Mike Dolt Enterprises
Mike Dolt Enterprises
User: Alex_rg15, Location: Texas
Asked on: Monday, March 17, 2025 at 10:41 pm EDT
Answered on: Monday, March 17, 2025 at 10:44 pm EDT
Question: (90-degree rated wire installed on 75-degree terminations can use the 90-degree column ampacity for the correction factor calculation)how many amps can run through seven-#1 rhh conductors when installed in rmc to an outdoor panel with 75-degree celsius temperature ratings where the ambient temperature is 88 degrees fahrenheit?I know the answer is 97.44 AmpsI got this byStep 1#1 RHH is 145 amps table 310.16 (b)Step 2Adjustment factor for more than three current-carrying conductors7-9 70% table 310.15(B)(3)(a)Step 3Ambient factortable 310.15(b)(1)(1)Column 90 C0.96Step 4 math145 amps X 70% X 0.9697.44 ampsMy question is why I need select the 0.96 from Column 90 C, and not 0.94 from Column 75 C from table table 310.15(b)(1)(1)When the the panel is installed at 75 C temperature ?
You've correctly calculated the ampacity, but your question is valid. Here's the breakdown of why you use the 90°C column for the ambient temperature correction factor: Understanding the Principle
The principle behind using the 90°C column for the ambient temperature correction factor is that the wire itself is rated for 90°C. This allows you to use the higher temperature rating of the wire to determine how much derating is needed due to the ambient temperature. However, the final ampacity cannot exceed the ampacity of the lowest rated component in the circuit (in this case, the 75°C termination).
Reasoning for 90°C Column for Correction
The 90°C rating of the RHH wire allows the wire to withstand a higher operating temperature. This means that the insulation can handle more heat before it degrades. When the ambient temperature is higher than the standard temperature (usually 30°C or 86°F), the wire will naturally run hotter. The 90°C column lets you calculate how much to reduce the ampacity based on the wire's higher heat tolerance.
75°C Termination Limitation
The 75°C termination rating at the panel means that the conductors connected to the panel must not exceed this temperature. This is why you use the 75°C column for the final ampacity selection from Table 310.16 but you can use the 90°C column for the ambient temperature correction factor.
NEC Guidance
NEC 310.15(B)(1) & (3) Specifies the adjustment factors to be applied based on the temperature rating of the conductor In your calculations, you are applying an adjustment factor due to ambient temperature and for more than 3 current-carrying conductors in a raceway or cable.
This article permits using the 90°C column in table 310.16 to determine your adjustment factor. The final calculated ampacity cannot exceed the ampacity associated with the temperature rating of the terminations or the conductor. The lowest value is selected.* In summary
Start with the 90°C ampacity of the conductor.2. Apply the ambient temperature correction factor using the 90°C column.3. Apply the adjustment factor for the number of conductors.4. The final ampacity cannot exceed the ampacity for the 75°C rating of the termination.Therefore, you were correct in using the 90°C column for the ambient temperature correction factor.Important Note: Always refer to the latest edition of the NEC and consult with a qualified electrician or electrical engineer for specific applications to ensure compliance with all applicable codes and regulations. Flag for review