Kilowatts to Electric Horsepower for IEC to NEMA Swaps

Respecifying an IEC Machine's Motors for a North American Order

A machine designed in Europe carries its drive ratings in kilowatts — 0.75, 1.5, 4, 11 kW straight off the IEC catalogue. The week it is sold into a plant in Ohio or Ontario, every one of those figures has to become a motor size a North American distributor keeps on the shelf, and that shelf is stocked in horsepower. Electric horsepower carries the translation, because it is the round 746 W figure a NEMA nameplate is written against.

Conversion factor: 1 kW = 1.340482574 ehp, since electric horsepower is defined as exactly 746 W. The 5.5 kW gear motor on your machine schedule is 7.3727 ehp, so the nearest stocked NEMA size is 7.5 hp — 5.595 kW, a 1.7 % step up you get for nothing.

What Actually Changes When the Machine Crosses the Atlantic

The 746-watt convention

Electric horsepower fixes one horse at exactly 746 W instead of the mechanical 745.69987 W. The rounding keeps catalogue arithmetic clean, and it is the figure sitting behind the hp number on a NEMA plate.

Two size ladders drawn a century apart

IEC preferred ratings climb 0.55, 0.75, 1.1, 1.5, 2.2, 3, 4, 5.5, 7.5 kW. NEMA climbs in fractions and whole horses: 1/2, 3/4, 1, 1.5, 2, 3, 5, 7.5 hp. Neither ladder was ever drawn to meet the other.

A 60 Hz supply waiting at the far end

Nothing in the crate changes, but synchronous speed does. A 4-pole motor that turned 1,500 rpm at 50 Hz turns 1,800 rpm at 60 Hz, and every belt, screw and impeller geared off it moves 20 % faster.

Kilowatts on the drawing, horsepower on the order

Gearbox selection, heat rejection and the mechanical drawings all stay metric. Only the motor line items get restated, which is why the conversion happens once — at the export bill of materials.

Walking a Motor Schedule Through the Export Bill of Materials

Most machines carry a handful of drives rather than one, so this is list work: take each kilowatt rating in turn, find the horsepower size it maps to, and mark the ones where the substitution is not clean.

Enter each drive's kilowatt rating

Type the figure exactly as the schedule lists it — 0.37, 2.2, 18.5. The electric horsepower appears as you type, with as many decimals as the comparison needs. A comma decimal separator works if that is how the document was written.

Round to a size the distributor stocks

The exact answer is almost never a catalogue size. 2.2 kW converts to 2.9491 ehp, which buys a 3 hp motor. 4 kW converts to 5.3619 ehp — and the next NEMA rung up is 7.5 hp, not 5.5.

Turn it round when the quotation comes back in horsepower

Press the swap button (↔) to run ehp → kW and see what a supplier's 7.5 hp offer means against your 5.5 kW line: 5.595 kW. That is the direction you work in while reading a quotation rather than writing one.

Copy the plain figure into the parts list

The copy button hands over the bare number with no unit and no spaces, which is what a spreadsheet cell or an ERP part description expects. Ctrl + C inside a field does the same thing.

Check the driven load, not only the motor: on a 60 Hz supply a fan or pump impeller sized for 50 Hz turns 20 % faster and, by the affinity laws, asks for roughly 1.7 times the power. The motor can be the right size on paper and still trip on site.

Where the IEC Kilowatt Ladder and the NEMA Horsepower Ladder Part Company

Standard IEC ratings with their exact electric-horsepower value and the nearest NEMA size a North American distributor holds. The last column is the real story: how much bigger or smaller that substitution leaves the drive.

IEC rating	Exact equivalent	Nearest stocked NEMA size	What the substitution costs
0.75 kW	1.0054 ehp	1 hp (0.746 kW)	0.5 % smaller
1.5 kW	2.0107 ehp	2 hp (1.492 kW)	0.5 % smaller
3 kW	4.0214 ehp	5 hp (3.73 kW)	24.3 % larger — no 4 hp rung exists
4 kW	5.3619 ehp	7.5 hp (5.595 kW)	39.9 % larger — the worst gap in the range
5.5 kW	7.3727 ehp	7.5 hp (5.595 kW)	1.7 % larger
11 kW	14.7453 ehp	15 hp (11.19 kW)	1.7 % larger
18.5 kW	24.7989 ehp	25 hp (18.65 kW)	0.8 % larger
30 kW	40.2145 ehp	40 hp (29.84 kW)	0.5 % smaller — the nearest size sits under the original

Most of the ladder tracks within a couple of per cent, close enough that nobody argues. The 3 kW and 4 kW rows are where it breaks: NEMA has no 4 hp or 5.5 hp rung, so those ratings fall through to the next size and arrive 24 % and 40 % oversized, dragging a bigger frame and starter with them. At the other end, 30 kW converts to 40.21 ehp against a 40 hp motor — fractionally smaller than what left the factory, worth a second look if that drive was already working hard.

What This Page Gives You on an Export Job

Step down the drive list without clearing anything

Both fields stay live, so you can overtype 0.37, then 2.2, then 18.5 and read each answer immediately — the pace a motor schedule is actually worked through.

Reverse the direction when the vendor sets the terms

Writing a specification runs kW → ehp; reading a North American quotation runs the other way. The swap button covers both without leaving the page.

Mechanical hp and metric PS from the same dropdown

Searchable unit lists on both sides carry every power unit, so an export sheet mixing ehp on the motor plates, PS in a supplier brochure and BTU/h on a heater stays on one page.

Figures that paste straight into the parts list

Results carry up to eight decimals with thousands spaced for readability, and the copy button strips all that formatting back to a clean number.

Export Motor Substitution Questions

Why do IEC kilowatt sizes and NEMA horsepower sizes not line up?

They were standardised independently and never reconciled. IEC preferred ratings follow a roughly geometric series in kilowatts; NEMA grew out of fractions and whole numbers of horses. Some rungs land almost on top of each other — 11 kW is 14.75 ehp against a 15 hp motor — and some have no partner at all, because nothing exists between 3 and 5 hp, or between 5 and 7.5 hp, for 3 kW and 4 kW to land on.

What happens to a 50 Hz motor's rating on a 60 Hz supply?

Synchronous speed rises 20 %. If the voltage rises in step — a 400 V/50 Hz winding fed 480 V/60 Hz — flux and torque are unchanged and shaft power goes up by that same 20 %. On the common 460 V/60 Hz supply the volts-per-hertz ratio falls about 4 %, so flux and pull-out torque drop slightly. Either way the kilowatt figure stamped on the plate no longer describes what the motor will do, which is why exporters respecify rather than simply reship.

Will a NEMA motor bolt into an IEC machine's mounting?

Rarely without an adaptor plate. IEC frames are named for shaft height in millimetres (100L, 132S, 160M) and NEMA frames by a code number (143T, 182T, 254T). Foot-hole spacing, shaft diameter, key size and flange patterns — IEC B5/B14 against NEMA C-face and D-flange — all differ, and one family is metric while the other is inch. The practical choices are to keep the IEC frame with a 60 Hz winding, or to design the bedplate and coupling around the NEMA frame from the outset.

Which voltage should the substituted motors be wound for?

North American industrial supplies are typically 208, 230, 460 or 480 V at 60 Hz, with 575/600 V common across much of Canada, against 400 V at 50 Hz in Europe. For machinery, 460 V is the usual default, and a dual-voltage 230/460 V winding leaves the installer a choice. The pick has to agree with the control transformer, overload settings and cable sizing in the panel, so it is settled alongside the motor size rather than afterwards.

Should I always round up to the next NEMA size?

Round up freely when the step is small — nobody is harmed by 7.5 hp on a 5.5 kW duty. Jumping a whole extra rung is a different decision: the 4 kW to 7.5 hp move brings a larger frame, higher starting current, a poorer power factor at part load and possibly a bigger starter or drive. And where the exact figure sits just above a size, as 30 kW does at 40.21 ehp, check the measured shaft duty before assuming the 40 hp motor cannot cope.

Kilowatts to Electric Horsepower