HVACDirect.com Equipment Guide

How to Size a Commercial Packaged Rooftop Unit

A commercial packaged rooftop unit must match the building’s cooling and heating loads, ventilation airflow, duct resistance, electrical service, roof curb, and controls. Use this guide to understand nominal tons, estimate a preliminary capacity, and record the voltage and phase required before comparing equipment.

Determine the commercial cooling load
Match voltage, phase, airflow and heat type
Verify curb, duct and electrical requirements

Do not select a commercial rooftop unit from square footage alone. Occupancy, lighting, equipment, outdoor air, operating schedule, envelope, glass, orientation, climate, humidity and duct conditions can materially change the load. A qualified HVAC professional should perform an accepted commercial load calculation and verify the complete design before equipment is ordered.

Commercial Rooftop Unit Tonnage Estimator

Use a calculated design cooling load whenever available. If you only know the floor area, the square-footage method provides a broad budgeting estimate—not a final equipment selection.

Preferred: Enter a Design Cooling Load

Use the total peak cooling load from a commercial load calculation, including required outdoor air.

Design cooling load (BTU/h) When entered, this result takes priority over the area estimate.

Planning Only: Estimate from Floor Area

Choose a preliminary load density. This does not account for the many inputs required by a commercial load calculation.

Conditioned floor area (sq. ft.)

Preliminary cooling-load density

Record the Building Power

These choices do not calculate electrical capacity; they identify which product configuration may be compatible with the service at the unit.

Available voltage

Available phase

— nominal tons

Enter a cooling load or floor area

Your preliminary capacity will appear here automatically.

Estimated load—

Nominal airflow—

Selected powerNot selected

Final capacity and airflow must be selected from manufacturer performance data at the actual design conditions and external static pressure.

Shop Commercial Rooftop Units

Estimator assumptions: 12,000 BTU/h per nominal ton, common half-ton rounding for the displayed starting size, and approximately 400 CFM per nominal ton. Actual product sizes, sensible heat ratios, airflow settings and delivered capacities vary. Loads above the available single-unit range may require multiple rooftop units or a different system design.

Start with the Commercial Cooling Load

Nominal tonnage is a capacity class, not a complete description of how a unit will perform on a particular roof. Commercial load calculations evaluate the building and its use at local design conditions.

Nominal tons = design cooling load in BTU/h ÷ 12,000

Nominal size	Nominal cooling capacity	Approximate starting airflow
3 tons	36,000 BTU/h	About 1,200 CFM
5 tons	60,000 BTU/h	About 2,000 CFM
7.5 tons	90,000 BTU/h	About 3,000 CFM
10 tons	120,000 BTU/h	About 4,000 CFM
12.5 tons	150,000 BTU/h	About 5,000 CFM
15 tons	180,000 BTU/h	About 6,000 CFM
20 tons	240,000 BTU/h	About 8,000 CFM
25 tons	300,000 BTU/h	About 10,000 CFM

One large building does not automatically mean one large rooftop unit. Multiple units may provide better zoning, scheduling, redundancy, duct distribution and part-load performance. The design team should divide the building into appropriate thermal zones before selecting unit quantities and sizes.

Calculate envelope loads

Include roof, walls, glass, doors, infiltration, orientation, solar exposure and local outdoor design conditions.

Add internal and ventilation loads

Account for occupants, lighting, plug loads, process equipment, required outdoor air and operating schedules.

Check actual unit performance

Verify total and sensible capacity at design temperature, airflow and altitude—not only the catalog’s nominal tonnage.

Match Voltage and Phase Exactly

Commercial rooftop equipment is offered in multiple electrical configurations. The voltage and phase at the installation point must match the complete model number and nameplate. They are not field-selectable preferences and must not be inferred from tonnage.

Common configuration	Where it may appear	Selection note
208/230V, single phase, 60 Hz	Some smaller light-commercial packaged units	Availability decreases as capacity and electric heat requirements increase.
208/230V, three phase, 60 Hz	Common light-commercial and commercial rooftop equipment	Confirm whether the facility service is 208V or 240V and follow the unit wiring instructions.
460V, three phase, 60 Hz	Common commercial and larger rooftop equipment	Usually draws lower current than a comparable lower-voltage model, but requires a compatible 480V-class service.

208V/230V and 460V equipment are not interchangeable. A qualified electrician or contractor should measure and identify the service, confirm phase and frequency, and verify voltage imbalance, disconnecting means, grounding, conductor size and overcurrent protection. Three-phase equipment should also have correct phase sequence and protection against phase loss or reversal where required.

MCA and MOCP Are Nameplate Requirements

MCA is the minimum circuit ampacity used when sizing supply conductors. MOCP is the maximum permitted fuse or circuit-breaker rating. Neither value can be calculated reliably from nominal tons alone; compressors, indoor motors, accessories, electric heat and factory options affect the published electrical data. Use the exact unit nameplate and manufacturer tables.

Choose the Correct Packaged Unit Type

Packaged Air Conditioner

Provides cooling and may accept an electric-heat accessory. Use when a separate heating system exists or electric heat is part of the design.

Gas/Electric Rooftop Unit

Combines air conditioning with a gas furnace section. Verify gas type, supply pressure, input/output capacity, venting and combustion requirements.

Packaged Heat Pump

Provides compressor heating and cooling, often with optional auxiliary electric heat. Check low-ambient capacity and defrost requirements.

Heating must be sized separately from cooling. Gas heat is selected by rated input and output; electric heat is selected in kilowatts; heat pumps require capacity checks at the winter design temperature. Do not assume the cooling tonnage determines the correct heat size.

Airflow, Outdoor Air and Static Pressure Matter

A preliminary cooling airflow often begins near 400 CFM per nominal ton, but the correct value depends on sensible heat ratio, humidity control, supply-air temperature, ventilation, equipment limits and the duct system. The blower must deliver the required airflow against the actual external static pressure.

Supply and return ducts

Verify duct dimensions, transitions, fittings, filters, diffusers and return-air paths. An existing duct system may not support a larger replacement unit.

Outdoor ventilation air

Calculate required outdoor air for occupancy and space use. Outdoor air adds sensible and latent load and may require an economizer or powered exhaust.

External static pressure

Select the blower, motor, drive or tap that delivers design CFM at the calculated resistance without exceeding motor or unit limits.

Economizer operation

An economizer can provide outdoor-air cooling when conditions permit. Verify controls, sensors, relief or exhaust and code requirements.

Humidity and latent capacity

Restaurants, assembly areas and humid climates may need more latent performance than a simple sensible-load estimate reveals.

Altitude and ambient limits

Capacity, airflow, combustion and controls may require correction or accessories at high altitude or extreme outdoor conditions.

Replacing an Existing Rooftop Unit

Matching nominal tons is only the beginning. Photograph the entire nameplate and record the existing unit, curb and utility details before ordering.

Replacement item	What to verify	Why it matters
Model and full electrical data	Capacity, voltage, phase, frequency, MCA and MOCP	Prevents electrical and performance mismatches.
Roof curb	Outside dimensions, opening, supply/return orientation and structural condition	Different brands and generations may need an engineered curb adapter.
Airflow arrangement	Downflow or horizontal; supply and return locations	Must align with the duct system and selected factory configuration.
Operating weight	Unit, curb adapter, accessories and service loads	The roof structure must support the complete installation.
Heating utilities	Gas type/pressure or electric-heat circuit requirements	Heating accessories can substantially change electrical data.
Controls	Thermostat, staging, economizer, BAS protocol and smoke shutdown	Existing controls may not be compatible with the new sequence.
Refrigerant generation	Current product refrigerant and associated safety provisions	Newer low-GWP equipment may require updated installation practices and controls.

Plan the lift and installation before the equipment arrives. Confirm crane access, pick weight and center of gravity, roof access, weather, rigging points, curb readiness, shutdown time, condensate drainage, service clearances and permits with qualified contractors.

Before You Select a Commercial Packaged Rooftop Unit

Verify	What to record	Why it matters
Commercial design loads	_____ BTU/h cooling; _____ BTU/h heating	Determines capacity at actual design conditions.
Unit quantity and zoning	_____ units serving _____ zones	Affects comfort, scheduling, redundancy and duct distribution.
Electrical service	_____ volts / _____ phase / 60 Hz	Must match the complete model and nameplate exactly.
Exact electrical ratings	MCA _____ A; MOCP _____ A	Controls conductor and overcurrent-device selection.
Required airflow	_____ CFM at _____ in. wg	Determines blower and drive selection.
Outdoor airflow	_____ CFM	Adds load and affects economizer, relief and exhaust design.
Heating type and size	Gas _____ BTU/h; electric _____ kW; heat pump	Heating load and utilities are separate selection requirements.
Curb and airflow orientation	Dimensions; downflow/horizontal; adapter required?	Ensures physical alignment and a weather-tight installation.
Controls and accessories	Stages, economizer, VFD, BAS, smoke detector and sensors	Factory options affect compatibility, wiring and operation.

Commercial Rooftop Unit Terms to Know

Nominal ton: A capacity class equal to approximately 12,000 BTU/h of cooling.
RTU: Rooftop unit; packaged heating, cooling and air-moving equipment installed outdoors.
CFM: Cubic feet of supply, return or outdoor airflow per minute.
External static pressure: Duct-system resistance the unit blower must overcome.
MCA: Minimum circuit ampacity used to size supply conductors.
MOCP: Maximum overcurrent protection permitted for the unit.
IEER: Integrated Energy Efficiency Ratio, representing commercial cooling performance at multiple loads.
Economizer: Controls outdoor air to provide cooling when outdoor conditions are suitable.

Need Help Selecting a Commercial Rooftop Unit?

Have the design cooling and heating loads, required airflow and static pressure, voltage, phase, existing model number and curb dimensions ready. An HVACDirect.com specialist can help you compare compatible packaged rooftop equipment.

Shop Commercial Rooftop Units