Complete PLC Control Cabinet Components List (BOM Guide 2026)

A PLC control cabinet is far more than a PLC in a box. Between the incoming power supply and the field devices lies a carefully engineered assembly of protection devices, controllers, I/O modules, relays, terminal blocks, and climate control hardware. Specifying the wrong component — whether an undersized circuit breaker or an unprotected power supply — can compromise safety and reliability. This guide breaks down every component category inside a modern PLC cabinet, with a complete BOM example for a typical 64 I/O system.

1. What Goes Inside a PLC Control Cabinet? The Complete Anatomy

A standard industrial control cabinet can be divided into five functional zones:

• Power distribution zone: Main disconnect, circuit breakers, surge protection, power supplies, and UPS
• Controller zone: PLC CPU, communication modules, and (optionally) HMI
• I/O interface zone: Digital/analog I/O modules, signal conditioners, and isolation
• Load switching zone: Contactors, relays, motor starters, and solid-state relays
• Field termination zone: Terminal blocks, cable glands, and wiring infrastructure

Each zone must be physically separated for safety and EMC. At SENTRADO, we use galvanized steel partition plates between the power and I/O zones, maintaining at least 50mm of clearance for heat dissipation.

2. Primary Controller: The PLC CPU

The CPU is the brain of the cabinet. Selection depends on I/O count, program complexity, communication requirements, and redundancy needs. For Siemens-based systems, the three most common choices are:

For OEMs building serial machines, the S7-1200 offers the best price-performance ratio. For process plants or water treatment where uptime is critical, an S7-1500R redundant CPU eliminates single-point-of-failure risk. We cover this decision in detail in our S7-1500 vs S7-1200 selection guide.

3. Power Supply Units

The 24V DC power supply is the most critical support component — its failure takes down the entire control system. Our specifications always include:

• Primary 24V DC PSU: Sized at 1.3× the total calculated load (PLC + I/O + relays + sensors). For a 64 I/O cabinet, that's typically a 24V/10A unit (e.g., Siemens SITOP PSU6200 or Phoenix Contact QUINT4). We always use industrial-grade PSUs with NEC Class 2 compliance.
• Redundant power supply: For critical applications, install two PSUs in redundancy configuration using a redundancy module (e.g., SITOP RED1200). If one PSU fails, the other takes over without interruption. This adds about $300-500 but eliminates the #1 cause of PLC downtime.
• UPS (uninterruptible power supply): A 24V DC UPS module (e.g., SITOP UPS1600 with battery module) provides 10-30 minutes of ride-through during power dips. The PLC uses this time to execute a controlled shutdown, saving retentive data and closing valves safely. For pump stations, a UPS is non-negotiable — uncontrolled stops cause water hammer.

4. I/O Modules

The I/O modules connect the PLC to the physical world. For a typical 64 I/O system:

• Digital input (DI) modules: 16- or 32-channel, 24V DC sinking/sourcing. For S7-1500, we use DI 32x24V DC HF (high feature) modules with channel diagnostics. Each channel can detect wire breaks and short circuits.
• Digital output (DO) modules: 16-channel transistor outputs (0.5A per channel) for solenoid valves and indicators. For higher currents, use relay output modules or interposing relays — never drive contactors directly from PLC DO cards.
• Analog input (AI) modules: 8-channel, 16-bit resolution for 4-20mA / 0-10V signals. Specify isolated modules (channel-to-channel and channel-to-bus) for noisy environments. HART-enabled AI modules are required for smart transmitters in process applications.
• Analog output (AO) modules: 4- or 8-channel for VFD speed references and control valve positioners.
• Specialty modules: Counter modules for flow meters, serial communication modules (RS485/Modbus RTU), and safety I/O modules (F-DI/F-DO) for SIL-rated emergency stop circuits.

5. Circuit Breakers and Protection Devices

Protection devices safeguard wiring and equipment from overcurrent and short circuits. The hierarchy follows a cascading coordination principle:

• Main disconnect switch: A load-break switch (e.g., Siemens 3LD2) rated for the cabinet's full incoming current, with door interlock and lockout/tagout capability. Required by IEC 60204-1.
• Main circuit breaker (MCCB): Provides short-circuit and overload protection for the entire cabinet. For a 63A supply, use a 3-pole MCCB with adjustable thermal and magnetic trips (e.g., Siemens 3VA1).
• Miniature circuit breakers (MCB): Individual branch protection for each major load: PLC PSU (6A C-curve), HMI (6A B-curve), contactors (10A C-curve), convenience outlets (16A C-curve). Use B-curve for electronic loads and C-curve for inductive/motor loads.
• Residual current device (RCD/GFCI): A 30mA Type B RCD for circuits where personnel may contact equipment, especially in wet environments. Type B detects both AC and DC residual currents (important with VFDs).
• Surge protection device (SPD): Type 2 SPD on the main incoming supply for all cabinets. For field cables running outdoors (sensor cables between buildings), add Type 3 SPDs at the cabinet entry.

6. Contactors and Relays

While PLCs provide logic outputs, they don't switch high-power loads directly. The interface between PLC and power requires:

• Interposing (intermediate) relays: 24V DC coil relays with SPDT or DPDT contacts (e.g., Phoenix Contact REL-MR or Omron MY2). These isolate the PLC's transistor outputs from the contactor coil and allow one PLC output to switch multiple circuits. We use socket-mounted relays with LED indication and manual override/test buttons.
• Power contactors: For switching motors, heaters, and other high-current loads. Size according to AC-3 utilization category (motor duty). A 7.5kW motor at 400V draws approximately 15A full-load current — use a 25A contactor (e.g., Siemens 3RT2026) for service factor margin.
• Safety relays: For emergency stop circuits, light curtains, and safety gate monitoring, use certified safety relays (e.g., Pilz PNOZ s7 or Siemens 3SK1). These are required for PL e / SIL 3 applications and cannot be replaced by standard relays or PLC logic.
• Solid-state relays (SSR): For high-frequency switching or applications where mechanical relay wear is a concern (heater control, frequent valve cycling). SSRs have no moving parts but require heat sinking and fusing on the load side.

7. HMI and Operator Interface

Most PLC cabinets include at least a basic HMI for local operation and diagnostics:

• Basic panel: 7-10 inch touch panel (e.g., Siemens KTP700 Basic) for machine-level control, alarm display, and parameter adjustment. Connects directly to the PLC via PROFINET.
• Comfort panel: 12-19 inch panels (e.g., TP1200 Comfort) with recipe management, trend displays, and web server functionality for complex machines.
• Panel mounting: Use a NEMA 4/4X sealed mounting kit when the HMI is mounted on the cabinet door. Ensure the door reinforcement supports the panel weight and maintains the enclosure IP rating.

8. Communication Equipment

Modern PLC cabinets are network nodes. Typical communication hardware includes:

• Industrial Ethernet switch: Managed switches (e.g., Siemens SCALANCE XB208) with 8 ports for connecting PLC, HMI, VFDs, remote I/O, and uplink to SCADA. Use managed switches for VLAN segmentation, SNMP diagnostics, and ring redundancy (MRP).
• Protocol gateway: When connecting devices with different protocols (Modbus RTU to PROFINET, EtherNet/IP to PROFINET), use a protocol converter (e.g., Anybus X-gateway or Moxa MGate).
• 4G/LTE router: For remote sites without wired infrastructure, an industrial cellular router (e.g., Teltonika RUT956) provides VPN-secured remote access. Always use a fixed private APN or VPN tunnel — never expose PLC ports directly to the internet.
• Media converter: Fiber optic media converters for long-distance communication (over 100m copper limit) or high-noise environments. Single-mode fiber can run 10-40 km without repeaters.

9. Terminal Blocks and Wiring

Terminal blocks provide the organized interface between internal cabinet wiring and field cables:

• Feed-through terminals: Standard screw or spring-cage terminals for general I/O. We prefer spring-cage (push-in) terminals for vibration resistance and faster wiring — they reduce assembly time by 30% compared to screw terminals.
• Fuse/disconnect terminals: For field sensor power distribution, each 24V field device gets its own fused terminal with a 5×20mm glass fuse. A single sensor short won't take down the entire 24V bus.
• Multi-level terminals: Double- or triple-level terminals save DIN rail space for high-density I/O. A 2.5mm² triple-level terminal provides three connections in the same width as a single-level block.
• Shield connection terminals: Direct shield clamping terminals (e.g., Phoenix Contact SK series) provide 360° shield termination directly to the grounded DIN rail — no pigtail wires.
• Wire and ferrules: Use stranded copper wire with XLPE insulation (rated 90°C) for all internal wiring. Every conductor gets a properly crimped bootlace ferrule. Minimum size: 1.0mm² for control, 1.5mm² for power.

10. Enclosure and Climate Control

The enclosure protects all internal components from the environment:

• Enclosure: Rittal VX, Hoffman, or equivalent modular enclosure. For indoor factory use, IP54 painted steel is standard. For outdoor or washdown environments, specify IP65 SS304 minimum. We cover material selection in detail in our harsh environment cabinet design guide.
• Filter fan: For clean indoor environments where internal heat load exceeds natural dissipation. Size for the calculated heat load plus 30% margin. Use a filtered exhaust with a matching outlet filter.
• Cabinet air conditioner: For sealed enclosures in high ambient temperatures or dusty/wet environments. A 1000W AC unit (e.g., Rittal TopTherm) maintains internal temperature at 35°C even at 50°C ambient.
• Enclosure heater: For cabinets in cold or humid climates, a 50-100W PTC heater with thermostat prevents condensation when the cabinet is idle. Critical for outdoor installations in climates with high diurnal temperature swing.
• Cable glands: Nickel-plated brass for general use, SS316 for marine/chemical, EMC glands for shielded cables. Every cable entry must maintain the enclosure's IP rating.

11. Emergency Stop and Safety Circuits

Every machine control cabinet must include a hardwired emergency stop circuit per IEC 60204-1 Clause 9.2.5.4 and ISO 13850:

• The e-stop circuit uses a dedicated safety relay with force-guided contacts, independent of the PLC
• E-stop mushroom buttons are red on yellow background, with positive-opening (normally-closed) contacts that mechanically open when pressed
• The circuit must be monitored: a welded contact or broken wire must be detected and trigger a safe state
• Reset requires an intentional manual action — the system cannot auto-restart after an e-stop

12. What Does a 64 I/O PLC Cabinet BOM Actually Look Like?

Below is a representative BOM for a 64 I/O (32 DI / 16 DO / 8 AI / 8 AO) Siemens-based cabinet controlling a water treatment skid:

13. Cost Breakdown by Category

For the 64 I/O cabinet above, the approximate component cost distribution is:

14. Where to Save vs Where to Never Compromise

After 25+ years building cabinets, here's our guidance on cost optimization:

You can save on:

• Enclosure finish: powder-coated steel is fine for indoor dry environments — don't over-spec stainless steel
• HMI size: a 7-inch Basic panel handles 90% of machine applications; 15-inch Comfort panels are only needed for complex recipes or SCADA-like local operation
• Terminal block brand: reputable second-tier brands (Dinkle, Weidmüller A-series) offer equivalent reliability at 20-30% savings vs premium lines

Never compromise on:

• Power supply: a cheap unbranded PSU is the #1 cause of unexplained PLC crashes. Use SITOP, Phoenix QUINT, or PULS.
• Circuit protection: UL/IEC-listed breakers from Siemens, ABB, or Schneider. Counterfeit breakers don't trip when needed.
• Safety components: safety relays, e-stops, and contactors must be certified components from reputable manufacturers. This is life-safety equipment.
• Wire quality: use 600V-rated stranded copper wire from a certified manufacturer. Cheap wire has inconsistent conductor diameter and insulation that degrades in 2-3 years.