Sizing Guide
How to Size an Enclosure Air Conditioner
The step-by-step formula for calculating the right cooling capacity for any industrial, telecom, or utility enclosure.
Quick Answer
Calculate your internal heat load in watts , multiply by 3.412 to convert to BTU/hr , add a 20-25% safety factor, and match to a cooling unit with that capacity at your maximum ambient temperature . For a 500W heat load at 104°F ambient, you need approximately 2,050 BTU/hr — a VoltAire AN02 (2,000 BTU/hr).
Enclosure Cooling Sizing Calculator
Add your components, set your conditions, and get an instant recommendation. Or scroll down to learn how to do it by hand.
1. Add Heat Sources
Click to add common components:
Or add a custom heat source — name it, enter the watts, click Add:
2. Installation Environment
Typical R-3 to R-7
Default: 12 sq ft
Wall conduction:
Top + sun-facing sides
Hottest expected outside temp at the enclosure
Max allowable temp inside the enclosure
Your Calculation
Recommended: Heat Exchanger
Ambient (°F) is °F below your target (°F) — no compressor needed.
Why not an AC? Your enclosure generates W of heat, and your ambient temp is well below the target. A heat exchanger moves this heat out passively — no refrigerant, no compressor, lower cost, less maintenance, and lower energy consumption.
Almost there — enter your temperatures
We need your max ambient and target interior temperatures to recommend the right cooling type. If ambient is cooler than your target, a heat exchanger can handle the job at lower cost (no compressor, no refrigerant). If ambient is hotter, you'll need an air conditioner.
Recommended: Air Conditioner
Ambient (°F) is at or above your target (°F) — active refrigeration required.
Your heat load needs of cooling capacity, but the largest VoltAire heat exchanger (HTC090B) provides only 90 W/°F. An air conditioner handles the load actively.
Close call: Your ΔT is only °F. A heat exchanger needs at least 10°F differential to work effectively. If you can tolerate a slightly higher internal temperature, a heat exchanger would save cost. Otherwise, go with the air conditioner.
Exceeds Single-Unit Capacity
Your calculated requirement of BTU/hr exceeds our largest single unit (AT19 at 19,000 BTU/hr). You may need multiple units or a custom solution.
Call (844) 925-5668Add your heat-generating components above to calculate your cooling requirement.
Then enter your ambient and target temperatures — we'll recommend either an air conditioner or a heat exchanger depending on your conditions.
This calculator provides an estimate. Actual requirements may vary based on enclosure construction, ventilation, and environmental conditions. Call (844) 925-5668 for engineering assistance.
Prefer to Do It by Hand?
The Manual Method — Step by Step
The calculator above does this math for you, but here's the full breakdown so you understand exactly what's happening under the hood.
The Formula
Step-by-Step Sizing Formula
Follow these steps to determine the correct enclosure air conditioner size for your application. No special tools required — just component datasheets and a calculator.
List All Heat-Generating Components
Inventory every device inside the enclosure that generates heat. Common sources include variable frequency drives (VFDs), programmable logic controllers (PLCs), power supplies, servo drives, relays, contactors, transformers, battery chargers, lighting, and door/panel heaters. Don't forget secondary sources like terminal blocks carrying high current and any instrumentation.
Sum Total Watts (Heat Dissipation)
Add the maximum power dissipation in watts for each component. Use manufacturer datasheets — they list maximum power dissipation or, for motor drives, input power minus output power equals heat. If datasheets are unavailable, assume 100% of input power becomes heat for resistive loads (heaters, lights) and use the efficiency rating to calculate waste heat for electronic loads.
Example
VFD (25 HP, 93% eff.) — 1,400W input, heat = 1,400 × 0.07 = 98W
PLC + I/O modules — 85W
24VDC power supply (480W, 89% eff.) — heat = 480 × 0.11 = 53W
Panel lighting — 40W
Contactors & relays — 30W
Total internal heat load = 306W
Convert Watts to BTU/hr
Enclosure air conditioners are rated in BTU per hour. To convert your wattage total:
BTU/hr = Watts × 3.412
306W × 3.412 = 1,044 BTU/hr
Add Solar Loading (Outdoor Enclosures)
For enclosures installed outdoors with direct sun exposure, solar radiation adds significant heat. The amount depends on enclosure color, material, and geographic location. Use these approximate values:
| Surface Type | BTU/hr per sq ft |
|---|---|
| Light color / reflective | 100 |
| Medium color (gray, beige) | 200 |
| Dark color (black, dark gray) | 300 |
Calculate the sun-exposed surface area in square feet (typically the top and one or two sides), then multiply by the appropriate factor. Indoor enclosures can skip this step.
Account for Cabinet Wall Conduction
Heat conducts through cabinet walls in whichever direction temperature drives it. For an outdoor cabinet sitting in 100°F ambient with a 75°F target interior, walls constantly leak ambient heat in — adding to your cooling load. For a cabinet hotter inside than outside, walls leak heat out — partially cooling the enclosure for free.
Q_walls = U × A × |ΔT|
- U = heat-transfer coefficient (BTU/hr · ft² · °F)
- A = cabinet surface area (ft²)
- |ΔT| = absolute difference between ambient and target temperatures (°F)
| Cabinet Construction | R-value | U (BTU/hr·ft²·°F) |
|---|---|---|
| Uninsulated painted steel (default) | ∼0 | 1.0 |
| 1″ foam-insulated NEMA 4X | R-5 | 0.17 |
| 2″ foam-insulated (extreme climate) | R-10 | 0.09 |
| Stainless steel sandwich (premium) | R-15 | 0.06 |
Worked example — outdoor cabinet, 100°F ambient, 75°F target (AC scenario):
Cabinet: 24″×24″×12″ outdoor NEMA 4X, 1″ foam insulated (R-5).
A = 12 ft², U = 1/(5+0.85) = 0.17, ΔT = 25°F.
Q_walls = 0.17 × 12 × 25 = 51 BTU/hr coming in through the walls.
Add this to your cooling load. An uninsulated equivalent (U = 1.0) would have Q_walls = 1.0 × 12 × 25 = 300 BTU/hr — nearly 6× larger.
For heat-exchanger scenarios (target hotter than ambient):
Walls leak heat out — passively helping cool the cabinet. Subtract Q_walls from your cooling load instead of adding it. Counterintuitively, insulation reduces this passive cooling effect, which means insulated cabinets need slightly more heat-exchanger capacity, not less.
How big is the effect? For most mid-to-large cabinets with meaningful equipment heat loads, wall conduction is 5–15% of the total. Not catastrophic to ignore, but enough to push borderline sizing decisions one way or the other — particularly for small RTU panels with low internal heat where wall conduction can dominate.
Determine Maximum Ambient Temperature
Identify the maximum summer temperature at the installation site. Use ASHRAE design data, local weather records, or the building's design conditions. For outdoor installations, use the 1% design dry-bulb temperature for your region — this is the temperature exceeded only 1% of the time during the cooling season. Common design temperatures range from 95°F (35°C) in the northern U.S. to 115°F (46°C) in the desert Southwest.
Important: Cooling capacity decreases as ambient temperature rises. A unit rated at 4,000 BTU/hr at 95°F may only deliver 3,200 BTU/hr at 115°F. Always check the capacity rating at your actual design ambient.
Apply Safety Factor
Add a margin for unknowns, future component additions, and capacity degradation over the unit's lifespan:
Indoor enclosures, well-documented heat loads, moderate climate
Outdoor enclosures, extreme ambient, mission-critical equipment, or uncertain heat loads
Required BTU/hr = (Internal + Solar ± Walls) × Safety Factor
(1,044 + 0 + 0) × 1.2 = 1,253 BTU/hr
Walls add to the load for AC scenarios (ambient hotter than target) and subtract from it for HX scenarios (target hotter than ambient).
Match to a VoltAire Model
Select the VoltAire model with a rated cooling capacity at or above your calculated requirement. For our 1,253 BTU/hr example, the VoltAire AN02 at 2,000 BTU/hr provides comfortable headroom. Don't jump to a much larger unit — moderate oversizing is fine, but excessive oversizing causes short cycling and reduces equipment life.
At a Glance
Quick Reference: Heat Load to Model
| Heat Load | Model | BTU/hr | Price |
|---|---|---|---|
| 200-300W | AN01 | 1,000 | $2,494 |
| 400-600W | AN02 | 2,000 | $2,634 |
| 800-1,200W | AN04 | 4,000 | $3,140 |
| 1,200-1,800W | AN06 | 6,000 | $3,526 |
| 1,800-2,400W | AN08 / AT08 | 8,000 | From $3,908 |
| 3,000-5,500W | AT19 | 19,000 | From $6,155 |
Prices shown are list price for standard configurations. 48VDC and NEMA 4X options may vary. In-stock units ship same or next business day.
Avoid These Errors
5 Common Sizing Mistakes
Ignoring solar gain on outdoor enclosures
Solar radiation can add 1,000+ BTU/hr to an outdoor enclosure. A sizing calculation that only accounts for internal components will undersize the cooling system by 20-40% in many outdoor installations. Always calculate sun-exposed surface area and apply the appropriate solar loading factor.
Using nominal instead of maximum ambient temperature
Cooling capacity must be sized for the worst-case scenario. Using an average summer temperature instead of the 1% design temperature means your system will fail on the hottest days — exactly when your equipment needs cooling most. Use ASHRAE design data for your region.
Forgetting altitude derating above 5,000 feet
Air-cooled systems lose approximately 1% of cooling capacity per 100 feet above 5,000 feet elevation. At 7,500 feet (common in the western U.S.), a unit rated at 4,000 BTU/hr effectively delivers only about 3,000 BTU/hr. Size accordingly or consider the next larger unit.
Not accounting for all heat sources
It is easy to miss secondary heat sources: panel lighting, door heaters (anti-condensation), terminal blocks carrying high current, and heat conducted through shared enclosure walls. A thorough component inventory prevents undersizing surprises after installation.
Oversizing the cooling unit
Bigger is not always better. An oversized air conditioner costs more upfront and causes short cycling — the compressor starts and stops too frequently, which reduces compressor lifespan, increases energy usage, and impairs dehumidification. Select a unit that matches your calculated load, not one two sizes up.
See It in Action
Real-World Sizing Examples
PLC Cabinet in a 95°F Factory
Components: PLC (85W), power supply (53W), I/O cards (120W), panel light (40W), contactors (30W), misc (172W)
Total heat load: 500W
BTU/hr: 500 × 3.412 = 1,706
Solar: None (indoor)
Safety factor: 1.2x = 2,047 BTU/hr
Recommended: AN02 (2,000 BTU/hr) — $2,634
Telecom Hut in Arizona
Components: Network switches, UPS, battery charger, fiber gear
Cabinet: 36″×36″×24″, NEMA 4X uninsulated, gray
Conditions: 115°F max ambient, 95°F target interior
Total heat load: 2,000W
BTU/hr: 2,000 × 3.412 = 6,824
Solar: 18 sq ft exposed × 200 = 3,600 BTU/hr
Walls: 1.0 × 36 ft² × 20°F = +720 BTU/hr
Subtotal: 11,144 BTU/hr
Safety factor: 1.25x (critical) = 13,930 BTU/hr
Recommended: AT19 (19,000 BTU/hr) — or two AT08 units in parallel
Motor Drive Enclosure
Components: Three 100HP VFDs (97% eff.), PLC, HMI, power distribution
Total heat load: 5,000W
BTU/hr: 5,000 × 3.412 = 17,060
Solar: None (indoor)
Safety factor: 1.2x = 20,472 BTU/hr
Recommended: AT19 (19,000 BTU/hr) — $6,155
FAQ
Frequently Asked Questions
What is the formula to size an enclosure air conditioner?
How do I calculate internal heat load for an electrical enclosure?
Do I need to account for solar loading on outdoor enclosures?
What happens if I oversize my enclosure air conditioner?
How does altitude affect enclosure air conditioner sizing?
Not Sure? Call Us.
Our team sizes enclosure cooling systems every day. Tell us your heat load, enclosure dimensions, and ambient conditions — we'll recommend the right unit in minutes.