NEC Ampacity - Code Interpretation

[cherryg222]

Recently read an article in EC&M from 2007 which said:

When correcting conductor ampacity for elevated ambient temperature, the correction factor used for THHN conductors is based on the 90°C rating of the conductor, based on the conductor ampacity listed in the 90°C column of Table 310.16 [110.14(C)].

When adjusting conductor ampacity, the ampacity is based on the temperature insulation rating of the conductor as listed in Table 310.16, not the temperature rating of the terminal [110.14(C)].

The first statement is easy and logical. The second statement requires an explanation. Why would we not use 75degC Column ampacity values for 75degC rated terminals, typically in LV?

 

[davidbeach]

You can derate based on the 90°C column but you can't exceed the rating in the 75°C column.

 

[cherryg222]

David...That's interesting. For.eg for 250kcmil cable, the listed ampacities at 75 and 90degC are 255 & 290A. What you are saying is the ampacity will be 290A x d OR 250A whichever is lower. Let us say the derating factor is 0.85. Then 290 x 0.85 = 246.5A. Since 246.5A < 250A, hence 246.5 will apply. Correct?

 

[davidbeach]

Yep.

 

[cherryg222]

thx David. I also wanted to know under what circumstances Table 310.15 (B)(20) can be used. For example, can it be used for tray laying? Again, three Single Insulated Conductors refers to triplexed cables? Does a trefoil on tray qualify for these ampacities?

 

[7anoter4]

Table 310.15(B)(20) (formerly Table 310.20) Ampacities of Not More Than Three Single Insulated Conductors, Rated Up to and Including 2000 Volts, Supported on a Messenger, Based on Ambient Air Temperature of 40°C (104°F).
Usually supported on a messenger it means "three cables close each other [in equilateral triangle-trefoil]and this could be triplexed also [there are not laying on cable tray but supported on a messenger]. For cable on cable tray ampacity see art.392.

 

[cuky2000]

The general rule related to the ampacity of the selected conductor is that should not exceed the ampacity of the weakest component on the cabling system (connector, termination, etc.)

Conductors with insulation temperature ratings higher than terminations temperature rating can be used for ampacity adjustment, correction, or both.

Temperature limitations per NEC fall into two categories:
(1) Circuits rated ≤100 A: If a termination point's temperature rating is unknown, the conductor's ampacity must be selected from the 60 degrees C column regardless of the insulation type.

(2) Circuits rated >100 A:
2.1) the lowest temperature rating of the termination points is 75 degrees C (or 60/75 degrees C), the conductor's ampacity can be selected from the 75 degrees C column.
2.2)The 90 degrees C column's ampacity rating is only permitted if all termination points are temperature rated at 90 degrees C

 

[7anoter4]

If we are following NEC Art.392 then in certain conditions the Table 310.15(B)(20) can be employed indeed.
See 392.80 Ampacity of Conductors
Ampacity of Cables, Rated 2000 Volts or Less, in Cable Trays.
(d) Where single conductors are installed in a triangular
or square configuration in uncovered cable trays, with a maintained
free airspace of not less than 2.15 times one conductor
diameter (2.15 × O.D.) of the largest conductor contained
within the configuration and adjacent conductor configurations
or cables, the ampacity of 1/0 AWG and larger cables
shall not exceed the allowable ampacities of two or three single
insulated conductors rated 0 through 2000 volts supported on
a messenger in accordance with 310.15(B).
Informational Note: See Table 310.15(B)(20).

 

[cherryg222]

7anoter4..Thank you for the elaboration. This is the application which I was expecting, where trefoil arrangement is followed with the required separation. I wish they had a cross reference to Sec 392.80 on this particular table.
Now, if we do not maintain this separation but go in trefoil,touching formation, would the derating factors in Table 310.15(B)(3)(a) apply??

If the above is true, where does that leave Table 310.15(B)(17)? Why is the reference to derating factor missing at the bottom of this table?

 

[cherryg222]

So the statement, 2.1) the lowest temperature rating of the termination points is 75 degrees C (or 60/75 degrees C), the conductor's ampacity can be selected from the 75 degrees C column and 90deg C ampacity to be considered for derating by David; are these self contradicting?

 

[7anoter4]

Table 310.15(B)(17) Allowable Ampacities of Single-Insulated Conductor in Free Air.
[If the conductor is far enough from other heat source -including other cables]
Art.398 [ 398.19 Clearances] recommends 2 inches clearance.
So the derating it is only for ambient air temperature.
IEC 60287-2-1 Table 2 indicates factors for up to 3 cables in open air.

 

[cherryg222]

In Art.392.80(1)(a),it has been indicated that...."....Adjustment factors shall be limited to the number of current carrying conductors in the cable and not to number of conductors in the cable tray". How is this statement to be interpreted? If I have 5 runs of 3C - #4 cables in a tray in random fill, what will be the total ampacity? Looking at 90degC column, single run of 3C #4 is 95A. 5 runs would mean 5 x 95 = 475A. Is that it? Or do we have to apply derating as per Table 310-15(B)(3)(a), where number of current carrying conductors would be 3 x 5 = 15, leading to derating factor of 50%. So the derated ampacity would be 475A x 0.5 = 237.5A.

 

[mparenteau]

For example, perhaps you have 5 conductors in the tray. 3 phase, 1 neutral and 1 ground. Obviously, the ground is not considered current carrying. Then, you determine the # of current carrying conductors taking into consideration if the neutral is considered current carrying - it's either 3 or 4. Then any adjustment factors are based off of that.

Mike

 

[7anoter4]

NEC Art 392.22 A (b) Where all of the cables are smaller than 4/0 AWG,[5 sets of 3*4awg copper.-081 inch diameter] the sum of the cross-sectional areas of all cables shall not
exceed the maximum allowable cable fill area in Column 1 of Table 392.22(A) for the appropriate cable tray width.
For width =4" 4.5 inch^2 or for Solid bottom 3.5 in^2
Total fill area 5*0.81^2=2.58 in^2
392.80 Ampacity of Conductors.
A(1) Multiconductor Cables. The allowable ampacity of multiconductor cables, nominally rated 2000 volts or less, installed according to the requirements of 392.22(A) shall be as given in
Table 310.15(B)(16) and Table 310.15(B)(18), subject to the provisions of (A)(1)(a), (b), (c), and 310.15(A)(2).
In your case you may use Table 310.15(B)(16)
But, since the current is less than 100 A [110.14 Electrical Connections( C)(1) 1) (1) Conductors rated 60°C (140°F).Eventually I=70 A
Note: According to
ANSI/NEMA WC51/2009 Table 5-4 [80% of indoor Ampacity according to IEEE 835] it is also 70 A

 

[cherryg222]

After listening to a presentation on ampacities and reading the details of Art. 310.15(B)(3), I am back to square 1. The Article says.........."where number of ......., or where single conductors or multicoductor cables are installed without maintaining spacing for a continuous length longer than 600mm and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3).
Now what does this mean?
Doesn't this mean that if I have 4 cables (3 conductor each) of size 4, in a tray without maintaining spacing (random fill), then the derating factor (3 conductors per cable x 4 cables = 12 current carrying conductors), I need to apply a derating factor of 0.5 (from 10-20 conductors)in Table 310.15(B)(3)(a)?
If yes, where does that leave the interpretation of Art.392.80(A)(1) which says that...."Adjustment factors will be limited to the number of current carrying conductors in the cable and not to number of conductors in tray"??
In my opinion, the two Articles contradict themselves.
Your opinion please.

 

[7anoter4]

cherryg222,you are right in a way. What ever the main article it is 392.80 A and part 1 of this article allows to use Table 310.15 A2[a and b] but nothing about A(3a) except :"see A(1 a,b,c) of the same article 392.80".Part A1a allows use a part of 310.15.A(3a) only: “taking in consideration the number of live conductors of the cable only”. So no contradiction is here.