Matching a Replacement Motor to the Nameplate on the Old One
When a motor burns out, the rating stamped on its plate is the specification the replacement has to hit. On North American equipment that figure is electric horsepower — a unit fixed at exactly 746 W so that motor catalogues, overload tables and conductor charts all rest on one round number. The mechanical horsepower a machine designer calculates in is 745.69987158227 W. Close, but not the same unit, and worth knowing which one you are holding.
What the Stamped Rating Actually Tells You
746 watts, rounded for the tables
Shaft output, never terminal input
Service factor sits above the number
The frame number travels with the rating
From the Old Nameplate to the Purchase Requisition
The plate is usually half painted over and the motor is already on the workshop bench, so the sequence is simple: read what you can, translate it, then check it against whatever the driven machine's documentation says.
Enter the rating stamped on the plate
Type the ehp figure on the left — 1, 7.5, 25, 150. The mechanical value fills in as you type. A comma works as a decimal point for fractional ratings, and any spaces you leave in are ignored.
Compare it with the driven machine's demand
The pump, fan or conveyor manual normally states absorbed power in mechanical hp or in kW. Put both onto the same footing before deciding whether the old motor was oversized or running right at its limit.
Reverse it when the spec sheet leads in mechanical hp
Press the swap button (↔) to run hp → ehp and see which standard motor size a mechanical figure lands on. A 30 hp absorbed load comes back as 29.9879 ehp, so a 30 ehp frame covers it with nothing left over.
Copy the plain figure onto the order
The copy button puts the number alone on the clipboard — no unit, no thousands spacing — ready to paste into a stores system or a supplier's enquiry form. Ctrl + C inside a field does the same.
NEMA Frame Sizes and What Each Stamped Rating Buys
Standard T-frame assignments for four-pole, 1 800 rpm totally-enclosed motors, with the nameplate rating shown both ways and the shaft output it stands for in watts.
| NEMA frame (1 800 rpm) | Nameplate (ehp) | Mechanical (hp) | Shaft output (W) |
|---|---|---|---|
| 143T | 1 ehp | 1.0004 hp | 746 W |
| 182T | 3 ehp | 3.0012 hp | 2 238 W |
| 184T | 5 ehp | 5.0020 hp | 3 730 W |
| 213T | 7.5 ehp | 7.5030 hp | 5 595 W |
| 215T | 10 ehp | 10.0040 hp | 7 460 W |
| 254T | 15 ehp | 15.0060 hp | 11 190 W |
| 284T | 25 ehp | 25.0101 hp | 18 650 W |
| 326T | 50 ehp | 50.0201 hp | 37 300 W |
Notice how the ratings step: 1, 3, 5, 7.5, 10, 15, 25, 50 — a catalogue ladder, not a continuous range. The mechanical column never strays more than a twentieth of a horsepower from the stamped figure even at 50 ehp, which is why nobody on the shop floor bothers to distinguish the two. The frame column is what actually decides whether a motor fits the bedplate you already have.
What This Page Does During a Motor Swap
Both boxes stay live down a motor schedule
Type in either field and the other follows, so you can run through every drive on a plant list without clearing anything between entries.
Turn a machine spec back into a motor size
The swap button reverses to hp → ehp, the direction you want when the driven equipment quotes absorbed power and you need the standard frame that covers it.
Every power unit against the same plate
Searchable dropdowns on both sides cover kW, W, PS and the rest, so an imported plate quoting kilowatts goes through the same field without leaving the page.
A clean figure for the stores system
Results carry up to eight decimals and the copy button hands over the bare number, which is what a requisition line or a maintenance record expects.
Questions from the Motor Room Floor
Why was electric horsepower fixed at exactly 746 watts?
Because the electrical trade needed a whole number to build tables around. Watt's mechanical definition of 550 ft·lb/s works out at 745.69987158227 W, an awkward figure to carry through motor catalogues, overload settings and conductor charts. Rounding it up to 746 W settled the electrical side on one clean constant, and the definition has stayed there ever since.
Does the plate rating mean the power going in or the power coming out?
Coming out. The stamped horsepower is the mechanical power available at the shaft with the motor at rated load, voltage and frequency. Electrical input is that figure divided by efficiency: 25 ehp is 18 650 W of shaft power, but at 93 % efficiency the supply is delivering just over 20 kW. The separate full-load amps line is the number to size cable and protection from — not the horsepower.
What does a 1.15 service factor actually allow on a 10 hp motor?
It permits 11.5 ehp — 8 579 W of shaft output — but only at rated voltage and frequency, within the stated ambient, and with the winding running hotter for it. Efficiency and power factor both fall off in the service-factor region, and every extra 10 °C on the insulation roughly halves its life. Treat it as headroom for occasional peaks, not as a spare 15 % of continuous duty.
Does the 0.04 % gap between the two horsepowers ever change what I order?
In practice, no. On a 100 ehp motor the difference is 0.04 hp — about 30 W, well inside manufacturing tolerance and dwarfed by the service factor and by the catalogue steps you have to choose between anyway. It matters when you are reconciling a calculation against a written standard, or explaining why 100 ehp appears as 74.6 kW in one document and 74.57 kW in another.
Which nameplate lines matter most when matching a burnt-out motor?
Rating and full-load speed first, because together they set the frame; then voltage and frequency, frame number and enclosure type, insulation class and service factor, full-load amps, and the design letter that fixes the starting-torque characteristic. Photograph the whole plate before the motor leaves site — shaft diameter and mounting dimensions are far harder to recover once it has gone to the rewind shop.
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