MV Cable in Tray Without Spacing

[cbark]

I am trying to determine the ampacity for the following installation.

12" Cable Tray
3 Parallel - 3/C 15 kV, 250 kcmil, cu. MV-105 cable (Okonite Okoguard Okoseal cables)

There is not room in the tray to install the cables with a 1 cable diameter between the cables. Section 392.13 addresses ampacity for multiconductor cables, but only if the cables are spaced. How do I determine the ampacity if the cables are installed without the required spacing?

 

[Smart $]

I am trying to determine the ampacity for the following installation.

12" Cable Tray
3 Parallel - 3/C 15 kV, 250 kcmil, cu. MV-105 cable (Okonite Okoguard Okoseal cables)

There is not room in the tray to install the cables with a 1 cable diameter between the cables. Section 392.13 addresses ampacity for multiconductor cables, but only if the cables are spaced. How do I determine the ampacity if the cables are installed without the required spacing?

392.13(B)???

...(B)(1) if not spaced

...(B)(2) if single conductor and spaced

...(B)(3) if triplexed meeting the spacing requirement

 

[cbark]

392.13(B)???

...(B)(1) if not spaced

...(B)(2) if single layer, single conductor and spaced

...(B)(3) if triplexed meeting the spacing requirement

391.13(B) is for single conductor cables, not with a 3 conductor cable. Am I missing something?

 

[pete m.]

I am trying to determine the ampacity for the following installation.

12" Cable Tray
3 Parallel - 3/C 15 kV, 250 kcmil, cu. MV-105 cable (Okonite Okoguard Okoseal cables)

There is not room in the tray to install the cables with a 1 cable diameter between the cables. Section 392.13 addresses ampacity for multiconductor cables, but only if the cables are spaced. How do I determine the ampacity if the cables are installed without the required spacing?

From what I have read you can't figure the ampacity unless you maintain the spacing. It appears to me that you need a wider tray or possibly smaller cable if the calculated load will allow.

Pete

 

[cbark]

From what I have read you can't figure the ampacity unless you maintain the spacing. It appears to me that you need a wider tray or possibly smaller cable if the calculated load will allow.

Pete

This is an existing installation and I am trying to determine what the cables are rated for...

 

[Smart $]

391.13(B) is for single conductor cables, not with a 3 conductor cable. Am I missing something?

Okonite's website lists MV-105 15kV Okoguard Okoseal as single conductor cables... at least that's all I saw.

http://www.okonite.com/Product_Catalog/medium_voltage_cables.html

PS: There is no provision for a compliant installation of multiconductor MV cable without maintaining spacing in uncovered tray. If not spaced, the tray has to be covered (for at least 6' and unventilated) to be compliant and the ampacity is adjusted accordingly.

 

[Smart $]

PPS: And I do understand there should be a provision for not spaced and uncovered, and also for ventilated covers.

 

[pete m.]

PPS: And I do understand there should be a provision for not spaced and uncovered.

I agree. Sounds like the makings for a good proposal although I don't have the technical background to substantiate it.

Pete

 

[weressl]

391.13(B) is for single conductor cables, not with a 3 conductor cable. Am I missing something?

392.22 Number of Conductors or Cables.
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(C) Number of Type MV and Type MC Cables (2001​

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Volts or Over) in Cable Trays.

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The number of cables rated 2001 volts or over permitted in a single cable tray shall not exceed the requirements of this section.​

The sum of the diameters of single-conductor and multiconductor cables shall not exceed the cable tray width, and the cables shall be installed in a single layer. Where single conductor cables are triplexed, quadruplexed, or bound together in circuit groups, the sum of the diameters of the single conductors shall not exceed the cable tray width, and these groups shall be installed in single layer arrangement.

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So the above recognizes that the normal installation would be placing 3/c cables side-by-side and it defines the limits of that installation.​

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(B) Ampacity of Type MV and Type MC Cables (2001 Volts or Over) in Cable Trays.

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The ampacity of cables, rated 2001 volts, nominal, or over, installed according to 392.22(C) shall not exceed the requirements of this section.

(1) Multiconductor Cables (2001 Volts or Over). ​

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The allowable ampacity of multiconductor cables shall be as given in Table 310.60(C)(75) and Table 310.60(C)(76), subject to the following provisions:
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The above tables can be used to determine the ampacity of the cables when you install them to the original provision of side-by-side, but it is subject to two provisions if the basic installation is modified:
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(a) Where cable trays are continuously covered for more than 1.8 m (6 ft) with solid unventilated covers, not more than 95 percent of the allowable ampacities of Table 310.60(C)(75) and Table 310.60(C)(76) shall be permitted for multiconductor cables.

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So if you have the cable tray covered, you need to reduce the ampacity as there will be more heat trapped,​

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(b) Where multiconductor cables are installed in a single layer in uncovered cable trays, with maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the allowable ampacities of Table 310.60(C)(71) and Table 310.60(C)(72).​

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and if you choose to leave space between the cables then you can use the 71 and 72 tables, rather than multiplier modification in (a) for the 75/76 tables.​

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To summ it up, you can use tables 75 and 76 for the side-by-side installed MV cables.​