Commercial HVAC Load Calculator

A commercial HVAC load calculator organizes the electrical load data from a rooftop unit, split system, package unit, or similar equipment submittal into a preliminary MCA screen and estimated kVA. The primary result is the Preliminary MCA screen, which provides an early conductor-load benchmark for branch-circuit or feeder review before finalizing conductor ampacity, raceway layout, panel capacity, and voltage-drop calculations.

The calculation uses the entered motor full-load current, electric heater load, system voltage, and phase basis. It does not perform Manual J load calculations and does not replace the model-specific Manufacturer MCA reference, Manufacturer MOCP reference, equipment nameplate, installation instructions, or AHJ-approved design.

Equipment Load Inputs

Use one electrical equipment boundary for the values entered. For example, include the supply-fan motor and all other motors serving the same packaged unit, but do not combine unrelated rooftop units merely because they are supplied from the same panelboard.

Input	Electrical use
Largest motor FLA (A)	Full-load current of the largest motor within the equipment boundary
Other motor FLA total (A)	Combined full-load current of all remaining motors in that same equipment boundary
Electric heater load (kW)	Electric heat or auxiliary heater power shown on the equipment submittal
System voltage (V)	Voltage used to convert heater kW to current and to calculate estimated kVA
Phase	Phase basis used for the heater-current calculation
Manufacturer MCA reference (A)	Optional exact MCA from the unit nameplate or approved submittal
Manufacturer MOCP reference (A)	Optional protective-device value retained for review; it does not select an overcurrent protective device

Largest motor FLA should be the motor current specified for the equipment, not a measured operating current taken under an unknown load condition. Other motor FLA total is the sum of the remaining motor FLA values. Keeping these two entries separate allows the calculation to apply the largest-motor treatment while still including the remaining motor load.

Electric heater load is entered in kW. The calculator converts that heating power to amperes from the selected phase basis and the entered System voltage (V). A 480 V three-phase heater therefore produces a different line current than an equivalent kW heater supplied at a different voltage or phase arrangement.

Preliminary MCA Screen

The calculator produces these results:

• Heater current: current associated with the entered Electric heater load (kW) at the selected voltage and phase basis
• Entered load current sum: the direct sum of Largest motor FLA (A), Other motor FLA total (A), and Heater current
• Preliminary MCA screen: a preliminary conductor-load screen that applies a 125% multiplier to the largest motor FLA, then adds the other motor and heater currents
• Estimated load: apparent power in kVA based on the entered load-current sum, voltage, and phase basis
• Margin to entered MCA: comparison against the optional Manufacturer MCA reference (A); it remains 0 A when no manufacturer MCA is entered

For the three-phase motor-and-heater screen shown by the calculator, the preliminary MCA calculation is:

\(\displaystyle \text{Preliminary MCA screen} = 1.25 \times \text{Largest motor FLA} + \text{Other motor FLA total} + \text{Heater current}\)

The entered current total is:

\(\displaystyle \text{Entered load current sum} = \text{Largest motor FLA} + \text{Other motor FLA total} + \text{Heater current}\)

For balanced three-phase equipment, estimated apparent load is:

\(\displaystyle \text{Estimated load (kVA)} = \frac{\sqrt{3}\times V \times I}{1000}\)

where (V) is System voltage (V) and (I) is the Entered load current sum.

The preliminary MCA screen is useful during equipment coordination because it gives a calculated load target before selecting conductor AWG or kcmil size. It can also expose a mismatch between input assumptions and the Manufacturer MCA reference on the exact equipment submittal.

Calculation Example

Assume a 480 V balanced three-phase commercial HVAC unit with these entered values:

Field	Value
Largest motor FLA (A)	20 A
Other motor FLA total (A)	5 A
Electric heater load (kW)	0 kW
System voltage (V)	480 V
Phase	Balanced three-phase
Manufacturer MCA reference (A)	0 A
Manufacturer MOCP reference (A)	0 A

With no electric heat entered, the heater-current result is:

\(\displaystyle \text{Heater current} = 0\text{ A}\)

The direct current sum is:

\(\displaystyle 20\text{ A} + 5\text{ A} + 0\text{ A} = \text{25 A}\)

The preliminary MCA screen is:

\(\displaystyle (1.25 \times 20\text{ A}) + 5\text{ A} + 0\text{ A} = \text{30 A}\)

The estimated three-phase load is:

\(\displaystyle \frac{1.732 \times 480 \times 25}{1000} = \text{20.7846 kVA}\)

The results are therefore:

Result	Value
Heater current	0 A
Preliminary MCA screen	30 A
Entered load current sum	25 A
Estimated load	20.7846 kVA
Margin to entered MCA	0 A

A 30 A preliminary MCA screen is not, by itself, a final conductor selection. The actual conductor ampacity can change with conductor material, insulation temperature rating, terminal rating, ambient conditions, adjustment factor, correction factor, number of current-carrying conductors, and other installation conditions.

Conductor and Raceway Review

The Preliminary MCA screen supports an initial equipment-circuit review:

• Compare the calculated screen with the exact Manufacturer MCA reference (A) from the unit nameplate or approved submittal.
• Use the governing manufacturer MCA and applicable installation requirements when selecting branch-circuit conductors.
• Verify terminal temperature ratings before using conductor ampacity values based on a particular insulation temperature rating.
• Apply required correction factor and adjustment factor calculations separately where ambient temperature or current-carrying conductors affect allowable ampacity.
• Carry the estimated kVA into panelboard, feeder, transformer, generator, and service-load coordination where applicable.
• Use the load current and actual conductor length for a separate voltage-drop review.
• Select raceway size through a separate raceway-fill calculation after conductor quantity, AWG or kcmil, insulation type, equipment grounding conductor, and routing are known.

The Manufacturer MOCP reference (A) is provided for documentation and review only. It is not a calculated overcurrent protective-device selection, and the calculator does not determine breaker or fuse type, rating, or compliance with the equipment listing.

Field Verification

Verify the exact equipment configuration before releasing conductors, disconnects, breakers, fuses, or raceway. Commercial HVAC equipment may include factory-installed accessories, electric heat stages, variable-speed drives, controls transformers, crankcase heaters, multiple compressors, or other components that alter the nameplate values and installation requirements.

Use the manufacturer’s current submittal and equipment nameplate as the final source for MCA and MOCP. Confirm the actual voltage, phase, conductor terminals, field-installed options, disconnect arrangement, conductor routing, and AHJ requirements separately from the calculator arithmetic.

Related workflows: Heat Pump Electrical Load Calculator and Commercial Load Calculator.