Mike Dolt Enterprises
Mike Dolt Enterprises
User: Jpflex, Location: Victorville
Asked on: Wednesday, April 2, 2025 at 2:34 pm EDT
Answered on: Wednesday, April 2, 2025 at 2:34 pm EDT
Question: NEC 310.15B2Says to add a 60 degrees F to the ambient temperature when calculating and correcting conductor's ampacity, when it is in a cable or raceway with its bottom less than 3/4 from a roof or exposed to direct sunlight.My 2023 handbook provides an exampleA feeder runs less than 3/4 inches above a commercial building roof. This feeder supplies a calculated load of 175 A with an ambient temperature of 95 degrees ferenheight. None of the loads are continuos and the neutral is not considered current carrying (bundling effect)QUESTION:What is the size of THWN-2 ALUMINUM FEEDER rated at 90 degrees C is needed for this installation?ANSWER:New ambient temperature95 degrees F + 60 degrees F = 155 deg fTable 310.15B11THWN-2 insulation rated 90 degrees c. Therefore, a 0.58 reduction to the conductor ampacity at 90 degrees c is required175 ampere / 0.58 = 302 ampere aluminum conductor is to be selected from table 310.16A 400 kcmil ALU conductor at 305 amperes at 90 degrees c is chosenThen the book says according to 110.14 C, we must select a wire for 75 degree c when terminating onto equipment rated for 75 deg cHowever, instead of including the 60 deg F adder to ambient temp as with the wire subject to 90 degree c limit, they do NOT include this adder to the 75 degree limit.The book says the 400 kcmil aluminum THWN-2 can be used here for termination because at 75 deg F, the ampacity is 270 i amperes and is enough for the 175 i ampere load.What does not make sense is that if NEC assumes that the condition of use by subjecting wire in direct sunlight or less than 3/4 above a roof causes elevated conductor temperature for a given ampere limit, then the ampacity is either lowered or a larger size wire is selectedWhy then is nec not adding 60 degree adder to the wires ampacity at 75 degrees F and increasing the conductor size under the same condituons of direct sunlight ar distance from a roof?If direct sunlight or distance above a roof less than 3/4 inch causes wire temperature to increase above its 90 degree C insulation temp limit at the 310.16 tables ampacity shown, it will surely do the same for the tables ampacity limit for the conductor at 75 degrees C
You've hit on a very common point of confusion in NEC calculations, and your reasoning is absolutely sound. Here's the breakdown
The Core Issue:
You are correct in questioning why the 60°F adder isn't applied when evaluating the conductor's ampacity at the 75°C termination point. Logically, if the environment is causing the conductor to run hotter, that impact should be considered regardless of the temperature rating being evaluated.
Why the Code is the Way It Is (Even if It's Imperfect)
The NEC is a consensus-based standard, and its rules are often a balance between theoretical accuracy, practical application, and the accumulated experience of the electrical industry. Here's the likely rationale behind the current approach:
Focus on Worst-Case Scenarios
The NEC prioritizes safety and assumes the "weakest link" will govern the system's performance. In this case, that's the 75°C termination. The assumption is that if the conductor at the termination isn't exceeding its 75°C limit under normal termination temperatures and ambient conditions, that is "good enough".
Complexity vs. Benefit
Factoring in the 60°F adder again for the 75°C termination calculation would add another layer of complexity to an already intricate calculation. While theoretically more accurate, the code makers might feel the added complexity doesn't justify the incremental improvement in safety, especially since:
Safety Margins
The code already contains several built-in safety factors, such as derating for multiple conductors in a raceway, and not including neutral for current carrying ampacity deration.
Real-World Application
The termination point is inside of an electrical panel. Electrical panel are to be placed within a space that can be maintained at 90 degrees F in commercial spaces. In residential spaces, the panel could be put in a basement that might be much cooler than the 95 degrees + 60 degrees F.
Practical Implications & Why You're Right to Question It
Even though the NEC doesn't explicitly require it, your instinct to consider the added heat at the 75°C termination point is valid.
Over-Sizing
The best practice in situations where the conductor is subjected to direct sunlight is to upsize the conductor. It's a more costly option, but it ensures there is no over-heating of the conductor.
Cooling/Shading
Consider methods to reduce the impact of direct sunlight. For example, shading the raceway with a physical barrier or using a raceway designed to minimize solar heat gain.*
Manufacturer Recommendations
Check with the equipment manufacturer. They may have specific instructions or limitations regarding conductor sizing and temperature ratings in high-ambient conditions.
In summary: While the NEC example provided adheres to the code, it's good practice to factor this into your considerations as an engineer or electrician during installation and design to ensure the life time of your equipment. Flag for review