Motor Frame Sizes

NEMA Motor Frame Dimensions (inches unless noted)

Four dimensions decide whether a motor fits, and all four must match: shaft height D, foot bolt spacing 2E (front to back along the shaft) and 2F (side to side), and shaft diameter U. A motor that matches on shaft height and diameter but not on bolt spacing will not bolt down, and one that matches on the feet but not the shaft will not couple.

The shaft height rule is exact and worth memorising. For any T-frame, the first two digits divided by four give D in inches. It holds on all 76 frames in this table — which is also how the values were verified.

The 'T' suffix is not decoration. It marks a frame that follows the 1964 NEMA restandardisation. A pre-1964 motor with the same number but no T has different dimensions, and the frames listed here without a T — 42, 48, 56 and their H variants — are the older small frames that kept their original numbering.

2E and 2F are bolt spacing, not overall size. They locate the four foot holes, and they are why a larger motor will not fit an existing base even when the shaft height matches. Where a base must accept a different frame, the usual answer is an adapter plate drilled for both patterns.

Shaft extension length and shoulder dimensions are defined by NEMA MG 1 but are not given here, because the published tables disagree on how those two columns are labelled. Confirm them from the specific motor's drawing rather than from a summary table.

Checking Whether a Replacement Motor Fits

Matching a motor is a dimensional exercise before it is an electrical one. Work through the mechanical checks first, because a motor that does not fit cannot be used regardless of its rating.

Shaft height and bolt spacing locate the motor on its base. All three come from the frame number, so if the frame matches, the mounting does too. If the frame differs, the 2E and 2F columns tell you whether the existing base can accept it.

Shaft diameter and keyway decide whether the existing coupling or pulley fits. A coupling bored for a 7/8 in shaft will not take a 1-1/8 in shaft, and re-boring it is often possible but not always — there has to be enough material left around the bore.

Frame length is the dimension that usually has clearance. The two frames in a pair — 143T and 145T, 182T and 184T — share shaft height, bolt spacing and shaft diameter but differ in frame length, so swapping between them normally works if there is room at the non-drive end.

Three things the frame number does not tell you, and which must be checked separately: the mounting configuration (footed, C-face, D-flange), the enclosure type, and the electrical characteristics. A C-face motor and a footed motor can share a frame number and have nothing in common mechanically.

Frequently Asked Questions

How do I find a NEMA motor frame size?

For a T-frame, divide the first two digits by four to get the shaft height in inches — 143T is 3.5 in, 284T is 7 in. The frame number also determines the foot bolt spacing and shaft diameter, all of which are in the table above. Frames without a T are the older series.

What dimensions must match to replace a motor?

Shaft height, foot bolt spacing in both directions (2E and 2F), and shaft diameter. All four come from the frame number, so matching the frame matches them. Also check the mounting configuration — footed, C-face or D-flange — which the frame number does not specify.

What does 2E and 2F mean on a motor frame?

They are the foot bolt spacings. 2E is the distance between bolt holes measured along the shaft direction; 2F is the distance measured across it. Both are centre-to-centre dimensions, so they are used to lay out the mounting holes rather than to describe the motor's size.

What does the T mean in a NEMA frame number?

It marks a frame following the 1964 NEMA restandardisation, which changed the dimensions assigned to several frame numbers so higher horsepower could fit the same envelope. A motor numbered 284 and one numbered 284T are not dimensionally interchangeable.

What is the shaft diameter of a 143T motor?

Seven-eighths of an inch, or 22.2 mm, with a 3/16 in square keyway. The 145T shares the same shaft height, bolt spacing and shaft diameter and differs only in frame length, so the two are often interchangeable where there is clearance at the non-drive end.

Why does a larger frame number mean a bigger motor?

Because the number encodes the shaft height, which sets the frame's physical size bracket. Within a size bracket the horsepower depends on the frame length and the speed — the same 284T frame carries roughly 20 to 25 HP at 1800 rpm, about half that at 3600 rpm and around double at 900 rpm.

Related

• Motor Full-Load Current
• Keyway Sizes
• Breaker Sizes Chart
• Bolt Circle Calculator
• NEMA Enclosure Types

Value Sources

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

Data Sources

Verification scope

No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.

Derived values — the following values on this page are calculated, not taken directly from the standard:

Dimensions are the NEMA standard frame dimensions. Shaft extension length and shoulder dimensions are defined by NEMA MG 1 but are omitted here because published tables differ in how those columns are labelled — take them from the motor's own drawing. Mounting configuration and enclosure type are independent of the frame number and must be confirmed separately.

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