British horsepower (often called mechanical or imperial horsepower) is a unit of power defined as exactly 550 foot-pounds per second, which translates to 745.7 watts of electrical power. When you are wiring a motor or programming a variable frequency drive (VFD), confusing British HP with metric HP (PS) or ignoring the conversion to watts entirely changes your calculated full-load ampacity, potentially leading to undersized conductors, excessive voltage drop, and nuisance tripping on your motor branch circuit.

While software engineers and automotive enthusiasts toss the term around loosely, electrical professionals must treat horsepower as a strict mechanical output rating that requires specific derating and efficiency calculations to determine the actual electrical input required from your panel.

The Core Numbers: British vs. Metric and Electrical Horsepower

Before pulling wire or setting a breaker, you need to know exactly which flavor of horsepower is stamped on the equipment nameplate. The global standardization of motors means you will frequently encounter metric, electrical, and British ratings on the same jobsite. Below is the definitive conversion matrix you need for circuit calculations.

Table 1: Horsepower Standards and Electrical Equivalents
Standard Type Exact Wattage ft-lbf/s Primary Region Typical Application
British (Mechanical) 745.69987 W 550 US, UK, Canada Industrial motors, HVAC compressors, pumps
Metric (PS / CV) 735.49875 W 542.4 EU, Asia, Auto European imported CNCs, VFD parameter sets
Electrical 746 W 550.2 US (NEC) Nameplate ratings, NEC Article 430 tables
Boiler 9809.5 W 7233 Global Steam boilers, industrial heating elements
Bench Note: For standard US electrical calculations, the National Electrical Code (NEC) and NIST guidelines treat British Mechanical HP (745.7W) and Electrical HP (746W) as functionally identical. The 0.3W difference per HP is negligible in branch circuit sizing, so we use 746W as the baseline multiplier in the field.

Worked Example: Sizing a Circuit for a 5 HP British Motor

Let us look at what British horsepower changes in a real circuit installation. You are tasked with wiring a 5 HP, 3-phase, 230V AC induction motor (British HP rating) for a workshop air compressor.

The Fatal Mistake: Raw Wattage Conversion

A common error among junior technicians is to convert the mechanical horsepower directly to electrical current using basic Ohm's law, ignoring motor efficiency and power factor. Here is how that mistake happens:

Incorrect Math: 5 HP × 746 W = 3,730 W.
Current (I) = 3,730 W / (230V × √3) = 3,730 / 398.37 = 9.36 Amps.

If you size your wire and breaker for 9.36 Amps, the motor will trip the breaker on startup, and the wire will overheat under load. Why? Because 5 HP is the mechanical output at the shaft, not the electrical input from the panel. Motors are not 100% efficient, and inductive loads suffer from a lagging power factor.

The Correct Method: Using NEC Article 430

Instead of doing raw wattage conversions, the NEC Article 430 requires us to use standardized Full-Load Current (FLA) tables that already account for typical efficiency and power factor losses.

  1. Find the FLA: According to NEC Table 430.250, a 5 HP, 3-phase, 230V motor has a standard FLA of 15.2 Amps. (Notice how much higher this is than our incorrect 9.36A calculation).
  2. Size the Conductors: NEC 430.22 requires motor branch circuit conductors to be sized at 125% of the FLA.
    15.2A × 1.25 = 19.0 Amps.
    Looking at the 75°C column of NEC Table 310.16, 12 AWG THHN copper is rated for 25A, which is sufficient. However, for mechanical strength and voltage drop mitigation over longer runs, 10 AWG THHN is the standard jobsite choice.
  3. Size the Overcurrent Protection: For an inverse-time breaker, NEC Table 430.52 allows up to 250% of the FLA to handle the locked-rotor inrush current.
    15.2A × 2.50 = 38.0 Amps.
    Per NEC 240.6, we round up to the next standard breaker size: a 40 Amp 3-pole breaker.
Safety Caveat: Always verify the motor nameplate FLA. If the nameplate lists a specific FLA (e.g., 14.8A) that differs from the NEC table, use the nameplate value for thermal overload heater sizing (NEC 430.32), but continue using the NEC table value for conductor and breaker sizing to maintain standardized safety margins.

Where You Meet British Horsepower in Practice

You will rarely see 'British Horsepower' explicitly written on a modern schematic. Instead, you will encounter its effects in three specific scenarios:

1. Programming Variable Frequency Drives (VFDs)

When commissioning a VFD (like a Yaskawa GA800 or an Allen-Bradley PowerFlex), the drive needs to know the motor's nominal power to calculate its internal thermal protection model. If you are installing a European-imported CNC machine, the motor nameplate might read '7.5 kW' or '10 PS' (Metric HP). If the VFD interface defaults to British HP, entering '10' will tell the drive the motor can handle 7,460 Watts, when it is actually only rated for 7,355 Watts. Over time, this 1.4% thermal miscalculation can degrade the winding insulation. Always convert Metric PS to kW first, then let the VFD handle the internal HP translation.

2. Sizing Generators for Motor Starting

When sizing a standby generator for a facility, you must account for the starting kVA of large British HP-rated motors. A 50 HP British motor (37.3 kW output) might require a starting kVA of 150 kVA due to the 6x inrush current multiplier. Generator sizing software requires you to input the exact HP standard; selecting Metric HP in the software for a US-built compressor will slightly undersize the alternator's transient response capability, leading to severe voltage sag when the motor kicks on.

3. Legacy Industrial Panels

In older US and UK manufacturing plants, you will find disconnect switches and motor starters labeled strictly in HP (e.g., 'NEMA Size 2: 10 HP at 230V'). These NEMA ratings are strictly tied to British Mechanical Horsepower. You cannot safely swap a 10 HP British-rated contactor for a 10 kW European-rated load without verifying the exact amperage, as the kilowatt load will draw roughly 34% more current than the contactor's HP rating implies.

Frequently Asked Questions

What do people commonly confuse British Horsepower with?

The most common confusion is between British HP and Metric HP (Pferdestärke or PS). A 100 HP US muscle car engine and a 100 PS European hatchback engine do not produce the same power; the British HP engine produces about 1.4% more wattage. In electrical terms, confusing the two leads to mismatched VFD parameters and improperly scaled thermal overload relays.

Is British HP the same as Electrical HP?

For all practical electrical wiring purposes, yes. British (Mechanical) HP is 745.7 Watts, while Electrical HP is defined as exactly 746 Watts. The 0.3-watt difference per horsepower is entirely swallowed by the standard safety margins and derating factors required by the NEC. You will not find a multimeter or breaker that can distinguish between the two.

Why does my equipment manual use kW instead of HP?

Kilowatts measure real electrical power directly, bypassing the mechanical abstraction of horsepower. Modern IEC-standard equipment (common in Europe and increasingly in global industrial automation) uses kW because it directly correlates to the electrical input required, making circuit design, cable sizing, and energy consumption calculations much more straightforward. For a quick field reference, remember that 1 kW is roughly equal to 1.34 British HP.

For deeper reading on unit conversions and mechanical standards, the Engineering Toolbox horsepower guide provides an excellent cross-reference for boiler, metric, and hydraulic horsepower variants you might encounter in specialized plant environments.