A P&ID says what instruments a plant needs, but it never says where the installer should bolt each one. The location drawing turns tags into real positions, heights and cable paths that construction crews can build without guessing.
An instrument location plan places every field instrument on a scaled plan view with its tag, coordinates and elevation. It also shows junction boxes and cable routes, so it links the P&ID to the hook up drawings and the cable schedule.
What Is an Instrument Location Plan?
An instrument location plan is a scaled plan view of a plant area that shows where each field instrument is installed, with its tag number, plant coordinates and elevation, along with junction boxes, air manifolds and main cable routes. It is prepared during detailed engineering from the piping and instrumentation diagram and the piping layout.
The drawing is also called an instrument location layout or instrument location drawing. Construction teams use it to fix stands and route cables, while maintenance teams use it years later to find an instrument quickly.
An ISA Autoquiz, based on the ISA book Maintenance of Instruments and Systems, states that the drawing shows the location, elevation and tag number of each instrument. Specification numbers belong on instrument lists, and wiring details belong on conduit schedules or cabling diagrams.
Why an Instrument Location Plan Matters on Site
A transmitter placed in the wrong spot can end up behind a pipe rack column, above a hot line or out of reach without scaffolding. Fixing such errors after piping is erected costs far more than moving a symbol on a drawing.
The drawing also decides the length of every field cable, the size of the junction boxes and the number of cable tray runs. That makes it an input to the bill of materials, to cable tray fill calculation and to the construction schedule.
ISA points out that the instrument location plan is mainly for new plant installation. It tells the installer exactly where each instrument goes and at what height, such as grade, a platform or a point on a process line.
Inputs Needed Before You Start
Automation Forum lists the reference documents used to prepare an instrument location plan. Collect the latest revision of each one before placing the first symbol.
When a piping isometric moves a tapping point, the instrument location plan must follow it. The same tapping point affects impulse line routing, explained in impulse line installation slope, so both documents need checking together.
What an Instrument Location Plan Shows
- Instrument tag number with symbol for each device.
- Plant coordinates, east and north, of the instrument stand.
- Elevation of the instrument, stand and process tap.
- Junction box tag, position and elevation.
- Instrument air manifold location and elevation.
- Main cable tray or trench routes with tags.
- Hook up drawing reference for each instrument.
- Notes on access, platforms and special supports.
Automation Forum adds that the drawing may list the instrument type, measured variable, range or set point and the control system it connects to. On large projects these extra fields often move to the instrument index so the plan stays readable.
Inline items, such as control valves, flow elements and magnetic flow meters, are shown at their pipe location with a tag only. Their exact position is fixed by the piping layout, not by the instrument designer.
Coordinates, Elevations and Drawing Scale
Most plants use a plant grid of east and north coordinates referenced to a project datum, with plant north shown on every sheet. Many projects set finished grade at a notional elevation such as EL 100.000 so that no level in the plant becomes a negative number.
Elevations on an instrument location plan usually refer to the centre of the instrument or the top of the stand base. State the convention in the general notes, because a 300 mm mistake on a platform level can put a gauge out of sight.
Tweed Shire Council specification EL04 requires that all drawings be to scale. Seattle Public Utilities guidance suggests detailed site plans at 1 inch to 20 feet or larger where possible, which shows how much detail a crowded area needs.
Show each instrument at its stand position, not at the tapping point, when the two differ. Add a small leader to the tap so the installer sees both locations clearly.
Placing Instruments for Access and Maintenance
Automation Forum advises placing the instrument on a stand as close as practicable to the tapping point, or where the operator needs it for reading and maintenance. The Tweed EL04 specification sets an optimum working height of 1500 mm above grade or walkway.
The same specification says field instruments must be serviceable or replaceable without a ladder or scaffold, unless the owner approves otherwise. Dial thermometers and pressure gauges must be plainly visible and accessible from grade or a platform.
Keep instruments out of walkways, vehicle routes and lifting zones, and avoid hot surfaces and vibration. Transmitter mounting advice from pressure transmitter installation best practices applies directly when you choose each stand position.
Junction Box Locations and Cable Routes
Junction boxes collect individual field cables and send multicore cables to the control room or marshalling cabinet. Place them at the centre of their instrument cluster, at an accessible height and away from drain points and steam leaks.
Tweed EL04 asks for 316 stainless steel external junction boxes rated IP65 or better, with gland plates and terminal rails sized for 25 percent spare. It also separates power and instrument cables by at least 300 mm and keeps 2100 mm head clearance over floors and walkways.
Separate intrinsically safe and non intrinsically safe cables, and analog from digital and power, as set out in shielded cable and twisted pair practice. In classified areas, check the zone boundaries in hazardous area classification before fixing each box.
A single change of a junction box position can alter dozens of cable lengths in the cable schedule. That is why designers freeze JB locations early and revise them only with a formal change note.
7 Smart Steps to Prepare an Instrument Location Plan
Many EPC contractors produce these drawings in a 3D plant design tool linked to the instrument database, but the logic is the same on a 2D sheet. Clashes with cable trays, discussed in cable tray sagging, are easier to catch in the 3D review.
Cable Length From Coordinates
The instrument location plan gives coordinates and elevations, so a designer can estimate cable length to the junction box before the cable schedule is final. Cables follow trays at right angles, so the route length is the sum of the differences, not the straight line distance.
Cable length = Route × (1 + Allowance ÷ 100)
Instrument at E 125.5, N 340.0, EL 104.2
JB at E 110.0, N 352.0, EL 101.5
Route = 15.5 + 12.0 + 2.7 = 30.2 m
Cable length = 30.2 × 1.15 = 34.7 m
Instrument to JB Cable Length Calculator
Use the result for early estimates only, and confirm the final value from the routed model. Long analog runs also need a check on loop resistance, as explained in analog input stability and cable length.
Add the cable tray or trench tag at every change of direction on the plan. Installers then follow the route without guessing which tray level to use.
Links to Hook Up Drawings and the Cable Schedule
Each instrument symbol carries the number of its hook up drawing, which shows the stand, tubing, valves and fittings. The cable schedule then takes the JB tag and route from the plan to list cable number, type, size and length.
After installation, the instrument location plan supports the instrument loop checking procedure, because the commissioning team must find every device in the field. It is also updated to as built status and kept with the other instrumentation and control standards documents of the plant.
- Installers know exact position and height.
- Cable lengths and tray loads can be estimated early.
- Access and maintenance problems are caught on paper.
- Maintenance teams find instruments quickly for years.
- Out of date when piping moves taps.
- Crowded sheets become hard to read.
- Wrong elevation reference causes rework.
- Missing JB or tray tags slow down installation.
Common Mistakes and Review Checks
The most frequent error is placing an instrument from an old piping revision, so the tag sits on a pipe that has moved. The next is forgetting platform levels, which leaves gauges hidden above head height or inside handrails.
Tweed EL04 also requires that as constructed changes be clouded on the drawing during construction and approved before handover. A disciplined as built update keeps the drawing useful for the life of the plant.
Tweed EL04 Installation Specification
Instrument and JB Location Layout Video
Instrument Location Plan FAQ
It is a scaled plan drawing that shows where each field instrument is installed in a plant area. Each symbol carries the tag number, coordinates and elevation.
It also shows junction boxes, air manifolds and the main cable tray or trench routes. Installers during construction and maintenance teams later both rely on it to find devices in the field.
The main inputs are the P&ID, plot plan, piping plans and piping isometrics. The instrument index, data sheets and hook up drawings add type and mounting details.
Loop diagrams and control narratives show which control system each device connects to. Always use the latest revision of every input document to avoid costly rework on site.
An ISA Autoquiz lists location, elevation and tag number as the core content of each entry. Without these three items, an installer cannot place the instrument correctly in the field.
Specification numbers are kept on instrument lists and installation details instead of this plan. Wiring details belong on conduit schedules, cable schedules and cabling diagrams.
The Tweed EL04 specification gives an optimum working height of 1500 mm above grade or walkway. It also asks that instruments be serviceable without a ladder or scaffold.
Your own project specification may give a different figure, so follow it first. In every case, local gauges and indicators must be readable from grade or a fixed platform.
Place them near the centre of their instrument group at an accessible height. Keep them away from drains, steam leaks, hot lines and vehicle routes.
External boxes are usually stainless steel with at least IP65 protection, as Tweed EL04 requires. Leave about 25 percent spare terminals and gland space for future instruments in the area.
The plan gives each instrument its junction box and its tray or trench route. The cable schedule uses that route to record cable number, size and length.
Any change of JB position on the plan must therefore be carried into the schedule straight away. A formal change note keeps both documents consistent through every revision.
It is updated during construction whenever an instrument or junction box is moved in the field. Changes are clouded and approved before handover to the plant owner.
The final as built issue is then stored with the other plant records and drawings. Maintenance teams use that version for many years of troubleshooting and plant modifications.
Related Articles
- Piping and Instrumentation Diagram
- Marshalling Cabinet DCS Wiring
- Cable Tray Fill Calculation
- Hazardous Area Classification Zone
- Instrument Loop Checking Procedure
External References
- EL04 Installation of Electrical and Instrumentation Works, Tweed Shire Council
- Instrument Location Layout, Automation Forum
- Piping and Instrumentation Diagram, Wikipedia
What We Learn Today
- An instrument location plan places every field instrument on a scaled plan with its tag number, plant coordinates and elevation for installers and maintenance teams.
- It also shows junction boxes, air manifolds and cable routes, which feed the cable schedule, tray sizing and early cable length estimates.
- Good placement keeps instruments near their taps yet reachable from grade or platforms, ideally around 1500 mm high without ladders or scaffolds.
