A frequently misunderstood subject related to electric motors is insulation class and temperature ratings. This paper tries to describe, in basic terms, the temperature relationships that are meaningful in standard AC induction motors. Some of the same information can be applied to DC motors but DC motors are more specialized and some of the ratings are slightly different.

Perhaps the best way to start is to define the commonly used terms.



Ambient temperature is the temperature of the air surrounding the motor or the room temperature in the vicinity of the motor. This is the “threshold point” or temperature that the entire motor would assume when it is shut off and completely cool.


Temperature rise is the change in temperature of the critical electrical parts within a motor when it is being operated at full load. For example: if a motor is located in a room with a temperature of 78° F, and then is started and operated continuously at full load, the winding temperature would rise from 78° F to a higher temperature. The difference between its starting temperature and the final elevated temperature, is the motor’s temperature rise.


Since the most common method of measuring “temperature rise” of a motor involves taking the difference between the cold and hot ohmic resistance of the motor winding*, this test gives the average temperature change of the entire winding including the motor leads and end turns as well as wire placed deep inside the stator slots. Since some of these spots are bound to be hotter than others, an allowance factor is made to “fudge” the average temperature to give a reflection of what the temperature might be at the hottest spot. This allowance factor is called the “hot spot allowance”.

*The formula for determining temperature rise by resistance is given in the appendix.


Insulations have been standardized and graded by their resistance to thermal aging and failure. Four insulation classes are in common use. For simplicity, they have been designated by the letters A, B, F, and H. The temperature capabilities of these classes are separated from each other by 25° C increments. The temperature capabilities of each insulation class is defined as being the maximum temperature at which the insulation can be operated to yield an average life of 20,000 hours. The rating for 20,000 hours of average insulation life is as shown below.

Insulation Class Temperature Rating
A 105° C
B 130° C
F 155° C
H 180° C


There are a number of insulating components used in the process of building motors. The obvious ones are the enamel coating on the magnet wire and the insulation on the leads that come to the conduit box. Some less obvious components of the “system” are the sleeving that is used over joints where leads connect to the magnet wire, and the lacing string that is used to bind the end turns of the motor. Other components are the slot liners that are used in the stator laminations to protect the wire from chafing. Also, top sticks are used to hold the wire down in place inside the stator slots. Another important component of the system is the varnish in which the completed assembly is dipped prior to being baked. The dipping varnish serves the purpose of sealing nicks or scratches that may occur during the winding process. The varnish also binds the entire winding together into a solid mass so that it does not vibrate and chafe when subjected to the high magnetic forces that exist in the motor.

Much like a chain that is only as strong as its weakest link, the classification of an insulation system is based on the temperature rating of the lowest rated component used in the system. For example, if one Class B component is used along with F and H components, the entire system must be called Class B.


Now that the basic terms have been identified, we can move on to understand the total picture and how the factors of temperature go together in the motor rating.

The basic ambient temperature rating point of nearly all electric motors is 40° C. This means that a motor, rated for 40° C ambient, is suitable for installation in applications where the normal surrounding air temperature does not exceed 40° C. This is approximately 104° F. A very warm room. This is the starting point.

When the motor is operated at full load, it has a certain amount of temperature rise. The amount of temperature rise is always additive to the ambient temperature. For example, U frame motors were designed for Class A insulation and a maximum temperature rise by resistance of 55° C. When operated in a 40ˆ C ambient temperature, this would give a total average winding temperature of 40° (ambient) + 55° (rise) or 95° C. The ten degree difference between 95° C and the 105° C rating of Class A insulation is used to handle the “hot spot allowance”. Now, if you use the same motor design but change the system to Class B, there is an extra 25° C of thermal capability available. This extra thermal capability can be used to handle:

a =>
higher than normal ambient temperatures,
b =>
higher than normal temperature rise brought on by overloads, or
c =>
the extra capability can be used to extend motor life and make it more tolerant of overheating factors caused by high or low voltages, voltage imbalance, blocked ventilation, high inertia loads, frequent starts, and any other factors that can produce above normal operating temperatures.

For example: if a motor with Class A “design” (55° C) temperature rise is built with Class B insulation, then it could be expected to give a normal insulation life even when subjected to ambient temperatures of 65° C.

Most “T” frame motors are designed for use with Class B insulation. In a “T” frame motor with Class B insulation, the extra 25° of thermal capacity (Class B compared to Class A), is utilized to accommodate the higher temperature rise associated with the physically smaller “T” frame motors.

For example: a standard T frame, open drip proof motor might have the following rating: 40° C ambient, 80° C temperature rise, and a 10° hot spot allowance. When these three components are added together, you will find that the total temperature capability of Class B insulation (130° C) is used up.


By taking a Class B, totally enclosed fan cooled, T frame motor, and building it with Class F insulation, it is usually possible to increase the service factor from 1.0 to 1.15. As mentioned previously, this same change of one insulation class can be used to handle a higher ambient temperature or to increase the life expectancy of the motor. Th same change could also make the motor more suitable for operation in high elevations where thinner air has a less cooling effect.


Over the years, great improvements have been made in insulating materials. With these improvements have come cost reductions. As a result of these changes, most motor manufacturers use a mixture of materials in their motors, many of which have higher than required temperature ratings. For example, Baldor does not use Class A materials. This means that even though many fractional horsepower motors are designed for Class A temperature rise, the real insulation is Class B or better. Similarly, many motors designed for Class B temperature rise actually have insulation systems utilizing Class F and H materials. This extra margin gives the motor a “life bonus”. At the present time, Baldor has standardized on ISR (Inverter Spike Resistant) magnet wire in all three phase motors 1 HP and larger. this wire has a Class H temperature rating and excellent resistance to high voltage spikes.

As a rule of thumb, insulation life will be doubled for each 10 degrees of unused insulation temperature capability. For example: if a motor is designed to have a total temperature of 110° C (including ambient, rise, and hot spot allowance), and is built with a Class B (130° C) system, an unused capacity of 20° C would exist. This extra margin would raise the expected motor insulation life from 20,000 hours to 80,000 hours. Similarly, if a motor is not loaded to full capacity, its temperature rise will be lower. This automatically makes the total temperature lower and extends motor life. Also, if the motor is operated in a lower than 40° C ambient temperature, motor life will be extended.

The same ten degree rule also applies to motors operating at above rated temperature. In this case, insulation life is “halved” for each 10° C of overtemperature.


Motor surface temperature is frequently of concern. The motor surface temperature will never exceed the internal temperature of the motor. However, depending upon the design and cooling arrangements in the motor, motor surface temperature in modern motors can be high enough to be very uncomfortable to the touch. Surface temperatures of 75° to 95° C can be found on T frame motor designs. These temperatures do not necessarily indicate overload or impending motor failure.


Insulation life is affected by many factors aside from temperature. Moisture, chemicals, oil, vibration, fungus growth, abrasive particles, and mechanical abrasion created by frequent starts, all work to shorten insulation life. On some applications if the operating environment and motor load conditions can be properly defined, suitable means of winding protection can be provided to obtain reasonable motor life in spite of external disturbing factors.


U frame 184 through 445U frames, were designed based on using Class A insulation. Temperature rise was not precisely defined by the resistance method. Temperature rise by thermometer for Class A, open drip proof motors was 40° C. This was generally thought to be equivalent to approximately 50° C by resistance. U frame motors were the industry standard from 1954 to 1965 and are still preferred in some industries and plants. T frame, 143T through 449T motors are generally designed based on using Class B insulation with temperature rises by resistance of approximately 80° C. Production of T frame motors started in the mid-sixties and they continue to be the industry standard at this time.


A key ingredient in motor life is the insulation system used in the motor. Aside from vibration, moisture, chemicals, and other non-temperature related life-shortening items, the key to insulation and motor life is the maximum temperature that the insulation system experiences and the temperature capabilities of the system components.

Table 1 shows the temperature ratings, temperature rise allowances and hot spot allowances for various enclosures and service factors of standard motors.

Table 2 shows a listing of temperature related life-shortening factors along with symptoms and cures. You may find this table useful.

Insulation System Class A B F H
Temperature Rating in Degrees Centigrade 105° 130° 155° 180°
Temperature Rise Allowance by Resistance (Based on 40° C Ambient Temperature)
All Motors with 1.15 Service Factor
(Hot Spot Allowance)
Totally Enclosed Fan Cooled Motors
(Hot Spot Allowance)
Totally Enclosed Non-Ventilated Motors
(Hot Spot Allowance)
Motors other than those listed above
(Hot Spot Allowance)
* When operating at service factor loading the hot spot temperatures can actually exceed the insulation rating resulting in shortened motor life.

* Bearing lubrication must also be matched to high operating temperature.
**Reduced voltage starting method and motor characteristics must be matched to the load requirement.
Temperature Related Life-Shortening Factors
Low Voltage Overload Tripping
High current
Short motor life
Correct power supply or match motor to actual power supply voltage rating.
High Voltage Overload Tripping
High current
Short motor life
Correct power supply or match motor to actual power supply voltage rating.
Unbalanced phase currents
Overload tripping
Determine why voltages are unbalanced and correct.
Overload Overload Tripping
High current
Short motor life
Determine reason for overload. Increase motor size or decrease load speed.
High Ambient Temperatures Short motor life * Rewind motor to higher class of insulation.Oversize motor to reduce temperature rise.Ventilate area to reduce ambient temperature.
Short motor life
Runs hot
Amperage o.k.
Clean lint and debris from air passageways or use proper motor enclosure for application.
Frequent Starts Short motor life ** Use a reduced voltage starting method. Upgrade class of insulation.
High Inertia
Short motor life
Overload tripping during starting
Oversize motor frame.
Use higher class of insulation
** Use a reduced voltage starting method.


Temperature Rise by Resistance Method

Degrees C Rise = Rh – Rc/ Rc (234.5 + T)

Where Rc = Cold Winding Resistance in Ohms

R h = Hot Winding Resistance in Ohms

T = Cold (ambient) Temperature in Degrees Centigrade

Note: This formula assumes that the ambient temperature does not change during the test.

Example: A small motor has a cold temperature of 3.2 ohms at 25° C (77° F) ambient temperature. After operating at full load for several hours, the resistance measures 4.1 ohms and the ambient has increased to 28° C.

Calculate the temperature rise:

Apparent rise = 4.1 – 3.2/3.2 (234.5 + 25) = 73° C

Correcting for 3° C increase in ambient:

Actual rise = 73° – 3° = 70° C

Centigrade Fahrenheit Conversions
Actual Temperatures

To change Fahrenheit to Centigrade:

C° = (F° – 32) 5/9

To change Centigrade to Fahrenheit:

F° = (C° x 9/5 ) + 32

Rise Values Only

Degrees “C” Rise = °F (Rise) x .56

Degrees “F” Rise = °C (Rise) x 1.8

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