A junction box terminal drawing tells you exactly where every instrument conductor connects and where it continues toward the control panel. It must identify both ends of each connection, including cable numbers, core or pair identifiers, and terminal references. A cable route alone is not enough. The drawing should support installation, isolated continuity checks, fault tracing, and later modifications without relying on assumptions about conductor colors or terminal positions.
What the terminal drawing connects
The junction box is an intermediate termination point between field instruments and the control-panel wiring. Individual instrument cables usually enter the box and terminate on identified terminal blocks. An outgoing multicore or multipair cable carries those circuits onward.
The terminal drawing records those electrical connections. Each conductor needs a traceable path through these references:
- Instrument tag and instrument terminal designation.
- Incoming cable identifier and conductor identifier.
- Junction box tag, terminal-strip designation, and terminal number.
- Outgoing cable identifier, pair number, and core identifier.
- Destination cabinet tag, terminal strip, and terminal number.
- Continuation reference when the circuit proceeds onto another drawing.
Use the identifiers actually marked on the equipment and cables. If cable cores carry printed numbers, record those numbers rather than substituting colors. For paired cables, distinguish the pair number from the two conductor identifiers within that pair.
A terminal number identifies a connection point, not necessarily an entire circuit. Multi-level terminals also need a level designation whenever the numbering system does not uniquely identify each level.
The instrument junction box selection article covers choosing the enclosure around these connections. The terminal drawing addresses the connections themselves.
Follow one signal from instrument to panel
The following example shows connection mapping for a two-conductor instrument circuit. All tags and terminal assignments are illustrative, not product identifiers or a complete loop design.
Assume the instrument documentation identifies its terminals as 1+ and 2−. The project cable schedule identifies the outgoing pair conductors as A and B.
| Instrument terminal | Incoming cable and core | Junction box terminal | Outgoing cable | Pair and core | Destination terminal |
|---|---|---|---|---|---|
| FT-101:1+ | IC-101, core 1 | JB-01, X1:01 | MC-01 | Pair 01, A | MAR-01, X10:101 |
| FT-101:2− | IC-101, core 2 | JB-01, X1:02 | MC-01 | Pair 01, B | MAR-01, X10:102 |
The first row traces one conductor from the instrument through the junction box to the marshalling cabinet. The second row traces its return conductor. Neither row alone establishes where loop power originates or how the input channel connects.
Add a reference to the loop diagram or panel wiring sheet showing the circuit beyond MAR-01, X10. That document should establish the power source, input channel, polarity, and any intervening isolator or barrier.
Define the terminal drawing's viewing direction, such as looking into the open box from the front. Label field and outgoing sides, especially when the terminal symbol does not make their relationship obvious.
If a connection uses an installed bridge, show its exact terminal span and purpose. Adjacent terminal symbols must not imply an electrical bridge that is absent from the physical assembly.
Show screens separately
Cable screens and drain wires need their own treatment. State whether each screen passes through insulated terminals, terminates locally, or continues to a designated panel shield bar.
Do not assume every screen bonds to the junction box. The correct arrangement depends on the equipment instructions and the project's grounding and EMC design. Record intentional bonds and intentional isolation explicitly.
Protective bonding is a separate function. Use the enclosure grounding and bonding guide for the enclosure's fault-current path rather than treating an instrument screen as its protective conductor.
What the drawing verifies and what testing must establish
A drawing review checks whether the documented connection path is complete and internally consistent. It can reveal duplicate terminal assignments, missing destinations, inconsistent polarity, and unassigned outgoing pairs.
It does not prove that the installed conductors follow those assignments. Physical verification requires inspection and suitable electrical checks against the approved drawing.
Before continuity testing, isolate the relevant circuits and verify the safe condition under the site's electrical safety procedure. Disconnect or otherwise protect connected electronics as their manufacturers require. Parallel paths through devices or bridges can produce misleading continuity results.
Record checks against identifiable endpoints, for example FT-101:1+ to MAR-01, X10:101. Also check for unintended connections between conductors and to earth where the circuit design requires isolation.
Keep these verification limits clear:
| Check | What it establishes | What it does not establish |
|---|---|---|
| Drawing review | Documented connections and identifiers agree | Physical wiring matches the document |
| Visual inspection | Markers, terminations, and hardware match the drawing | Reliable end-to-end electrical continuity |
| Isolated continuity check | Intended endpoints have a conductive path | Correct instrument operation or analog scaling |
| Functional loop check | The assembled loop responds as specified | Enclosure sealing or environmental performance |
Continuity and insulation resistance are different tests. Do not apply an insulation-resistance test voltage across connected transmitters, input electronics, or surge devices unless their documentation expressly permits it.
A terminal drawing also cannot establish ingress protection, chemical compatibility, or enclosure heat performance. Those require separate specifications and verification.
How the mapping changes the physical layout
Finish the connection schedule before fixing terminal-strip length and cable-entry positions. Otherwise, the enclosure can fit the terminal blocks but leave inadequate space for the actual wiring.
Count physical terminal positions, not just signals. Feed-through terminals, screen terminals, protective-earth terminals, disconnect terminals, and spare positions can require different amounts of rail space. Accessories also occupy space.
Keep pairs together and preserve the cable manufacturer's permitted bend radius. Place entries so conductors reach their assigned terminals without crossing unrelated circuits or obstructing access.
Document signal groups where segregation matters. Analog signals, digital circuits, and power wiring should not become interchangeable simply because spare terminals are available. The article on mixing analog and digital signals explains the layout considerations behind that decision.
Use the backplate and DIN rail layout guide for rail, duct, and service-space fundamentals. On the terminal drawing, identify the strip arrangement clearly enough to locate each connection in the enclosure.
Reserve spare pairs deliberately. Mark whether they terminate on allocated spare terminals or remain insulated and secured. An unused core should not appear connected merely because it lies beside an occupied terminal.
Keep electrical mapping separate from fabrication
The terminal drawing and enclosure fabrication drawing answer different questions. They can share references, but neither should substitute for the other.
| Document | Main purpose | Essential content |
|---|---|---|
| Terminal drawing | Define conductor connections | Tags, cable cores, pairs, terminal assignments, bridges, and destinations |
| Internal layout | Locate installed equipment | Terminal strips, rail, duct, bonding points, and access clearances |
| Fabrication drawing | Define the enclosure and openings | Body dimensions, material, mounting details, cutout dimensions, and locations |
The terminal drawing may identify an incoming cable without defining its gland hole. The fabrication drawing may locate that hole without identifying any conductor inside the cable.
Tie these documents together through a cable-entry schedule. Each entry should identify its cable, fitting, enclosure face, and location reference. Confirm the fitting's sealing range against the actual cable outside diameter.
The gland plate and split cable-entry comparison addresses entry-system choices. The enclosure drawing verification article covers checking mechanical details before fabrication.
A later cable change can affect both documents. A replacement multipair cable may need a different gland, more bending space, or revised screen terminations. Review those consequences before changing only the cable number.
What to specify in the issued drawing package
Specify a terminal drawing that provides an unbroken connection record, not just a sketch of terminal blocks. Include the junction box tag, drawing number, revision, sheet references, and the documents used for instrument terminal designations.
The issued package should contain:
- A conductor-by-conductor terminal map with unique endpoint references.
- A cable schedule defining core numbers, pair identifiers, and screen construction.
- Explicit screen, protective-bonding, bridge, and spare-core arrangements.
- An internal layout coordinated with terminal hardware and cable bending space.
- A fabrication drawing coordinated with cable entries and mounting details.
- A continuity record using the same endpoint identifiers as the terminal map.
Update the connection record after approved installation changes. Keep the final drawing aligned with cable markers, terminal labels, and recorded checks. An undocumented terminal swap makes later fault tracing unreliable even when the circuit currently works.
For the enclosure order, specify material, outside height, width, depth, mounting arrangement, and coordinated cutouts separately from the electrical mapping. The custom stainless enclosure and cutout page explains the mechanical information needed for drawing review.
Our wall-mount enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Terminal mapping does not preserve that rating by itself. Review the installed entry components, seals, and assembly details independently.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 6 × 6 × 4 in | 304 stainless | 6 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 8 × 6 × 4 in | 304 stainless | 8 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 10 × 8 × 4 in | 304 stainless | 10 x 8 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
What is the purpose of a junction box terminal drawing?
It defines the connection path from each instrument terminal through the junction box to the outgoing cable and destination cabinet. Cable identifiers, conductor or pair identifiers, and terminal references make that path usable for installation, continuity checks, and fault tracing.
How is a terminal drawing different from a loop diagram?
A loop diagram shows the complete instrument circuit, including power, input connections, and other devices. A terminal drawing concentrates on physical termination assignments. Reference the two documents together so the junction box connections remain consistent with the complete circuit.
Should the drawing include cable screens and spare cores?
Yes. State where screens connect, where they remain isolated, and how unused conductors are secured or assigned to spare terminals. Screen bonding must follow the equipment instructions and project design. Do not assume the instrument screen provides protective bonding for the enclosure.
Can the terminal drawing be used to fabricate the enclosure?
No. A terminal drawing defines electrical connections but usually lacks the dimensions needed for fabrication. Supply a coordinated mechanical drawing with enclosure dimensions, material, mounting details, and cutout locations. Use a cable-entry schedule to connect the electrical cable information with the mechanical openings.
