Understanding NEMA Frames: A Reference Guide


NEMA Frame/Shaft Sizes

Frame numbers are not intended to indicate electrical characteristics such as horsepower. However, as a frame number becomes higher so in general does the physical size of the motor and the horsepower. There are many motors of the same horsepower built in different frames. NEMA (National Electrical Manufacturers Association) frame size refers to mounting only and has no direct bearing on the motor body diameter.

By NEMA definition, two-digit frame numbers are fractional frames even though 1 HP or larger motors may be built in them. Three-digit frame numbers are by definition integral frames. The third numeral indicates the distance between the holes parallel to the base. It has no significance in a footless motor. Refer to NEMA Standard Dimension Chart.

NEMA Suffixes

C = NEMA C face mounting (specify with or without rigid base)
D = NEMA D flange mounting (specify with or without rigid base)
H = Indicates a frame with rigid base having an F dimension larger than that of the same frame without the suffix H. For example, combination of 56H base motors have mounting holes for NEMA 56 and NEMA 143-5T and a standard NEMA 56 shaft.
J = NEMA C face, threaded shaft pump motor
JM = Close-coupled pump motor with specific dimensions and bearings
JP = Closed-coupled pump motor with specific dimensions and bearings
M = 6 3/4" flange (oil burner)
N = 7 1/4" flange (oil burner)
T, TS = Integral horsepower NEMA standard shaft dimensions if no additional letters follow the "T" or "TS."
TS = Motor with NEMA standard "short shaft" for belt driven loads
Y = Non-NEMA standard mount; a drawing is required to be sure of dimensions. Can indicate a special base, face or flange.
Z = Non-NEMA standard shaft; a drawing is required to be sure of dimensions.

NEMA Prefixes

Letters or numbers appearing in front of the NEMA frame number are those of the manufacturer. They have no NEMA frame significance. For example, the letter in front of LEESON's frame number, L56, indicates the overall length of the motor.

Mounting

Unless specified otherwise, motors can be mounted in any position or any angle. However, unless a drip cover is used for shaft-up or shaft-down applications, drip-proof motors must be mounted in the horizontal or sidewall position to meet the enclosure definition. Mount motors securely to the mounting base of equipment or to a rigid, flat surface, preferably metallic.

Types of Mounts


Rigid base

Is bolted, welded or cast on main frame and allows motor to be rigidly mounted on equipment.

Resilient base

Has isolation or resilient rings between motor mounting hubs and base to absorb vibration and noise. A conductor is imbedded in the ring to complete the circuit for grounding purposes.

NEMA C face mount

Is a machined face with a pilot on the shaft end which allows direct mounting with a pump or other direct coupled equipment. Bolts pass through mounted part to threaded hole in the motor face.

NEMA D flange mount

Is a machined flange with rabbet for mountings. Bolts pass through motor flange to a threaded hole in the mounted part. NEMA D flange kits are stocked by some manufacturers, including LEESON.

Type M or N mount

Has special flange for direct attachment to fuel atomizing pump on an oil burner. In recent years, this type of mounting has become widely used on auger drives in poultry feeders.

Extended through-bolt

Have bolts protruding from the front or rear of the motor by which the driven load is mounted. This is usually used in applications involving small direct drive fans or blowers.

Types of Enclosures


Drip-proof

Vents in endshield and/or frame are to prevent drops of liquid from falling into motor within a 15 degree angle from vertical. Designed for use in areas that are reasonably dry, clean, and well ventilated (usually indoors). If installed outdoors, it is recommended that the motor be protected with a cover that does not restrict the flow of air to the motor.

Totally enclosed air over (TEAO)

Dust-tight fan and blower duty motors designed for shaft mounted fans or belt driven fans. The motor must be mounted within the airflow of the fan.

Totally enclosed non-ventilated (TENV)

No vent openings, tightly enclosed to prevent the free exchange of air, but not airtight. Has no external cooling fan and relies on convection for cooling. Suitable for use where exposed to dirt or dampness, but not very moist or hazardous (explosive) locations.

Totally enclosed fan cooled (TEFC)

Same as TENV except has external fan as an integral part of the motor, to provide cooling by blowing air around the outside frame of the motor.

Totally enclosed, hostile and severe environment motors

Designed for use in extremely moist or chemical environments, but not for hazardous locations.

Totally enclosed blower cooled motors

Same as TEFC except external fan must run on a power supply that is independent of the inverter output. Cooling per MG 1.6 (IC 46).

Explosion-proof motors

Have bolts protruding from the front or rear of the motor by which the driven load is mounted. This is usually used in applications involving small direct drive fans or blowers.

CLASS I (Gases, Vapors)

Group A Acetylene
Group B Butadiene, ethylene oxide, hydrogen, propylene oxide
Group C Acetaldehyde, cyclopropane, diethel ether, ethylene, isoprene
Group D Acetone, acrylonitrite, ammonia, benzene, butane, ethylene dichloride, gasoline, hexane, methane, methanol, naphtha, propane, propylene, styrene, toluene, vinyl acetate, vinyl chloride, xylene.

CLASS II (Combustible Dusts)

Group E Aluminum, magnesium and other metal dusts with similar characteristics.
Group F Carbon black, coke or coal dust
Group G Flour, starch or grain dust

The motor ambient temperature is not to exceed +400C or -250C unless the motor nameplate specifically permits another value, and is noted on the nameplate and in the literature. LEESON explosion-proof motors are approved for all classes noted except Class I, Groups A & B.

NOTE: All above data is for reference purposes only
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