— A Practical Guide for Engineers, EPCs, and Project Teams
Instrument cables carry the nervous system signals of a process plant — 4-20 mA loops, thermocouple millivolt signals, RTD resistance values, and digital fieldbus communications. These signals are low-energy, low-voltage, and highly susceptible to corruption by electrical noise, physical damage, and environmental factors.
The physical routing of these cables — how they are separated from power cables, how they are supported in trays, and how they are installed — determines whether that signal arrives at the control system intact or degraded. This guide covers the three interconnected elements of instrument cable routing: separation, tray layout and selection, and installation practices.
1. Separation: The Overarching Principle
The single most important rule in instrument cable routing is separation from power cables. Instrument signals are low-level and noise-sensitive; power cables are high-energy noise sources. The distance between them is not a suggestion — it is a design requirement.
1.1 Why Separation Matters
A 150/250 V instrumentation cable is dimensioned for phase-to-earth stress of 150 V in continuous operation-28. If a 690 V power conductor faults to the tray or to a bonded structure, the resulting transient voltage can easily exceed the dielectric withstand of the 150/250 V jacket, causing insulation breakdown and arcing into the signal pair-28. This is the core concern driving segregation rules.
Beyond safety, separation prevents:
Capacitive coupling — noise transferred through electric fields between adjacent cables
Inductive coupling — noise transferred through magnetic fields from high-current conductors
Ground loop currents — unwanted currents flowing through shield grounds
1.2 General Separation Hierarchy
From most to least restrictive, instrument cables should be routed:
| Priority | Routing Method | Application |
|---|---|---|
| 1. Dedicated tray | Instrument cables only | Best practice for critical signals |
| 2. Shared tray with metal divider | Instrument cables on one side, power on the other | When dedicated trays are impractical |
| 3. Separate conduits | Instrument cables in one conduit, power in another | Short runs, limited space |
| 4. Shared tray without divider | Only when separation distances are met | Least preferred |
Instrument cables should not share the same raceway with power cables, control cables, or telephone cables-2. Where co-routing is unavoidable, physical barriers or dedicated separation distances must be provided.
1.3 Recommended Separation Distances
The required separation between instrument and power cables depends on voltage, current, and the length of parallel run.
Minimum separation for parallel runs (typical industry practice) :
| Power Wiring | Minimum Separation |
|---|---|
| 125V or 10A | 250 mm |
| 250V or 50A | 500 mm |
| 440V or 200A | 750 mm |
| 3.3kV or 500A | 1250 mm |
| 11kV or 800A | 4000 mm |
Source: BHEL engineering design basis-3
For shared tray with metal divider: A continuous grounded metal divider inside a single ladder tray converts it into two effectively separate raceways at one-third to one-half the cost of two trays-6.
General heuristics when no specific value is specified-6:
Default separation: 6 inches (150 mm) per 100 feet of parallel length
Double the result when no shielding is available
When crossing is unavoidable, cross at right angles (magnetic coupling at 90° is zero by symmetry)-6
Shorten parallel runs wherever possible-6
For vertically stacked trays--2:
Arrange from top to bottom: highest voltage at the top, lowest voltage at the bottom
Instrumentation trays should always be at the bottom
Provide at least 12 inches (300 mm) of clear space between tray levels
For horizontally separated trays--1:
Minimum horizontal separation of 3 feet (900 mm) if no physical barrier exists
If separation is less than 3 feet, install a fire barrier extending 1 foot above and below the tray
1.4 Intrinsically Safe (IS) Circuit Separation
IS circuits require additional segregation:
IS and non-IS circuits shall not be contained within the same cable or marshalled in the same junction box-3
IS and non-IS cables may be run together in the same tray/duct or trench but must be separated into different bundles as far apart as possible-3
IS cables in the field are recognised by their light blue-coloured sheath (similar to RAL 5015)-
Dedicated blue conduit or tray with metal divider is recommended-6
1.5 Crossovers and Transitions
Crossovers that bring signal and power cables into proximity shall be made at right angles-3
The minimum separation at the point of crossover is 250 mm-3
Separation requirements may be relaxed at entries to instruments and panels-3
2. Cable Tray Layout and Selection
Cable trays are the primary support system for instrument cables in industrial plants. Proper tray selection and layout ensure mechanical support, maintain separation, and allow for future expansion.
2.1 Tray Types
| Tray Type | Characteristics | Best For |
|---|---|---|
| Ladder tray | Rungs provide cable support; ventilated | General industrial; allows heat dissipation |
| Ventilated trough | Solid bottom with ventilation slots | Moderate cable density; indoor/outdoor |
| Solid bottom | Continuous solid surface | Dusty environments; sensitive cables |
| Solid bottom with cover | Enclosed tray | Low-level instrumentation cables — installed in solid, non-ventilated trays with solid covers or rigid conduits- |
2.2 Tray Material Selection
| Material | Advantages | Best For |
|---|---|---|
| Hot-dipped galvanised steel | Strong, durable, cost-effective | Most industrial applications-2 |
| Stainless steel | Corrosion-resistant | Marine, chemical, offshore |
| FRP (Fiber Reinforced Polymer) | Lightweight, corrosion-resistant, non-magnetic, electrically insulating | Chemical plants, wastewater, coastal installations-18 |
Tray strength requirements-2:
Tray strength should be verified per NEMA VE 1
Design for a dead cable load of 50 pounds per foot, plus a single live load of 200 pounds applied at any point
Supports should be capable of spanning a 2-metre length unsupported-
2.3 Tray Sizing and Fill Capacity
Fill ratios (typical design targets) -18:
| Cable Type | Typical Fill Ratio | Notes |
|---|---|---|
| Power cables | 40% – 50% of tray cross-section | Leave room for heat dissipation |
| Control and instrumentation cables | 50% – 70% of tray cross-section | Smaller diameters allow higher fill |
| Future expansion | 10% – 25% spare capacity | Reserve space for additional circuits |
Tray fill calculation-18:
Sum the total cable cross-sectional area in each tray section
Divide by the allowable fill ratio
Select a tray size that meets or exceeds the minimum internal area
Example: If the cable bundle requires 6000 mm² of area and the design fill ratio is 50%, the minimum tray internal area should be 12,000 mm²-18.
NEC requirements-:
Multiconductor only (power, control, instrumentation): 50% of tray fill area
Multiconductor + any single conductors: 40% of tray fill area
Spare capacity: While NEC does not mandate spare space in cable trays-, industry best practice is to reserve 10-25% spare capacity for future additions-.
2.4 Tray Support Spacing
| Location | Typical Support Span | Notes |
|---|---|---|
| Indoor straight runs | 1.5 m – 3 m between supports | Common spacing for standard loads-18 |
| Outdoor columns or building steel | Up to 6 m between vertical posts | Check wind, snow, and ice loads-18 |
| Bends, tees, reducers | Support within 300 mm – 600 mm of each fitting | Add support close to each fitting-18 |
Vertical spacing between trays--2:
12 inches (300 mm) clear space between tray levels, measured from the bottom of the upper tray to the top of the lower tray
9 inches (225 mm) minimum clearance between the top of a tray and beams, piping, etc.
2.5 Tray Routing Best Practices
| Practice | Why |
|---|---|
| Instrument trays should always be at the bottom in stacked tray arrangements | Lowest voltage, most sensitive signals should be farthest from noise sources--2 |
| Route trays away from high fire hazard areas | Avoid routing over main lubricating oil reservoirs, diesel generator sets, etc.-2 |
| Use fire stops at floor and wall penetrations | Prevents fire spread between compartments-2 |
| Avoid routing through areas with combustible fluid accumulation | Where unavoidable, use enclosed conduit and only one separation group-1 |
| Openings in solid floors for vertical runs shall be sealed with fire-resistant material-1 |
3. Installation Practices
3.1 Cable Installation in Trays
| Practice | Why |
|---|---|
| Do not exceed tray fill capacity | Prevents overheating and allows future additions |
| Bundle cables by type and route | Facilitates identification and maintenance |
| Leave service loops at termination points | Allows re-termination without re-pulling |
| Protect cables from mechanical damage | Use covers where needed-3 |
| Route cables in preformed reinforced concrete trenches underground | Provides mechanical protection and access-3 |
| Crossings beneath roads by ducts encased in concrete-3 |
3.2 Cable Glands and Terminations
Cable screens shall be electrically continuous throughout the cable run and earthed at one point only — typically the panel reference bar-3
The design of the earthing system shall avoid the creation of earth loops caused by duplication of earthing paths-3
Cable glands should be selected to match the cable type (armoured/unarmoured) and environmental rating
3.3 Underground Cable Installation-3
Cables routed underground shall be run in pre-formed reinforced concrete trenches
Crossings beneath roads or access ways shall be by means of ducts encased in concrete
The routes of all main trenches shall be shown on the detail drawings
4. Verification and Commissioning Checklist-6
Before placing a cable routing system into service:
| Check | What to Verify |
|---|---|
| Visual inspection | Confirm tray and conduit runs match the drawings; verify no undocumented crossings exist |
| Spacing measurement | Verify as-built separation matches the design specification at every parallel section. Tolerance: ±1 inch for separations ≤6 in; ±2 in for larger separations |
| Barrier continuity | For shared trays with dividers, verify the divider runs full depth and full length, with no gaps at splices or supports |
| Grounding | Verify cable screens are earthed at one point only; confirm tray continuity to earth |
| Identification | Verify cable tags and tray labels match the schedule |
5. Why Choose Anhui Tiankang for Instrumentation Cables?
Anhui Tiankang (Group) Co., Ltd. has been manufacturing industrial cables and instrumentation for nearly five decades. Our instrumentation cables are designed for reliable signal transmission in the most demanding industrial environments.
Instrumentation cable portfolio:
Shielding configurations: IS (individual screen), OS (overall screen), and IS+OS for maximum protection
Sheath materials: PVC, LSZH, oil-resistant, and SHF2 (mud-resistant for offshore)
Fire performance: IEC 60332 (flame retardant), IEC 60331 (fire resistant), hydrocarbon fire resistant
Armour options: STA (steel tape), SWA (steel wire), and braided options
Ex certification: Intrinsically safe cables with low capacitance, blue LSZH sheath, Ex-ia certified
Core advantages:
CNAS-accredited laboratory for full electrical and fire performance testing
Comprehensive certifications: CCC Ex, ATEX, IECEx, CCS marine
Proven track record: long-term supplier to CNPC, Sinopec, CNOOC, and international EPC projects
6. Conclusion
Instrument cable routing is not an afterthought — it is a fundamental engineering decision that determines whether signals arrive at the control system intact. The three pillars of good routing design are:
| Element | Key Principle |
|---|---|
| Separation | Keep instrument cables away from power cables. Use dedicated trays, metal dividers, or specified separation distances. Instrument trays at the bottom. |
| Tray layout | Select the right tray type and material. Size for 50-70% fill with 10-25% spare capacity. Support at proper intervals. |
| Installation | Install per best practices. Protect from mechanical damage. Ground shields at one point only. Verify with a commissioning checklist. |
Remember: The cable route is as important as the cable itself. A properly specified cable installed in a poorly designed route will fail. A properly designed route protects the cable and ensures signal integrity for the life of the plant.
Contact Us
For instrumentation cable selection, routing advice, or project quotations, please contact:
Yin Shuangjie
International Sales Manager
📧 Email: [email protected]
📱 WhatsApp / Zalo: +86 17856068126
🌐 Website: http://www.tiankang-global.com/
Anhui Tiankang – Your partner for reliable instrumentation and cable solutions.
