Code Quiz: Calculating Conductor Ampacity

April 10, 2019

A look at how location affects ampacity of a 3-phase feeder to a remote building.

Question: What is the maximum ampacity of a 3-phase feeder consisting of 3 - 4/0 and 1 – No. 4 GND in a 2-in. PVC conduit? The feeder is installed underground to a remote building. All conductors are XHHW Copper. The lugs at both ends are rated at 90°C.

See the answer below.

Answer: 230A

Explanation: The lugs on both ends are rated for 90°C but the XHHW insulation is only rated for 75°C in a wet location. The 90°C rating is for dry locations only. NEC Sec. 300.5(B) classifies the interior of underground raceways as a wet location.

Lupacchino is a senior design engineer with Gaylor Electric, Inc. in Noblesville, Ind. and serves as an alternate on NEC Code-Making Panel No. 3. He is currently licensed in more than 80 jurisdictions. He can be reached at [email protected].

About the Author

John Lupacchino | Senior Design Engineer

John began his career in electrical construction as an apprentice and worked his way through the trade. He is currently licensed in more than 80 jurisdictions. With more than 35 years of experience, he now plays a vital role in Gaylor Electric’s Engineering Department, where he serves as a senior design engineer. With a focus on design engineering, quality assurance, and code compliance, he has earned the highest level of respect from industry professionals alike. John currently serves as an alternate on NFPA 70 Code Making Panel No. 3. In addition, he enjoys sharing his electrical knowledge through education. As an Associated Builders and Contractors instructor, he continues to help grow a new generation of high-performing construction leaders.