Mike Dolt Enterprises
Mike Dolt Enterprises
User: Jpflex, Location: Victorville
Asked on: Wednesday, March 26, 2025 at 1:18 pm EDT
Answered on: Wednesday, March 26, 2025 at 1:18 pm EDT
Question: As currently worded in NEC, various ambient temperatures may quailfy to be used in either table because ambient temperatures may be simultaniously other than 30 degrees celcius table 310.15B11 and or other than 40 degrees celcius table 310.15 B12Therefore, the NEC is not clear on which to use.For example:A mid point ambient temperature between 30 and 40 degrees celcius could be 35 degrees celciusAn ambient temperature of 35 degrees celcius is both "other than 30 degrees celcius" table 310.15B11 and is also "other than 40 degrees celcius"Therefore, which table does NEC expect the electrician to use for ambient temperature correction factors and wire ampacity determination based on their wording?I do understand that the second table includes higher ambient temperatures but this still does answer which table to use according to the info within table titlesfollowing is an example by the 2023 NEC handbook. Notice that the answer is based on a 90 degree THHN copper conductor insulation which we know cannot terminate on most utilization equipment typically rated for 75 degrees maximum2) NEC hand book exampleDetermine the ampacity of 2 AWG THHN copper conductors to be installed in a raceway in an ambient temperature of 50 degrees celciusNEC AnswerTable 310.16Start at 130 ampere THHN at 90 degree insulationNext NEC chose table 310.15B111, instead of table 310.15B12?130i x 0.82 (derating factor based on 90 degree insulation) = 106.6 amperesHowever if not simply terminating to a connector, bus or power block rated for 90 degrees celcius, should the ampacity of the wire be reduced to its maximum current value of 115 amperes (table 310.16) for a 75 degree utilization device termination point?Also if the second table 310.15B12 were used instead we would have had an original wire ampacity of 130i x 0.89 = 115.7 amperes but NOT the 106.6 amperes shown by the NEC hand book as an answer.Thanks
You've hit on a point of potential confusion in how the NEC wording can be interpreted. Let's break down the issue and clarify the intent.
The Core Issue: Overlapping Ambient Temperature Ranges
You are absolutely correct that the literal wording of the table titles in 310.15(B)(1) can lead to ambiguity. The phrase "other than 30°C" and "other than 40°C" creates an overlap, especially in temperatures between 30°C and 40°C.
The NEC's Intended Hierarchy: Use the Table that Most Accurately Reflects the Conditions
While the wording isn't perfect, the intent of the NEC is that you should use the table that most closely aligns with the actual ambient temperature. Here's the implicit hierarchy:1. Direct Table Match
If the ambient temperature exactly matches a temperature listed in Table 310.15(B)(1), use that correction factor directly.
2.
Closest Table
If the ambient temperature is between listed temperatures, use the table that contains values that bracket the actual ambient. This typically means interpolating between the provided values.In your example of 35°C: It's other than 30°C and other than 40°C. However,
Table 310.15(B)(1) gives you correction factors that are closer to 35°C than Table 310.15(B)(2) does.Why the NEC Handbook Example Uses Table 310.15(B)(1)
The NEC Handbook example, with a 50°C ambient, chose Table 310.15(B)(1) correctly. It gives correction factors for temperature ranges that bracket 50 degrees,Your Concern about 75°C Terminations
You raise a critical point about the final ampacity determination in relation to termination temperature ratings. Here's the correct application:1. Start with the Corrected Ampacity
You've correctly performed the temperature correction. For 50°C with 90°C THHN, you found a corrected ampacity of 106.6A (using Table 310.15(B)(1)).
2.
Account for Termination Temperature Limits
The NEC requires you to choose the lowest ampacity from the following: The ampacity from the insulation temperature rating (after corrections). In our case, 106.6 A (2 AWG THHN at 90°C and 50°C ambient temp) The ampacity based on the termination temperature rating per 110.14(C). In most cases that would limit the ampacity to 115A at 75°C.
3.
Final Ampacity
Since the 75°C termination limitation (115A) is higher than the corrected ampacity, you use 106.6A as the final allowable ampacity in this instance.
Important Clarification
You only need to adjust downward to comply with termination temperature limits. You cannot increase the ampacity beyond the corrected value based on insulation type. The 90°C rating of THHN allows you to correct for ambient temperature, but it doesn't allow you to violate the termination temperature limits.
In Summary
When selecting the temperature correction table, choose the one that provides correction factors that are most relevant to the actual ambient temperature, giving closest bracketing. Always consider the termination temperature limits (110.14(C)) and use the lowest* ampacity rating.
Recommendation
The NEC has been notified of the confusion over lap, I do hope the over lapping can be clarified soon. Flag for review