Which correction factor for temp.?

[stephenherman]

I am an electrical instructor and have a problem that I cannot not find the answer too. The question is: An electric annealing oven is located in an area with an ambient temperature of 125?F. The oven contains a 50 kW electric heating element and is connected to 480 volts. The conductors are to be copper with type THHN insulation. The termination temperature is not known. The furnace is expected to operate more than 3 hours continuously. What size conductor should be employed to make this connection?

50,000 / 480 = 104.167 amperes (Increase 125% because of continuous load, 104.167 x 1.25 = 130.2 amperes.) Although the insulation type is located in the 90 degree C column, the size must be selected from the 75 degree C column because termination temperature (110.14 C). The next step is to use the temperature corrections factors located at the bottom of table 310.16 because the oven is located in an area with 125 degrees F. Do I use the correction factor in the 75 degree column since the conductor size is chosen form that column, or do I use the correction factor in the 90 degree column since the insulation type is located in that column? The answer is 3/0 AWG if I use the correction factor from the 75 degree column and 2/0 AWG if I use the correction factor from the 90 degree column.

 

[cpal]

three phase ?? or single??


use the correction factor in the column that aligns with the insulation.


max load current may not exceed the 75 deg col.

 

[infinity]

You would use the correction factor and the ampacity based on the insulation type of the conductor. In this case THHN is rated for 90 degrees C so you would find both the ampacity and correction factor in the 90 degree column.

 

[charlie b]

three phase ?? or single??

Very important question! If it is three phase, then you must divide the amps (104) by the square root of three (1.732). That gives you a much smaller wire.

By the way, you should not bother to carry calculation results to 6 significant digits (i.e., 104.167). Neither of the two values you used to obtain those amps (i.e., 50kW and 480V) is that precise, and the result cannot be more precise than the input data.

 

[hillbilly]

I used to design and build industrial ovens, so I have a few thoughts.
If the ambient temperature is 125F and the conductors are in metal conduit that's attached to the exterior surface of the oven, the conductor temperatures could be reach 90C without any electrical load.
What is the operating temperature of the oven? What type of wall insulation?
What type of raceway will you use? Is it attached to the oven?
You may require high temperature conductors.
Pat close attention to the terminal connections, especially the ones that are connected to conductors that enter the oven and supply the electric heaters.
That sounds like a pretty severe environment.
Just a thought.
steve

 

[kingpb]

I think it would be wise to treat this environmemt like a boiler area, and use high-temp wire. The ambient is just over 50 deg C, but Steve points out that where it is installed could actually be much higher.

Here is a example of what you may need.

http://www.r-scc.com/PDF/Rocktherm-H1.pdf