Derating Based on Temp and Voltage Drop

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necnotevenclose

Senior Member
Between ambient temp and voltage drop my process has been to calculate ambient temp first than look at voltage drop than based on the two values go with largest size? Is that correct?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Between ambient temp and voltage drop my process has been to calculate ambient temp first than look at voltage drop than based on the two values go with largest size? Is that correct?
I do temperature derating,with rooftop adder if applicable and conduit fill if applicable, then voltage drop, and take the larger of the two.
 

kwired

Electron manager
Location
NE Nebraska
Does it really matter which is done first? Either way you are going to use the larger conductor if you come up with two possible sizes.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I don’t quite understand the question, or the first reply. You don’t derate on the basis of voltage drop, and voltage drop is not a function of ampacity. Here is what I think you are saying, and what I would do, if the situation arose in one of my designs:
  1. Calculate the load.
  2. Select a conductor that has an ampacity at least as high as the calculated load.
  3. Calculate the derated ampacity of the selected conductor, based on ambient temperature. If the results are still at least as high as the calculated load, then you can use that wire. Otherwise, go back to step 2, and pick a larger wire.
  4. Whatever wire size you have selected, after getting past step 3, is the wire size you use for the voltage drop calculation. If the VD is acceptable, you are done. Otherwise, select a larger wire and redo the VD calculation. You don’t have to rethink the temperature derating question; that consideration has already been addressed.

And yes, you can address these two considerations in the other order, as kwired has pointed out. You can start by picking a wire that gives you an acceptable VD, then apply the temperature derating, and finally look to see if the calculated ampacity is at least as high as the calculated load.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I don’t quite understand the question, or the first reply. You don’t derate on the basis of voltage drop, and voltage drop is not a function of ampacity.
I may not have been unambiguous with my answer. In no particular order, I decide what the minimum wire size would be from a required ampacity perspective taking into account the deratings for ambient temperature, elevation from roof, and conduit fill, whichever are applicable. I also calculate what the minimum wire size would be from an allowed voltage drop perspective. After I have done those two calculations, I pick whichever is the larger of the two results. Sometimes ampacity wins, sometimes voltage drop wins.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
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