Horsepower to Electric Horsepower for Drive Sizing

Choosing a Drive When the Load Is Quoted in Mechanical Horsepower

A panel builder rarely gets to pick the units. The machine designer hands over an absorbed power in mechanical horsepower, the drive supplier's catalogue is organised in the electrical horsepower of 746 W, and the enclosure schedule wants kilowatts. All three describe the same duty, so the first job in any drive selection is getting them onto one line before anything is priced.

Conversion factor: 1 hp = 0.9995976831 ehp. A load absorbing 60 mechanical horsepower is 59.9759 ehp, so the 60 ehp line in the catalogue covers it — and that line is really a 77 A output at 460 V, which is the number that decides the selection.

How a Drive Catalogue Is Really Organised

Horsepower is shorthand for amps

A drive has no shaft. Its real limit is continuous output current into the motor, and the horsepower heading is just the motor size that current normally serves at a stated voltage.

Two ratings per chassis

The same hardware is listed twice — heavy or constant-torque duty with 150 % overload for a minute, and normal or variable-torque duty with 110 % for fans and centrifugal pumps.

The kilowatt column beside it

Most makers publish one worldwide chassis with both a NEMA horsepower column and an IEC kilowatt column, and the two rarely convert cleanly into each other because each follows its own standard size ladder.

Frame breaks cost money

Ratings cluster into physical chassis sizes. Crossing one break adds width, weight, cooling airflow and often a different terminal arrangement inside the panel, so it is worth knowing where you sit in a band.

Working a Selection from Absorbed Power to a Part Number

Treat the conversion as the first of four moves, and never as the last word — the motor nameplate current always gets the final say.

Put the mechanical figure in the left field

Enter the absorbed or rated horsepower from the machine documentation — 7.5, 25, 60, 200. The electrical equivalent appears as you type, with a comma accepted as the decimal separator and spaces ignored.

Land on the catalogue line above it

Ratings come in fixed steps, so round up, never down. Anything between 15 and 20 electrical horsepower is bought as a 20 ehp drive — there is no half size to hide in.

Flip it when the catalogue leads and the load follows

Press the swap button (↔) for ehp → hp when you already have a drive on the shelf and want to know the mechanical duty it will carry. A 40 ehp unit answers 40.0161 hp.

Move the number onto the panel schedule

The copy button lifts the bare value with no unit attached, which is what a bill of materials cell or a supplier configurator wants. Ctrl + C inside a field copies the same way.

Finish the selection on current, not on power: a motor rewound for an odd voltage, a long cable run, an altitude above 1 000 m or a 50 °C enclosure can all leave a nominally correct drive short. Compare the motor's full-load amps against the drive's continuous output current in the duty class you actually need.

Drive Ratings, Their Electrical Equivalent and Output Current

A typical 460 V three-phase drive range, showing what each mechanical horsepower figure becomes in electrical horsepower, the kilowatt value printed on the same catalogue line, and the continuous output current for normal-duty service.

Load (hp)	Electrical (ehp)	Catalogue kW	Output current at 460 V
5 hp	4.9980 ehp	3.73 kW	7.6 A
7.5 hp	7.4970 ehp	5.59 kW	11 A
10 hp	9.9960 ehp	7.46 kW	14 A
15 hp	14.9940 ehp	11.19 kW	21 A
25 hp	24.9899 ehp	18.64 kW	34 A
40 hp	39.9839 ehp	29.83 kW	52 A
60 hp	59.9759 ehp	44.74 kW	77 A
100 hp	99.9598 ehp	74.57 kW	124 A

The middle two columns barely move — a 100 hp load is 99.9598 ehp, four hundredths short of the round figure — while the current column doubles between 25 and 60 hp. That is the whole point: the horsepower heading tells you where to look in the catalogue, and the ampere column tells you whether the drive will survive the load once it is running.

What This Page Handles While You Specify

Type either end while pricing a line-up

Both fields are live, so you can work through a whole panel of drives — one per conveyor, one per fan — without resetting the page between them.

Read a shelf drive against a new duty

Swapping the direction answers the question that comes up in the stores aisle: what mechanical load will the unit already sitting on the rack actually carry?

Reach the kilowatt column from the same box

The searchable dropdowns hold every power unit on the list, so the IEC side of a dual-standard catalogue is one selection away rather than a second calculation.

Bare values for the bill of materials

Results run to eight decimals and copy without a unit or thousands spacing, so nothing needs cleaning up after it is pasted into a schedule.

Drive Selection Questions from the Panel Shop

Should I select a drive by horsepower or by the motor's full-load amps?

By amps. Horsepower narrows the search; current settles it. The same motor shows very different full-load amps on different voltages — a nameplate reading 230/460 V might list 13.4 A and 6.7 A — so a drive chosen on the power heading alone can be badly wrong for the connection you are actually making. Read the amps off the plate and check them against the drive's continuous output current.

What changes between a constant-torque and a variable-torque rating?

The overload allowance, and therefore the horsepower the same chassis is sold as. A conveyor, extruder or positive-displacement pump needs full torque at any speed, so it takes the heavy-duty rating with roughly 150 % overload for 60 seconds. A centrifugal fan or pump follows the affinity laws — torque falls with the square of speed — so it can use the normal-duty rating with about 110 % overload, one or two sizes higher on the same hardware.

Why does one drive carry both a horsepower figure and a kilowatt figure?

Because it is sold into both markets from one production line. The electronics are identical; only the labelling changes. North American motors come in 5, 7.5, 10, 15 ehp steps while IEC motors come in 4, 5.5, 7.5, 11 kW steps, and those ladders do not line up, so a drive that heads a 15 hp column may head an 11 kW column even though 15 hp is 11.19 kW.

How much extra drive does a high-inertia load need?

Enough to hold the accelerating current for the whole run-up, which is a time problem rather than a power problem. A large centrifuge, hammer mill or induced-draught fan can take a minute or more to reach speed, and if the ramp outlasts the drive's overload window the drive trips before the load ever arrives. Either step up one or two sizes so the starting current sits inside the continuous rating, or lengthen the ramp and accept the slower start.

Do harmonics or a hot enclosure force me to derate?

Both can. Ambient temperature above the rated figure, altitude above roughly 1 000 m and a raised switching frequency each cut the continuous current the drive may deliver, and the published derating curves are multiplicative — a hot panel at altitude loses on two counts. Harmonics are a separate matter: the drive's own input current distortion loads the supply transformer and may call for a line reactor, a DC choke or a filter, none of which change the motor-side rating but all of which change what the installation needs upstream.

Horsepower to Electric Horsepower