Conductor Ampacity Calculation – Part Seven ~ Electrical Knowhow

In Article " Conductor Ampacity Calculation – Part Six ", I indicated Some special cases for the application of adjustment factor of table 310.15(B)(3)(a) where is More Than Three Current-Carrying Conductors in a Raceway or Cable, which were:

Bare or Covered Conductors,
Neutral Conductor,
Grounding or Bonding Conductor,
Main feeders to Dwelling units,
Conductors contained in sheet metal auxiliary gutter,
Conductors contained in Nonmetallic Auxiliary Gutters,
Conductors in Metal Wireways,
Conductors for Remote Remote-Control, Signaling, And Power-limited Circuits,
Conductors for Fire Alarm Systems,
Conductors and Cables in Cable Trays.

Today, I will explain Last Special Case for Conductor Ampacity Calculations: Conductors and Cables in Cable Trays as follows.

For more information and good following, please review the following articles:

Conductor Ampacity Calculation – Part One

Conductor Ampacity Calculation – Part Two

Conductor Ampacity Calculation – Part Three

Conductor Ampacity Calculation – Part Four

Conductor Ampacity Calculation – Part Five

Conductors and Cables Ampacity Calculations in Cable Trays

Rule#1: Ampacity of Multiconductor Cables, Rated 2000 Volts or Less, installed in Cable Trays

As per 392.80(A)(1), the Ampacity of Multiconductor Cables, Rated 2000 Volts or Less, installed in Cable Trays using the allowable fill areas as per Section 392.22(A), shall be as given in Table 310.15(B)(16) and Table 310.15(B)(18) subject to the three below provisions.

Provision#1

If there are more than 3 current carrying conductors in a multiconductor cable, derate cable ampacity per section 310.15(B)(3)(A).

Note to Provision#1

Adjustment factors shall be limited to the number of current-carrying conductors in the cable and not to the number of conductors in the cable tray.

Provision#2

Where cable trays are continuously covered for more than 1.8 m (6 ft) with solid unventilated covers, use 95% of the ampacity values shown in Table 310.15(B)(16) and Table 310.15(B)(18) for multiconductor cables.

Note to Provision#2

Where cable tray covers are to be used, it is best to use raised or ventilated covers so that the cables can operate in a lower ambient temperature.

Provision#3

If multiconductor cables are placed in a single layer in uncovered trays, with a maintained spacing of not less than (1) cable diameter between cables, the ampacity of the cables shall not exceed the allowable ambient temperature-corrected ampacities of multiconductor cables with not more than (3) insulated conductors in free air in accordance with Section 310.15(C) and Table B.310.15(B)(2)(3). (see below image)

Note to Provision#3

By spacing the cables one diameter apart, the engineer may increase the allowable ampacities of the cables to the free air rating as per Section 310.15(C) and Table B.310.15(B)(2)(3)in Appendix B. Notice that the allowable fill of the cable tray has been decreased in this design due to the cable spacing.

Notes to Rule#1

The ampacities in Table 310.15(B)(16) are based on an ambient temperature of 30˚ C. While Conduit and cable tray wiring systems are often installed in areas where they will be exposed to high ambient temperatures. For such installations, some designers and engineers neglect using the Ampacity Correction Factors listed in table 310.15(b)(2)(a) which results in the conductor insulation being operated in excess of its maximum safe temperature. These correction factors must be used to derate a cable for the maximum temperature it will be subjected to anywhere along its length.

The ampacities in Table B.310.15(B)(2)(3) are based on an ambient temperature of 40˚ C, You must use the ambient ampacity correction factors, listed in Table 310.15(B)(2)(b), for ambient temperatures other than 40°C (104°F).

The provisions of 310.15(A)(2) will be applied for of Multiconductor Cables, Rated 2000 Volts or Less, installed in Cable Trays as per Rule#2 and its exception in below.

Rule#2: Selection Of Ampicity For Multi-Ampacity Conductors

As per 310.15(A)(2), Where more than one ampacity applies for a given circuit length, the lowest value shall be used. (see below image)

Exception Rule#2

A higher ampacity is permitted, if the length of the reduced ampacity doesn’t exceed 10 ft and is not longer than 10% of the total length of the circuit.

Example#1:

Five nos., 4 conductor multi cables, 1/0 AWG, THHN are stacked together in a solid-bottom cable tray with two nos., 4 conductor multi cables, 350 Kcmil, THHN installed as a single layer. What are the ampacity of these multiconductor cables noting that all conductors in cables are current carrying conductors?

Solution:

Step#1: From table 310.15(B)(16), find the allowable ampacity for these multiconductor cables

Allowable ampacity For 350 Kcmil THHN = 350 A Allowable ampacity For 1/0 AWG THHN = 170 A

Step#2: Applying Adjustment Factors from table 310.15(B)(3)(a) for More Than Three Current-Carrying Conductors in a Raceway or Cable

Since, Adjustment factors shall be limited to the number of current-carrying conductors in the cable and not to the number of conductors in the cable tray.

Nos. of current carrying conductors in cable = 4
So, the Adjustment Factors from table 310.15(B)(3)(a) = 80%

Derated allowable ampacity For 350 Kcmil THHN = 350 A x 80% = 280 A

Derated allowable ampacity For 1/0 AWG THHN = 170 A x 80% = 135 A

Example#2:

Same cables as in example#1, but the cable tray has a solid cover, What are the ampacity of these multiconductor cables noting that all conductors in cables are current carrying conductors?

Rule#3: Ampacity of Single-Conductor Cables Cables, Rated 2000 Volts or Less, installed in Cable Trays

As per 392.80(A)(2), for Single-Conductor Cables or single conductors cabled together (triplexed, quadruplexed, etc.), Rated 2000 Volts or Less, installed in Cable Trays according to the requirements of 392.22(B), The adjustment factors of 310.15(B)(3)(a) shall not apply , and the ampacity shall comply with the below Table.

Ampacity of Cables Rated 2000 Volts or Less in Cable Tray (single conductor cables)
cable size	solid unventilated cable tray cover (for more than 1.8 m (6 ft) )	applicable ampacity tables	multiply Ampacity Table values by	special conditions
600 K cmil and larger	no cover allowed (**)	310.15(b)(17) and 310.15(B)(19)	0.75
600 K cmil and larger	yes	310.15(b)(17) and 310.15(B)(19)	0.7
1/0 AWG through 500 Kcmil	no cover allowed (**)	310.15(b)(17) and 310.15(B)(19)	0.65
1/0 AWG through 500 Kcmil	yes	310.15(b)(17) and 310.15(B)(19)	0.6
1/0 AWG & Larger in single layer	no cover allowed (**)	310.15(b)(17) and 310.15(B)(19)	1	maintained spacing of one cable diameter
Single conductors in triangle configuration 1/0 AWG and Larger (see below image)	no cover allowed (**)	310.15(B)(20) (see NEC section 310.15(B))	1	Spacing of 2.15 x one conductor O.D. between cables (***)
(*) The ambient ampacity correction factors must be used.
(**) At a specific position, where it is determined that the tray cables require mechanical protection, a single cable tray cover of six feet or less in length can be installed.
(***) O.D. of the largest conductor contained within the configuration and adjacent conductor configurations or cables, the spacing of 2.15 times one conductor diameter is to be maintained between circuits. Two interpretations of this statement are possible as follows: Interpretation #1: The 2.15 times one conductor diameter is the distance between the centerlines of the circuits (the center lines of the conductor bundles). Interpretation #2: The 2.15 times one conductor diameter is the free air distance between the adjacent cable bundles. The use of the word “circuit” is unfortunate as its presence promotes Interpretation #1. An installation based on Interpretation #1 is not desirable as a free air space equal to 2.15 times one conductor diameter between the cable bundles should be maintained to promote cable heat dissipation.

Example#3:

Four single Copper conductors 700 Kcmil THHN installed in uncovered cable tray as a single layer, what is the allowable ampacity for these conductors?

Solution:

Step#1: From table 310.15(B)(17), find the allowable ampacity for these single conductors

Allowable ampacity For 700 Kcmil THHN = 855 A

Step#2: Applying 392.80(A)(2)(a), Derate ampacity by 75%

Derated allowable ampacity For 700 Kcmil THHN = 855 A x 75% = 641 A

Example#4:

Same as example#3, but the cable tray is covered, what is the allowable ampacity for these conductors?

Solution:

Step#1: From table 310.15(B)(17), find the allowable ampacity for these single conductors

Allowable ampacity For 700 Kcmil THHN = 855 A

Step#2: Applying 392.80(A)(2)(a), Derate ampacity by 70%

Derated allowable ampacity For 700 Kcmil THHN = 855 A x 75% = 598.5 A = 599 A

Rule#4: Ampacity for Combinations of Multiconductor and Single-Conductor Cables in the same cable tray

As per NEC 392.80(A)(3), Combination of multiconductor and single- conductor cable in the same cable tray must not exceed the fill requirements for each type of cable as if it were installed in its own tray and all installation requirements and ampacity limits for each cable type apply, provided that the following conditions apply:

The sum of the multiconductor cable fill area as a percentage of the allowable fill area for the tray calculated in accordance with 392.22(A), and the single-conductor cable fill area as a percentage of the allowable fill area for the tray calculated in accordance with 392.22(B), totals not more than 100 percent.

Multiconductor cables are installed according to 392.22(A) and single-conductor cables are installed according to 392.22(B) and 392.22(C) and (D).

In the next Article, I will explain the second method: Under engineering supervision, as provided in 310.15(C). Please, keep following.

Back To EE-4: Basic Electrical design course – Level III

Conductor Ampacity Calculation – Part Seven

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