The short answer
- Plan on the mounting panel outline, not the box size: it is smaller than the inside of the enclosure.
- Most devices clip onto 35 mm top-hat rail to IEC 60715, in a 7.5 mm or 15 mm deep profile. The rail is mechanical support, not a conductor you can rely on.
- Run wire duct between the rail rows and size it so it is no more than half full: duct makers base their fill tables on a 50% maximum.
- Respect each device's clearances, keep heat-sensitive parts low and away from heat sources, and route power and signal wiring apart.
- Check what the door carries and the room in front of the enclosure before you finalize.
Start with the mounting panel
The mounting panel (also called the backplate or subpanel) is the removable plate the devices bolt to. It stands off the back wall on studs, clear of the side walls and the door flange, so it is always smaller than the inside of the enclosure. That is the area you lay out on.
An example from our catalog: the 24 x 20 x 8 in 304 stainless wall-mount enclosure has, per its reference drawing, an inside of 606.6 x 505.0 x 200.2 mm nominal, a mounting panel of 545.6 x 444.0 mm (about 21.5 x 17.5 in) and a usable depth of 184.9 mm from the face of the panel to the inside of the closed door. Every NEMA4 Enclosure wall-mount enclosure has a removable galvanized mounting panel on welded studs, so you can take it out, build the layout on a bench and put it back.
Work in this order: draw the panel outline at full scale or in CAD, place the rail rows and the wire duct between them, then fill the rows with devices from their datasheet dimensions. A sum of device areas always underestimates the space, because devices only fit in whole rows and duct takes a large share of the panel. The sizing guide covers spare space and depth; this guide covers the layout itself.
DIN rail profiles to IEC 60715
"DIN rail" is the common name for the standardized metal rails that switchgear and controlgear clip onto. The current standard is IEC 60715:2017, "Dimensions of low-voltage switchgear and controlgear - Standardized mounting on rails for mechanical support of switchgear, controlgear and accessories". It replaced the 1981 first edition and its 1995 amendment. The profiles you will meet:
| Profile | Size (width x depth) | Where it is defined | Typical use |
|---|---|---|---|
| Top hat, standard | 35 x 7.5 mm | IEC/EN 60715 (formerly EN 50022, DIN 46277-3) | Terminal blocks, breakers, relays, power supplies, PLCs |
| Top hat, deep | 35 x 15 mm | IEC/EN 60715 | The same devices where a deeper rail is specified |
| Miniature top hat | 15 x 5.5 mm | EN 50045 (DIN 46277-2) | Small terminals and accessories |
| G-type rail | - | EN 50035 (withdrawn), DIN 46277-1 | Older heavier, higher-power components |
Rails are usually cold-rolled carbon steel with a zinc-plated or chromated finish. They are meant for mechanical support and are not used as a busbar, though they may give a device its chassis ground connection. The 2017 edition of IEC 60715 notes that the electrical function of the rail for protective earthing is covered by the relevant product standard, so do not assume a rail grounds a device unless its manual says so. See grounding and bonding an enclosure for how to tie the rail and the panel into the protective earth.
Fix each rail to the mounting panel along its length with machine screws into tapped holes or nuts on studs, and keep rails horizontal unless a device manual allows otherwise. Some devices are orientation-sensitive: one DIN-rail power supply maker requires the input terminals at the bottom and a reduction in output current for any other orientation.
Rows and wire duct
The classic layout alternates rail rows and slotted wire duct, with incoming power and the main disconnect at one end and field terminal blocks near the cable entries. Leave each row long enough for its devices plus end stops and spare positions.
- Duct fill. One wiring-duct maker sets the maximum wire fill in all its fill tables at 50% of the duct's interior cross-section, citing NFPA 79-2012 section 13.5.2, "Percentage Fills of Raceways (Ducts)", and explains that in practice 50% is all a duct can hold because of the air gaps between round cables. Treat 50% as a ceiling for the finished panel, not a target on day one.
- Duct material. The same maker states that under NFPA 79-2015 section 13.3.1, nonmetallic duct is permitted inside enclosures only when made of flame-retardant material, with flame retardance defined by the IEC 60332-1 test.
- Spare space. Control panel engineers describe 20 to 25% spare space as typical. Keep it as empty rail and duct capacity in the rows where additions are likely.
- Bending room. Large conductors need bending space at their terminals; NEC 312.6 sets it for cabinets and cutout boxes. Leave the most room at the main power terminals.
Device clearances come from the device manuals
Power supplies, drives, PLCs and computers specify free space around them for airflow and for reaching their terminals. These figures differ by product and frame size, so take them from each manual. As an example, one 24 V DIN-rail power supply datasheet asks for:
- 40 mm above, 20 mm below and 5 mm on the left and right sides;
- 15 mm at the sides where the neighbor is a heat source, such as another power supply;
- side clearance that may drop to zero only when the unit is permanently loaded below 50%;
- no obstruction of airflow or of the ventilation grid.
The space above and below a device is often where the wire duct runs, so check that the duct edge is outside the clearance zone, not inside it. Clearances also add up across a row: a row of four power supplies loses more width to side gaps than people expect.
Heat zones: what goes high and what goes low
Warm air collects at the top of a closed enclosure. Application engineers writing in Control Design give these layout rules:
- Mount heat-sensitive components in the lower portion of the cabinet, where it is cooler, and keep them as far as possible from heat-generating components.
- Keep heat-generating devices together, which makes the heat easier to manage.
- Where possible, mount heat-producing components such as transformers, line reactors and drives outside the panel.
- Follow the spacing recommended by the component maker, and position components so they impede airflow as little as possible. Standoffs that raise rails off the mounting panel, and devices on the side walls, can gain air space.
In a sealed NEMA 4X-type enclosure the heat has to leave through the walls, so the layout cannot fix an enclosure that is too small for its load. Check the total first with the enclosure heat calculation, then use the layout to keep hot spots away from the parts that mind them.
Keep power and signal wiring apart
Noise couples from power wiring into signal and communication cables that run alongside it. A widely used wiring and grounding guideline from a major automation maker (1998 edition) sorts conductors into categories: category 1 for control and AC power, category 2 for analog, low-power digital and communication cables, category 3 for interconnections between system components inside the enclosure. For category 2 cables it gives these minimum distances from category 1 conductors:
| Power conductors | In contiguous metal wireway or conduit | Not in metal wireway or conduit |
|---|---|---|
| Category 1 conductors under 20 A | 0.08 m (3 in) | 0.15 m (6 in) |
| AC power lines 20 A or more, up to 100 kVA | 0.15 m (6 in) | 0.3 m (1 ft) |
| AC power lines over 100 kVA | 0.3 m (1 ft) | 0.6 m (2 ft) |
The same guideline says signal cables that must cross power lines should do so at right angles, and that its spacings are for noise immunity only: local codes govern safety. Inside one enclosure that usually means a separate duct for analog and network cables, on the opposite side of the rail from the power wiring, and field terminals grouped by category. Where a device's own terminals are closer together than the spacing, the guideline accepts that.
Door devices and live parts on the door
Pushbuttons, pilot lights, selector switches, meters and displays on the door reach back into the enclosure. When the door closes they must clear every device on the mounting panel, and their wiring needs a flexible loop across the hinge side. Measure the depth behind the door of each device against the usable depth, and keep tall devices on the panel away from the area behind the door devices. The cutouts and glands guide covers hole sizes for 22 and 30 mm operators and HMI cutouts.
Live terminals on the inside of a door are also a touch hazard when it is open. According to a summary of NFPA 79-2024 by a testing laboratory, the clarified scope of clause 6.2.3.1 explicitly includes live parts mounted on the inside surface of doors or covers, with guarding judged effective when a 12.5 mm (1/2 in) jointed test finger cannot touch them; a new clause 6.2.3.1.1 expects protection against unintentional direct contact even where access is controlled by a key, tool or interlocked disconnect. Finger-safe terminals, shrouds and barriers solve it. Where devices are mounted on a conductive door, it also needs a bonding jumper; see the grounding guide.
Service space inside and out
A layout that fits is not finished until someone can work on it. Inside, leave room to reach every terminal screw with a screwdriver, to read labels, and to replace a device without removing its neighbors. Outside, check the door swing and the working space in front of the enclosure. NEC 110.26(A) (2020 and 2023 editions) requires working space in front of equipment likely to be examined, adjusted, serviced or maintained while energized, including at least a 90 degree opening of equipment doors; the depth depends on voltage and conditions. Our 24 x 20 x 8 in example has a door swing radius of 511.3 mm (about 20.1 in) on its reference drawing. Check the edition your authority having jurisdiction enforces.
Before you order, drop your layout onto the mounting panel outline from the reference CAD on any product page, check the depth against the door, and leave spare rail and duct. The custom enclosure specification guide lists what to send when no catalog size fits.
Custom builds. NEMA4 Enclosure builds custom enclosures from one piece in 304 and 316L stainless, powder-coated carbon steel, aluminum, polycarbonate and fiberglass, delivered in 10 business days from approval of the drawing. If your layout needs openings in the door or the walls, we cut the openings for it; send the positions with your quote request. Wall-mount enclosures in 304 and 316L are manufacturer-rated NEMA 4X-type / IP66 - not UL listed.
Frequently asked questions
What size DIN rail is standard?
The 35 mm top-hat rail defined in IEC 60715, either 35 x 7.5 mm or the deeper 35 x 15 mm profile. Smaller 15 x 5.5 mm miniature rail exists for small accessories.
How full can wire duct be?
Duct makers base their fill tables on a 50% maximum of the interior cross-section, citing NFPA 79. In practice round cables leave air gaps, so plan for less than that and keep spare capacity for changes.
Can a DIN rail be used as the ground?
Not by default. Rails are mechanical supports, and IEC 60715:2017 leaves the protective earthing function of a rail to the relevant product standard. Bond the rail to the mounting panel and ground bus, and follow each device manual.
How far apart should power and signal cables be?
One widely used automation guideline gives 3 in from power conductors under 20 A when the signal cable is in contiguous metal wireway, and 6 in when it is not, with larger distances for heavier power lines. Cross power lines at right angles.
Where should hot devices go on the backplate?
Group heat sources together and keep heat-sensitive devices low and away from them, or move transformers and drives outside the enclosure. Always keep the clearances the device makers specify.
Sources
- IEC 60715:2017, Dimensions of low-voltage switchgear and controlgear - Standardized mounting on rails (title, edition, technical changes)
- Wikipedia, "DIN rail" (profile designations, material, mechanical support only)
- Panduit, Comprehensive Wiring Duct Solutions, WDSG03 (50% fill per NFPA 79 13.5.2, flame-retardant duct per NFPA 79 13.3.1)
- PULS, CS5.241 24 V DIN-rail power supply datasheet (installation clearances, orientation)
- Control Design, "Crowded panels: heated discussion" (component placement for heat)
- Control Design, "Experts weigh in on best practices for control panel housings" (spare space)
- Rockwell Automation, Industrial Automation Wiring and Grounding Guidelines, publication 1770-4.1 (conductor categories and spacing)
- Intertek, NFPA 79-2024 update, part 3 (live parts on doors, clauses 6.2.3.1 and 6.2.3.1.1)
- NFPA 70, National Electrical Code (free read-only access): 110.26, 312.6
- EC&M, "NEC requirements for working clearances" (110.26 door opening)