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Sizing Motor Overload Protection

There are several types of devices that can be used to provide overload protection and the sizing procedure can vary depending on the type of device used.

C It is important to keep differences in the procedures separate and understood well so as not to

install overloads that do not provide adequate protection to the motor.

• The simplest and most straightforward sizing procedures for motor overload protection are

applied when sizing overload relays using the cover of the motor starter, control center, or manufacturer’s catalog.

• The National Electrical Code specifies methods to calculate the maximum size motor

overload protection for specific motors if a manufacturers chart is not available.  Installations relying on fuses and circuit breakers as back-up overload protection must be calculated using the NEC method.

NEC Calculations

The NEC in general requires the maximum size overload device be set to open at 115% or 125% of the motor’s full-load current rating, depending upon the service factor and/or temperature rise of the motor.  There are however, exceptions.

• 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 40EC 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:

Find the maximum size overload device to provide overload protection to a 3 phase, 230 Volt, 10 horsepower motor with FLA of 28 amps if:

Ambient Temp = 40EC, S.F.=1.15: 28 amps X 125% = 35 amps Ambient Temp = 40EC, S.F.=1.00: 28 amps X 115% = 32.2 amps Ambient Temp = 50EC, S.F.=1.15 28 amps X 115% = 32.2 amps Ambient Temp = 50EC, S.F.=1.00 28 amps X 115% = 32.2 amps

If use of the size calculated results in the motor tripping off line when started, the overload device may be increased to a maximum of:

Maximum size allowable:

28 amps X 140% = 39.2 amps

Electronic Overloads

Electronic overloads sense the load current and the heating effect on the motor is computed.  If an overload condition exists, the sensing circuit interrupts the power circuit.

C The tripping current can be adjusted to suit the particular application.

C Electronic overloads often perform additional protective functions such as ground fault and

phase loss protection.

Fuses

Fuses have limited application as the primary means of overload protection for motors but can be effectively used to provide back up overload protection.

• Single-element fuses are not designed to provide overload protection.

C Their basic function is to protect against short circuits and ground faults.

C If sized to provide overload protection, they would blow when the motor starts due to high

motor inrush current.

• Dual-element fuses can provide motor overload protection, but they have to be replaced

when they blow which can be a disadvantage.

• There is a risk of single-phasing damage to the motor when only one fuse blows unless

single-phase protection is provided.