De-Rating of Conductors

[Designer371141]

I have always used 75 degree C ampacity ratings when performing de-rating, regardless of the wire type because typical lugs are all 75c rated and seldom 90c. Everywhere I have worked uses the same theory to error on the side of the more stringent criteria. I recently was requested and told that when it comes to derating of wire due to conduit fill that you always use the 90c so you get higher ampacity ratings. IS this right, I have never heard of it, every example I have seen in the code as well as Ugly's uses Article 310 tables and 75c rated ampacities.

What is the industry standard, or if not what is a good rule of thumb, I should think the more stringent is the safest way to go

 

[JFletcher]

You can use the 90*C column for derating. Your final ampacity cannot exceed the value for the 75*C column due to 75*C terminations, and for NM you use the 60*C column for final. You also have to abide by 240.4(D) for conductors 10ga and smaller. Also see 110.14.

 

[JFletcher]

Here's a good article on it:

http://electriciantesting.blogspot.com/2009/12/wire-ampacity-and-conduit-fill.html

If you put 16 #12 THHN in a 3/4" EMT, the max # of wires for that size and type of conduit, you have to derate to 50%. #12 THHN @ 90*C is good for 30A; half of that is 15A. You'd be limited to a 15A breaker instead of a 20 that's normally used.

I think most cases the EC would run 2 3/4" conduits (8CCC/pipe = 70% derating x 30A = 21A) to be able to use 20A breakers than eat the 50% derating involved in that particular example.

 

[Designer371141]

I know you CAN use the 90c; the question was "Is is standard to use the 75c column", I guess I would also like to know why use the 90c if all the ratings cannot exceed that of the 75c column due to the lugs themselves, seems like its trying to bypass the intent of the code.

 

[JFletcher]

I know you CAN use the 90c; the question was "Is is standard to use the 75c column", I guess I would also like to know why use the 90c if all the ratings cannot exceed that of the 75c column due to the lugs themselves, seems like its trying to bypass the intent of the code.

No, it is not standard to start derating from the 75*C column.

Why use the 90*C column? Because wire rated for 90*C can carry more current that wire rated for 75*C.

Running 15A thru #12 wire rated for 90*C and terminals made for 75*C will not result in any part of either exceeding 75*C, provided conduit fill and ambient temperature derating is taken into consideration. In other words, it is not unsafe to use 90* wire on 75* terminations, because the final ampacity is limited to the 75* tables/charts. The Code knows the weakest link here is the terminations; it's already taken into consideration.

If you could find 90* terminations (and they exist), you can use 90* wire and also the 90* column. Such would be the case say from j-box to j-box using Polaris connectors. If you needed 460A, you could run 1 set of 600MCM @ 90* vs 750MCM @ 75* or two sets of 4/0 Cu or 300MCM AL. Then run the latter to your breaker or disco which would be rated at 75*C.

You can always build beyond/more stringent than code, tho ampacity ratings are already pretty conservative. Only if I was borderline between wire sizes (eta: or planning for some increased future use) after all conduit fill, ambient temp, and voltage drop factors are taken into account would I consider upsizing to the next size wire.

 

[Julius Right]

In my opinion, if the clearance from the end of conduit to the circuit breaker terminals it is more than 24"-usually it happens- then the cable will be in "free open air" in this place.
For instance, if for 4-6 wires k=0.8 let's take an example #6 copper 90oC insulation. From Table 310.15(B)(16) for #6 copper 90oC insulation 75A
75*.8= 60A. Table 310.15(B)(17) in Free Air #6 will be 80 A for 60oC.
that means if we take the ampacity for reduced current in conduit for 90oC the conductor temperature “in open air” will be less than 60oC.

 

[infinity]

I have always used 75 degree C ampacity ratings when performing de-rating, regardless of the wire type because typical lugs are all 75c rated and seldom 90c. Everywhere I have worked uses the same theory to error on the side of the more stringent criteria. I recently was requested and told that when it comes to derating of wire due to conduit fill that you always use the 90c so you get higher ampacity ratings. IS this right, I have never heard of it, every example I have seen in the code as well as Ugly's uses Article 310 tables and 75c rated ampacities.

What is the industry standard, or if not what is a good rule of thumb, I should think the more stringent is the safest way to go

If your conductors have a 90° C insulation (THHN, THWN-2, XHHW, etc.) you can use that ampacity for derating calculations. I see nothing wrong with do that. If for your own peace of mind you want to use the 75° C ampacity you may but IMO I don't really see it improving safety. Just for the record we always use the 90° C ampacity for our calculations.

 

[Smart $]

...I would also like to know why use the 90c if all the ratings cannot exceed that of the 75c column due to the lugs themselves, seems like its trying to bypass the intent of the code.

Think of it like this...

• Derating using the 90°C rating is for the condition(s) of use between terminal enclosures.
• Using the 75°C rating is for the condition of use (termination) within the terminal enclosure.
• The latter is padded by 25% for the continuous portion of the load.