Why 1 TR = 4.71 HP and also ≈ 1.25 HP in HVAC? Both are correct, but mean different things.

Why 1 TR = 4.71 HP and also ≈ 1.25 HP in HVAC? Both are correct, but mean different things.
This confusion happens because Horsepower (HP) is used to describe two completely different parameters in HVAC systems:
Cooling Output vs Electrical Input (Motor Power)
✦ Step 1: What is 1 TR?
1 Ton of Refrigeration (TR) is a unit of cooling capacity.
1 TR = 12,000 BTU/hr = 3.517 kW (cooling output)
⚠️ TR represents heat removal capacity, NOT electrical consumption.
✦ Step 2: Why 1 TR = 4.71 HP (Cooling Output)
This comes from pure unit conversion:
• 1 TR = 3.517 kW
• 1 HP = 0.746 kW
• 3.517 ÷ 0.746 = 4.71 HP
‣ Meaning:
1 TR = 4.71 HP of cooling output
〤 This is NOT compressor motor size
✓ Used only for capacity comparison
✦ Step 3: Why 1 TR ≈ 1.25 HP (Electrical Input)
This is the actual compressor/motor power, based on system efficiency.
Using COP (Coefficient of Performance):
COP = Cooling Output ÷ Input Power
Example (COP = 3.8):
Input power = 3.517 ÷ 3.8 = 0.925 kW
Convert to HP:
0.925 ÷ 0.746 = 1.24 HP
✓ Result:
1 TR ≈ 1.2–1.5 HP motor input (typical real systems)
✦ Real-World Example: 10 TR Chiller
Cooling output = 35.17 kW = 47 HP (output)
Electrical input ≈ 11.7 kW = 16 HP (motor)
✓ Same unit
✓ Different meaning
⚠️ Critical Engineering Rule
〤 Never size cables, MCC, or generators using 4.71 HP/TR
✓ Always use kW/TR, COP, or actual input kW
Typical values:
• Air-cooled chiller → 1.0–1.3 kW/TR
• Water-cooled chiller → 0.6–0.9 kW/TR
• High-efficiency systems → <0.6 kW/TR
‣ Final Engineering Truth
✓ 1 TR = 4.71 HP → Cooling capacity (output)
✓ 1 TR ≈ 1.2–1.5 HP → Electrical motor input
Same unit. Different physics.