THHN/THWN-2 Question

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designer82

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When use of THHN/THWN-2 cables is specified, do they mean either one is ok, or are they referring to some special cable that has both?

I would definitely its the first one but I would like to verify.


Thanks
 
Most manufacturer conductors of that type have multiple ratings.
Here is Southwire:
These conductors have multiple ratings. Depending upon the product application, allowable temperaturesare as follows: •THHN or T90 Nylon- Dry locations not to exceed 90° C •THWN-2- Wet or dry locations not to exceed 90° C or locations not to exceed 75° Cwhen exposed to oil •THWN- Wet locations not to exceed 75° C or dry locations not to exceed 90° C or locations not to exceed 75° C when exposed to oil •TWN75- Wet locations not to exceed 75° C •MTW- Wet locations or when exposed to oil at temperatures not to exceed 60° C or dry locations not to exceed 90° C (with ampacity limited to that for 75° C conductor temperature per NFPA 79) •AWM- Dry locations not to exceed 105° C only when rated and used as appliance wiring material
 
THHN and THWN are two overlapping wire types, basically one now.

The HH means higher temp, and the HW means heat and wet.

The -2 suffix means the wet temperature rating is equal to the dry rating.

The T means thermo-set plastic, and the N means an outer nylon layer.
 
designer82 said:
When use of THHN/THWN-2 cables is specified, do they mean either one is ok, or are they referring to some special cable that has both?
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THWN-2 is the most inclusive rating, and is what most wire in this product family will be, for what you can get today. The summary of the term's meaning is that it is 90C rated in both wet and dry locations, and has the nylon outer jacket. The reason it is also marked with the lesser ratings, is to indicate backwards compatibility with previous generations of products.

Some datasheets only carry the THWN-2 in larger sizes, such as #8 and larger, so if important to your application, you have to check the datasheet carefully. The small conductor rule will also come in to play on circuits 30A and less, which is probably why there is a good chance that #10 and smaller may not carry the full THWN-2 rating. THHN/THWN without the "-2" rating is what #10 and smaller could be, which means wet locations are limited to 75C ampacity as the starting point for your adjustment/temperature corrections, and dry locations allow the 90C rating.
 
This is now the cable insulation is marked. THHN / TWWN- 2 , 600 / 1000 VOLT. 75 / 90 C. You still base the amp rating on 75 C, BUT any amp derating you use the 90 C, AMP RATING.
 
Amp derating use 90 C. Amp rating in Dry locations, but use the 75 C amp rating to derate in wet locations ? I never new that. Where is the code related reference ?
 
Table 310.104 lists the conductors and applicable ampacity values (that's one advantage of " --2", it's 90Ä wet or dry)
110.14 and 240.4 will limit the circuit size after derating.
 
I a design specifies THHN/THWN-2 then you need to use conductors which meet both requirements.

As has been mentioned the requirements for the different ratings are very similar; I've never bought THHN that wasn't also THWN, but depending upon supplier some might be THWN and some might be THWN-2.

1) I wonder if the designer really knows what the rating means or if they simply copied it from a supplier or a spool of wire. For a design I would specify several different acceptable types that meet the needs of the project, not a combined type.

2) Since THHN/THWN is pretty much the most common wire type out there a supplier might see that designation as meaning 'wire' and not double check for the -2 at the end. So make sure you get the right stuff delivered.

Jon
 
Are there any applications where it would require THWN and makes use of THWN-2 invalid?
I think most mfg are making the dual rated wire as it avoids making multiple single rated wires, just a cost saving measure for mfg. Only single rated I've come across recently was some new/old stock that I was given that had some atleast +30yr old straight THHN.
 
Fred B said:
Are there any applications where it would require THWN and makes use of THWN-2 invalid?
I think most mfg are making the dual rated wire as it avoids making multiple single rated wires, just a cost saving measure for mfg. Only single rated I've come across recently was some new/old stock that I was given that had some atleast +30yr old straight THHN.
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No, because THWN-2 automatically includes the previous ratings of THHN and THWN. You'll see the print legend and datasheet indicate all three designations, so that this fact is clear.
 
Carultch said:
No, because THWN-2 automatically includes the previous ratings of THHN and THWN. You'll see the print legend and datasheet indicate all three designations, so that this fact is clear.
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I didn't think there should be an issue, just wasn't aware if there was anything that would require use of 75C wire instead of 90C wire? I know there is for the opposite direction.
 
I can't imagine any situation where THWN-2 would be required yet THHN be prohibited, unless it is a customer who has a phobia of double letters :)

My point is that if the installation requires a wet rated 90C conductor then THWN-2 should be specified, even though what gets purchased will be THHN/THWN-2/....

If the installation doesn't require wet rated conductors then THHN should be specified even though it will mean buying the same wire.

Jon
 
winnie said:
I can't imagine any situation where THWN-2 would be required yet THHN be prohibited, unless it is a customer who has a phobia of double letters :)
Jon
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unbelievably there is such a phobia:::: well actually for long words.... Hippopotomonstrosesquippedaliophobia
 
Little Bill said:
Shouldn't you be out on a bike or something?:p
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Soon. Very soon.
ricky.gif
 
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