide, undamaged, and have anchors that match the bolt pattern. Plan the pad for every unit you install now and any you expect to add, and call 811 before you dig.
For the full pedestal story, including replacements and decorative covers, see CBU pedestals explained.
Mailbox kiosks and shelters
A mailbox kiosk is a roofed shelter over a row of CBUs. It keeps residents dry while they collect mail and gives the HOA a spot for a notice board and lighting. Kiosks are built locally to your design. Confirm the clear height under the roof against the unit height (62", or about 72" for Regency) and leave room for the carrier to swing the master doors fully open.
Site planning: where the CBUs can go
USPS approves the site, not only the hardware. PO-632 section 3-5.1 says the local growth manager or designee must approve the CBU sites and type of equipment, and that a mailbox is typically within one block of the residence. When you pick a location, think about:
- Carrier access: the carrier needs a safe place to stop. A pull-off or curb cut beside the pad is common.
- Resident access: a sidewalk to the pad, lighting for winter evenings, and accessibility under your local building code (PO-632 defers to local codes on this).
- Clearance: space in front of the units for the master doors to open fully and for residents to stand clear of traffic.
- Drainage: keep the pad out of low spots and away from sprinkler throw.
- Growth: leave pad length for future units if the community is built in phases.
How to buy and install CBUs: step by step
- Contact USPS during design. Ask your local post office for the growth manager or the person who handles new delivery. Get the mode of delivery confirmed in writing.
- Count delivery points. Include every home, plus amenity buildings or offices that will receive mail.
- Choose the CBU types and OPLs using the table and ratio above.
- Get site and equipment approval from USPS before you order.
- Request a quote with your unit count, types, finish and delivery address.
- Pour or prepare the pad to the manufacturer's instructions and let it cure.
- Install the pedestals and units with the anchor kit, level and bolted.
- Ask USPS to install the master lock. USPS supplies its own master access lock, which developers cannot pre-install (PO-632 section 3-3). Your growth manager coordinates it.
- Hand out keys. Each customer door comes with a lock and three keys. Under PO-632 the builder or property owner provides lock and key service for CBUs, so keep a key log.
USPS access vs private access units
Every CBU we list is a USPS Access unit: the carrier door is ready for the USPS master lock. If the mail is delivered by a private staff member instead of USPS, such as on a gated campus or a military base, Salsbury fits a master commercial lock (part 3375) instead. Tell us which you need when you ask for a quote, because the lock is factory-installed.
Common mistakes on CBU projects
- Ordering before USPS approval. A growth manager can ask for a different location, more parcel lockers or a different mix of types. We have seen this on small jobs too: a builder ordered six post-mount mailboxes for a new project, then learned from the post office that the site needed multi-family units.
- Too few parcel lockers. Packages that do not fit go back to the post office, and residents blame the HOA.
- A pad sized for today only. Breaking out and re-pouring a pad in phase two costs more than pouring it longer the first time.
- Ignoring door-swing clearance. Master doors need room to open fully, especially inside a kiosk.
- Choosing postal gray for new construction. Salsbury sells gray for replacement units only.
Get a quote for your cluster mailbox project
We supply complete CBU projects: standard and Regency units, outdoor parcel lockers, pedestals, anchor kits and door labels. Send us your unit count and we will quote the full project. Request a quote and include the number of homes, any USPS requirements you have been given, and your preferred finish.
CBU questions from HOAs and builders
Do you offer trade pricing on CBUs?
Volume pricing is shown on our site. For larger orders, generally 10 units or more, we can issue a custom quote, and the price can depend on the shipping location.
How many mailboxes are in a cluster box unit?
Standard CBUs hold 4, 8, 12, 13 or 16 customer mailboxes depending on the type (I to VI), plus one to four parcel lockers. For larger communities you install several units side by side on one pad.
Who has to approve a new cluster mailbox?
Your local USPS growth manager or postmaster approves both the location and the type of equipment. Get that approval during design and before you order, because USPS can ask for changes to the site or the mix of units.
Do cluster mailboxes need a concrete pad?
Yes. CBU pedestals bolt to concrete with anchor bolts. Salsbury calls for 3,000 psi reinforced concrete, 4" to 8" thick depending on pad size. An existing pad can be reused if it is at least 48" wide and in good condition.
What is the difference between CBU types?
The type defines the number and size of doors and parcel lockers. Type III has 16 doors and 2 lockers, Type IV has 13 taller doors and 1 locker, and Type V has 4 double-height doors and 2 lockers.
Can we choose a color other than gray?
Yes. Salsbury CBUs come in sandstone, bronze, green, black and white. Postal gray is sold for replacement units only. Confirm the color with your HOA guidelines and local post office before ordering.
Who installs the lock the mail carrier uses?
USPS does. The master access lock belongs to USPS and is not available for builders to pre-install. Once the units are bolted down, contact your growth manager or local post office to schedule it.
******It is always recommended to contact your local postmaster prior to installation. While most of our mailboxes are USPS-approved your local postmaster may have specific preferences in terms of mailbox types and installation location******