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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 |