705.28 Conductor Ampacity for Power sources.

A

ACDCBag420

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705.28Circuit Sizing and Current.​

  1. (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.
  2. (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. (1)The maximum currents in 705.28(A) multiplied by 125 percent without adjustment or correction factors
    2. (2)The maximum currents in 705.28(A) with adjustment and correction factors
    3. (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 feeder

Once 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.
 
Your right im getting confused with the wording. Spec sheets give the value for the load as a "continuous output" or "continuous current" so yea it would be just 1.25 times that. Thanks for the help.
 
ACDCBag420 said:

705.28Circuit Sizing and Current.​

  1. (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.
  2. (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. (1)The maximum currents in 705.28(A) multiplied by 125 percent without adjustment or correction factors
    2. (2)The maximum currents in 705.28(A) with adjustment and correction factors
    3. (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 feeder
. . .

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.
Click to expand...
Checks (2) and (3) are based on the conductor ampacity, and so would allow you to use the 90 degree column for 90C insulated wire. While Check (1) is a termination check, and you are limited to the termination temperature rating, usually the 75C degree column.

Cheers, Wayne
 
PWDickerson said:
BTW, you don't "choose from the three options". You must calculate all three options and design to the highest resulting value.
Click to expand...
Just to be clear, all three methods determine a minimum wire size and you must use the largest of the three. The maximum of the three minimums, so to speak.
 
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