What Is Brake Horsepower (BHP) in Electric Motors: Sizing and Circuit Guide

Brake horsepower (BHP) is the actual usable mechanical power delivered by a motor or engine to its output shaft after subtracting internal friction, windage, and parasitic losses. When you read "5 HP" on an electric motor nameplate, that number represents the BHP rating—the guaranteed continuous mechanical work the shaft can deliver to a load without exceeding the motor's thermal insulation limits.

Understanding BHP is the dividing line between guessing and engineering a motor circuit. Many hobbyists and junior technicians assume that a 1 HP motor draws exactly 746 watts from the wall. It does not. 746 watts is the mechanical power leaving the shaft (the BHP). The electrical power entering the motor must be higher to account for heat and friction. Confusing BHP with electrical input power is the most common reason DIYers undersize variable frequency drives (VFDs) and trip breakers on startup.

The Core Difference: Brake vs. Input vs. Indicated Horsepower

To size conductors, overloads, and disconnects correctly, you must separate the mechanical output from the electrical input. Think of BHP as your take-home pay, while the electrical input power is your gross salary. The "taxes" are the inefficiencies lost as heat inside the motor windings and bearings.

• Indicated Horsepower (IHP): The theoretical electromagnetic power generated inside the motor's air gap. You will almost never see this on a nameplate; it is a design metric for motor engineers.
• Brake Horsepower (BHP): The IHP minus mechanical losses (bearing friction, cooling fan drag). This is the nameplate HP.
• Input Horsepower (Electrical): The BHP divided by the motor's efficiency. This is the actual electrical power drawn from your panel, which dictates your wire gauge and breaker sizing.

Below is a data-dense reference table for standard NEMA Premium efficiency 3-phase induction motors operating at 460V. Notice how the electrical input kW always exceeds the mechanical BHP output.

Note: Full Load Amps (FLA) are derived from NEMA MG-1 standards and codified in NEC Table 430.250 for branch circuit sizing. Actual motor nameplate FLA may vary slightly by manufacturer and power factor.

Worked Example: Sizing a Circuit for a 4.2 BHP Conveyor Load

Let's look at what BHP changes in a real installation. Suppose you are wiring a new aggregate conveyor belt. The mechanical engineer calculates that the loaded belt requires exactly 4.2 BHP to turn the drive pulley at the required speed.

Step 1: Select the Motor
You cannot buy a 4.2 HP motor. You must select the next standard size up, which is a 5 HP (5 BHP) NEMA frame motor. The motor nameplate reads: 5 HP, 460V, 3-Phase, 60Hz.

Step 2: Calculate True Electrical Input
If you mistakenly assumed 5 HP = 3,730 watts drawn from the line, your calculations would be wrong. Using the efficiency formula ($P_{in} = \frac{BHP \times 746}{\text{Efficiency}}$), and assuming a standard 89.5% efficiency for a 5HP premium motor:

Step 3: Size the Branch Circuit (NEC Article 430)
The National Electrical Code (NEC) does not want you calculating wire size based on the exact 4.2 BHP load. It requires you to size based on the motor's nameplate FLA to handle starting currents and thermal mass.

• Conductor Sizing (NEC 430.22): Conductors must carry 125% of the motor FLA. Using NEC Table 430.250, a 5HP motor at 460V has an FLA of 7.6A.
  7.6A × 1.25 = 9.5 Amps.
  A 14 AWG THHN copper wire (rated 20A at 75°C) is technically sufficient, though many industrial specs mandate 12 AWG minimum for mechanical durability.
• Breaker Sizing (NEC 430.52): An inverse-time breaker for a standard AC motor can be sized up to 250% of FLA to allow for locked-rotor starting current.
  7.6A × 2.50 = 19 Amps.
  The next standard breaker size up is a 20A 3-pole breaker.
• Overload Relay (NEC 430.32): The thermal overload protects the motor from running overloads. It is dialed to 115% of the actual nameplate FLA (not the table value). If the physical nameplate says 7.2A, you dial the overload to 8.28A.

Where You Meet BHP in Practice

You will encounter BHP limitations in three specific scenarios on the jobsite or the workbench:

1. Variable Frequency Drive (VFD) Sizing

VFDs are sold by HP ratings, but their internal IGBTs actually switch current. If you use a 5 HP VFD to drive a 5 HP motor, but the application is a "constant torque" load (like a hoist or conveyor) running at low speeds, the motor's internal cooling fan slows down. The motor can no longer dissipate the heat generated by delivering 5 BHP at low RPM. In practice, you must either buy a VFD rated one size up (7.5 HP) or add a shaft-mounted blower to the motor.

2. Generator and Inverter Sizing

When sizing an off-grid inverter or a diesel generator to run a well pump, you must account for the BHP starting surge. A 2 BHP submersible pump might draw 1,800 watts while running, but the locked-rotor current required to break the static friction and reach full BHP output can spike to 6,000 watts for 2 seconds. Your inverter must have a "surge" or "peak" rating that covers this BHP startup transient, not just the continuous running wattage.

3. The "Peak HP" Marketing Trap

Cheap consumer equipment (air compressors, shop vacs, table saws) often advertise "Peak HP" or "Developed HP." This is a marketing fabrication measuring the electrical input power for a fraction of a second before the thermal breaker trips. A consumer air compressor labeled "5 Peak HP" usually has a true continuous BHP rating of about 1.5 to 2.0. Always look for the continuous duty BHP or the actual 120V/240V amp draw to know what you are actually buying.

Frequently Asked Questions