Mike Dolt Enterprises
Mike Dolt Enterprises
User: ACDCBag420, Location: Rutlant Vermont
Asked on: Thursday, May 22, 2025 at 12:24 am EDT
Answered on: Thursday, May 22, 2025 at 12:24 am EDT
Question: 705.28Circuit Sizing and Current.(A)Calculation of Maximum Circuit Current. Where not elsewhere required or permitted in this Code, the maximum current for the circuit shall be the continuous output current rating of the power production equipment.(B)Conductor Ampacity. Where not elsewhere required or permitted in this Code, the circuit conductors shall be sized to carry not less than the largest of the following:(1)The maximum currents in 705.28(A) multiplied by 125 percent without adjustment or correction factors(2)The maximum currents in 705.28(A) with adjustment and correction factors(3)Where connected to feeders, if smaller than the feeder conductors, the ampacity as calculated in 240.21(B) based on the over-current device protecting the feederOnce you have your wire sized for continuous load 705.28 requires you to choose from three options. Assume you are not connecting to feeders and there are no adjustment factors you must multiply your continuous loads by 1.25 a second time. When you get that number are you allowed to use your conductors ampacity from the 90 degree column if it rated for 90 degrees?Example: Two continuous loads with no adjustment factors (45.8 + 33.65) x 1.25 = 99.31 amps. Apply 705.28 B 1 99.31 x 1.25 = 124.13. My enclosures are rated 75 degree and my wire is rated 90 degrees. Would I choose my wire from the 75 degree column or 90? 75 degree would mean using #1 copper, 90 degrees rated number 2 copper is rated 130.
You've correctly identified the key points and the potential confusion. Here's the breakdown
Understanding the Code Requirements
705.28(A)
Establishes the starting point: the continuous output current rating of the power production equipment.
705.28(B)
Outlines how to size the conductors based on that current. It requires you to choose the largest result from the listed calculations.
705.28(B)(1)
Maximum current from (A) multiplied by 125%.
705.28(B)(2)
Maximum current from (A) with adjustment and correction factors. You're assuming there are none in your scenario.
705.28(B)(3)
Irrelevant in your scenario since you're not connecting to feeders.
Your Calculation and the Question
Your calculation is accurate:1. Combined continuous load: 45.8 + 33.65 = 79.45 amps2. Multiply by 1.25 (from 705.28(A)): 79.45 1.25 = 99.31 amps3. Multiply again by 1.25 (from 705.28(B)(1)): 99.31 1.25 = 124.14 ampsThe crucial question is: Can you use the 90°C ampacity column after this calculation?The Answer: You must use the 75°C column.
Here's the reasoning, based on the NEC and common interpretations:1. Termination Temperature Ratings
The NEC 110.14(C) states you must select the conductors based on the lowest temperature rating of the terminations, which you stated in your problem is 75°. This is also true if the gear itself is only rated for 75°. This applies even if the conductor insulation is rated higher (90°C).
2.
Purpose of the 90°C Column
The 90°C column is primarily for derating purposes (adjustments for ambient temperature or conduit fill). The 90°C rating allows for greater derating before reaching the ampacity limit of the conductor.
In Summary
Since your enclosures and terminations are rated for 75°C, you must use the 75°C ampacity column. Therefore, you would choose #1 copper, as you noted.
Important Considerations and Best Practices Consult with a Qualified Electrician
These calculations are critical for safety. Always consult with a licensed and experienced electrician to ensure your installation meets all applicable codes and safety standards.
AHJ Approval
Your local Authority Having Jurisdiction (AHJ) has the final say on code interpretation. It's always a good idea to discuss your design with them, especially in complex cases like renewable energy systems.
Documentation: Keep detailed records of all calculations, conductor sizes, and equipment ratings for future reference and inspections. Flag for review