Adjustment Factors & Ampacity

[eager2learn]

I'm ahving a hard time understanding how to apply this logic in the field. When we derate a circuit how does this determine the size of the conduit. Example : 6 - #3 THHN = 110 AMPS, which then = 1-1/4" Sch 40 PVC

When we derate the conductors by 80% = 88 AMPS - What do I do from here?

Would the new conductor size be #4 THHN = Which would then mean I could still utilize 1-1/4" Sch 40 PVC for proper installation?

Or am I doing something backwards

Please Advise

 

[charlie b]

Step one is to determine the load. In your example, if you did put that many #3 conductors in the same conduit (let’s forget about the conduit size, for now), the conductors would be good for only 88 amps. So the next question would be, is the load less than or equal to 88 amps? If so, you can use the #3’s, and you can put them into any size conduit for which that many conductors does not exceed 40% fill.

If not, then you can’t use #3. You would then see if #2’s would suffice. Using your same process, we take 80% of 130, and get an ampacity of 104 amps. Now we ask if that is enough ampacity for the amount of calculated load.

Once you establish the minimum size wire needed to serve the load, then and only then do you figure out how big the conduit has to be, in order to fit in that many wires of that size.