NEC Ampacity Tables : 310.15(B)(16) thru (21)

[D!NNy]

Hello,

can someone help in understanding these tables: Currently i am sizing conductors from a switchgear to motor via cable tray for 500HP motor of 549.4FLA.

Based 110.14 (C) (1) (b), for temperature Limitations: For the circuits over 100A conductor size to be based on 75 Deg C. using table 310.15(B)(16) as mentioned in 110.14 (C) (1).

But In article 392.80 (2) (d) Ampacity of conductors to be selected based on the table 310.15(B)(20) which has very high ampacity when compared to the 310.15(B)(16).

• But none of the manufacturer provide the ampacities for the cable tray rated cables to match with 310.15(B)(20)?
• southwire Cable tray rated cable link http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet67
• Example 500MCM has 380 amp per 310.15(B)(16) and 496 amps per 310.15(B)(20) at 30 DEg C and 40 Deg C temperatures respectively.
• Why the ampcities of these two tables are having huge difference in ampacities? is it just because of the ventilation?

Thanks for the help in advance.

 

[kwired]

Hello,

can someone help in understanding these tables: Currently i am sizing conductors from a switchgear to motor via cable tray for 500HP motor of 549.4FLA.

Based 110.14 (C) (1) (b), for temperature Limitations: For the circuits over 100A conductor size to be based on 75 Deg C. using table 310.15(B)(16) as mentioned in 110.14 (C) (1).

But In article 392.80 (2) (d) Ampacity of conductors to be selected based on the table 310.15(B)(20) which has very high ampacity when compared to the 310.15(B)(16).

• But none of the manufacturer provide the ampacities for the cable tray rated cables to match with 310.15(B)(20)?
• southwire Cable tray rated cable link http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet67
• Example 500MCM has 380 amp per 310.15(B)(16) and 496 amps per 310.15(B)(20) at 30 DEg C and 40 Deg C temperatures respectively.
• Why the ampcities of these two tables are having huge difference in ampacities? is it just because of the ventilation?

Thanks for the help in advance.

I would say yes, open air around the conductors allows for more dissipation of heat and therefore the ability to carry more current without overheating the insulation. That is what ampacity is all about - what temperature can it operate before the insulation becomes compromised, the conductor itself can operate at higher temperatures.

 

[D!NNy]

• But none of the manufacturer provide the ampacities for the cable tray rated cables to match with 310.15(B)(20)?
• southwire Cable tray rated cable link http://www.southwire.com/ProductCata...prodcatsheet67

This is my main concern.

 

[captainwireman]

The manufacturers cite using the 2011 code still refers to 310.15(B)(16) as 310.16, but the numbers are the same on the manufacturers site as well as 310.15(B)(16). Am I understanding the question?

 

[captainwireman]

This product is a 3 conductor cable. Even though it may be supported by a messenger cable, it still is a three conductor cable, not three single conductors.

If this is installed in an uncovered cable tray, the conductors need to be single conductors bundled in triangular or square fashion, maintained free air space from other single conductors or cables, ect. This product is still a cable assembly so it would not apply, as I see it.

 

[texie]

Hello,

can someone help in understanding these tables: Currently i am sizing conductors from a switchgear to motor via cable tray for 500HP motor of 549.4FLA.

Based 110.14 (C) (1) (b), for temperature Limitations: For the circuits over 100A conductor size to be based on 75 Deg C. using table 310.15(B)(16) as mentioned in 110.14 (C) (1).

But In article 392.80 (2) (d) Ampacity of conductors to be selected based on the table 310.15(B)(20) which has very high ampacity when compared to the 310.15(B)(16).

• But none of the manufacturer provide the ampacities for the cable tray rated cables to match with 310.15(B)(20)?
• southwire Cable tray rated cable link http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet67
• Example 500MCM has 380 amp per 310.15(B)(16) and 496 amps per 310.15(B)(20) at 30 DEg C and 40 Deg C temperatures respectively.
• Why the ampcities of these two tables are having huge difference in ampacities? is it just because of the ventilation?

Thanks for the help in advance.

As a practical matter you will have to use Table 310.15(B)(16) 75 degree column due to temperature rating of the terminations. There are exceptions to this of course and other ways around this in some cases, but your equipment temp. rating is likely going to be the determining factor here.

 

[kwired]

This product is a 3 conductor cable. Even though it may be supported by a messenger cable, it still is a three conductor cable, not three single conductors.

If this is installed in an uncovered cable tray, the conductors need to be single conductors bundled in triangular or square fashion, maintained free air space from other single conductors or cables, ect. This product is still a cable assembly so it would not apply, as I see it.

I agree, if a multiconductor cable is what is being used then 310.15(B)(16) is the correct table, with adjustments needed if there are more then three current carrying conductors in the cable - or if more then three current carrying conductors in an enclosed cable tray whether individual or multiconductor cables are used.

 

[D!NNy]

FYI all,

Here is what i found:

392.80(A)(1)(c) Multi-conductor cables: Where multiconductor cables are installed in a single layer in uncovered trays, with a maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the allowable ambient temperature-corrected Ampacities of multiconductor cables, with not more than three insulated conductors rated 0 through 2000 volts in free air, in accordance with 310.15(C).
Informational Note: See Table B.310.15(B)(2)(3).

Manufacturers said that cables do provide higher ampacities than listed in 310.15(B)(16) but they are not included in the catalog. Per this table I have 2 X 500MCM, = 2 X 416A = 832 A minimum conductor ampacity needed is 686 A for a 500 HP with a 549 FLA.

Thanks for your input.

 

[Smart $]

FYI all,

Here is what i found:

392.80(A)(1)(c) Multi-conductor cables: Where multiconductor cables are installed in a single layer in uncovered trays, with a maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the allowable ambient temperature-corrected Ampacities of multiconductor cables, with not more than three insulated conductors rated 0 through 2000 volts in free air, in accordance with 310.15(C).
Informational Note: See Table B.310.15(B)(2)(3).

Manufacturers said that cables do provide higher ampacities than listed in 310.15(B)(16) but they are not included in the catalog. Per this table I have 2 X 500MCM, = 2 X 416A = 832 A minimum conductor ampacity needed is 686 A for a 500 HP with a 549 FLA.

Thanks for your input.

The problem is 110.14(C) limits max' ampacity to the 75?C column value of Table 310.15(B)(16).

The only way around this is mentioned on page 4 of the following Square D Data Bulletin...

http://static.schneider-electric.us/docs/Power Management/0110DB9901.pdf

 

[kwired]

The problem is 110.14(C) limits max' ampacity to the 75?C column value of Table 310.15(B)(16).

The only way around this is mentioned on page 4 of the following Square D Data Bulletin...

http://static.schneider-electric.us/docs/Power Management/0110DB9901.pdf

Correct but 110.14(C) only limits the conductor to the 75C ampacity for the terminations, if you have insulation rated higher then that you can use that higher value for ampacity adjustments for multiple conductors in raceways, cables, etc. or high ambient temp.

 

[Smart $]

Correct but 110.14(C) only limits the conductor to the 75C ampacity for the terminations, if you have insulation rated higher then that you can use that higher value for ampacity adjustments for multiple conductors in raceways, cables, etc. or high ambient temp.

From what I gather, adjustment and correction is not an issue here. No adjustment required for multiconductor cable in cable tray for specific condition of installation. He's wanting to use the 90?C ampacity, and to do so requires converting the termination limitation to also be 90?C

 

[augie47]

Not directly relevant as to "what Table", but in the original post it appears he is trying to size his cable based on a nameplate FLA of 549.4.We don't have specifics on the motor or type starting, but would one not be required to size the cable per 430.250 at 590 amps ?

 

[kwired]

From what I gather, adjustment and correction is not an issue here. No adjustment required for multiconductor cable in cable tray for specific condition of installation. He's wanting to use the 90?C ampacity, and to do so requires converting the termination limitation to also be 90?C

I think we are in agreement, just not seeing the picture quite the same way.

Terminal temperature rating will limit size of conductor regardless of what temp rating the insulation is. Even a bare conductor would still need minimum size based on terminal temp rating.

 

[Julius Right]

I think you are right: none of the manufacturer indicates the ampacity according to art.310.15(B)(20) "Ampacities of Not More Than Three Single Insulated Conductors Supported on a MESSENGER".
The art.310.15(B)16 (in Raceway, Cable, or Earth (Directly Buried)) it is a conservative appreciation of the ampacity to be suitable for any type of installation, and as you said, for the worst heat dissipation conditions. You may use the ampacity shown in other table-including 310.15(B)(20) if the layout condition is as stated in this table.
Art. 392.17 Ampacity of Conductors (2)(d):
"(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).?
If the cable tray bottom is almost free [it is not a "solid bottom"] and the distance between groups of 3 cables will be at least the group diameter the air will flow free between groups facilitating the cooling.
In other standards- as IEC 60364-5-52 for instance- if the distance will be twice this above mentioned distance no derating is required at all.

 

[Smart $]

Not directly relevant as to "what Table", but in the original post it appears he is trying to size his cable based on a nameplate FLA of 549.4.We don't have specifics on the motor or type starting, but would one not be required to size the cable per 430.250 at 590 amps ?

Yes.

 

[Smart $]

I think we are in agreement, just not seeing the picture quite the same way.

Terminal temperature rating will limit size of conductor regardless of what temp rating the insulation is. Even a bare conductor would still need minimum size based on terminal temp rating.

That's right...

 

[Julius Right]

I think we are in agreement, just not seeing the picture quite the same way.

Terminal temperature rating will limit size of conductor regardless of what temp rating the insulation is. Even a bare conductor would still need minimum size based on terminal temp rating.

I am sure you agree with me:
The conductor temperature only will limit the ampacity but the insulation type will state:
1) if the insulation rated temperature is not less than this temperature
2) If the insulation specific thermal resistance and the thickness could avoid heat dissipation [may be also the outside color?] so it will require derating.