When is it applicable to use 310.15(B)(17) for cable ampacity?

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philly

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Are there specific cases where the cable ampacities in table 310.15(B)(17) can be used? My understanding has always been that based on 110.14 termination temperature ratings dictate that we are to use ampacities listed in 310.15(B)(16) regardless of application. 110.14(C) says that 310.15(B)(16) should be used and appropriately modified by 310.15(B)(17) (Not sure what that modification part intends to convey)

I see the ampacities in 310.15(B)(17) applied a lot in cable tray applications with the basis of cables in "free air" I've always been confused in these cases where the termination temperature limitation comes into play in these cases? Can this table only be applied if the cable terminations are also in "free air" or is this table applicable when cables are in free air and termination is in an enclosure (panelboard, switchgear etc...)?

Thanks
 
That table is only used for the conductors in free air not terminations. Of course termination comes into play also but if we just have a cable spanning between 2 buildings then we can use the ampacity of the conductor based on (B)(17). The termination to another cable into the building or wherever would limit the ampacity of the entire run unless 310.15(A)(2) comes into play.


(2) Selection of Ampacity. Where more than one ampacity
applies for a given circuit length, the lowest value shall be used.
Exception: Where different ampacities apply to portions of a circuit, the
higher ampacity shall be permitted to be used if the total portion(s) of
the circuit with lower ampacity does not exceed the lesser of 3.0 m
(10 ft) or 10 percent of the total circuit.
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So you could potentially have a smaller wire spanning the building tied to a larger wire and still have the ampacity of the larger conductor. Thus I could have a #8 wire in free air with a #6 terminating in the buildings and still be able to use 60 amps... Just an example it depends on many factors
 
Dennis Alwon said:
That table is only used for the conductors in free air not terminations. Of course termination comes into play also but if we just have a cable spanning between 2 buildings then we can use the ampacity of the conductor based on (B)(17). The termination to another cable into the building or wherever would limit the ampacity of the entire run unless 310.15(A)(2) comes into play.
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I'm not sure I quite follow the cable vs terminations here.

Lets say I had an outdoor panel where a 500MCM cable was terminated onto a 400A feeder breaker. That cable was then run in cable tray outdoors to a downstream load (load is less than 380A). In this case would the 620A ampacity in (B)(17) be able to be applied or would you be limited to the 380A in B(16)?
 
The conductor would be rated 620 amp however I don't think this table applies to cable tray. In free air is literally individual conductors spanning between structures.

That portion of the run would be rated 620--- if in free air-- but the final ampacity would be diminished based on termination and wiring method
 
philly said:
My understanding has always been that based on 110.14 termination temperature ratings dictate that we are to use ampacities listed in 310.15(B)(16) regardless of application.
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There's nothing in 110.14 that says that.

310.15 tells you how to determine conductor ampacity, and includes multiple Tables for different conditions.
 
philly said:
I'm not sure I quite follow the cable vs terminations here.

Lets say I had an outdoor panel where a 500MCM cable was terminated onto a 400A feeder breaker. That cable was then run in cable tray outdoors to a downstream load (load is less than 380A). In this case would the 620A ampacity in (B)(17) be able to be applied or would you be limited to the 380A in B(16)?
Click to expand...

Where are you getting permission to use a table other than the two mentioned in 110.14(c)(1)? Seems clear to me it is saying use the "standard" table or the "resi" table if applicable.
 
Dennis Alwon said:
So you could potentially have a smaller wire spanning the building tied to a larger wire and still have the ampacity of the larger conductor. Thus I could have a #8 wire in free air with a #6 terminating in the buildings and still be able to use 60 amps... Just an example it depends on many factors
Click to expand...

The problem arises, when you splice your #8 to your #6. The splice would most likely need to be in an enclosure, where 310.15(B)(16) would apply. Best case scenario, you take credit for the 90C rating for the splice, when the splice is in an otherwise-empty enclosure. The splice is still a chokepoint that limits you to the 310.15(B)(16) ampacity.

I suppose if you had a splice that was rated to be in free air, and was rated for the 310.15(B)(17) ampacities itself, perhaps 310.15(B)(17) ampacities would apply to it, in a manner that actually governs your conductor sizing. Service conductors are spliced to utility-owned conductors all the time in free air.
 
electrofelon said:
What I mean is for the purposes of determining the ampacity for the terminations. Isnt 110.14(c)(1) clear that for the terminations you have to use the standard table, or the residential table if applicable?
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Yes, the terminations. The cable ampacity is a separate matter.
 
Here is how it works. If you have more than 3 current carrying conductor's then you must derate however when using 90° C wire you can use #12 at 30 amps for de-rating purposes. Even with 9 current carrying conductor's the de-rating factor is 70% (30 amps * 70% = 21 amps). Same is true with 14 gauge used at 25 amps. So unless there are 10 current carrying conductor's then it is not an issue unless ambient temps come into play.

For NM
If you use NM, even though there is 90C wire we are limited to the 60C column for the overcurrent protective device because of 334.80, but we can still de-rate from the 90C column. As long as the overcurrent protective device & load does not exceed the ampacity in the 60C column then the design is compliant.

The same is true for any ampacity. We derate from the strongest link in the chain and we are only as strong as the weakest link. So if something in the circuit is 75 C and the wire is 90C then we de-rate from the 90C but our overcurrent protective device cannot be larger than the 75C ampacity
 
Suppose I run 200' of 3/0 copper at 90C. The wire is rated 225 amps. Now suppose I splice both ends with 90C split bolts and connect it to 4/0 into the panel which has 75C terminations. Now I can use the 3/0 at 225 and 4/0 is rated 230 amps at 75C so I am good to use a 225 amp breaker on this circuit or feeder.
 
Pitt123 said:
This is the part I am getting confused with. How do you determine final circuit ampacity considering both terminations and cable?

Can you elaborate more or provide an example?

Thanks
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Let's say we have a feeder routed as follows: panelboard->conduit->weather head and transition to a messenger supported wiring outside....

The conductors in conduit will be rated 90 degrees C, the panel board terminations will be rated 75 degrees using the (B)(16) ampacity, the splices up at the weatherhead will probably be rated 90 degrees,. The aerial cable will be in free air thus we can use the free air table so they will either have a higher ampacity or can be smaller than the conductors in conduit. The weakest link (assuming the aerial is appropriately sized and whatever is after that is appropriately sized) will be the (B)(16) value of the conductors in conduit at 75 degrees for the panel board terminations.
 
Carultch said:
The problem arises, when you splice your #8 to your #6. The splice would most likely need to be in an enclosure, where 310.15(B)(16) would apply. Best case scenario, you take credit for the 90C rating for the splice, when the splice is in an otherwise-empty enclosure. The splice is still a chokepoint that limits you to the 310.15(B)(16) ampacity.
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But 310.15(A)(2) applies. So even though a little bit of your #8 would be in an enclosure, most likely you get to ignore that.

Cheers, Wayne
 
Carultch said:
Terminations as part of equipment, are most commonly 75C.
Terminations that are individual products (split bolts, insulation piercers, distribution blocks, Polaris blocks) are commonly rated for 90C.
Click to expand...


Yes I agree.. Did I say otherwise? Maybe I could have been clearer.
 
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