College Project Question

Status
Not open for further replies.
C

crx722

New member
First post here,

Having some trouble with a project in my 300 level Electrical course.
It relates to choosing proper wire for a construction site.

Given: 50m (one way) of Stranded and coated 8 guage copper (rhw)
240v AC source voltage.
75c wire temp
31-35 c ambient temp


It asks: for a 25A draw is the proper wire?
Calculate Voltage across load
Is the voltage satisfactory

I feel that I had a pretty good grasp, but the temperature factors and whatnot are sending me sideways

Any help would be greatly appreciated.

Thank you ,

Jeff
 
Take a look at the NEC table 310.16, noting in particular the conditions it includes as assumed values at the top of the table, and the temperature correction factors underneath the table.

Welcome to the forum.
 
Considering it is a college course, I don't think they intend for him to use the NEC but actually calculate out the results.

In this case, what he needs to determine is:

  • Will the copper heat up above 75 degrees celsius during normal operation? This will be a more thermal dynamics question to determine the wattage/per length of heat in the wire and how fast the ambient air can dissipate this heat.
  • What is the resistance of the wire loop and what is the resistance of the load to create a 25 amp draw? This will be 3 series resistances, the resistance of the conductor to load, the resistance over the load, and the resistance of the return path of the conductor. We know the current and can calculate the resistance of the wire through resistivity calculations (adjusted for temperature) and can thus determine the load resistance and consider it a voltage divider to determine what the voltage over the load is.
  • Whether or not the voltage is satisfactory is entirely dependent on the load


Materials have a resistivity (resistance per foot) based on the temperature. Depending on what class you are taking, it might also be important to include reactances since this is an AC line. It will affect the voltage seen over the load.

Wikipedia has a pretty basic writeup for what I think you are looking for.

Unless, I am overthinking this as there is probably more information to the problem than what we are being told...
 
Last edited:
To me, the clue is he is a construction management major, which means the NEC probably is the context of the question. Beyond charlie b's pointing to Table 310.16, I would add a reference to Chapter 9, Table 8.

Between the two references, proper conductor ampacity and voltage drop can be readily determined from the rest of the information given. The only question that would be left open in my mind would be, "Is the voltage satisfactory?" The context of the question may be hinting at an acceptable percent voltage drop; but since there is no rigorus "acceptable percent voltage drop," in the NEC reference may need to be to something like 210.19(A) FPN4.
 
crx722 said:
First post here,

Given: 50m (one way) of Stranded and coated 8 guage copper (rhw)
240v AC source voltage.
75c wire temp
31-35 c ambient temp


It asks: for a 25A draw is the proper wire?
Calculate Voltage across load
Is the voltage satisfactory

I feel that I had a pretty good grasp, but the temperature factors and whatnot are sending me sideways

Jeff
Click to expand...


Jeff
I assume you have been taught how to calculate the voltage drop. Don't be confused by the temperature. The 75C is the rating of the insulation. As
Charlie said use NEC table 310.16 for cable ampacity. Look under the 75C col until you find a conductor that has an ampacity of at least 25 A. As Charlie indicated look at the top of the table and you will see the temperature C given for the table ampacity.
If you temp, 31-35 C, is different there are adjustment factors at the bottom of the table. Let us know what you find.
 
If you look at chapter nine table "8" its gives the resistance per km. you state that it is coated so make sure that you use that column and make sure you use resistance for stranded. This table is based on 75 degree "C" for a different temperature you use note "2" below the table. I calculated it. But I will let you come up with the answer.
 
Status
Not open for further replies.
Top