Mike Dolt Enterprises
Mike Dolt Enterprises
User: travski06, Location: Eastern Oregon
Asked on: Thursday, April 21, 2011 at 6:12 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:30 am EDT
Question: The Power company is making me provide the wire from the transformer to a 800 amp switch gear. The calculated load is 395 amps. I was going to pull parallel 500 mcm's based off of 310.16, and 240.4b however there is no overcurrent device. Is there any code reference's that would allow me to do so or would the calculated amperage of my wire have to be 800 amps or more. Thanks
Code References
NEC 310.15(B)(16)
"Conductors connected in parallel shall have an ampacity not less than the sum of the ampacities of the individual conductors."
NEC 240.4(B)
"Overcurrent protection shall be provided for conductors and equipment in accordance with their ampacity."
Analysis
The calculated load of 395 amps requires a conductor with an ampacity of at least 395 amps. Parallel 500 mcm conductors have an ampacity of 340 amps each (per Table 310.16). Two parallel 500 mcm conductors would have a combined ampacity of 680 amps.
Conclusion
Based on NEC 310.15(B)(16), the use of parallel 500 mcm conductors would be permissible as the combined ampacity (680 amps) is greater than the calculated load (395 amps).
However, NEC 240.4(B) requires overcurrent protection for conductors. In this case, there is no overcurrent device. Therefore, parallel 500 mcm conductors cannot be used without an overcurrent device that is rated for 680 amps or more.
Recommendation:
To comply with NEC 240.4(B), you should: Increase the conductor size to provide an ampacity of 800 amps or more.* Install an appropriate overcurrent device rated for the calculated load plus any future expansion or load growth (e.g., a 1200 amp circuit breaker). Flag for review