How do I size a breaker for an air handling unit? In the submittals, I have information regarding the individual components of an AHU such as the fans, motor, filters. Do I use the current rating of each piece of equipment and calculate the total load? Thanks.
Air handling unit breakers
- Thread starter zaid200
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Buck Parrish
Senior Member
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- NC & IN
Are you're talking about sizing the internal equipment to make a name plate for the AHU ?
How do I size a breaker for the entire AHU? I have specs but it is only for the equipment that make up an AHU such as the fans and filters.
Buck Parrish
Senior Member
- Location
- NC & IN
You know I am an electrician, I use the name plate on the equipment. It tells us the maximum breaker size and the minimum circuit size. I thought you were talking about sizing the individual componets to make the name plate.
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- Location
- New Jersey
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- Journeyman Electrician
The manufacturer will tell you what the MCA and MaxOCPD required for the unit
Buck Parrish
Senior Member
- Location
- NC & IN
If you're talking about an electric furnace. You can change sizes of the heating components. On residential this is ussually marked in the field by the hvac guy. So their would be no way to do that in the office.
The condenser breaker as Infinity said it's always sized by the manufacturer. The furnace too except to change out heat strips.
If you're drawing plans or a panel schedule. You would still need the kw of the heat strips then add the motor to get your breaker size.
If it's gas you would treat it as a motor calculation.
Perhaps an asterix and a note at the bottom stating to follow NEC rules on breaker sizing covers you.
Again I'm an electrician but the NC board of examiners test makes you feel like their testing you as if you're going to be an engineer.
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If I did have to calculate the breaker size, can I do that using the FLC or do i need the LRC too.
Buck Parrish
Senior Member
- Location
- NC & IN
430.6 Ampacity and Motor Rating Determination.
The image above shows a portion of NEC Table 430.250 and a three-phase motor nameplate.In order to begin calculating the branch circuit or feeder size for a motor, the full load current (FLC) rating of the motor must be determined. Section 430.7(A) in the NEC® provides a list of all required markings on a motor nameplate and the FLC is one of them.
Even though motors are marked with their FLC per 430.7, Section 430.6(A)(1) requires conductors to be sized using the full load current derived from the tables in Part 14 of Article 430 rather than the FLC from the motor nameplate for most installations:
430.6(A)(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.
It is important to select the correct table. Table 430.248 provides FLC values for single-phase alternating-current motors while Table 430.250 provides FLC values for three-phase alternating-current motors. The text near the top of the table makes it clear that the table allows voltages ranging from 110 to 120, 220 to 240, 440 to 480, and 550 to 600 volts.
Refer to the image of the three-phase motor nameplate. The motor can be used in 3 different voltage configurations. The motor is not a low speed (less than 1200 RPM). It is not a high torque motor. It only has one RPM value (revolutions per minute) so it is not a multispeed motor. To size conductors and overcurrent protection for this motor, NEC® Table
What about for motor feeder overcurrent device? Is that sized differently? Would we use table 430.52?430.6 Ampacity and Motor Rating Determination.
The image above shows a portion of NEC Table 430.250 and a three-phase motor nameplate.
In order to begin calculating the branch circuit or feeder size for a motor, the full load current (FLC) rating of the motor must be determined. Section 430.7(A) in the NEC® provides a list of all required markings on a motor nameplate and the FLC is one of them.
Even though motors are marked with their FLC per 430.7, Section 430.6(A)(1) requires conductors to be sized using the full load current derived from the tables in Part 14 of Article 430 rather than the FLC from the motor nameplate for most installations:
430.6(A)(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.
It is important to select the correct table. Table 430.248 provides FLC values for single-phase alternating-current motors while Table 430.250 provides FLC values for three-phase alternating-current motors. The text near the top of the table makes it clear that the table allows voltages ranging from 110 to 120, 220 to 240, 440 to 480, and 550 to 600 volts.
Refer to the image of the three-phase motor nameplate. The motor can be used in 3 different voltage configurations. The motor is not a low speed (less than 1200 RPM). It is not a high torque motor. It only has one RPM value (revolutions per minute) so it is not a multispeed motor. To size conductors and overcurrent protection for this motor, NEC® Table
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