Derating.......

Status
Not open for further replies.
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
O.K. folks..help me out here...after many years of inspecting I kinda know the Code, but someone asked a question which, I feel, involves more engineering than Code. The Notes to the ampacity tables give a adjustment factor for more than 3 current carrying conductors in
a raceway. Lets assume I have a 420 amp 3 phase load(non cont). I can feed that with a 600 MCM Cu.(420 amp) If on the other hand, I decide to feed with parallel conductors in ONE conduit, due to derating, I can't use 4/0's (260x2x.8). Yet the 4/0 is rated at 230, and from one point of view (2) 4/0s = (1) 600. Both situations produce approximately the same % conduit fill.
Why do we still need to derate if we a paralleling conductors?
 
There is less total copper in the parallel 4/0s (423.2kcmil) then in the single 600kcmil. The lesser amount of copper will have more voltage drop and produce more I^2R heat. The derating is because of the heat that is produced.
Don
 
I'll take it one step further.

Looking at a single phase, the current in each of the two 4/0 conductors will be about the half the current that a single 600 would have seen. That is because the difference in resistance is small, when compared to the resistance of the load, so it is the resistance of the load that determines the total current.

Looking at the table of conductor properties, we see that the resistance of a single 4/0 is more than double the resistance of a single 600.

Looking at the formula for heating (I2R, as Don pointed out), when you switch from a single 600 to a pair of 4/0 the heating becomes:
  • 2 times
  • (half the current) squared, times
  • more than double the resistance.
That is, you get 2 times (1/4) times (more than 2), so the answer is more than 1. This means the heating from a pair of 4/0 is higher than the heating would have been from a single 600.
 
derating

derating

love this site..............humbling though...you guys are brilliant
 
Yet the 4/0 is rated at 230, and from one point of view (2) 4/0s = (1) 600.
Click to expand...
If you use 2 4/0 per phase the 90C rating is 260 amps per conductor.

ampacity = 2 x 260 x 0.80 = 416 amps. 2 4/0's do not have the ampacity for your 420 amps load. You would have to use 2 250's and now you have a different problem.
 
Status
Not open for further replies.
Top