Mike Dolt Enterprises
Mike Dolt Enterprises
User: buggsy65, Location: Tennessee
Asked on: Monday, March 14, 2011 at 8:25 pm EDT
Answered on: Tuesday, May 14, 2024 at 2:48 am EDT
Question: I have a 100 hp 460 volt three phase motor 124A flc and a 5kva xfmr 10.4A fed by a 250 amp main breaker. How do I size the service entrance conductors correctly. My calculation tells me to use 2/0 phase conductors and a #4 grounded conductor and my coworker says that I need 4/0 phase conductors and a #2 grounded conductor. Please advise.
Your Calculation
Using the National Electrical Code (NEC) Table 310.16, for a 100 hp motor, the full-load current is 124A. For a 5kVA transformer, the full-load current is 10.4A. The total full-load current is 124A + 10.4A = 134.4A.Based on 134.4A, the NEC Table 310.15(B)(16) requires 2/0 AWG copper conductors for the phase conductors and #4 AWG copper for the grounded conductor.
Coworker's Calculation
Your coworker's requirement for 4/0 phase conductors and a #2 grounded conductor is based on the following: The 250 amp main breaker is the protection device for the entire service entrance and must be sized to handle the total load, including the motor and transformer. Assuming a 125% demand factor for the transformer, the total load is 100 hp motor + 5kVA transformer 1.25 = 134.4A 1.25 = 168A. Using NEC Table 310.15(B)(16), for a 168A load, 4/0 AWG copper conductors are required for the phase conductors. A #2 AWG copper conductor is required for the grounded conductor based on NEC Table 250.122(A).
Conclusion:
Both calculations are correct based on different assumptions and interpretations of the NEC. The choice of conductor size depends on the specific requirements of the installation and the available protection devices.In order to ensure compliance with the NEC and the safety of the installation, it is recommended to consult with a qualified electrician or refer to local regulations and requirements. Flag for review