The short answer
Wireways
Wireway is the slotted plastic duct that organizes the wiring inside a panel, and its design decides whether the panel can be wired, tested, and modified without cutting anything. The rules are simple: fill it to no more than about half its cross-section so wires can be added and traced, size it for the conductors that will actually run through it plus the growth the panel will see, keep power, control, and signal wiring in separate ducts or at least separate sections, route it so that every terminal has a wireway within reach and the wire lengths are short, and keep it away from the heat of drives and transformers and the noise of drive output cables. Wireway that is too small, too full, or shared between drive power and instrument signals is where a panel becomes both unmaintainable and noisy.
Key points
- Fill to about half; a full wireway cannot be traced, added to, or closed.
- Separate power, control, and signal wiring in separate wireways; drive output cables get their own path.
- Size from the conductor count and size with growth, not from what fits today.
- Every terminal strip has a wireway beside it; wires run from device to wireway to terminal, never across open space.
- Keep wireway out of the heat above transformers and drives and below the depth of the deepest component.
Fill and sizing
The conductor area in a wireway should not exceed about half of the internal cross-section, and less is better in a panel that will be modified. The area is computed from the number and size of conductors with insulation, and the sum is compared with the duct area. Growth of a quarter to a third is added at design. A common failing is a wireway sized for the schematic and then filled by the field wiring, the shield drains, the ferrules, and the slack, so that the cover will not close; the answer is a deeper or wider duct in the layout, not a hammer.
| Nominal wireway | Internal area, roughly | Practical conductor count at half fill |
|---|---|---|
| 1 by 2 inch | About 1.7 square inches | Around 40 to 50 of 16 AWG, fewer with ferrules and shields |
| 1.5 by 3 inch | About 4 square inches | Around 100 of 16 AWG, or a mixed bundle of 14 and 12 AWG control |
| 2 by 3 inch | About 5.5 square inches | Power feeders to several starters, or a busy control section |
| 3 by 3 inch | About 8.5 square inches | Main power runs and large motor leads |
Separation
- Power wiring above 120 volts in its own wireway, with the drive output cables routed separately and as short as possible.
- 120 volt control wiring in its own wireway, or separated from power by a partition.
- 24 volt discrete, analog signal, and network wiring together only if they are all low voltage and none is a noise source; analog and network away from anything switched.
- Intrinsically safe wiring in a dedicated, identified wireway with the required separation.
- Crossings at right angles where separation cannot be maintained, with the crossing kept short.
Routing
Wireway runs horizontally along the top and bottom of each row of components and vertically between columns, so that every device has a duct within a few inches of its terminals and every wire runs device to duct to terminal. Terminal strips have a duct alongside for the field wiring, sized for the field conductors, which are larger and stiffer than the panel wiring. The layout keeps ducts away from the top of transformers and drives, where the heat is, and clear of the depth of deep components so that the cover can be removed with the panel live. A duct that has to be removed to reach a device is a duct in the wrong place.
Details that matter
- Fingers
- The slots in the duct wall; the wire exits through the finger nearest the terminal, and fingers are broken out cleanly, never leaving sharp edges.
- Covers
- Covers stay on in a finished panel; a panel that ships with covers in a bag has a fill problem.
- Radius
- Duct corners and wire exits respect the conductor bend radius, especially for shielded and network cable.
- Wire dressing
- Wires enter the duct at right angles with a little slack, not pulled taut across the finger edge.
- Labeling
- Wire labels are placed where they can be read at the terminal, which means the label is outside the duct.
- Shield drains
- Shield drains are terminated at the ground bar close to where the cable enters, not run the length of the panel in the signal duct.
When not to use wireway
- Drive output cables of any size: route directly, short, and away from everything, in a shielded cable where the drive requires it.
- Large feeders: cable ties and standoffs, or a dedicated large duct.
- Fiber patch cords: a dedicated tray or a fiber management panel with radius control.
- Very small panels: point-to-point wiring with proper dressing can be cleaner than duct that fills the box.
Frequently asked questions
- Can 24 volt instrument wiring and 120 volt control share a wireway?
- They can be run together only where the code separation rules allow and the noise is acceptable, and in practice a shared duct makes the analog signals noisy. Separate ducts, or a partition, are the standard.
- How much growth should I allow?
- A quarter to a third of the fill at design, more on a panel for a plant that changes often. Spare terminals and spare I/O without spare wireway are wasted.
- Metal or plastic wireway?
- Plastic slotted duct for panel wiring, in a flame-rated grade. Metal wireway is an installation product for runs between enclosures and is not a substitute for panel duct.
- The wireway is full and I need to add three wires.
- Remove the cover, add the wires, and note in the change record that the duct is over fill; then plan a duct replacement at the next outage. Cutting the cover or leaving it off is how panels catch stray wires.
Related topics
- Component LayoutArranging a panel so it is safe to work in, cool enough to run, quiet enough for its signals, and possible to modify: power and control separation, heat, wireways, terminals, and the working space rules.
- TerminalsDesigning the terminal strips of a control panel: grouping by voltage and function, numbering that matches the drawings, fused and disconnect terminals for instrument loops, shield and ground terminals, spares, ferrules and torque, and the terminal schedule.
- Noise ProblemsElectrical noise as a panel problem: the four ways it couples, the design features that keep it out, the sources people forget such as unsuppressed coils and unbonded doors, and the sequence for finding where noise gets into a panel that should be clean.
- Enclosure Heat CalculationsEstimating the temperature inside a control panel from the power its contents dissipate and the surface that carries it out, what to do when the answer is too hot, and why drives change everything.
- Panel LayoutsThe drawing of what goes where inside a control panel: enclosure, back plate with every component to scale and keyed to the bill of material, wireways, terminals, door devices, and entries. The conventions, the required spacings, and the checks.
- Cable Routing ProblemsNoise that comes from where a cable runs: signal cables sharing a conduit, tray, or wireway with power or drive cables, long parallel runs, and shallow crossings. How coupling works, the separations that prevent it, and how to find the shared path.
Direct contact
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