Undersized Conduit and Derating in a Warm Ceiling
A commercial tenant improvement runs nine current-carrying No. 12 THHN copper conductors and one bare No. 12 ground through a 3/4 inch EMT run of 40 feet through a ceiling plenum where the ambient temperature reaches 45 C in summer. The circuits are 20 A and serve lighting and receptacle loads. The installer used the 75 C column for ampacity and did not apply any adjustment. The inspector notes the conduit is nearly full and the conductors are warm to the touch after the lights have run for an hour. The tenant wants to add a tenth current-carrying conductor to the same run for a new circuit.
1. Calculate the adjusted ampacity of the existing conductors and determine whether the run is compliant.
Read the worked answer
No. 12 THHN is rated 30 A at 90 C, but the termination rating limits the circuit to the 75 C column, which is 25 A for No. 12, and the overcurrent device is 20 A. Apply the ambient correction for 45 C: the 90 C correction factor is about 0.87, giving 30 x 0.87 = 26.1 A before bundle adjustment. Now apply the conduit fill adjustment: nine current-carrying conductors in a single raceway fall in the 7-to-9 conductor band, which carries a 0.70 factor. Combined: 30 x 0.87 x 0.70 = 18.3 A. That is below the 20 A overcurrent device, so the existing run is not compliant for a 20 A circuit. The installer's use of the 75 C column without adjustment is the error. The conductors being warm confirms the calculation. Options: reduce the number of current-carrying conductors, split the run into two raceways, upsize the conductors, or reduce the ambient by rerouting.
2. Can the tenth conductor be added? Show the math and state the remedy.
Read the worked answer
Adding a tenth current-carrying conductor moves the run into the 10-to-20 conductor band, which carries a 0.50 factor. Adjusted ampacity becomes 30 x 0.87 x 0.50 = 13.05 A. That is far below 20 A and cannot protect the circuit. Even before the tenth conductor, the run was already non-compliant at 18.3 A. The remedy is not to add a conductor but to correct the existing installation: split the circuits across two raceways so each carries no more than six current-carrying conductors (0.80 factor, giving 30 x 0.87 x 0.80 = 20.9 A, acceptable for a 20 A circuit), or upsize to No. 10 conductors and re-evaluate terminations, or reroute out of the 45 C plenum. After correction, re-verify the ambient correction and document the calculation. The tenth circuit should be run in the new raceway.