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Motor Service Factor (SF) Defined By NEMA

Motor Service Factor (SF) Defined By NEMA
Motor Service Factor (SF) Defined By NEMA

Permissible horsepower loading

Motor Service Factor (SF) is the percentage of overloading the motor can handle for short periods when operating normally within the correct voltage tolerances. This is practical as it gives you some ‘fudge‘ in estimating horsepower needs and actual running horsepower requirements.

It also allows for cooler winding temperatures at rated load, protects against intermittent heat rises, and helps to offset low or unbalanced line voltages.

BALDOR Open Drip Proof C-Face Foot Mounted motor - 1/3Hp-100Hp NEMA 56C-404TC
BALDOR Open Drip Proof C-Face Foot Mounted motor – 1/3Hp-100Hp NEMA 56C-404TC

For example, the standard SF for open drip-proof (ODP) motors is 1.15. This means that a 10-hp motor with a 1.15 SF could provide 11.5 hp when required for short-term use. Some fractional horsepower motors have higher service factors, such as 1.25, 1.35, and even 1.50.

NEMA defines service factor as a multiplier, when applied to the rated horsepower, indicates a permissible horsepower loading, which may be carried under the conditions specified for the service factor at rated voltage and frequency.

This service factor can be used for the following:

  1. To accommodate inaccuracy in predicting intermittent system horsepower needs.
  2. To lengthen insulation life by lowering the winding temperature at rated load.
  3. To handle intermittent or occasional overloads.
  4. To allow occasionally for ambient above 40°C.
  5. To compensate for low or unbalanced supply voltages.

NEMA does add some cautions, however, when discussing the service factor:

  1. Operation at service factor load for extended periods will usually reduce the motor speed, life and efficiency.
  2. Motors may not provide adequate starting and pull-out torques, and incorrect starter/overload sizing is possible. This in turn affects the overall life span of the motor.
  3. Do not rely on the service factor capability to carry the load on a continuous basis.
  4. The service factor was established for operation at rated voltage, frequency, ambient and sea level conditions.

Most motors have a duty factor of 1.15 for open motors and 1.0 for totally closed motors.

Traditionally, totally enclosed fan cooled (TEFC) motors had an SF of 1.0, but most manufacturers now offer TEFC motors with service factors of 1.15, the same as on ODP motors. Most hazardous location motors are made with an SF of 1.0, but some specialized units are available for Class I applications with a service factor of 1.15.

The service factor is required to appear on the nameplate only if it is higher than 1.0.

Reference: Understanding Motor Nameplate Information: NEMA vs. IEC Standards – Continuing Education and Development, Inc.

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

2 Comments


  1. mehrdad zamani
    Feb 12, 2017

    Dear Edvard
    when we have mechanical power eg 500 hp and want to heavy duty with 1.5 service factor this mean we should select 750 hp motor? and by 95% efficiency reached to 773 hp electrically is it correct.
    if this is correct then motor normally operate on 500 hp (mech) which is lower than 750 hp and is not good.
    or the manufacturer gives us 500 hp mechanical power output motor can work temporarily in 750 hp.
    thanks for your recommendation.


  2. ADNAN ALRADI
    Feb 13, 2014

    Dear Edvard,Hi

    I dint have NEMA with me,but is there a chance to knowwhat would be teh short time that we can rung the motor according to the SF.

    As far as I know if the SF=1.1 for example and the operation duty S1 we can run the motor on Full Load Current for long time( indefinitly) but we cannot run teh same motor on 110% for long time( indfinitly) am I correct?
    Please advise and thanking you rcooperation.

    Reagrds,
    Adnan

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