NEC calculation for overload sizing

Sizing for overload is important to avoid serious damage for electrical application such as motor starter,machinery or others electrical equipment.

According to NEC,the general requirement for overload sizing be set around 115% or 125% from full load ampere.We should setting the overload relay within this parameter to avoid electric motor from serious damage.

For calculation of overload sizing,it depend on the motor full load ampere current rating,the service factor and temperature for motor.I already explain in my last post about several factor effected for overload relay sizing.

NEC Calculations for overload relay sizing.

Below is exception for overload relay sizing :-

 

  • For motors rated 40EC with a Service Factor of 1.15 or greater, 125% of the motors FLA is used to calculate the maximum size device for overload protection.
  • For motors rated greater than 40 EC or unmarked, 115% of the motors FLA is used to calculate the maximum size device regardless of the motor’s Service Factor.
  • If use of the previous size rules results in the motor tripping off line during starting, the device can be increased to a maximum of 140% of the motors FLA.

Example:

Calculated sizing for overload device to provide overload protection to a 3 phase, 200 Volt, 15 HP motor with Full Load Ampere is 35 ampere,ambient temp : 40 EC and service factor : 1.15

Answer :-

Ambient Temp = 40 EC

S.F.=1.15

35 ampere x 125% = 43.7 ampere @ 44 ampere

If use of the size calculated results in the motor tripping off line when started, the overload device may be increased to a maximum size allowable for application : 35 amps X 140% = 49 ampere

Comments

  1. haitham hashim says:

    Useful information not prevent me anything

  2. Ricardo says:

    Hi, Dear

    I have a question Do we need a Overload protection for a heater unit that has a 240 VAC motor inside with 1/12 PH at 1750 rpm single face ?

    Unit operation specifications are: 240VAC, 60A single face with termostat

    Thanks a lot!

    • HI ricardo,
      it no need to install overload relay for heater unit because the rccb and circuit breaker have overload protection :D

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  1. [...] Overload relay should install in our motor starter application for extra protection.Overload relay can avoid serious damage for electric motor when overload happen to our system with proper setting. [...]

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