Brake Horsepower vs Nameplate HP on Air Compressors

The short answer: Nameplate horsepower is the rated input of the motor; brake horsepower (BHP) is the mechanical output at the shaft, which is what the compressor pump actually consumes. The two numbers differ by the motor’s efficiency, typically 4-7 percent on a NEMA Premium drive motor. The difference is why two compressors with identical nameplate HP can deliver materially different SCFM at the same pressure: pump efficiency, motor efficiency, and package losses all sit between the wall plug and the discharge port.

Definitions

Two terms with overlapping common-usage but different technical meanings:

• Nameplate horsepower is the rated input horsepower stamped on the motor nameplate. Under NEMA MG 1, the nameplate HP is the design point: the rated full-load output of the motor under defined voltage, frequency, ambient, and altitude conditions. Strictly, NEMA nameplate HP is the motor’s output rating, not its electrical input.
• Brake horsepower (BHP) is the mechanical horsepower measured at the shaft – the work the motor delivers to the driven equipment. On a compressor, BHP is the shaft power the airend actually draws from the motor. BHP equals motor input HP × motor efficiency.

Outside the world of motor specifications, “nameplate HP” is often used loosely to mean “the HP number printed on the compressor’s nameplate.” For compressors, that number is almost always the motor’s NEMA-rated output. The compressor’s actual shaft load (BHP) is generally a few percent higher than the motor’s continuous output rating because compressor packages are sized so the airend draws roughly nameplate HP at rated pressure and full load. This sounds circular; the practical point is that BHP, motor output HP, and motor input kW are three slightly different numbers that get conflated in conversation but diverge on the spec sheet.

For the short version, see the glossary entry on brake horsepower vs nameplate HP.

Why the difference matters

Two scenarios where the BHP distinction shows up in real procurement:

• Sizing the branch circuit. The electrical service must support the motor’s full-load input current, not BHP. A 50 HP motor at 94.5 percent efficiency draws about 50 / 0.945 = 52.9 input HP, or 39.5 kW at full load. The branch circuit, overload relay, and starter are sized on input.
• Computing real operating cost. The utility meter reads input kW, not BHP. Specific power figures (kW per 100 CFM at full load) are at the package terminals, not at the airend shaft. If you back into BHP from specific power, you have to divide by motor efficiency.
• Comparing compressors with mismatched nameplate ratings. A 50 HP IE2 motor and a 50 HP IE4 motor have identical nameplate HP but draw materially different input kW. The IE2 unit draws roughly 2 percent more input power at the same shaft load, which is the entire premise of the efficiency-class hierarchy.

The math

Conversion among the three numbers (input kW, nameplate HP, BHP) on a typical industrial motor:

Input kW = Nameplate HP × 0.746 / motor efficiency

BHP = Nameplate HP (when motor is operating at full nameplate load)

Specific power (kW/100 CFM) = Input kW / (SCFM / 100)

Worked example: a 25 HP IE3 / NEMA Premium screw compressor at 93.6 percent motor efficiency.

• Input kW at full load: 25 × 0.746 / 0.936 = 19.9 kW.
• BHP at the airend shaft, full load: 25 HP (this is the motor’s rated mechanical output).
• If the package delivers 99 SCFM at 125 PSI (catalog figure for a 25 HP fixed-speed screw), specific power = 19.9 / 0.99 = 20.1 kW per 100 SCFM.

Working in the other direction: given a CAGI data sheet that lists 20.1 kW per 100 SCFM and 99 SCFM at full load, you can back-compute that the package draws 19.9 kW at full load, the motor delivers 19.9 × 0.936 = 18.7 kW shaft = 25.0 HP of shaft work, and that matches the nameplate.

Reference BHP for typical compressor sizes (125 PSI, single-stage screw)

Approximate full-load values for fixed-speed oil-flooded rotary screw compressors at 125 PSI. Real catalog units vary; verify against the manufacturer’s CAGI data sheet for any procurement decision.

HP / kW02040608010012054.25.75 HP108.311.410 HP2519.928.425 HP5039.556.650 HP10078.7113.0100 HPNameplate HPInput kW at 125 PSIBHP at +20 PSI overpressure

Nameplate HP	Approx full-load BHP	Approx full-load input kW	Approx SCFM at 125 PSI
5	5	4.2	16-18
7.5	7.5	6.3	22-27
10	10	8.3	32-39
15	15	12.4	50-57
25	25	19.9	92-105
50	50	39.5	205-225
100	100	78.7	460-490

The kW values assume IE3 / NEMA Premium motor efficiency at full load (about 89-95 percent across this range). The SCFM bands reflect the differences across brands and stage configuration: two-stage units deliver SCFM toward the high end of each band, single-stage units toward the low end.

When this distinction matters

• When sizing branch electrical: use motor input kW, not BHP, and apply the NEC 125 percent rule to full-load amps.
• When computing operating cost: use input kW, because that is what the meter reads.
• When comparing two compressors with the same nameplate HP and different efficiency classes: the BHP is the same, the input is different, and the dollars-per-1000-SCF differs accordingly. See true cost of compressed air per 1,000 SCF for the cost math.
• When sizing a generator or a UPS for compressor backup: use starting kVA (often 6-7 times full-load kVA for across-the-line starts) and full-load kW input, not BHP.

Real-world examples from the catalog

• Quincy QGS-15: 15 HP rotary screw at 55.9 SCFM / 125 PSI. The IE3 drive motor draws roughly 12.4 kW at full load; the BHP at the airend shaft is 15 HP at the rated discharge pressure.
• Atlas Copco GA11-125: 15 HP screw at 55 SCFM / 125 PSI. Similar full-load input draw with a slightly different specific power because the airend efficiency and the motor efficiency at this size vary by manufacturer in the second decimal place.
• Sullair LS-100: 100 HP at 480 SCFM / 100 PSI. The lower 100 PSI discharge means slightly lower full-load BHP per SCFM than a 125 PSI unit at the same size, which is why the LS-100 lands at the favorable end of its size class on the CAGI specific-power column.
• Kaeser CSD 75: 100 HP at 461 SCFM / 110 PSI. At 110 PSI the BHP is slightly higher than the LS-100’s 100 PSI point, reflecting the higher compression ratio; the package specific power is comparable.

Common questions

Is BHP the same as shaft horsepower?

Effectively yes, in this context. Brake horsepower is named for the mechanical brake (dynamometer) historically used to measure shaft output; shaft horsepower is the modern term and refers to the same physical quantity. Both are mechanical output of the motor at the rotating shaft, before the airend’s own efficiency losses.

If my compressor’s nameplate is 50 HP, is the airend a 50 BHP airend?

At the rated discharge pressure, yes. Airend manufacturers size the airend so that at the package’s rated pressure, the shaft load equals the motor’s nameplate HP. Operating at a higher discharge pressure raises BHP and pushes the motor into service-factor headroom (see motor service factor); operating at lower discharge pressure reduces BHP below nameplate.

Why do CAGI data sheets list kW input but not BHP?

Because input kW is what the customer pays. The motor manufacturer publishes efficiency at full load, and the airend manufacturer’s BHP curve sits behind the package design. The buyer’s cost lever is at the wall plug, which is input kW, so that is what the data sheet reports.

Can two compressors at the same nameplate HP deliver different SCFM?

Yes, and routinely. Stage count (single vs two-stage), airend rotor profile, oil viscosity, intercooler geometry, and discharge-pressure point all change SCFM at a given BHP. A two-stage 25 HP screw at 125 PSI typically delivers 4-8 percent more SCFM than a single-stage 25 HP screw at the same pressure because of lower intra-stage leakage and lower discharge temperature.