Redundant PLC enclosure design: split the racks, not the rating

Short answer: build a redundant PLC pair as two enclosures, not one. Put the A rack and the B rack in separate boxes, feed each from its own breaker and its own pair of 24 V DC supplies, wire both DC inputs of every switch, and leave 30% of the DIN rail empty in each box. One box, one breaker, one supply or one ring cable can then fail without stopping the process. Keep the rating whole on both halves: each box carries the full rating its room calls for, because a pair never adds up to one. The worked example below sizes a rack that fills 1.0 m [39.4 in] of rail into a 20 x 32 x 10 in box; count your own rail before you order.

What each box has to survive alone

The split exists so no fault outside the process takes both racks at once. Write the fault list before the layout:

• a feeder breaker trips, or someone switches it off to work
• one 24 V DC supply in a box dies, or its ORing module fails
• a UPS set drops its load
• the room a box stands in loses power, floods or burns
• a ring cable between the boxes is cut or crushed

The controller pair already handles its own internals: the two chassis hold two copies of the process data, synchronize over dedicated cabling, and the rack you power up first becomes the primary, as the redundancy system's user manual describes. Your job is to keep that pair's power, network and enclosure paths independent. The redundancy stops at the field: a single photocell, control valve or motor feed stays single. Mark that boundary on the drawings so nobody credits the pair with more than it carries.

What you need per enclosure

Two mirrored boxes and two of everything each rack needs. Per box:

• an enclosure sized to the layout with 30% spare DIN rail (the worked example below sizes it)
• two 24 V DC DIN-rail supplies, each able to carry the full box load alone
• a two-input ORing or redundancy module, or a DC-UPS module where the spec sets a ride-through time
• a network switch with both DC power inputs brought out to terminals
• its own feeder breaker or fuses and terminals for a feed nobody else shares
• 35 mm [1.4 in] top-hat rail and wiring duct sized to the 50% fill figure

Box B repeats the list unchanged. Mirrored drawings make the second quote, the build and the spare-parts shelf simple.

Build each box in eight steps

1. Draw both boxes from one layout. List what each rack carries alone: the controller, its network modules, two 24 V DC supplies, the ORing module and its own switch. A mirrored pair of drawings makes the second box, the quote and the spare-parts shelf simple.
2. Lay out the rail with 30% spare. Two runs beat one long run in a wall-mount box; the worked example below sizes a rack whose drawn rail is 1.0 m.
3. Feed each box from its own breaker. Take box A and box B from different panels, different UPS sets or different rooms, isolating the feeds the way two power sources in one cabinet are isolated.
4. Fit the supplies and the ORing module. Orient the output diode with the current flow, as DC-UPS application notes instruct. A silicon diode drops about 0.7 V at load, so set the output trim to suit. A DC-UPS module carries its own battery set; its instructions set the charging temperature and the ventilation the cabinet gives it. Pick the pair with a DIN rail power supply selection guide rather than by wattage alone.
5. Wire both DC inputs of the switch from the box's ORed bus. A switch fed once is the single point that undoes everything else on this list.
6. Ring the network. Put both boxes' switches and the remote drops in one device level ring (DLR), the topology that heals any single break or node loss; a DIN rail enclosure for network switches shows the switch hardware and its clearances. Beyond 100 m [328 ft], the copper Ethernet span, run fiber between the rooms.
7. Land the field cables through glands matched to the box rating, size the grounding conductors per the next section, and label A and B at the door, the breaker and the terminals.
8. Energize rack A completely, then rack B. The rack powered first becomes the primary, so the switching order fixes which box leads.

Worked example: one rack with 1.0 m of drawn rail in a 20 x 32 x 10 in box

Inputs and where each comes from

• Rail the rack fills as drawn: 1.0 m [39.4 in]. Example input; count yours from the layout drawing.
• Spare rail: 30%. The allowance this guide builds to.
• Rail profile: 35 mm [1.4 in] top-hat DIN rail.
• Wiring duct: 2 x 2 in [50 x 50 mm], kept half full or less. Duct fill tables cap at 50% of the interior cross-section, citing NFPA 79-2012 section 13.5.2, Percentage Fills of Raceways (Ducts); the backplate and DIN rail layout guide carries the fill method.
• Layout margins: 2 in [50 mm] top and bottom, 2 in between each rail and its duct, 3 in [75 mm] between the two rail groups, 2 in at each end of a run. The guide's layout rules.
• Depth inputs: deepest module 4 in [100 mm], read off your datasheet, and 1.5 in [40 mm] clear between duct and door, by the same method the enclosure size for DIN rail devices article walks through.
• Catalog limits: standard stainless wall-mount boxes run from 6 x 4 x 4 in up to 60 x 36 x 16 in (H x W x D) in the size finder; custom boxes are cut from one piece at any size.

The rule

Fitted rail equals drawn rail times 1.30. A rail run fits a box when the run plus 4 in of end margins is inside the interior width. Outside size equals the interior stack plus about 2 in per dimension for walls, door and backplate; confirm the true interior on the drawing before you approve it.

The arithmetic

• Fitted rail: 1.0 m x 1.30 = 1.30 m [51.2 in] of rail per rack.
• One run needs 51.2 + 4 = 55.2 in of interior width. The widest standard stainless box gives about 34 in, so one run is out. Split it: 1.30 m / 2 = 0.65 m [25.6 in] per run.
• Width: 25.6 + 2 + 2 = 29.6 in interior, so order 32 in outside.
• Height: 2 + 2 + 1.4 + 2 + 3 + 2 + 1.4 + 2 + 2 = 17.8 in interior, so order 20 in outside.
• Depth: 4 + 2 + 1.5 = 7.5 in interior, so order 10 in outside.

Result and decision

Two custom boxes at 20 x 32 x 10 in (H x W x D), one per rack, ordered at any size from one piece with the cutout set each rack needs at the enclosure builder. The threshold: a single run fits the widest standard stainless box up to 30 in [762 mm] of run, which is 0.58 m [23 in] of drawn rail at the 30% rule. Under that, buy the catalog size; over it, split the rail into two runs as here or order the custom size. Custom boxes are delivered in 10 business days, counted from your approval of the drawing.

Bonding, working space and mounting heights

Size each equipment grounding conductor from the rating of the breaker ahead of it, using NEC Table 250.122; the grounding and bonding guide carries the full method. Bond the enclosure, backplate and raceways so they can carry the fault current imposed on them, as NEC 250.96(A) calls for, and strip paint from threads and contact surfaces at grounding connections per NEC 250.12.

Leave working space in front of both boxes. NEC 110.26(A) (2020 and 2023 editions) sets at least 3 ft (900 mm) of depth for 0-150 V to ground in Condition 1, 30 in of width or the equipment width, and a 90 degree door swing, per Table 110.26(A)(1); the enclosure sizing guide works that space into a layout. Mount so the highest breaker handle inside sits at most 2.0 m (6 ft 7 in) above the floor, per NEC 240.24(A) and the enclosure mounting guide. Then land the feeds dead: lock out, verify absence of voltage, and torque the lugs to the marking.

Keep the full rating on both halves

Split the racks, never the rating: each box carries on its own the Type its location calls for, and two half-rated boxes do not make one rated pair. Our stainless boxes are manufacturer-rated NEMA 4X / IP66 for the specified design and are not UL listed, and in US installations NEC 110.28 says an IP code does not carry a US enclosure type specification, so order by the NEMA type. Choose 304 stainless for indoor washdown and most outdoor sites, 316L for salt air, brine and chlorides.

Every hole keeps the figure. Size each cutout to its gland as the cutouts, glands and windows guide sets out, and mount through the feet so the wall side stays dry, as mounting a weatherproof box explains. The weakest part rules the assembly: a gland or accessory rated below the box drags the whole box down to its own number.

Commissioning checks that prove the split works

1. Energize rack A fully, then rack B, and confirm the HMI shows which rack leads.
2. Trip box A's feeder breaker with the process running. The pair swaps leadership; restore the breaker and watch the backup flag clear.
3. Kill supply A1 at its own breaker, then supply A2. The 24 V bus holds through the ORing module both times.
4. Unplug one ring cable between the boxes. The ring heals; note the recovery time the switches report against the process's tolerance.
5. Pull one DC input of a switch. The switch stays up, which proves both inputs were wired and live.
6. Run the ride-through test to the figure the spec sets; one plant's spec carried 30 controllers, every switch and the servers for 30 minutes.
7. Close both doors and check each gasket sits proud and even after the fault tests.

Mistakes that cost the redundancy or the rating

• One feeder for both boxes. The trip you built the split to survive now drops both racks at once; give each box its own breaker and its own path back to the source.
• Both switch inputs behind one fuse. Land them on separate branches so a blown fuse leaves one input live.
• A shared raceway between the rooms. Fire or a fallen tray takes both cable routes together; keep the paths apart the way the feeds are.
• Two supplies, doubled heat. Check the box's heat balance with both supplies running, since a supply near full load dissipates per its datasheet and two of them warm a compact box fast; the nested enclosure heat buildup article works an example.
• An accessory below the box rating, or an open drain on a sealed box. Drains belong on Type 3R boxes; on a Type 4X box an open drain is just a hole.
• Crediting the pair with more than it carries. Field instruments, valves and motor feeds outside the two boxes stay single unless the design says otherwise.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions