# Derating

#### kimrichi

##### Member
Do we use column 90 in table 310-16 for derating when ambient tempraturw is less than 86 f too
let us say we have 12 thhn wire terminals are 75 c ambient is 50 f what is the allowable ampacity
if it was thwn wire same details does it make a difference
appreciate the help

#### kwired

##### Electron manager
Do we use column 90 in table 310-16 for derating when ambient tempraturw is less than 86 f too
let us say we have 12 thhn wire terminals are 75 c ambient is 50 f what is the allowable ampacity
if it was thwn wire same details does it make a difference
appreciate the help
I think I get what you are asking. If the ambient temp is below 86F then your "ampacity adjustment" for ambient temperature is increased ampacity instead of decreased. You do everything the same as otherwise except the factor for ambient temp is greater than one instead of less than one. You still have to apply derations for number of conductors in raceway if there are more than three current carrying conductors in the raceway.

#### infinity

##### Moderator
Staff member
THHN is a 90? C conductor so 90? C ampacity can be used for the starting point of your derating calculation. THWN is a 75? C conductor so you would use 75? C ampacity for that stating point. Yes, lower than 86? F ambient temperatures will give you a small additive value to the ampacity.