In the 2005 handbook art. 220.40 example on page 108. When sizing OCPD & conductor size, when is it allowed to use conductors of different temperture ratings. the example states the over current device is rated @ 75 degrees C., so they use that column till the end, the example changes to the 90 degree C. column, wouldn't that conductor be to hot for that terminal ? The three temp columns have always kinda confused me, can any body explain?
? on 2005 handbook example
- Thread starter bamit
- Start date
- Status
- Location
- Logan, Utah
In the example, there are 4 current carrying conductors in the conducit so you must adjust the ampacity of the conductors in accordance with 310.15(B)(2)(a). Now when applying adjustment factors you can use the temperature rating of the type of conductor that you are using to start you adjustments. So in the example in the hand book you have 600 kcmil THHN conductors that are rated at 90 degrees C.
So to apply the derating factor you would take the ampacity of the conductor from the 90 degree column and multiply it by the factor given in Table 310.15(B)(2)(a) which for 4 current carrying conductors in a raceway would be 80%. So in the example you take an allowable ampacity of 475 amps and multiple it by 80% and you get an allowable ampacity of 380 amps. This ampacity is lower than the allowable ampacity of 600 kcmil conductos at the 75 degree column so we must use the lower number for the allowable ampacity of the conductors.
Hope I didn't confuse you too much.
Chris
So to apply the derating factor you would take the ampacity of the conductor from the 90 degree column and multiply it by the factor given in Table 310.15(B)(2)(a) which for 4 current carrying conductors in a raceway would be 80%. So in the example you take an allowable ampacity of 475 amps and multiple it by 80% and you get an allowable ampacity of 380 amps. This ampacity is lower than the allowable ampacity of 600 kcmil conductos at the 75 degree column so we must use the lower number for the allowable ampacity of the conductors.
Hope I didn't confuse you too much.
Chris
okay Raider 1, thanks, let me see if I got this right, If i'm applying an adjustment factor to a conductor in the 90 degree column & it's ampacity is not lower than the same size conductors ampacity in the 75 degree column [without derating] the 75 degree 600 kcmil thwn can be used. Or am I suppossed to change the designed thwn to the 90 degree thhn conductor.
- Location
- Logan, Utah
bamit said:
You must use whichever ampacity is lower.
Basically you are limited to the temperature rating of the terminations to which the conductors is going to be connected. So if I have a 90 degree rated conductor, the only time I can use the 90 degree column is for derating purposes. (This is due to the fact that there is no equipment or breakers that I am aware of that have a temp rating of 90 degrees.)
So lets look at the handbook example again. They use 600kcmil THHN copper conductors and the terminals to which the conductos are landed are rated at 75 degrees C. We have 4 current carrying conductors run in the conduit so we have to derate in accordance with 310.15(B)(2)(a). We can take the ampacity of the 600 kcmil THHN from the 90 degree column and multiply it by 80%. This would be 475 amps X .80 which would be 380 amps.
Now we look at the derated ampacity and compare it to the ampacity of 600 kcmil in the 75 degree column and see that the ampacity of 600 kcmil is 420 amps.
Now we must take the lowest ampacity rating which would be the adjusted rating of 380 amps and apply it to the conductors.
Chris
- Location
- Logan, Utah
bamit said:
If you use a thwn conductor you would have to start the adjustment factor at the 75 degree column so you have 420 amps multiplied by 80% and have an adjusted ampacity of 336 amps. That is why in the example they choose to use THHN conductors because you can use the 90 degree column to start your derating.
Chris
Raider, I'm with ya & will go that route if the wire type isn't specified. Except for the fact that if this is a test question and they say they're useing thwn I would stay in that column and use 750 kcmil thwn @ 380 amps just because thats what they say they're useing. If that doesn't make sense to you I must be missing something.
weressl
Esteemed Member
bamit said:
Maybe this simplifies it? Maybe not...
For the basic selection on which column you use the determining factor will be the lowest temperature ANY of the circuit components can take. Circuit components are any conductive or insulating elements with a temperature rating from one end of the unswitched line including any intermediate terminating points. On the 'other' side of the switch you start the excercise again. It is possible - but not likely - that on a circuit breaker you have 75C* terminals on the line side and 90C* rated terminals on the load side.
After you have selected the column you have to multiply the current rating from the column with the temperature adjustment factor below the table.
To further complicate it, if you have more than 3 current carying conductors in a raceway - excluding the neutral - you have to further derate the conductors ampacity.
Clear as mud?
- Status