Marshalling cabinet sizing from an I/O list. The rules that survive the field.

The PLC card-sizing math gets you to a card count and a CPU rack count. The cabinet count is a separate calculation that depends on terminal density, zoning, and the marshalling philosophy the owner has standardized on.

Marshalling cabinets versus system cabinets

These terms are sometimes used interchangeably, which causes confusion at the panel-builder scope stage. They are different things.

A marshalling cabinet, also called a junction cabinet or field-termination cabinet is the controlled disconnect point between field cabling and the PLC system. Field instruments run their cables to the marshalling cabinet. Terminal blocks provide the physical landing and labeling point for each conductor. Cross-wiring or a pre-built trunk cable then runs from the marshalling cabinet to the PLC system cabinet.

A system cabinet, also called a PLC cabinet or controller cabinet houses the CPU, I/O cards, power supplies, and communications modules. The PLC program runs in the system cabinet. No field cables terminate directly in the system cabinet on a two-tier marshalling architecture. The system cabinet sees only the trunk cables from the marshalling cabinet.

The distinction matters for the bid because.

• Marshalling cabinets are panel-builder work, structured cabling, terminal blocks, fuses, barriers. System cabinets are PLC vendor work or panel builder work from a vendor hardware kit. The cost-per-cabinet differs substantially.
• Some house standards define a 600 mm depth for marshalling cabinets and 800 mm depth for system cabinets. These cannot be swapped at FAT without rework.
• IS barriers live in the marshalling cabinet. If you size the system cabinet for IS protection, because you assumed built-in-IS cards, your panel layout will not match the design.

What goes in a marshalling cabinet

A marshalling cabinet contains.

• Terminal blocks for every field signal, one per conductor, so a two-wire 4-20 mA loop takes 2 terminals. A four-wire transmitter takes 4.
• Fuses on power-carrying terminals. 24 VDC discrete outputs and loop-powered transmitters in most cases.
• Surge protection devices on field-side conductors, depends on the operating company's lightning standard.
• Intrinsic-safety barriers on hazardous-area channels.
• Cross-wiring to the PLC cabinet via a discrete cable bundle.

Size each row of the I/O list to the terminals it consumes, sum them, divide by terminals per cabinet, round up. That is the cabinet count for the plant.

Terminals per channel

For a typical signal.

Single-tier is the lighter-weight design. Two-tier is the operating-company-friendly design that the major IOC and EPC standards converge on. Hazardous-area service, Zone 1, Zone 2 in IEC parlance, Class I Division 1, 2 in NEC drives the IS adder column. Plants with majority hazardous-area instrumentation can run 1.5-2x the terminal count of a notionally similar non-classified plant.

Terminal block density. What the rail actually holds

Terminal blocks come in several widths. The most common in process plants.

• **Phoenix Contact or Weidmuller 6 mm terminals, standard spring-cage. ** approximately 40-48 terminals per DIN rail meter. On a 2000 mm tall cabinet with 10 usable rail rows and 800 mm usable rail per row, that is roughly 320-384 terminals before accounting for fuse blocks, label carriers, and end stops.
• **10 mm terminals, used for 24 VDC power, fused outputs. ** approximately 28-32 per rail meter. Fuse-terminal combos, Weidmuller SAKG, Phoenix PT series run similar density but add the fuse-access height above the rail.
• **IS Zener barriers, MTL 700 series, Pepperl and Fuchs Z787. ** approximately 15-20 barriers per rail meter. Each barrier handles one channel but is 30-40 mm wide. This drives IS-heavy cabinets to double or triple the rail length of a comparable non-IS cabinet.
• **IS galvanic isolators, P and F KF series, MTL 5000. ** similar width to Zener barriers plus an additional power-supply bus rail for isolated power.

A practical terminal-density budget for a well-managed 800x800 mm cabinet is 200-250 usable signal terminals, accounting for mandatory group fuses, end brackets, cable duct, and door-internal cable routing. A 1200x800 mm cabinet reaches 350-400 usable terminals on a well-populated panel.

These numbers assume you do not use the cabinet for intermediate relays or power distribution. Relay banks, MCBs, and UPS modules consume rail space fast. If the cabinet doubles as an auxiliary relay cabinet, reduce the usable-terminal budget by 20-30 percent.

Cabinet count math

Once terminal counts are summed, the formula is straightforward. If the I/O list is not yet structured with area-classification and marshalling-tier columns, the I/O list creation guide covers how to set those up before running the cabinet math.

Required cabinets = ceiling(total_terminals / terminals_per_cabinet) + spare_cabinets

Terminals per cabinet depends on cabinet size and house convention.

• **600x800 mm. ** tight, 150-200 terminals plus fuses. Fine for small skids. Rare in mainline plants.
• **800x800 mm. ** workhorse. 200-300 terminals plus fuses. Most North American plants standardize here.
• **1000x800 mm or 1200x800 mm. ** 350-450 terminals. European preference for larger cabinets.
• 2200 mm tall is standard cabinet height in both regions.

Spare cabinets sit at 10-15 percent on top of the math. The reasoning is identical to the spare-channel argument at the card level. Brownfield-style growth during construction and operations consumes spares whether you bid them or not.

SIS vs. BPCS separation

IEC 61511 requires the safety instrumented system to be physically independent of the basic process control system. Most operating companies translate this into.

• Separate PLC cabinets for SIS and BPCS.
• Separate marshalling cabinets for SIS and BPCS.
• Separate field cabling for SIS and BPCS instruments. A single field instrument is wired to one or the other. Dual-use is a 61511 non-conformance.

The cabinet-sizing math runs as two parallel calculations. One for BPCS scope, one for SIS scope. The SIS subset on a petrochemical plant lands somewhere in the 10-25 percent range of total instrument count. Bid SIS cabinets at a higher per-cabinet build cost because the certified hardware is more expensive and the documentation discipline is stricter.

Small plants occasionally share a cabinet with an internal divider. Some auditors accept it if the separation is documented and labeled. Some do not. Consult the operating company's safety philosophy document before assuming a shared cabinet is acceptable.

Intrinsic-safety zoning

Hazardous-area instrumentation needs IS protection. Two approaches.

• Zener barriers at the marshalling cabinet, with grounding requirements that drive a separate IS earth bus.
• Galvanic isolators that handle isolation without a dedicated IS earth.

Both consume terminal space. A typical Zener barrier takes 1.5x the terminals of a non-IS channel. A galvanic isolator takes 2x. Plants with significant Zone 1, Zone 2 instrumentation should bid 20-30 percent more cabinet count than the same plant in unclassified service.

The trap. Bid engineers count instruments off the P&ID without flagging the area classification. The hazardous-area flag is on a separate area-classification drawing or the equipment list. Cross-reference before locking the cabinet count.

Worked example. 200 I/O points, mixed IS and non-IS

A process skid with 200 I/O has the following composition after I/O list extraction and classification.

The plant uses two-tier marshalling and galvanic isolators for IS channels. Terminal count.

**Non-IS channels, two-tier, no IS adder. **

**IS channels, two-tier plus galvanic isolator adder of plus 2 terminals each. **

Grand total. 440 plus 540 980 terminals.

Using 800x800 mm cabinets at 250 usable signal terminals each.

Required cabinets = ceiling(980 / 250) = 4 cabinets
Spare (10%): +1 cabinet
Total: 5 marshalling cabinets

If the same 200 I/O had been all non-IS single-tier, 2 terminals each, 800x800 cabinet at 300 usable terminals.

200 x 2 = 400 terminals
ceiling(400 / 300) = 2 cabinets + spare = 3 cabinets

The IS and two-tier architecture more than doubles the cabinet count on this example. That is the materiality of the IS-zoning assumption in the bid.

Fuse and power distribution density

A fuse block or terminal fuse combination, Weidmuller SAKS 6.2A, Phoenix PT 6-FSI series takes the same rail space as 1.5 standard terminals per fuse position. Operating company standards vary widely on the fusing scheme.

• **Shared group fuse per 8 outputs. ** common in North American plants. Fewer fuse positions, tighter terminal count.
• **Individual fuse per output channel. ** required by some European operating companies for discrete outputs to motor starters and solenoid valves. Doubles the fuse terminal count for DO channels.
• **Loop-power fuse per AI transmitter. ** required by some standards for loop-powered 4-20 mA transmitters on the 24 VDC supply rail. Adds 1 fuse terminal per AI loop-powered channel.

Identify the fusing scheme from the operating company's I&C standard before finalizing the terminal count. A switch from group-fused to individually-fused outputs on 30 DO channels adds roughly 30 fuse terminal positions, which on an 800x800 cabinet is a material fraction of the usable capacity.

Things that bite

Fuse-per-channel rules. Some operating company standards require an individual fuse per discrete output channel, plus a fuse on every loop-powered transmitter. That doubles the terminal count if you assumed shared fuses.

SPD scope. Surge protection devices for lightning protection consume terminal space and increase the per-cabinet hardware cost. Check the operating company's lightning standard before bidding.

Multi-core cable runs. Some panel builders standardize on 12-core or 24-core multi-core cables between marshalling and PLC cabinets. The per-cable terminal count is fixed. If your channel count is not a multiple of the multi-core size, you spend cores on spares whether you wanted to or not.

Cabinet HVAC. Large hot plants run cabinet air conditioning. Cabinet count includes the AC enclosures. AC enclosures carry a different price-per-cabinet than a stock instrument cabinet. Check whether the bid scope includes AC. Note that AC units also consume wall space inside the cabinet, reducing usable rail space on the side walls.

Label carriers and end brackets. Every terminal row needs end brackets and usually a label carrier above the terminal strip. On a dense cabinet, these consume 10-15 percent of the rail length. Include them in the usable-terminal budget reduction.

A practical setup

The terminal-count math wants to live in the I/O list itself, not in a separate cabinet-sizing spreadsheet. Add the columns once and the cabinet rollup falls out of a pivot table whenever the count changes. The I/O list template ships with the columns pre-built. Tagsight extraction produces an I/O list those columns can feed off directly. For the next step downstream, From I/O List to Wiring: Termination Schedules and Marshalling covers how each channel row becomes a cable, a terminal, and a card pin.

Get the spare-capacity rule, the marshalling philosophy, the IS zoning breakdown, and the fusing scheme confirmed in writing before bid lock. The math takes care of itself after that.

FAQ

What is the typical terminal density per marshalling cabinet.

200-400 terminals depending on cabinet width, panel-builder convention, and whether the cabinet hosts only marshalling or also intermediate relays and power distribution. An 800x800 mm cabinet holds 200-250 terminals plus space for fuses. 1200x800 mm gets you to 400 if you accept tight cable management. Your house standard already specifies. Follow it.

Do SIS and BPCS share marshalling cabinets.

Strictly. No. IEC 61511 requires physical separation, which most operating companies extend to separate marshalling cabinets in addition to separate PLC cabinets. Pragmatically. Small plants with light SIS scope sometimes share a cabinet with a divider. Auditors do not love it but it is not always a non-conformance if the separation is documented.

What is the difference between single-tier and two-tier marshalling.

Single-tier. Field cable lands directly on the PLC card terminal. Two-tier. Field cable lands in a marshalling cabinet, gets cross-connected to the PLC cabinet via a discrete cable bundle. Two-tier doubles the terminal count and adds the marshalling cabinet to the panel scope. It pays back during plant changes because the controlled disconnect point is in the marshalling cabinet, not at the PLC terminal.

How do intrinsic-safety zones affect cabinet sizing.

IS barriers, Zener or galvanic add per-channel hardware that consumes terminal space. Each IS-protected channel takes 1.5-2x the terminal space of a non-IS channel. Plants with extensive Zone 1, Zone 2 hazardous-area instrumentation see noticeably higher cabinet counts than the same plant footprint in non-classified service.

When should I add a spare cabinet rather than just spare terminal space.

Add a complete spare cabinet when the plant is brownfield, likely to have scope additions during construction or when the operating company's standard requires a minimum of one fully empty cabinet per marshalling row. Spare terminal space within existing cabinets absorbs small additions. Whole spare cabinets absorb the instrument additions that come in during as-built reconciliation on complex brownfield jobs.