- October 9, 2026
- Posted by: Aceget
- Category: RECD & Emission Control Devices
Every RECD has a small paradox built into it. The device is mounted on one of the hottest objects in the building, and yet it is watched by sensors whose cables are made of materials that soften, crack or melt at a fraction of that temperature. A thermocouple measuring 450 °C gas has a cable that, in ordinary PVC form, would be uncomfortable above about 70 °C. A pressure transmitter reading backpressure has electronics that dislike much more than the heat of a summer afternoon.
The job of cable routing is to cross that temperature gap safely: to bring cables from cool places to hot places for the shortest possible distance, with the right materials at each stage, and to keep them there for years while the pipe heats, cools, grows and vibrates beneath them.
This guide looks at cable routing around an RECD by heat zone, from the hottest surfaces outwards, then works through cable choice, support, segregation, movement and inspection.
What Cables Actually Run to an RECD?
Start by listing them. The number surprises people.
| Cable or line | Typical purpose | Sensitivity to heat |
|---|---|---|
| Thermocouple or temperature sensor cables | Inlet and outlet gas temperature | Sensor tip is designed for heat; the cable and connector are not |
| Pressure transmitter cable | Backpressure signal to the controller | Transmitter electronics usually need a modest ambient temperature |
| Pressure impulse tubes | Carry pressure from the tapping to the transmitter | Metal tubes tolerate heat; plastic tubes do not |
| Controller power cable | Supply to the controller | Ordinary building cable ratings |
| Heater power cable | Supply to electric regeneration heaters, where fitted | Carries significant current near very hot parts |
| Blower or pump power | Regeneration air or dosing, where fitted | Motor cables near hot surfaces |
| Alarm and interlock cables | Signals to the DG controller or building system | Low voltage, often long runs |
| Communication cable | Network or antenna lead for remote monitoring | Sensitive to heat and interference |
| Earth bonding conductors | Frame and housing bonding | Bare or insulated; must stay connected |
Not every installation has all of these. Those with electric heaters or electrostatic stages carry more power cabling close to the hot parts, and so need more care. Our companion guide on RECD controller and heater power supplies explains the electrical side, and our explainer on what an RECD is and how it works describes where sensors sit.
Think in Heat Zones
The space around a hot exhaust is not uniformly hot. It falls into zones, and each zone suits different cable treatment.
Zone 1: Contact
The surface of the device, pipe, flanges and bosses. Uninsulated sections can be at or close to exhaust gas temperature. Insulated sections have cooler outer surfaces, but flanges, sensor bosses and supports often poke through insulation and stay very hot.
Rule: no cable should touch these surfaces, except the sensor element itself and any high-temperature tail designed for this purpose.
Zone 2: Radiant
The space close to hot surfaces, where heat arrives as radiation. Radiant heat does not need contact. A cable 100 mm from a bare hot pipe can reach far higher temperatures than the surrounding air. How much higher depends on the surface temperature, the distance, whether the pipe is insulated and whether a shield is in the way.
Rule: cables passing through this zone must be rated for the temperature they will actually reach, or protected by a shield.
Zone 3: Plume
The air directly above a hot pipe. Hot air rises, so the zone above a horizontal exhaust is warmer than the zone beside or below it, often for a surprising height. Cable trays mounted above exhaust pipes, a common arrangement in crowded plant rooms, sit in the plume.
Rule: avoid routing above hot pipes. Where it is unavoidable, increase the clearance and add a shield.
Zone 4: Cool
Everywhere else: away from the pipe, below or to the side of it, behind walls or in separate trays. Ordinary building cable works here.
Rule: run as much of each cable as possible in this zone, and cross into hotter zones for the shortest possible distance.
Map the zones before routing
Draw the zones on the layout drawing, using the surface temperature data from the vendor or, on an existing installation, readings taken at full load with an infrared thermometer. Readings matter more than assumptions: an uninsulated flange or a missing piece of cladding can create a local hot spot that changes the routing.
Choosing Cable for Each Zone
Cable insulation materials have different maximum operating temperatures. The figures below are general indications; always use the manufacturer’s rating for the specific cable.
| Insulation type | General indication of maximum continuous temperature | Typical use near an RECD |
|---|---|---|
| PVC | around 70 °C, some grades higher | Cool zone only |
| XLPE or EPR | around 90 °C | Cool zone; moderate radiant zone with care |
| Silicone rubber | around 150 to 180 °C | Short runs in the radiant zone; tails near sensors |
| Fluoropolymer, such as PTFE or FEP | around 200 °C and above | Radiant zone; sensor tails |
| Glass fibre or ceramic fibre braided | several hundred °C | Thermocouple tails close to hot surfaces |
| Mineral insulated, metal sheathed | very high, depending on the sheath | Sensor elements and tails in the contact zone |
The mineral-insulated copper-clad cable article describes one family of high-temperature cables, and properties of silicone rubber explain why it is widely used for heat-resistant flexible leads.
The tail and transition approach
The standard solution is not to run expensive high-temperature cable all the way back to the controller. Instead:
- A short high-temperature tail leaves the sensor or heater, rated for the contact and radiant zones
- The tail ends at a junction box or connector mounted in the cool zone, on a bracket that stands off from the hot surface
- Ordinary cable runs from the junction box to the controller, entirely in the cool zone
The junction box is the key element. Place it where it stays cool, can be reached for testing and is protected from water and oil. Do not fix it directly to the device housing unless the vendor has designed it for that.
Thermocouples need the right extension cable
A thermocouple works by producing a small voltage that depends on the temperature difference between its two ends. Between the sensor and the controller, the extension or compensating cable must be of the matching type for the thermocouple. Using ordinary copper cable, or the wrong type, introduces errors. Connections should also be made at points of similar temperature to avoid extra errors. Ask the vendor which thermocouple type is used and specify the matching extension cable.
Pressure lines: tubes before cables
Backpressure is often measured by a transmitter mounted away from the pipe, connected by a small metal tube from the tapping. That keeps the transmitter cool. Route the tube with a slight fall back towards the tapping so condensate drains rather than collecting in the transmitter. Plastic tubing near hot surfaces softens and kinks. A blocked or kinked tube gives a false backpressure reading, which matters because that reading is how the site knows the filter is loading; our explainer on how exhaust backpressure affects a DG set explains why it is watched so closely.
Route Planning Rules
Run below or beside, not above
Where cables and hot pipes share a route, place the cables below or to the side. Above is the worst position because of the plume.
Keep clearance from bare surfaces
Leave as much space as the layout allows between cables and any uninsulated hot surface. Where clearance is limited, a shield is needed.
Cross at right angles
When a cable must cross a hot pipe, cross it at a right angle, so it spends the least possible length in the hot zone. Running a cable parallel to a hot pipe for metres, even at a moderate distance, exposes it to heat along its whole length.
Use heat shields where clearance runs out
A simple metal shield between the hot surface and the cable, with air gaps on both sides, reduces radiant heat greatly. Fix the shield to the structure, not to the hot pipe. Our companion post on RECD insulation and heat shields covers shield design in more detail.
Avoid flanges, bosses and drains
Flanges and sensor bosses are often hotter than the insulated pipe around them, and they are opened during maintenance. Drains release hot condensate. Route cables away from all of them.
Keep cables off the RECD frame where it is hot
A device support frame warms near the saddles. Cables clipped to the frame close to the device can overheat. Use separate brackets or stand-off supports.
Plan the route through penetrations
Where cables leave the plant room alongside the exhaust, they should pass through a separate sleeve, not share the exhaust’s sleeve. The exhaust sleeve is hot and moves with thermal growth.
Supports and Fixings
The small parts that hold cables in place fail first.
| Item | Problem near heat | Better choice |
|---|---|---|
| Nylon cable ties | Become brittle and snap | Stainless steel ties, coated where they contact cable |
| Plastic saddles and clips | Soften and release | Metal saddles with suitable liners |
| PVC conduit | Softens and sags | Metal conduit or flexible metal conduit where needed |
| Adhesive fixings | Lose grip | Mechanical fixings |
| Cable tray with plastic accessories | Accessories deform | All-metal tray and accessories in warm areas |
Cables should be supported at intervals that stop them sagging towards hot surfaces as fixings age. A cable that was 150 mm from the pipe on installation day and has drooped to 30 mm three years later is a common finding.
Movement and Vibration
An exhaust system moves. It grows when hot and shrinks when cold. It also vibrates with the engine, especially near the engine bellows.
Leave loops
Wherever a cable attaches to something that moves, such as a sensor on the device housing, leave a service and movement loop so growth does not pull the cable or the connector. Make the loop in the cool zone if possible, or use high-temperature cable for it.
Do not anchor cables to two things that move differently
A cable fixed to the device at one end and to a building wall at the other will be stretched and relaxed with every heat cycle. Use flexible sections, loops or a junction box at the transition.
Watch for chafing
Vibration rubs cables against edges. Use grommets at entries, edge protection on brackets and gentle bends.
Our article on how to prevent generator vibration issues explains the sources, and the flexible bellows guide explains how vibration is isolated at the engine end.
Segregation and Interference
Signal cables near a generator share space with power cables carrying high currents, and with the alternator itself, a powerful source of electromagnetic fields.
Separate power and signal
- Run sensor and communication cables in a separate tray or compartment from power cables, or with adequate spacing
- Cross power cables at right angles where they must cross
- Keep heater power cables apart from thermocouple cables, especially near the device where both converge
Use screened cables for signals
Low-level signals such as thermocouple voltages and analogue transmitter outputs are susceptible to electromagnetic interference. Screened cables, with the screen earthed at the point the vendor specifies, reduce noise. Noisy signals produce jumpy readings and nuisance alarms.
Keep away from the alternator and main cables
Route signal cables away from the alternator terminal box and the main power cables leaving the set. Our article on how a diesel generator’s alternator works explains why it generates strong fields.
Water, Oil and Dirt
Heat is not the only enemy.
- Water runs along cables into junction boxes and connectors. Use drip loops before entries and glands with suitable sealing.
- Oil mist from the engine softens some cable materials over time. Choose oil-resistant sheathing near the engine.
- Soot collects on everything near the exhaust. Make junction boxes easy to open and clean.
- Wash-down in food and process plants requires suitable ingress protection.
Humid sites need particular attention; see our guide on RECDs for water treatment plants.
Rooftop and Outdoor Routes
Outdoors, cables near an RECD face sun, rain, wind and sometimes birds and rodents.
- Protect cables from direct sunlight with trays, covers or UV-resistant sheathing
- Avoid routes where standing water collects
- Fix cables against wind movement
- Use lightning and surge protection where cables enter the building
- Near the sea, choose stainless fixings and corrosion-resistant glands; our guide on choosing an RECD for coastal salt-laden environments explains why salt air attacks mixed metals
Our guide on rooftop RECD installation discusses exposed locations.
The Route Survey: A Practical Method
Before finalising the route on an existing installation, walk it with the generator at full load. Take:
- An infrared thermometer
- The layout drawing
- A tape measure
- A camera
At each point where a cable will pass, measure the surface temperature of the nearest hot surface and the air temperature at the proposed cable position, and note the distance. Record the results directly on the drawing. Repeat at the hottest time of year if possible, since ambient temperature adds directly to cable temperature. Our article on assessing an RECD for very hot ambient conditions covers summer conditions.
A sample survey record
| Point | Nearest hot surface | Surface temperature | Distance to cable | Air temperature at cable | Cable proposed | Action |
|---|---|---|---|---|---|---|
| A: transmitter tube near inlet tapping | Device inlet flange | very hot, uninsulated | 120 mm | high | Stainless tube | Re-route tube below flange |
| B: thermocouple tail exit | Sensor boss | very hot | 0 mm | not applicable | Braided high-temperature tail | Junction box on stand-off bracket 400 mm away |
| C: tray crossing over pipe | Insulated pipe cladding | warm | 300 mm above | moderate to high | XLPE in tray | Re-route tray below pipe |
| D: tray along wall | Insulated pipe cladding | warm | 600 mm beside | close to room temperature | XLPE in tray | Acceptable |
Values are illustrative. The pattern is what matters: the survey turns the route from guesswork into a list of measured points, each with a decision.
Labelling, Records and Maintenance
Label everything
Each cable should carry a label at both ends and at junction boxes, matching the cable schedule. When a technician replaces a thermocouple in three years, labels save hours.
Record the route
The general arrangement drawing should show the cable route, junction box positions and heat zones. Our companion guide on what an RECD general arrangement drawing should include explains how interfaces appear there.
Inspect periodically
Add cable checks to the routine:
- Discoloured, hardened or cracked insulation near hot surfaces
- Snapped ties and sagging cables
- Water inside junction boxes
- Loose connectors on sensors
- Chafed sheaths at brackets
Our RECD maintenance guide and the monthly maintenance checklist for diesel generators are natural places for these checks.
A Walkthrough: Engineering Workshop in Chennai
This is an illustrative case.
The site
- A 320 kVA set in an enclosed room at a fabrication and machining workshop
- RECD mounted horizontally on a floor frame, about 2.5 m from the engine, with the exhaust then running along the ceiling to an outer wall
- Two thermocouples, one pressure transmitter and a controller on the room wall
- An existing cable tray ran along the ceiling, directly above the exhaust route, carrying lighting and power circuits for the room
The symptoms
Within the first summer, the inlet temperature reading began to jump erratically and the pressure reading drifted. A lighting circuit in the overhead tray tripped twice. The maintenance supervisor found the thermocouple extension cable’s sheath stiff and cracked where it ran across the device housing, and the pressure transmitter mounted directly on the device bracket was too hot to touch.
The survey findings
| Finding | Effect |
|---|---|
| Thermocouple extension cable laid across the device insulation to reach the controller | Sheath overheated where it crossed a hot flange |
| Pressure transmitter mounted on the device bracket | Electronics above rated temperature |
| Plastic impulse tube looped near the inlet cone | Softened and partly kinked |
| Nylon ties along the device frame | Snapped; cables sagged onto the cladding |
| Overhead tray above the exhaust | Cables in the hot plume |
| Thermocouple cables in the same tray section as a heater supply cable for a parts washer | Interference on the temperature signals |
The correction
- Thermocouple tails replaced with braided high-temperature leads, ending at a junction box on a stand-off bracket below the device
- Matching thermocouple extension cable run in a dedicated screened route along the wall, below the exhaust level
- Pressure transmitter moved to the wall, connected by a stainless tube with a fall back to the tapping
- All ties near the device replaced with stainless steel ties with protective sleeves
- The ceiling tray re-routed to the opposite wall, away from the exhaust, with a heat shield where the exhaust passes a structural beam beside the remaining tray
- Signal and power cables separated into different trays
The readings settled immediately, and no further trips occurred. Workshops with heavy machinery and dust bring their own installation needs; our guide on RECD selection for dusty industrial locations covers them. The 320 kVA DG set RECD page shows this rating.
Common Cable Routing Mistakes
| Mistake | Typical consequence |
|---|---|
| Cables laid across insulation or cladding | Overheating at flanges and hot spots |
| Trays above the exhaust | Cables cooked in the plume |
| Transmitter mounted on the device | Electronics failure |
| Wrong thermocouple extension cable | Reading errors |
| Nylon ties near heat | Cables sag onto hot surfaces |
| No movement loops | Pulled connectors and broken cores |
| Power and signal cables together | Noisy readings and nuisance alarms |
| Cables through the exhaust sleeve | Heat damage and fire-stop failure |
Several of these appear in our wider list of common RECD installation mistakes.
Compliance Note
CPCB’s System and Procedure for Emission Compliance Testing of RECDs covers device approval rather than site wiring. Cable routing is an installation quality matter, but damaged sensor cables lead to false readings, missed alarms and gaps in records. Keep the cable schedule, route drawing and survey with your installation file, alongside your environmental compliance records.
Frequently Asked Questions
How far should cables be from a hot exhaust pipe?
As far as the layout allows, and below or beside rather than above. The right distance depends on the surface temperature, insulation and shielding. Measure temperatures at the proposed cable position and choose cable rated above them.
Can ordinary PVC cable be used near an RECD?
Only in the cool zone, away from hot surfaces and the plume. Near the device, use high-temperature tails ending in a junction box in a cool position.
Where should the pressure transmitter be mounted?
Away from the hot device, connected by a metal impulse tube, with the tube falling back towards the tapping so condensate drains.
Why are my RECD temperature readings jumping around?
Common causes include damaged extension cables, the wrong extension cable type, poor connections and interference from nearby power cables.
Are nylon cable ties acceptable near the exhaust?
Not near hot surfaces. They become brittle and break. Use stainless steel ties with protective sleeves.
Should cables share the exhaust pipe’s wall sleeve?
No. Give cables their own sleeve or penetration, away from the hot and moving exhaust.
Keep the Hot Part Short
The best cable route around an RECD is one that spends as little length as possible near heat. Map the zones, use high-temperature tails only where needed, transition to ordinary cable at a cool junction box, run below rather than above, cross at right angles, fix with metal, leave loops for movement, keep signal and power apart and inspect the hot end regularly. Do that, and the sensors on the device will keep reporting accurately for as long as the device itself lasts.
Aceget plans sensor positions, junction boxes and cable routes as part of every RECD layout, and supplies high-temperature tails and matched extension cable with its devices. To review cable routing on your installation, reach out to Aceget, or explore the retrofit emission control device range.