【正文】 f the motor’s fullload 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 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. Ambient Temperature Compensation The ambient temperature in which a starter and motor is located must be considered when selecting overloads because a high ambient temperature reduces overload trip time. ? Reduced overload trip time can lead to nuisance tripping if a motor is located in a cooler ambient temperature than the starter and lead to motor burnout when the motor is located in a hotter ambient temperature than the starter. ? Most thermal overload devices are rated for use at a maximum temperature of 40 degrees C which is about 104 degrees F. ? The overload device trips at less than 100 percent rated current when the ambient temperature exceeds 104 degrees F which can result in “nuisance tripping”. ? If the temperature is significantly below 104 degrees F, the overload device allows significantly more current through than it is rated for resulting in potential motor overload and failure without the overload tripping the motor off. ? A higher overload heater can be selected when the ambient temperature at the starter is higher than the temperature at the motor and a lower value selected when the ambient temperature at the starter is lower than the temperature at the motor. ? If the temperature varies widely during the year, the motor may not be protected when the temperature swings dramatically the other way unless the original overloads are switched back. Ambient Compensated Heaters For this reason, special Ambient Compensated Heaters which have a much “flatter” temperature response should be used in most outdoor applications and where ambient operating temperatures are significantly different. Sizing Motor Protection Systems Given the following motor, size the conductors, motor overcurrent and motor overloads to adequately protect the motor and conductors. Nameplate Info: FLA = 22 Service Factor = Ambient = 40 C STEP 1: Determine the motor’s FLA (full load amps) ? Go to the appropriate NEC Table to find the design FLA ? NEC Table 430150 for 3 phase: For 10 Hp, 230 Volt Motor = 28 amps STEP 2: Determine the size of branch circuit conductor required. ? NEC 43022 says the conductor ampacity equals the FLA x 125% ? Conductors supplying a single motor used for a continuous duty load must have a current carrying capacity of not less than 125% of the motor’s full load current (FLA) rating as given in NEC tables 430148 or 430150. ? Conductor Ampacity = 28 amps X = 35 amps ? Use NEC Table 31016 to select the conductor with the required ampacity ? From NEC Table 31016: 8 AWG Copper ? The NEC procedure requires use of the 8 AWG conductor so it will be large enough for any motor of the same size in the future. STEP 3: Determine the branch circuit overcurrent device size. ? The maximum branch circuit overcurrent device size is calculated based on the type of protective device selected (standard fuse, timedelay fuse, instantaneous breaker, inverse time breaker) and percentage multiplier from NEC Table 430152. ? Multiply the motors design FLA by the appropriate percentage in NEC Table 430152. ? 1. When the value found does not match a standard fuse/breaker size the NEC permits the next higher STANDARD size for a branch circuit overcurrent device. STEP 4: Determine the required size for the motor running overload protection. 1. Use the nameplate FLA directly to find the appropriate overload device heater on the motor starter cover or from manufacturers’ tables. 2. Use the nameplate FLA and NEC Section 43032 to calculate the maximum size for the motor overload protection in amps. ? NEC Section 43032 specifies the maximum overload protection size for most installations if nameplate amps aren’t available. (FLA X 115% or FLA X 125% depending on criteria). ? Since the motor’s ambient rating was 40 deg C and the . was , use 115%. For Ambient of 40 deg C and . = : 22 amps X 115% = amps ? NEC Section 43034 specifies the maximum size if th calculated value in Section 43032 will not allow the motor to start consistently. (Motor FLA X 140%). 22 amps X 140% = amps MAXIMUM Motor control centers In many mercial and industrial applications quite a few electric motors are required, and it is often desirable to control some or all of the motors from a central location. The apparatus designed for this function is the motor control center (MCC). Motor control centers are simply physical groupings of bination starters in one assembly. A bination starter is a single enclosure containing the motor starter and the fuses or circuit breaker. NEMA Definition: NEMA defines a motor control center in ICS182022 as being a floormounted assembly of one or more enclosed vertical sections having a horizontal mon power bus and principally containing bination motor control units. These units are mounted one above the other in the vertical sections. The sections normally incorporate vertical buses connected to the mon power bus, thus extending the mon power supply to the individual units. Power may also be supplied to the individual units by bus bar connections, by stab