When sizing a disconnect for a motor do you use the Table amps in the code or do you use the nameplate. I have a 20HP 3Phase motor at 240V. The table Amps is 54A. So 54 x 115%= 62.1A. Can you go down to a 60A or go up to a 100A Nonfusible disconnect General duty safety switch.
Sizing Disconnects for motors
- Thread starter wmeek
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masterinbama
Senior Member
- Location
- Huntsville Alabama
You have to go by the HP rating of the disconnect.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
You may want to see what the horsepower rating of the disconnect you wish to use is.
Most likely you will need a 100 amp switch for a 20 hp 230 volt motor.
Was looking at Square D catalog for a fused switch you will need a 100 amp switch - for one it is hard to fit a 80 or 90 amp fuse in a 60 amp fuseholder.
If you are using a non fused switch their 60 amp heavy duty switches are rated for 20 hp.
Most likely you will need a 100 amp switch for a 20 hp 230 volt motor.
Was looking at Square D catalog for a fused switch you will need a 100 amp switch - for one it is hard to fit a 80 or 90 amp fuse in a 60 amp fuseholder.
If you are using a non fused switch their 60 amp heavy duty switches are rated for 20 hp.
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
I agree. Our shop has had disconnects installed in the past that were oversized for the amp rating but turned out to actually be undersized due to the HP rating of the disconnect. I know this because they failed prematurely and we had to replace them with the next size up. Don't learn the hard way like us....:roll:
electures
Member
- Location
- South Jersey
Size at 115% of the FLA of the motor.
Size at 115% of the FLA of the motor.
If it is a single motor then it has to be sized at 115% of the FLA of the motor per table 430.250. A 100A switch is compliant.
430.6 Ampacity and Motor Rating Determination. The size of conductors supplying equipment covered by Article 430 shall be selected from the allowable ampacity tables in accordance with 310.15(B) or shall be calculated in accordance with 310.15(C). Where flexible cord is used, the size of the conductor shall be selected in accordance with 400.5. The required ampacity and motor ratings shall be determined as specified in 430.6(A), (B), and (C).
(A) General Motor Applications. For general motor applications, current ratings shall be determined based on (A)(1) and (A)(2).
(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.
430.110 Ampere Rating and Interrupting Capacity.
(A) General. The disconnecting means for motor circuits rated 600 volts, nominal, or less shall have an ampere rating not less than 115 percent of the full-load current rating of the motor.
Size at 115% of the FLA of the motor.
If it is a single motor then it has to be sized at 115% of the FLA of the motor per table 430.250. A 100A switch is compliant.
430.6 Ampacity and Motor Rating Determination. The size of conductors supplying equipment covered by Article 430 shall be selected from the allowable ampacity tables in accordance with 310.15(B) or shall be calculated in accordance with 310.15(C). Where flexible cord is used, the size of the conductor shall be selected in accordance with 400.5. The required ampacity and motor ratings shall be determined as specified in 430.6(A), (B), and (C).
(A) General Motor Applications. For general motor applications, current ratings shall be determined based on (A)(1) and (A)(2).
(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.
430.110 Ampere Rating and Interrupting Capacity.
(A) General. The disconnecting means for motor circuits rated 600 volts, nominal, or less shall have an ampere rating not less than 115 percent of the full-load current rating of the motor.
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