# 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

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

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!

• lemau says

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

3. Hrag ghazarian says

Hey i need to make a soft starter for a 3phase motor that will be connected between the ups and the motor. In my country the electricty is available only for 10 hours a day thus we have generators, and i need the motor to keep on running, but the motor is so big that when the electricity shuts off the ups must work until the generators start, but when the motor is going to work on the ups it cuts off take into consideration the ups power is about 25kva. So i need the best type of soft starter to keep the motor running on the ups until the generators turn on.

4. Laverne says

Regards for helping out, good info.

5. Mcebo Mawandla says

HOW TO SELECT correct step down Transformer (1000/110) that can supply the inruch VA of 710 for few seconds and the correctly rated conductor (for both control and power/main circuits)

Contactors x2
Inrush VA  350 VA x 2  700 VA
Sealed VA  18 VA x 2  36 VA

Timing Relays x2
Inrush VA  1.3 VA x 2  2.6 VA
Assuming Inrush VA = Sealed VA  2.6 VA

Lights x4
Max Amps = 20 mA
P = IV = 0.02 x 110 = 2.2 W
P = S, W=VA = 2.2 VA where cosѲ = 1
2.2 x 4 = 8.8 VA

Control Circuit VA
Total Sealed VA = 36 + 2.6 + (2.2 x 3)
= 45.2 VA
Inrush VA drawn by loads as soon as power is switched on
1. Power status light
2. 2 x on-delay timers
22.2 +2.6 = 4.8 VA
INRUSH VA DRAWN AFTER 10secs
1. 2 x contactors 350 VA x 2 = 700
2. 2 x Motor Status Lights  2.2 x 2 = 4.4 VA
i. 700+4.4 = 704.4 VA

TWO MOTORS EACH RATED AT 920V,31 FLAmps and 37 kW