The short answer
Component Layout
A good layout keeps power and control apart, puts the heat where it can leave, keeps analog and network wiring away from drives and contactors, and leaves room to work and to grow. The incoming power and the disconnect at the top or the side where the feeder enters, drives and starters in their own region, the controller and its I/O well away from them, terminals along the bottom or the sides where field wiring arrives, and wireways between every row with space to spare. Draw it to scale before anything is mounted.
Key points
- Separate power and control into regions with wireway between them, and keep field wiring off the control side until it reaches the terminals.
- Put drives and other heat sources where their heat rises out, not under the controller.
- Route analog and network cables in their own wireway, crossing power at right angles if they must cross at all.
- Terminals at the entry side, grouped by voltage and by field circuit, with spares.
- Leave twenty percent of the panel empty and every device reachable with the door open.
Regions
The first decision is to divide the back panel into regions by function and voltage, and to keep them separated by wireway. A typical arrangement for a pump panel puts the incoming disconnect and the main protection at the top corner where the feeder arrives, the motor starters or drives across the top or down one side, the control transformer and the power supplies near them, the controller and its I/O in the middle or on the opposite side, and the terminal strips along the bottom and the entry side. The regions give a technician a map: power is here, control is there, the field lands along the bottom.
| Component | Where | Why |
|---|---|---|
| Disconnect and main breaker | Top, at the feeder entry, with the handle through the door | Shortest feeder run; the handle is where a person expects it |
| Drives, soft starters, starters | Top or upper side, their own region | Heat rises out; keeps their noise and their line-side voltage away from control |
| Control transformer, power supplies | Near the power region | They belong to the power distribution and they are warm |
| Controller and I/O | Middle or lower, away from drives | Cooler, quieter, and the field wiring reaches it easily |
| Network switch, radio, modem | Near the controller, away from drives | Short patch cables and less interference |
| Terminal strips | Bottom and entry side | Field conduits enter from below; short runs from the field |
| Surge protection | At the entry of what it protects | A surge device on a long lead is a surge device on a resistor |
| Door-mounted devices | Front, in reach, grouped by function | Operators use them; wire them through a flexible loom at the hinge |
Heat
Heat rises, so the heat sources go high and the components that dislike heat go low. A drive mounted above a controller pours warm air over the controller all day. Drives need the clearance above and below that their manuals specify for their own airflow, and side by side they need the spacing stated for adjacent drives. A panel where the heat calculation is marginal is a panel where layout matters most, and the calculation is done before the layout, not after.
Noise
The output cables of a drive radiate high-frequency noise, and contactors and relay coils make sharp transients when they switch. Analog signal wiring, network cable, and communication cable pick both up. The layout keeps them apart: the drive and its cables in their region, the analog and network wiring in a separate wireway on the control side, and where a signal cable has to cross a power conductor, it crosses at a right angle and does not run parallel. Shields land on the ground bar at the control side. A well-laid-out panel needs no filters on its analog inputs; a badly laid-out one needs them and still reads noise.
Wireways and wiring
Wireway runs between every row of components and along every edge where wiring turns, sized so that it is no more than about half full when the panel leaves the shop, because it will be fuller later. Separate wireways, or a divided wireway, carry power and control. Wiring is color coded by function under the panel standard, UL 508A and the owner’s specification, so that a technician can tell a 120 VAC control wire from a 24 VDC one from a foreign voltage without a meter. Every wire is labeled at both ends with the number on the drawing.
Terminals
The terminal strips are the interface between the panel and the field, and they are where the technician spends the most time. Group them by voltage, with barriers or space between groups, and by circuit within a group, in the order the drawing shows. Fused terminals for field circuits that need individual protection, disconnect terminals for loops that will be tested, and spare terminals of every type. Terminals for intrinsically safe circuits are blue and separated from everything else by the distance the barrier manufacturer requires. Label every terminal, and mount the strip where the field conductors reach it without crossing the control region.
Working in it
- Every device is reachable with the door open and nothing removed. A relay behind a wireway cover behind a drive is a relay that never gets replaced.
- The back of the door is reachable when the door is open, with a loom long enough and a hinge clear of the wall.
- A dead-front covers live parts where operators will open the outer door to reach a display or a switch.
- Drawings, in a pocket on the door, current.
- The working space in front, per NEC 110.26, is marked on the layout and, once installed, on the floor.
- Twenty percent of the back panel and of every terminal strip is empty on the day it ships.
Frequently asked questions
- Where should the PLC go in a panel with VFDs?
- As far from the drives as the panel allows, and below them, with wireway between. The drives heat and radiate noise; the controller and its analog inputs want neither.
- Can power and control wiring share a wireway?
- Not if the layout can avoid it. Separate wireways, or a divided one, keep the drive and contactor noise off the signal wiring and keep a technician from finding line voltage beside a 24 volt wire. Where they must cross, cross at a right angle.
- How full should a wireway be?
- About half when the panel ships. It will be fuller after every change order, and a wireway that is full on day one cannot take the next wire without the cover coming off for good.
- How much spare space should a panel have?
- Twenty percent of the back panel and of the terminal positions is the usual rule. It costs a slightly larger enclosure and it saves the next enclosure.
Related topics
- Enclosure SelectionChoosing the box: NEMA and UL 50 type ratings, materials for corrosive and outdoor sites, sizing for heat and wire bending space, and the details at the conduit entries and the door that decide whether the rating survives installation.
- 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.
- WirewaysPanel wireway design: fill limits and the space terminations need, separating power, control, and signal wiring, routing around heat and drives, sizing rules, the cases where wireway is the wrong choice, and the habits that keep a panel serviceable.
- 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.
- Ground Loops in InstrumentationTwo grounds at different potentials, one signal circuit between them, and a reading that will not sit still. How to find it and how to prevent it.
- UL 508A Industrial Control PanelsWhat the listing actually covers, what has to appear on the label, and the requirements that most often cause a panel to fail inspection.
Direct contact
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