310.15 (B)(16)

[electricalist]

When I'm sizing my conductors do I start with wire type. I.E.
THWN is in the 75 degree column
XHHW-2 is in the 90 degree column
Or do I start with the terminal rating.


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[roger]

You start any adjustment from the highest rating of a particular conductor (see 110.14(C)) but you are limited to all the other components highest rating such as terminals and connectors which will most likely be 75 deg.

In your question THWN has a 75 deg rating and that is the highest ampacity for that insulation type whereas XHHW has a rating of 90 deg so you can start with that number but would be limited to all the other components as far as OCP.

Roger

 

[electricalist]

So the 75 degree column is where I should usually start.
This is how I have usually done it although I got in a debate about #2 XHHW-2 being good for 125 amps.
I'd say it's only good for 115


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[roger]

So the 75 degree column is where I should usually start.
This is how I have usually done it although I got in a debate about #2 XHHW-2 being good for 125 amps.
I'd say it's only good for 115


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No, using XHHW you start with the 90 deg column.

For example, if you were to put (9) #12 XHHW in a conduit you would start with 30A and then go to Table 310.15(B)(3)(a) where you have to adjust the ampacity to 70%

So,

30 X 70% = a 21 amp conductor.

If you had started with the 75 deg column you would have

25 X 70% = a 17.5 amp conductor.

Roger

 

[kwired]

When I'm sizing my conductors do I start with wire type. I.E.
THWN is in the 75 degree column
XHHW-2 is in the 90 degree column
Or do I start with the terminal rating.


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you need to figure both and final selection is whichever one comes up with the larger conductor.

If you have 90C conductors and don't have any temperature or number of conductors in raceway adjustments, the 75C selection is going to be larger every time and you can just go straight to that column in that situation. If you have a conductor that is only 75C rated, you have no business being in the 90C column at all.

Majority of thermoplastic conductor you find, at least in sizes 14-10 AWG, these days is rated THHN/THWN/THWN-2 though, and is rated for 90C in any application. All the XHHW I recall ever seeing also had the -2 rating.

 

[electricalist]

So #2 XHHW -2 SEC is good for 130 amps.
The panel that it lands in has 75 degree terminal rating.
Is my wire still good for 130 or 115 amps


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[Dennis Alwon]

So the 75 degree column is where I should usually start.
This is how I have usually done it although I got in a debate about #2 XHHW-2 being good for 125 amps.
I'd say it's only good for 115


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You are correct that #2 is only good for 115 amps however if there are situations that require derating , like temperature or the number of wires in a conduit then you can start at 130 amps (#2 @ 90C). Even after you derate the overcurrent protective device cannot be larger than 115 excpt where 240.4(B) applies

 

[Carultch]

When I'm sizing my conductors do I start with wire type. I.E.
THWN is in the 75 degree column
XHHW-2 is in the 90 degree column
Or do I start with the terminal rating.


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It is rare to have terminations that are rated 90C or more, especially those inside equipment. Not just the lug needs the temperature rating, but the entire piece of equipment needs it, unless it is a field-installed termination that is installed outside of equipment. Genearlly speaking, most terminations are rated 75C. For 100A and less, you do have the onus of proof to see that they are listed and labeled for 75C on both ends, because the default is 60C for 100A and less.

So it is a good habit to first check the column for termination rating first, knowing that the wire will be no smaller than that, and then think about the wire's rating. Correction & adjustment factors do not apply to terminations, only to wire ampacity. This is the part of your calculations where taking credit for a 90C wire's rating is most common, when 75C terminations are present. Both conditions must be met, termination ampacity without derates, and wire ampacity with derates, and whichever conductor is larger is the one you use.

Another place you would take credit for a 90C rating, is if you install a separate enclosure for a splice at each end, and then use 90C wire with 75C sizing to connect to the equipment. You might do this to remediate a mistake, or as a value engineering decision.

 

[kwired]

So #2 XHHW -2 SEC is good for 130 amps.
The panel that it lands in has 75 degree terminal rating.
Is my wire still good for 130 or 115 amps


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75 degree terminal rating limits you to 115 amps. If you have a high ambient temperature or more then 3 current carrying conductors you can make those adjustments from the 90C rating though. Your minimum conductor size after adjustments must still be at least whatever is in the 75C column because you have 75C terminals.

Where this gets tricky with 2 AWG is you could have up to 115 amps of load (with 125% for continuous load factored in if a continuous load) and still protect it with next standard size higher overcurrent device of 125 amps. If you needed a 80% ampacity adjustment though then the 130 value from the 90C column gets adjusted to 104, in that case the load could not be more then 104 amps (with continuous duty factor where applicable, and the maximum overcurrent protection could not be over 110 amps.

Certain loads like motors, air conditioning, capacitors - are exceptions that may allow higher overcurrent device settings.

 

[electricalist]

You are correct that #2 is only good for 115 amps however if there are situations that require derating , like temperature or the number of wires in a conduit then you can start at 130 amps (#2 @ 90C). Even after you derate the overcurrent protective device cannot be larger than 115 excpt where 240.4(B) applies

Yes sir. When derating I start with the respective type amperage.
Although I'm not advanced enough to understand how the full ampacity can be greater than a terminal rating due to wire type.


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[Dennis Alwon]

Yes sir. When derating I start with the respective type amperage.
Although I'm not advanced enough to understand how the full ampacity can be greater than a terminal rating due to wire type.


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The wire is rated more because the insulation is better therefore it can handle more heat. You are not adding more amps then the terminal as I said the overcurrent protective device cannot be greater than the 115 amps excluding 240.4(B).

So it is not the terminal ampacity that we are looking at but rather the temperature rating. We have not gone above the 75C rating at a specific ampacity

 

[electricalist]

310.15 (B)(16)

Ty sir.
I appreciate everyones time and teaching.


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[kwired]

Yes sir. When derating I start with the respective type amperage.
Although I'm not advanced enough to understand how the full ampacity can be greater than a terminal rating due to wire type.


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There is two ampacities to look at, one for the terminal temp rating one for the conductor insulation rating. If you have 75C insulation, there is less confusion because you are looking at the 75C ampacity column only. Otherwise you must figure both and pick whichever gives you the larger conductor. Most of the time with 75C conductors and 90C insulation the conductor from the 75C column will be the larger conductor. If you have more then just one 80% ampacity adjustment involved though then the adjusted 90C conductor will be larger.

Raceway with no more then 6 current carrying conductors and no temp adjustments necessary - the value in the 75C column still needs to be met for terminal rating.

Raceway with more then 9 current carrying conductors has 50% deration factor, the terminal temp is still satisfied with your example with 2AWG @ 75 C, but now that 130A @ 90C insulation rating needs adjusted to 65 amps - a larger conductor will be required at any point in the run where the 50% adjustment factor applies because of the potential heat developed in that bundle of 9+ current carrying conductors.