Technical Reference
Double Skin Rooftop Units - Model Code & Electrical Wiring Overview
Reference example: URT/K/EC/WP/MS 101 S.K/IM/RE/CC/IS/CP/PF/AT-C/IAV/IAS/IDL/NS
Electrical diagram reference: FG-000022_363_ED | Created 19/08/2025
This article explains how to interpret the model code and provides a high-level overview of the key electrical and communication wiring points for double skin packaged rooftop units with scroll compressors, EC inverter plug-fans and a mixing box.
Applies To
Double skin packaged rooftop units with scroll compressors, EC inverter plug-fans and mixing box.
| Reference model | URT/K/EC/WP/MS 101 S.K/IM/RE/CC/IS/CP/PF/AT-C/IAV/IAS/IDL/NS |
| Diagram reference | FG-000022_363_ED |
Purpose
This article is intended to help service, technical support and field teams identify fitted options, understand the control architecture, and locate key wiring references before attending site.
Model Code Breakdown
The model code identifies the base unit, configuration and fitted accessories.
Example:
URT/K/EC/WP/MS 101 S.K/IM/RE/CC/IS/CP/PF/AT-C/IAV/IAS/IDL/NS
URT/K/EC/WP/MS 101 S.K/IM/RE/CC/IS/CP/PF/AT-C/IAV/IAS/IDL/NS
Base Unit
| Code | Meaning |
|---|---|
| RTA/K | Cooling only with radial fan |
| RTA/K/WP | Reverse cycle heat pump with radial fan |
| RTA/K/EC | Cooling only with EC inverter plug-fan |
| RTA/K/EC/WP | Reverse cycle heat pump with EC inverter plug-fan |
For the attached example, RTA/K/EC/WP indicates a reverse cycle heat pump rooftop unit with EC inverter plug-fans.
Section Code
| Code | Meaning |
|---|---|
| MS | Mixing chamber |
| ECO | Economiser |
| ECO/REC-FX | Economiser with cross-flow heat recovery |
| ECO/REC-WH | Economiser with wheel heat recovery |
For the attached example, MS indicates the unit is fitted with a mixing chamber.
Common Accessory Codes
| Code | Description |
|---|---|
| CR | Remote control panel |
| CDT | Touchscreen remote control panel |
| IM | Automatic circuit breakers |
| RE | Maximum/minimum voltage and phase sequence control relay |
| CC | Condensing control down to -20°C |
| CT | Condensing control down to 0°C |
| IS | Modbus RTU, RS485 serial interface |
| IST | Modbus TCP/IP, Ethernet |
| ISB | BACnet MSTP, RS485 |
| ISBT | BACnet TCP/IP, Ethernet |
| ISS | SNMP, Ethernet |
| CP | Potential free contacts |
| PF | Filter differential pressure switch |
| AT | Constant air flow regulation |
| AT/P | Constant static pressure regulation |
| AT/C | Air flow adjustment according to load |
| IAV | Remote setpoint via 0-10 V signal |
| IAA | Remote setpoint via 4-20 mA signal |
| IAS | Remote signal for second setpoint activation |
| IDL | Demand limit from digital input |
Electrical Supply Overview
| Item | Detail |
|---|---|
| Main power supply | 400 V / 50 Hz / 3 phase + PE |
| Auxiliary circuit | 230-24 V / 50 Hz |
| Reference standard | CEI EN 60204-1 |
The customer/installer is responsible for the incoming power supply and upstream protection. The example includes recommended protection and cable sizing information, including a note to use a Type B differential switch ≥ 0.3 A where required.
Main Control System
The unit uses a Danfoss MCX20B2 programmable controller as the main microprocessor.
| Function | Examples |
|---|---|
| Compressor control | Compressor contactors and protection inputs |
| Fan control | Supply fan and condenser fan outputs |
| Safety monitoring | High pressure, low pressure, fan protection, phase relay and emergency stop |
| Analogue inputs | Pressure transducers, air temperature sensors and setpoint inputs |
| Communication | CAN bus, RS485 and optional gateway interfaces |
Communication Network Layout
| Network | Purpose |
|---|---|
| CAN bus | Local communication between controller, display and expansion modules |
| RS485 | Remote touchscreen/controller or BMS interface, depending on fitted option |
| Ethernet | Used when TCP/IP-based options are fitted, such as Modbus TCP/IP, BACnet TCP/IP or SNMP |
CAN Bus Wiring Notes
- CAN bus cable should have 120 Ω impedance.
- Termination must be fitted at the first and last local network units.
- Termination should be made as close as possible to the connector.
- Cable size and baud rate depend on cable length.
| Cable Length | Baud Rate | Minimum Cross-Section |
|---|---|---|
| Up to 100 m | 500 kbit/s | 0.34 mm² |
| 100-250 m | 250 kbit/s | 0.34 mm² |
| 250-300 m | 125 kbit/s | 0.34 mm² |
| 300-500 m | 125 kbit/s | 0.5 mm² |
| 500-1000 m | 50 kbit/s | 0.5 mm² |
Remote Controller & Display Wiring
| Device | Description |
|---|---|
| D1 / DR1 | Danfoss MMIGRS2 control panel |
| CDT1 | Touchscreen remote control panel |
The touchscreen remote controller is shown connected via RS485, with supply provided by the client/installer where applicable.
Compressors
The example wiring shows three scroll compressors on circuit 1 for the relevant model range. Each compressor includes:
- Three-phase power supply
- Compressor contactor
- Motor protection device
- Crankcase heater circuit
- Safety/protection feedback to the controller
The compressor page identifies Copeland scroll compressors and compressor motor protection settings in the example schematic.
Supply Fans
The example unit is fitted with EC inverter plug-fans.
- Supply fans are EC type.
- Fan speed can be changed by parameter.
- Path: Main Menu > Parameters > Rooftop > Supply Fan > Setup > SF2 "Maximum Speed"
- Default maximum speed is shown as 75% in the example.
This adjustment should only be carried out by suitably trained personnel with the appropriate access level.
Condenser Fans
Depending on fitted options, the wiring may include:
- Condensing control accessory CC
- Condenser fan motor protection
- Speed regulator
- External electrical panel connections
Important: Where the CC accessory is not fitted, the diagram includes jumper requirements. Always confirm the installed accessory configuration before modifying wiring.
Safety Circuit Overview
| Safety Device / Input | Purpose |
|---|---|
| Emergency stop | Stops unit operation via safety circuit |
| Phase sequence relay | Monitors phase sequence and supply condition |
| High pressure switch | Refrigeration circuit high pressure protection |
| Compressor motor protection | Compressor overload/protection feedback |
| Supply fan protection | Fan protection feedback |
| Air differential pressure switch | Airflow/filter-related monitoring depending on configuration |
| Filter differential pressure switch | Fitted when PF option is present |
| Oil level sensor | Fitted on applicable versions |
If a safety input is open or active, the controller may inhibit operation or generate an alarm depending on the configured logic.
Potential Free Contacts
Where the CP option is fitted, the unit provides potential free contacts / dry contacts for external signalling. These contacts are typically used for external status or alarm integration. Confirm the exact terminal reference from the site-specific wiring diagram before connection.
BMS Integration
| Code | Interface |
|---|---|
| IS | Modbus RTU via RS485 |
| IST | Modbus TCP/IP via Ethernet |
| ISB | BACnet MSTP via RS485 |
| ISBT | BACnet TCP/IP via Ethernet |
| ISS | SNMP via Ethernet |
| ISL | LonWorks via FTT-10 serial interface |
For the attached model example, IS is present, indicating a Modbus RTU RS485 interface.
Important Service Notes
- Confirm the complete model code from the unit nameplate.
- Use the model legend to identify fitted options.
- Check the wiring diagram revision against the unit documentation.
- Confirm whether remote controller, touchscreen, BMS gateway, filter pressure switch, demand limit and remote setpoint options are actually installed.
- Do not assume accessories are present unless shown in the model code or confirmed on the wiring diagram.
- For CAN bus or RS485 faults, inspect cable polarity, shielding, termination resistors and cable length.
- For no-run faults, check the safety chain before replacing control components.
- Electrical testing must be completed by qualified personnel only.
Common Support Checks
| Symptom | Recommended Checks |
|---|---|
| Unit has no display | Check 24 V supply, display wiring, CAN connection and controller power |
| Remote controller not communicating | Check RS485/CAN wiring, polarity, termination and accessory configuration |
| BMS cannot communicate | Confirm fitted interface option, protocol, address, baud rate and wiring |
| Supply fan not running | Check fan enable output, EC fan supply, analogue speed signal and fan protection input |
| Compressor not starting | Check safety chain, compressor contactor output, motor protection, pressure switches and controller demand |
| Filter alarm active | Check PF option, differential pressure switch tubing, filter condition and switch setting |
| Phase fault alarm | Check incoming supply, phase sequence relay and phase loss/voltage condition |
Escalation Information to Collect
- Full model number
- Serial number
- Wiring diagram reference and revision
- Active alarm code and description
- Controller/display photos
- Incoming voltage readings
- 24 V control voltage readings
- BMS protocol and wiring details, if applicable
- Photos of electrical panel, terminal strips and any modified wiring
- Site history, including recent electrical works or controller changes
Safety: Electrical testing and wiring changes must only be completed by qualified personnel. Always use the site-specific wiring diagram and confirm the installed option configuration before making changes.