Motor Frame Size Chart
NEMA motor frame size chart: shaft height and bolt patterns.
Motor Frame Size Chart
Replacing an electric motor on an industrial line or commercial HVAC air handler is a straightforward task—provided the mechanical footprint matches perfectly. An electric motor with the incorrect shaft height, shaft diameter, or base bolt pattern will not mate to the driven equipment without custom fabrication, expensive adapter plates, or structural base modifications.
This guide acts as a master reference for NEMA (National Electrical Manufacturers Association) standard frame sizes from 48 through 326T. It outlines critical mounting dimensions, decodes NEMA's frame numbering formulas, and explains how to handle specialized frame suffixes.
NEMA Motor Frame Dimensions Chart
Dimensions conform to NEMA MG 1 standards for T-frame motors. Shaft height (D) is from the base to shaft centerline. Bolt hole spacing (2E) is the distance between mounting holes lengthwise. Shaft length (BA) is from mounting face to shaft extension.
| Frame | D (shaft ht) | 2E (bolt sp) | F (bolt lng) | BA (shaft) | HP Range | Common Application |
|---|---|---|---|---|---|---|
| 48 | 3" | 2.12" | 2.75" | 3.00" | 1/20–1/3 HP | Small pumps, fans, appliances |
| 56 | 3.5" | 2.44" | 2.75" | 3.75" | 1/4–3/4 HP | General purpose, small machinery |
| 56C | 3.5" | 2.44" | 2.75" | — | 1/4–3/4 HP | C-face mounting, close-coupled pumps |
| 143T | 3.5" | 2.75" | 4.00" | 5.50" | 1–2 HP | General purpose motors |
| 145T | 3.5" | 2.75" | 5.00" | 6.62" | 1.5–3 HP | HVAC, conveyors, compressors |
| 182T | 4.5" | 3.75" | 4.50" | 7.25" | 3–5 HP | Light industrial machinery |
| 184T | 4.5" | 3.75" | 5.50" | 8.25" | 5–7.5 HP | Compressors, conveyors, pumps |
| 213T | 5.25" | 4.25" | 5.50" | 9.50" | 7.5–10 HP | Industrial motors, conveyor drives |
| 215T | 5.25" | 4.25" | 7.00" | 10.62" | 10–15 HP | Medium industrial motors |
| 254T | 6.25" | 5.00" | 6.25" | 12.38" | 15–25 HP | Large industrial motors |
| 256T | 6.25" | 5.00" | 8.25" | 13.25" | 20–30 HP | Large industrial and process motors |
| 284T | 7" | 5.88" | 7.25" | 15.25" | 25–40 HP | Large motors, compressors |
| 286T | 7" | 5.88" | 9.50" | 16.25" | 30–50 HP | Heavy industrial motors |
| 324T | 8" | 6.75" | 8.25" | 18.00" | 40–60 HP | Heavy industrial drives |
| 326T | 8" | 6.75" | 10.50" | 19.00" | 50–75 HP | Very large industrial motors |
How to Read a NEMA Frame Number
The NEMA frame number encodes the shaft height directly: D = Frame Number ÷ 16 inches. A 256T motor has a shaft height of 256 ÷ 16 = 16", divided by... wait — actually for T-frames: the first two digits divided by 4 give shaft height in quarter-inches. Frame 256T: 25 ÷ 4 = 6.25" shaft height. Frame 213T: 21 ÷ 4 = 5.25". The letter suffix (T = T-frame, C = C-face, H = high-efficiency) indicates the mounting type.
In practice, use the chart directly rather than calculating — but understanding the numbering helps you quickly assess whether a frame number represents a small, medium, or large motor.
The Base Length Meaning (Third Digit)
The third digit of a NEMA frame number indicates the distance between the mounting holes along the side of the base ($F$ dimension). While there is no direct math formula for this number, a higher third digit indicates a longer base frame. For instance, a 145T motor will have a bolt spacing pattern that is exactly 1 inch longer than a 143T motor (5.00" vs 4.00"), allowing it to house a physically longer, more powerful internal stator core.
Common NEMA Frame Suffix Letters
The letter at the tail end of a frame size designation tells you exactly how the shaft and mounting faces are constructed.
- T: The modern current industry standard for foot-mounted electric motors (established in 1965).
- U: The vintage industry standard (common from 1952 to 1965). U-frame motors are physically larger, heavier, and utilize a thicker shaft diameter than modern T-frame motors of equivalent horsepower.
- C: Indicates a C-Face motor. This face features a machined concentric pilot circle with threaded holes designed for bolting directly to a pump face, automotive transmission flange, or speed reducer gear box.
- D: Indicates a D-Flange motor. Similar to a C-face, but the mounting bolt holes pass cleanly through a flange rim that extends beyond the physical body of the motor frame shell.
- JM / JP: Specialized close-coupled pump mountings featuring longer shafts and specialized seals engineered specifically for the water treatment and fluid circulation industries.
- Y / Z: Indicates special, non-standard mechanical dimensions. Usually, this means the mounting base follows NEMA code, but the shaft extension or keyway diameter has been customized for an OEM machinery manufacturer.
T-Frame vs. U-Frame Motors
T-frame is the current NEMA standard, adopted in 1965. U-frame motors are older and are no longer manufactured — they have a larger frame for the same HP because they use less efficient materials. T-frame motors are not direct-fit replacements for U-frame motors of the same HP without an adapter plate or base modification.
When replacing a U-frame motor, identify the frame number from the nameplate and source a matched T-frame motor from a manufacturer's cross-reference, or use an adjustable motor base to accommodate the different shaft height.
C-Face and D-Flange Mounting
C-face (NEMA C): A standardized face-mounting pattern with a machined register that locates the motor precisely on the pump or driven equipment. Common for close-coupled pump drives and gear reducers. The "C" suffix indicates C-face.
D-flange (NEMA D): Similar to C-face but with a different mounting pilot diameter. Less common than C-face in general industrial applications.
Enclosure Styles and Environmental Considerations
Even if two motors share an identical NEMA 56 or 215T frame base, their total outer bulk can vary wildly based on their Enclosure Classification:
ODP (Open Drip-Proof)
Features ventilation slots that allow outside air to blow directly across the internal copper windings for maximum cooling efficiency. They are strictly limited to clean, dry, indoor environments (like an enclosed HVAC mechanical room) because moisture or conductive dust passing through the vents will quickly short out the motor.
TEFC (Totally Enclosed Fan-Cooled)
Completely sealed from the external environment with no open internal air vents. A mechanical fan is mounted outside the sealed frame under a protective metal shroud, blowing ambient air across external cooling fins. These are the undisputed standard for outdoor industrial operations, sawmills, car washes, and dusty manufacturing environments.
How to Properly Capture Motor Nameplate Data
Before ordering a replacement motor, never guess based on size alone. Take a clean photo or write down these mandatory variables directly from the metal nameplate riveted to the motor body:
- Frame Size (FRAME): (e.g., 184T, 56C)
- Horsepower (HP): Dictates the raw capacity of the motor.
- RPM / Poles: Ensure you match the rotational speed (typically 3600, 1800, or 1200 RPM for standard industrial settings).
- Voltage & Phase: (e.g., 230/460V Three-Phase vs. 115V Single-Phase).
- Service Factor (SF): The built-in overload margin of safety (typically 1.15 for industrial applications).
Sources & Industry References
- Official NEMA Standards: All frame dimensions and mechanical classifications conform directly to the core ANSI/NEMA MG 00001 (formerly MG 1) Motors and Generators Standard, which dictates standard motor dimensions across North America.
- Engineering Revisions: For a technical breakdown of how these NEMA sizing standards are organized and maintained, see the EASA Summary of NEMA Revisions.
- Manufacturer Reference Specs: Physical base layouts, shaft configurations, and legacy frame alignment measurements can be cross-verified through the Regal Rexnord / Marathon Electric Motor Dimension Portal as well as the industrial Baldor-Reliance NEMA Quick Reference Dimensions Layout.
Product Compliance and Suitability
The statements contained in this guide are intended for general informational purposes only. Such statements do not constitute a product recommendation or representation as to the appropriateness, accuracy, completeness, correctness, or currentness of the information provided. Information provided in this guide does not replace the use by you of any manufacturer instructions, technical product manual, or other professional resource or adviser available to you. Always read, understand, and follow all manufacturer instructions. Portions of this article were generated in part by AI.
