Ambient Temperature Question

[bcorbin]

Let's say I am going to feed an 800 amp switchboard through some existing conduits, which happen to be in a underground concrete ductbank for at least part of the run. The remaining feeder would be routed through ceiling cavities and chases of a healthcare facility. I would prefer to use two parallel sets of three hots and a ground; I have two spare 4" conduits available.

I would look at Table 310-16 and see that 600MCM's are good for 420 amps at 75C rating and 30C ambient temperature. At first glance, two sets of these would look to be okay. But if I have to derate them by a factor of even 0.94, they will be good for only 394.8A. And that's just for an increase of just one degree C.

How would I know how hot it's going to get inside the conduits in order to pick a temperature for derating the ampacity?

 

[charlie b]

How would I know how hot it's going to get inside the conduits in order to pick a temperature for derating the ampacity?

You don't. You need to know how hot it will get outside the conduits, in the areas through which it will be (or in this case, is already) routed. The word "ambient" refers to the world outside the conduit.

It has been, and will likely continue to be, a matter of debate as to whether the ambient temperature to use is (1) The highest temperature that area will see during a typical year, or (2) The average temperature that area will see throughout a typical year, or (3) Something else. Your local authority will likely have their own answer to that mystery. My own opinion is that item (2) is appropriate, but neither I nor anyone else can use the existing wording of the code to prove that it is required to be one way or the other or the other other.

 

[bcorbin]

Thanks, Charlie! I don't mind being confused, as long as I know there's a good reason for it!

 

[weressl]

You don't. You need to know how hot it will get outside the conduits, in the areas through which it will be (or in this case, is already) routed. The word "ambient" refers to the world outside the conduit.

It has been, and will likely continue to be, a matter of debate as to whether the ambient temperature to use is (1) The highest temperature that area will see during a typical year, or (2) The average temperature that area will see throughout a typical year, or (3) Something else. Your local authority will likely have their own answer to that mystery. My own opinion is that item (2) is appropriate, but neither I nor anyone else can use the existing wording of the code to prove that it is required to be one way or the other or the other other.

I believe that the engineering calculations for thermal life is based on 8 hour mean maximum temperature. At least that is what is used for motor and transformer design.

You are also allowed to select the next highest standard OCPD rating.