Table 310.16

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Castrovinci

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NJ
Hello,
I am going out for my liscense in oct and I thought I had table 310.16 down but now I am confused. While reviewing some code questions I ran across a question like this.

A 120/208 wye service has a computed load of 350 amps the neutral is current caring (52% non linear). The service will be installed overhead in rigid metal. What is the smallest THHN Conductor to be used.

Now where my confusion is....is (FOR NEC Testing purposes) do I use the 90 degree column of the 75 degree column? according to section 110.14c you must use the 75 degree column? I'm not sure I fully understand this section and its frustrating too me, I would imagine that in real world you would just stick to the 75 degree column for rather then 90 to be safe but my test question ask for the smallest possible not ness. what one would do in the real world.

If I have a 75 degree termination I still find my amps and derating out of the 90 degree column??

"Conductors with temp ratings higher then specified for terminations shall be used for ampacity adjustment,correction, or both"
WHAT does this mean?

Thanks for any assistance!
MrCastrovinci
 

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
You always have to limit your ampacity based on the weakest link in the system. Most panelboards and terminations today are rated 75C so if you have 90 C wire you are still limited to 75C because of the termination. You may use the 90C for derating purposes.
 

Dennis Alwon

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The computed load of 350 amps does not tell me if that is the neutral computed load or the service computed load. The neutral may be as small as the GEC for the building if the computed load is that low. If it is a feeder then the neutral may not be smaller than the EGC.
 

Castrovinci

Member
Location
NJ
You always have to limit your ampacity based on the weakest link in the system. Most panelboards and terminations today are rated 75C so if you have 90 C wire you are still limited to 75C because of the termination. You may use the 90C for derating purposes.

Thanks for your reply, this is what is confusing to me....if I am limited to 75 degree because of termination why can I still use the 90 degree column for my derating? Wouldn't I start the derating at 75 degree column?
 

raider1

Senior Member
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Location
Logan, Utah
Thanks for your reply, this is what is confusing to me....if I am limited to 75 degree because of termination why can I still use the 90 degree column for my derating? Wouldn't I start the derating at 75 degree column?

Because ampacity adjustments are required due to the heat that may be generated in a raceway or cable due to more than 3 CCC's in a raceway or ambient temperatures. Because the conductors themselves are capable of withstanding temperature of 90 degrees C then we are able to start with the 90 degree C ampacity for adjustments.

Hope this makes sense.:)

Chris
 

charlie b

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You start the derating at the 90C column. The text you quoted is intended to say that, although I will agree it is not at all clear in its wording. The reason you can start at the 90C column is that the conductor itself is rated to handle 90C above ambient. That is what the pair of "H" code letters within "THHN" is telling us. The "T" means thermoplastic, with a base temperature rating of 60C. Each "H" adds one "level of heat" at 15C. So two "H" letters means 60C + 15C + 15C, and that is why this conductor shows up in the 90C column. Since it can take 90C, without damage to its insulation, you can start with the ampacity in that column, and be confident that whatever the derated value might be, the conductor itself would not be damaged by a current at that level. Then you have to take into consideration the terminations, and that is what limits you to the value in the 75C column.
 

Castrovinci

Member
Location
NJ
Charlie,

So for (thhn) wire sizing I choose the 90% column...I am then good for ampacity. I now understand this


Now if I know that I only have 75 degree terminations I must start in the 75 degree column then derate to pick out the wire size.....Correct?

Thanks,
Mr. Castrovinci
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Charlie,

So for (thhn) wire sizing I choose the 90% column...I am then good for ampacity. I now understand this


Now if I know that I only have 75 degree terminations I must start in the 75 degree column then derate to pick out the wire size.....Correct?

Thanks,
Mr. Castrovinci

Not sure you have it.. Take nm cable. We know that the rating of the conductor is 90C so we start at the 90C column. Let's look at #6 THHN-- we see that at 90C it is good for 75 amps. Now suppose I need to derate at 80% for some reason. 75*.8= 60 amps. So we are good at 60 amps. Nm cable itself is limited by 334.80 to 60C so now we must look at 60C column. It states 55 amps. So even though we derated to 60 amps the cable itself makes us us 55 amps as the max load on the wire.

If the wire were in conduit -- no nm cable, then we see at 75C we are good for 65amps but the derating took us to 60 amps. In this case the conductor could be used at 60 amps.
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
You can be confident that the terminations are not going to be rated for 90C. So you can never use a "final value" of ampacity (i.e., after derating) that is higher than the 75C column. Here is an example:
  • You have a calculated load of 64 amps.
  • You have 3 current-carrying conductors in a raceway. That does not require a derating factor.
  • The conductor type is THHN.
  • The ambient temperature is 33C. That gives a derating factor of 0.96 (from 90C column).
  • Let's consider AWG # 6. In the 90C column, it shows an ampacity of 75. Multiply 75 times the derating factor of 0.96, and you get 72 amps.
  • Since the calculated ampacity of 72 is higher than the value posted in the 75C column, you declare that the ampacity, under this ambient temperature conditions, is 65.
  • That is high enough to handle the calculated load. You conclude that #6 THHN is acceptable for this application.
 

Castrovinci

Member
Location
NJ
Thank you gentlemen.......


I finally understand it.

It really is just a matter of proper wire size and whats actuallyusable on that wire...in sum

Its funny because I did 4 years schooling and another refresher course and not once did they show you how to correctly read that, and being in industrial we almost always went up the next size anyway for future or had it already calculated out on prints by PE or the manager.
 
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