PLC card sizing from a P&ID. The math, the rules, and what to bake into your bid.

Once the I/O count off the P&ID set is locked, card sizing is mechanical. The math is simple, the trap is that the simple math hides three or four assumptions that have to be right or the cabinet does not fit.

This is the procedure for every bid. You can run it in Excel or run it through extraction tooling that produces the rollup directly. Either way, the math is the same and the assumptions are the same.

The base calculation

Take the I/O count from the bid drawing set, broken out into AI, AO, DI, DO. Apply the spare-capacity uplift. Divide by channels per card, rounding up. That is the card count.

Required AI cards = ceiling((AI count * (1 + spare_uplift)) / channels_per_AI_card)

Same for AO, DI, DO. Sum the cards, group them into racks, group the racks into cabinets. That is the cabinet count. Multiply by the per-cabinet build cost and you have the panel scope of the bid.

The structured math is unforgiving on three numbers. The I/O count, covered in How to estimate PLC I/O count, the spare-capacity uplift, and the channels-per-card divisor. Each one shifts the cabinet count. The upstream step of organizing the instrument list into a structured, column-complete format is covered in the I/O list creation guide.

Spare-capacity uplift

20-25 percent is the conventional greenfield number. The math. Roughly 2-3 percent gets used during commissioning for field changes the as-built P&ID picked up, another 5-10 percent burns over the life of the plant for instruments added during operations, the remainder is genuine spare for future scope.

Brownfield jobs need more. 30-40 percent is the working number on a retrofit because the as-built drawings are wrong in a non-trivial fraction of cases and the field walk during commissioning will surface new tags. If you bid brownfield at the greenfield uplift you will run out of channels at FAT.

The trap is that the uplift applies at the card level, not the channel level. If you have 105 AI tags and you bid 8-channel cards, the math is ceiling(105 * 1.25 / 8) = 17 cards = 136 channels, which gives you 31 spare channels, 30 percent uplift on the count, 23 percent uplift on the channels. The card-level granularity always rounds up. The actual spare capacity is higher than the spare-capacity uplift suggests. Some house standards add a separate "spare cards" rule on top of channel uplift. Check whether yours does before double-counting.

A common rule breakdown by signal class.

AO and DO carry lower spare ratios because control valve and on-off valve additions require mechanical as well as electrical work and tend to go through formal MOC. DI additions, clipping a status contact into an existing circuit are more informal and accumulate faster.

Channels per card. Vendor-by-vendor reference

This is vendor-dependent and SKU-dependent. The numbers below cover the platforms that appear most often in greenfield and brownfield migration scope.

Siemens S7-1500 and ET 200SP

The S7-1500 CPU rack takes standard S7-1500 I/O modules. Distributed I/O on PROFINET uses ET 200SP BaseUnits with plug-in I/O modules.

ET 200SP modules are smaller and snap onto a BaseUnit rail. The same signal classes apply. Module density is the same but the form factor is narrower, which matters for cabinet depth. ET 200SP is the dominant distributed I/O choice on new Siemens PROFINET installations.

Rockwell ControlLogix 1756 and Compact 5069

ControlLogix uses 13- or 17-slot chassis. The 5069 CompactLogix line uses a smaller chassis with fewer slots but compatible module families.

For Rockwell L5X export from the P&ID extraction, the module types above match what Studio 5000 imports. When the tag table is generated from the I/O list, the module-type field carries the part number. Slot assignments are made in Studio 5000 after import.

Schneider Electric Modicon M580

The M580 is the current mainstream Schneider platform for medium-to-large process applications.

Schneider's safety line, Modicon Quantum Safety carries 4-channel AI and 8-channel DI for SIL 2 service, consistent with the other vendors.

Generic IEC 61131-3 estimate

When the vendor is not specified at bid time, use these defaults.

These match the conservative end of what all major vendors offer. If you later learn the owner has standardized on 16-channel AI cards, recalculate. The bid letter should note which channel densities were assumed.

Intrinsic-safety effects on card count

Instruments in hazardous areas, IEC Zone 1, Zone 2, NEC Class I Div 1, Div 2 require intrinsic-safety protection. IS affects card sizing in two ways.

IS-rated input cards. Some vendors offer AI cards with built-in IS barriers per channel. These typically run at half the density of a standard AI card, 4 channels instead of 8 and cost more per channel. Siemens F-AI and Rockwell 1756-IF8I are in this category. If your IS instrumentation uses these cards, the AI card count for IS channels doubles relative to the standard card count.

External IS barriers. If you use Zener barriers or galvanic isolators in a separate marshalling cabinet, see Marshalling cabinet sizing, the PLC card can be a standard non-IS card. The IS protection happens at the barrier, not at the card. In this case, card count is unchanged but marshalling cabinet size increases.

The choice between built-in-IS cards and external-barrier architecture is a project-level decision driven by the operating company's standard. Most large petrochemical and refining owners have standardized on one approach. Confirm before bid lock.

Redundancy effects

Redundant CPU configurations, 1oo2 or 2oo3 SIS architectures, hot-standby BPCS CPUs do not always double the I/O card count. The typical approach.

• **CPU redundancy, hot standby. ** Both CPUs share the same I/O cards. Card count does not change. CPU cost roughly doubles.
• **Full duplex I/O, SIL 3 voted systems. ** Each voting leg has its own card. A 1oo2 voted analog input needs two AI cards carrying the same signal. Card count doubles for voted channels.
• **Partial redundancy. ** Critical loops run redundant cards. The rest run standard. Card count increases for the redundant subset only.

Identify which loops require voted I/O before running the card-sizing math. This information comes from the safety requirements specification or the preliminary HAZOP, LOPA outcome, not from the P&ID alone. The P&ID may mark SIS-tagged instruments, but the voting architecture is in the SRS.

Worked example. 4-page P&ID set, 80 instruments

A mid-size gas processing skid with four P&ID sheets yields the following instrument count after extraction and review.

• AI. 32, flow transmitters, pressure transmitters, temperature transmitters
• AO. 8, control valves
• DI. 28, position switches, motor status, manual call points
• DO. 12, on-off valves, motor starters
• Total. 80 instruments

Of the 80, the HAZOP identifies 12 SIS instruments, 8 AI on safety-critical loops, 4 DI on ESD triggers. These run on a Triconex TMR SIL 3 logic solver, separate from the BPCS.

**BPCS card sizing, Siemens S7-1500, standard density. **

Nine cards fit in a single S7-1500 rack with room for the CPU, power supply, and communications modules. One 24-inch cabinet covers the BPCS.

**SIS card sizing, Triconex TMR, SIL 3 rated. **

Triconex TRICON uses a 1oo3 voting architecture at the module level. Each AI input card carries 8 channels across 3 legs, the chassis handles the voting internally. For 8 SIS AI channels, 1 TRICON AI module is sufficient, each module provides 8 channels with internal TMR voting. For 4 SIS DI channels, 1 TRICON DI module. Add 30% SIS spare. Still 1 module each.

Two Triconex modules plus the main chassis. One separate SIS cabinet, per IEC 61511 physical separation requirements.

**Total panel scope. ** one BPCS cabinet, one SIS cabinet.

If the SIS instruments had been run on the BPCS cards instead, which is non-compliant, the BPCS card count would be the same but the SIS separation requirement would not be met. The two-cabinet split is the IEC 61511, ISA 84 baseline.

Cabinet and rack-level sizing

Once card count is locked, group cards into racks. A typical Siemens S7-1500 rack accepts 8-12 I/O cards plus the CPU and power supply. A typical Rockwell ControlLogix 13-slot chassis accepts 11-12 I/O cards, reserving 1 slot for the CPU and 1 for power. Round up to whole racks per zone of the plant.

Each rack lives in a cabinet. A 24-inch enclosure typically holds 1-2 racks plus terminal blocks. A 36-inch enclosure holds 2-3 racks. Cabinet count is your terminal-density rule applied to the channel count. Usually 200-400 terminals per cabinet, depending on whether the panel builder routes terminals at the back or only the front.

Multiply cabinet count by per-cabinet build cost, your panel builder gives you this number on a project-by-project basis and that is the panel scope on the bid.

Things that bite at FAT

HART overlay scope changing late. Owner decides three months into the project to put HART on every AI channel. AI cards re-spec from 16-channel non-HART to 8-channel HART, channel density drops, cabinet count goes up. Write the HART scope assumption into the bid letter before the bid locks.

SIS scope appearing where you assumed BPCS. The owner's safety review reclassifies a few loops to SIS scope late in engineering. Each reclassified loop moves from a BPCS card, cheap, dense to a SIS card, expensive, sparse. Net effect on the bid. A few percent margin disappears. Build a small contingency for this.

Voting hardware misclassified. A 1oo2 redundant pair counted as one transmitter at bid time becomes two cards-worth of channels at build time. Cross-check your bid count against the equipment list and the safety requirement specification before you lock the number.

Marshalling between field and PLC. Some house standards do single-tier marshalling, field cable goes straight to the PLC card terminal. Others do two-tier, field to marshalling cabinet, marshalling cabinet to PLC cabinet. Two-tier doubles the terminal count and adds a separate marshalling cabinet to the panel scope. Confirm the marshalling philosophy with the owner before bid lock. See Marshalling cabinet sizing for the terminal-count math.

Run the card-sizing math three times on every bid. Once on the napkin during the kickoff call, once on the structured I/O list, Tagsight extracts the count so the column math is already there in the template, and once by yourself before lock. The first two are the math. The third one is where you catch the assumption that does not hold up.

The math is mechanical. The assumptions are the part that bites at FAT.

FAQ

What spare-capacity uplift should I apply to a bid I/O count.

20-25 percent is the conventional baseline for greenfield, 30-40 percent for brownfield where field changes during construction are expected. Your house standard probably defines this. Do not change it without telling your reviewer. The uplift covers spare channels at the card level, not full spare cards. Those get sized separately.

How many channels per card for a typical Siemens or Rockwell platform.

Siemens S7-1500 and Rockwell ControlLogix both run 8-channel and 16-channel AI, AO cards as the dominant SKUs, with 32-channel DI, DO. SIS-rated cards, Siemens F-series, Rockwell GuardLogix typically run 4-channel AI and 8-channel DI for SIL 2-3 service. The card-density math depends on which family you bid against.

Do I need separate cards for SIS and BPCS.

Yes. IEC 61511, ISA 84 prohibits mixing SIL-certified safety scope with regulatory scope on the same card. Separate cards live in separate racks, often in separate cabinets. Card-sizing math runs as two parallel calculations.

What about HART devices on AI cards.

HART-enabled AI cards cost more per channel but provide secondary variables and diagnostics back through the same wires. Whether to bid HART on every AI channel or only on critical loops is a project-level decision. Most operating companies have a standard. Follow theirs.

How do I handle instruments inside packaged equipment skids.

Packaged skids, compressors, air separation units, proprietary heat exchangers typically come with their own local control panel. The skid vendor's panel handles internal I/O. The main PLC connects via a fieldbus interface or a discrete signal summary, run permissive, fault contact. Count only the interface signals in your PLC I/O, not the full internal skid count. Ask the vendor for their signal interface list during the bid phase.