THHN/THWN properties

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J. Brinkley

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Does anyone know what the minimum temperature rating for using THHN/THWN is? Experience has shown that the insulation cracks and splits somewhere below 32 deg. f. Didn't the code book use to give min. temp. parameters in the conductors properties?
 
If there's a lower limit, I've never heard any talk about it.

I know that in the winter we normally set the wire that is going to be pulled later on near the heaters so that they don't crack during installation. After they're installed, low temperatures aren't going to make them spontaneously crack. If they were installed in a very cold ambient, the whipping about during a fault may well make them crack. Hard to say. I would hope that after a fault that certain due diligence would take place before the circuit is placed back in service.
 
From Southwire:

Will Southwire's new SIMpull THHN reduce the occurrence of nylon splitting in the field? Yes. Southwire's new SIMpull THHN results in less stress on the nylon during installation resulting in less splitting. It should be noted that the major contributor to nylon splitting is installing THHN in cold weather.
If installing in cold weather, the cable should be stored in a heated environment overnight prior to installation. Cold temperatures approaching freezing can cause the insulation to become stiff and possibly brittle. Care
should be taken to mitigate the effects of cold weather when installing in these situations. The cables should be kept in heated storage for at least 24 hours prior to installation and only removed from heated storage just prior to installation. Cables installed in cold weather should be handled more carefully and pulled more slowly. Even given these mitigating methods Southwire does not recommend installing thermoplastic insulated cables at temperatures below –10C (14F).
 
I pulled in some Sim-pull on a 15 degree day. The outer jacket split in 5' sections just pulling it off of the reel.
 
The 14 degree minimum is not just for SIMpull. It is found in many manufactures publications for PVC jacketed conductors. If you want to pull in the cold temperatures you need to look at XHHW or some other type of insutlation.
 
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