Code Quiz

Nov. 27, 2007
What is the maximum allowable ampacity of a 10 AWG THHN or THWN-2 insulated conductor installed in a schedule 80 PVC raceway (90°C rated) installed underground from the branch-circuit panelboard to the luminaires installed for parking lot lighting?

Q. What is the maximum allowable ampacity of a 10 AWG THHN or THWN-2 insulated conductor installed in a schedule 80 PVC raceway (90°C rated) installed underground from the branch-circuit panelboard to the luminaires installed for parking lot lighting? There are four current-carrying conductors in this raceway. Disregard the terminals and the branch-circuit rating for this example.

  1. 32A
  2. 30A
  3. 28A
  4. 24A

Answer: A

10 AWG THHN or THWN-2 is a 90°C rated conductor per Table 310.13(A). From Table 310.16, 90°C column, a 10 AWG THHN or THWN-2 is a 40A rated conductor at 30°C. Per 310.8, 10 AWG THHN or THWN-2 is permitted to be used in dry, dry and damp, and wet locations (the THWN-2 rating is used for wet locations).

40A x 0.8 (an adjustment factor for four current-carrying conductors [from 310.15(B)(2)(a)]) = 32A maximum allowable ampacity, per this set of conditions. We are not considering the terminals [(110.14 (C)] or the branch-circuit overcurrent protective device rating or setting at this time.

The answer is based on changes in the 2008 NEC [300.5(B), 310.8, 310.13(A), 310.15(B)(2)(a), 310.16, 352.10, and 352.12]. Even though it has been known that conductors within raceways installed in wet location are considered to also be in a wet location, this change in the NEC makes that point clear.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at [email protected].

About the Author

Steven Owen

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations

Electrical Conduit Comparison Chart

CHAMPION FIBERGLASS electrical conduit is a lightweight, durable option that provides lasting savings when compared to other materials. Compare electrical conduit types including...

Don't Let Burn-Through Threaten Another Data Center or Utility Project

Get the No Burn-Through Elbow eGuide to learn many reasons why Champion Fiberglass elbows will enhance your data center and utility projects today.

Considerations for Direct Burial Conduit

Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, direct buried, encased...

How to Calculate Labor Costs

Most important to accurately estimating labor costs is knowing the approximate hours required for project completion. Learn how to calculate electrical labor cost.