adjustment factor
- Thread starter kimrichi
- Start date
- Status
- Location
- Massachusetts
We don't use a temp rating hire than the conductor rating. If you where to use TW for instance you would be stuck with the 60 column.
What we do is uses THHN and the 90 C column to derate even when we have just 60 or 75 C terminals.
hardworkingstiff
Senior Member
- Location
- Wilmington, NC
Pretty much most of the conductors you purchase will have a 90C rating. If you happen to have a 60 or 75C rated conductor you need to use that column at the start of your calculations.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
For THHN/THWN you would use the 75? C ampacity in a wet location unless the conductor were also marked THWN-2.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Strictly speaking you only use the 90C column if the wire is THHN-2 or THWN-2, or something else with '-2' at the end, or something else that otherwise appears in the 90C column.
Hardworkingstiff is right that nowadays most stuff you will find to buy is rated for 90C.
Conductor ampacity factoring affected by the bundling and temperature conditions can be more comprehensible by referencing the cable(wire) insulation applications initially. Table 310.13 [2005 & 08] or Table 310.104[2011].
Selecting the distinct insulation type application range in those Tables is the key to the ampacity (wet-dry-temp) adjustments needed.
Note that NM and SER/SEU cabling, not found in the (T 310), is rated at 60c, but the cable conductor wiring is actually 90c rated.
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Can I ask why? Why does wet location change the de-rating column?
Also, I'm pretty sure THWN is rated for wet locations I was told that is what the "W" stood for. So if it is rated for wet locations why would you have to de-rate from a lower column?
Thanks Mike
- Location
- Illinois
- Occupation
- retired electrician
That only applies if you are using the conductors in a wet location. THHN without the -2 is suitable for 90?C use in dry locations.
- Location
- Illinois
- Occupation
- retired electrician
The THWN without the -2 is listed for 75?C in wet locations and 90?C in dry locations. See Table 310.13(A) in the 2008 code or 310.104(A) in the 2011.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
Don, I think he was asking why there is a different temp for wet and dry on the same wire. It's a good question, I've not looked into that before.
ETA: Sorry, I see he was asking two questions - but the other one has me interested.
ETA: Sorry, I see he was asking two questions - but the other one has me interested.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Like Don said look at the headings in T310.16.
The 75 deg column includes THWN the 90 deg column does not. The 90 deg column does incude THWN-2.
Most of conductor sold today has multiple ratings and usually includes THWN-2 in the rating. People don't even realize they are calculating wrong yet their result is correct because they were not actually wrong. (If that makes any sense)
The 75 deg column includes THWN the 90 deg column does not. The 90 deg column does incude THWN-2.
Most of conductor sold today has multiple ratings and usually includes THWN-2 in the rating. People don't even realize they are calculating wrong yet their result is correct because they were not actually wrong. (If that makes any sense)
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
Bad example - look at XHHW on table 310.104(A). Same conductor, different ratings.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Same situation with THHW. 75? wet, 90? dry.
Reality in most cases is conductors have multiple ratings marked on them and there is a good chance you have a 90 degree conductor in most cases anymore wet or dry. But you do need to check to make sure. I think this is mostly because that is what installers are demanding from suppliers so that is what they usually are going to stock.
Reality in most cases is conductors have multiple ratings marked on them and there is a good chance you have a 90 degree conductor in most cases anymore wet or dry. But you do need to check to make sure. I think this is mostly because that is what installers are demanding from suppliers so that is what they usually are going to stock.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
If you don't know the answer to my question, it's all right.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
If you don't know the answer to my question, it's all right.
I may know the answer. Is it all right if I don't know the question?
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
:lol:
derating is not wet or dry
derating is not wet or dry
A wet location does not change the derating (adjustment) column. The 90c column is the ampacity factoring line for conductor amp adjustments needed to compensate for bundling and ambient corrections for wiring insulation rated at 90c.
Any insulation rated for less than 90c must have a conductor re-sizing depending on the temperature operating conditions. If the insulation is not rated for 75c or more then the 60c column is used.
The wet or dry condition actually dictates the conductor insulation type suited for particular locations. (I.e. In Chicago, emt inside a structure would normally be dry compared to the same emt outside which would likely be determined as a wet location.)
Again, location can accept either 60, 75, or 90c rated conductors as long as the insulation type is rated for the wet or dry ambient location.
derating is not wet or dry
A wet location does not change the derating (adjustment) column. The 90c column is the ampacity factoring line for conductor amp adjustments needed to compensate for bundling and ambient corrections for wiring insulation rated at 90c.
Any insulation rated for less than 90c must have a conductor re-sizing depending on the temperature operating conditions. If the insulation is not rated for 75c or more then the 60c column is used.
The wet or dry condition actually dictates the conductor insulation type suited for particular locations. (I.e. In Chicago, emt inside a structure would normally be dry compared to the same emt outside which would likely be determined as a wet location.)
Again, location can accept either 60, 75, or 90c rated conductors as long as the insulation type is rated for the wet or dry ambient location.
insulation
insulation
The question may be reworded to get a better answer. Thhn or thwn insulation does not have a different 90c insulation rating as other type insulations do. See above response.
insulation
The question may be reworded to get a better answer. Thhn or thwn insulation does not have a different 90c insulation rating as other type insulations do. See above response.
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