Conductor Temperature

Status
Not open for further replies.

Dale Hayes

Senior Member
If a 6Ga. 90C THHN copper conductor reaches 194F at 75 Amps (based on ambient temperature of 30C (86F), what is the 6Ga. 90C THHN copper conductor temperature if only 30Amps is applied?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
If a 6Ga. 90C THHN copper conductor reaches 194F at 75 Amps (based on ambient temperature of 30C (86F), what is the 6Ga. 90C THHN copper conductor temperature if only 30Amps is applied?

you will only generate 16% of the heat when you have 30A versus 75A.

an interesting question. is this on a test?
 

Smart $

Esteemed Member
Location
Ohio
you will only generate 16% of the heat when you have 30A versus 75A.

an interesting question. is this on a test?
I imagine you calculated 30?/75? = 0.16 =16%.

That's a generally accepted determination... but heat generated is I?R, and the R value changes also. This is a common mistake I see with voltage drop calculations, too.
 

Smart $

Esteemed Member
Location
Ohio
Smart...

Thanks for your significant post revealing that "insignificant" is more significant than "nil!":lol:

Phil
I said perhaps! It would not be significant if the wires are only a few feet long. It would be significant if the wires are a thousand feet long. OP did not state circuit length, purpose for the analysis, etc. Lacking details makes it kind of hard to say what is and isn't significant...


Or are you the type to just make willy-nilly assumptions no matter what the case may be... ;)
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
As usually I am late, sorry!:ashamed1:
According to NEC Art. 310.15 Ampacities for Conductors Rated 0?2000 Volts.
C.Engineering Supervision. Under engineering supervision, conductor ampacities shall be permitted to be calculated by means of the following general equation:
I=SQRT((Tc-Ta)/Rca/Rdc/(1+Yc)) where:
Tc = conductor temperature in degrees Celsius (?C)
Ta = ambient temperature in degrees Celsius (?C)
Rdc = dc resistance of conductor at temperature Tc
Yc = component ac resistance resulting from skin effect and proximity effect
Rca = effective thermal resistance between conductor and surrounding ambient
Rdc depends on temperature, Yc depends on Rdc.
Rac usually does not depend on temperature [except for the trapped air in conduit and ducts and the heat evacuation into surrounding air.].
As all these parameters will decrease with temperature for a conservative appreciation could be considered constant.
Then 30A/75A=SQRT((Tc30A-Ta)/(Tc75A-Ta))
Tc30A=( 30/75)^2*(Tc75A-Ta)+Ta
Tc30A=( 30/75)^2*(90-30)+30=39.6oC[103.28 oF] as gar said.
 

Phil Corso

Senior Member
Smart...

I now uderstand the debate over "non-significant/nil!"

The resistance "change" I referred to was the "extra" resistance (meaning in addition to its DC resistance) due to Skin and Proximity effects!

Phil
 
Status
Not open for further replies.
Top