The short answer
How to Size an Enclosure
To size an enclosure, start from the component list with the dimensions and clearances of every device, arrange them on a layout with power at the top or one side, control below, terminals at the bottom or the side where the field conduits arrive, and wireway between every row and column sized for its fill; add a quarter to a third of the area as spare, because the panel will grow; compute the heat load from the losses of every component and the solar and ambient conditions, and check that the enclosure surface can shed it within the allowed internal temperature, adding a fan, a heat exchanger, or an air conditioner where it cannot; choose the enclosure type for the environment, indoor, outdoor, washdown, or corrosive; then round up to the next standard size in height and width, check the depth against the deepest component plus the wireway and door devices, check the door swing and the mounting, and confirm the whole against the listing requirements for spacing.
Key points
- Lay out every component with its clearances and the wireway before choosing a size.
- Add a quarter to a third spare area; a full panel on day one is a rebuild in year three.
- Compute the heat load and check the surface area; add cooling where the rise is too high.
- Choose the type rating for the location; round up to a standard size.
- Check depth, door swing, mounting, and listing spacing before ordering.
What you need
- The component list with dimensions, clearances, and heat losses from the data sheets
- The panel layout drawing template
- The enclosure manufacturer catalog with standard sizes and thermal data
- The site conditions: ambient temperature range, sun exposure, indoor or outdoor, corrosive or washdown
- An enclosure heat load calculator or the manufacturer thermal software
Procedure
- 1
List the components
Every device with its width, height, depth, mounting, required clearances from the data sheet and the listing standard, and its heat loss in watts.
- 2
Arrange the layout
Power section at the top or on one side, with the disconnect at the door handle height; drives and transformers where their heat rises away from electronics; the controller and I/O in the middle; terminals at the bottom or the side where conduits enter; the network and telemetry devices near the controller; wireway between every row and column.
- 3
Size the wireway
From the conductor count and size for each duct at half fill with growth; the duct dimensions become part of the layout.
- 4
Add spare
A quarter to a third of the mounting area empty, with wireway and terminals to use it; a spare rack slot; spare terminals.
- 5
Compute the heat load
Sum the component losses; add solar gain for an outdoor enclosure in the sun; take the maximum ambient. Compute the internal temperature rise for the enclosure surface area and material using the manufacturer method, and compare with the lowest component rating, often 40 to 50 degrees Celsius.
- 6
Decide the cooling
If the rise is acceptable, a sealed enclosure. If not: a filtered fan for clean indoor air, a heat exchanger for dirty or outdoor air, or an air conditioner for a hot location or a large load. A heater for condensation in cold or humid locations.
- 7
Choose the type
Type 12 indoors; Type 4 outdoors or washdown; Type 4X for corrosive or chemical rooms; Type 3R for outdoor with only rain protection; per the code and the site. Material accordingly: painted steel, stainless, or non-metallic.
- 8
Pick the size
The layout height and width rounded up to the next standard size, keeping the spare. Depth from the deepest component plus the wireway height plus the door-mounted devices, plus clearance.
- 9
Check the details
Door swing at the installation location; working space in front; conduit entry locations against the terminals; mounting, wall or free-standing or a leg kit; lifting; the sub-panel size for the mounting.
- 10
Check the listing
Spacings between live parts and the enclosure, between power and control, and around the disconnect, per the panel standard; the enclosure listed for the type.
ΔT = P ÷ (k × A)
- ΔT = internal temperature rise above ambient, °C
- P = total heat dissipated inside the enclosure, W
- A = effective enclosure surface area, m², per the manufacturer method for the mounting
- k = heat transfer coefficient, roughly 5.5 W/m²·°C for painted steel in still air; use the manufacturer value
| Location | Type | Cooling usually needed |
|---|---|---|
| Indoor control room | Type 12 | Sealed or a filtered fan |
| Indoor process area, wet | Type 4 or 4X | Sealed or a heat exchanger |
| Outdoor, shaded | Type 4 or 3R | Heat exchanger; heater in winter |
| Outdoor, in the sun | Type 4 with a sun shield | Air conditioner for drives; heater in winter |
| Chemical room | Type 4X non-metallic or stainless | Sealed with a heat exchanger |
Frequently asked questions
- How much spare space?
- A quarter to a third of the mounting area on a plant panel, more on a panel for a process that will change. Spare space costs a few inches of steel; a second enclosure costs a project.
- Can I mount components on the door?
- Operator devices, displays, and light devices, with the door bonded and the depth accounted for. Not heavy devices, and nothing that puts live parts within reach when the door opens without a dead front.
- Fan or air conditioner?
- A filtered fan where the ambient air is cool, clean, and dry enough to cool the panel; an air conditioner where it is not or where the load is large; a heat exchanger between. The heat calculation and the site conditions decide, and a fan in a dusty room is a filter maintenance program.
- Why is the panel hotter than the calculation said?
- A component loss underestimated, usually a drive or a power supply at load; a sun exposure not counted; a filter clogged; or a component mounted in the hot zone above a transformer. Measure the temperature at the top and the drive losses at load.
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.
- 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.
- 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.
- 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.
- Panel OverheatingA panel that runs hot: drives tripping on temperature in the afternoon, controllers faulting in summer, components discolored, a fan that never stops. The causes from an undersized enclosure to a clogged filter, measuring the rise, and the fixes.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.