If wire rated for 90 degree celsius (thhn)which can be used for derating purposes,but we are limited to the 60 or 75 degree celsius column because the terminations on most devices are only rated for this,what happened? Did they produce a better wire and insulation with the thhn? And if so why didnt they design a termination rated for 90 degree celsius so we could use the proper column with the proper termination? I guess what I'm asking is it seems we can never use the 90 degree column itself as a end result because of the termination ampere rating,so why not put the thhn in the 75 degree column with the 90 degree ampere rating and say derate as necessary until a 90 degree termination becomes common? (This may sound stupid but thats what I get when I think and type on the fly.)
conductors
- Thread starter jap2525
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- Location
- Illinois
- Occupation
- retired electrician
Re: conductors
If we start running the equipment at 90?C we will receive a lot of complaints. That is hot...194?F. Remember that if the conductor is loaded to the maximum permitted by the table and under the conditions specified, that the wire will reach this temperature. OSHA would require insulation for personal protection on our conduits if the conductors are running that hot. I think that the OSHA standard requires protection from anything that exceeds 140?F (60?C).
Also look at the wasted energy if we all start running things at 90?C. The temperature is a result of the I^2R heating and is wasted watt hours. We really should be looking at running at a lower temperature. Often times the pay back for installing larger conduit and conductors to serve a load is less than a year.
Don
If we start running the equipment at 90?C we will receive a lot of complaints. That is hot...194?F. Remember that if the conductor is loaded to the maximum permitted by the table and under the conditions specified, that the wire will reach this temperature. OSHA would require insulation for personal protection on our conduits if the conductors are running that hot. I think that the OSHA standard requires protection from anything that exceeds 140?F (60?C).
Also look at the wasted energy if we all start running things at 90?C. The temperature is a result of the I^2R heating and is wasted watt hours. We really should be looking at running at a lower temperature. Often times the pay back for installing larger conduit and conductors to serve a load is less than a year.
Don
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: conductors
The ?T? in ?THHN? means ?thermoplastic insulation (i.e., PVC),? and the ?N? means ?nylon jacketed.? This combination (like many others) is good for 60C. Each of the two ?H? symbols means ?add 15C (i.e., one 'unit of heat') to the temperature rating.? So by definition, the conductor named T-HH-N is good for the 60C plus 15C plus another 15C, for a total of 90C. That is why there is a separate column. That is also why TW is in the 60C column, and THW is in the 75C column, and THHW is in the 90C column. It represents a guarantee from the manufacturer of the insulation system?s ability to withstand high temperatures.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: conductors
If you limit the current to the ampacity value shown in the 75C column, then the temperature rise experienced within the cable, added to the ambient temperature (assumed to be 30C) will not result in a temperature, at the terminations or anywhere else along the run, in excess of 75C. That is how they selected the ampacity numbers that appear in the column. The same goes for the 60C column.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: conductors
Termination temperatures are part of the UL heat rise tests. It is allowed (and common) to have the circuit conductors act as heat sinks for overcurrent protective devices. If the conductors were to operate at the higher level of 90C then they would no longer sink heat from the OCPD, in fact they would act as heat sources instead.
Side note: one reason "foreign" equipment seems to run hotter than NEMA stuff is the temperature rise of the connections. IEC starters are usually rated for 75C minimum, while NEMA starters can handle 60C conductors.
Termination temperatures are part of the UL heat rise tests. It is allowed (and common) to have the circuit conductors act as heat sinks for overcurrent protective devices. If the conductors were to operate at the higher level of 90C then they would no longer sink heat from the OCPD, in fact they would act as heat sources instead.
Side note: one reason "foreign" equipment seems to run hotter than NEMA stuff is the temperature rise of the connections. IEC starters are usually rated for 75C minimum, while NEMA starters can handle 60C conductors.
lady sparks lover
Senior Member
Re: conductors
This is information I didn't know.
jeff43222
Senior Member
- Location
- Minneapolis, Minnesota
Re: conductors
This brings up a good point. Does anyone know of a definitive list of what all the letters on wires stand for? I know a lot of them, but some have remained elusive (e.g., what does the 'X' in XHHW stand for?). I find it much easier to remember and understand abbreviations if I know what each of the letters represents.
This brings up a good point. Does anyone know of a definitive list of what all the letters on wires stand for? I know a lot of them, but some have remained elusive (e.g., what does the 'X' in XHHW stand for?). I find it much easier to remember and understand abbreviations if I know what each of the letters represents.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: conductors
It means "cross linked." Almost all thermosets - like rubber - are cross linked where used as insulation. Type "X" is usually cross linked polyethylene but it may be ethylene-propylene rubber. Many conductor insulation materials have multiple ratings and could be either RHW or XHHW, depending on insulation thickness.
[ September 15, 2004, 05:50 PM: Message edited by: rbalex ]
It means "cross linked." Almost all thermosets - like rubber - are cross linked where used as insulation. Type "X" is usually cross linked polyethylene but it may be ethylene-propylene rubber. Many conductor insulation materials have multiple ratings and could be either RHW or XHHW, depending on insulation thickness.
[ September 15, 2004, 05:50 PM: Message edited by: rbalex ]
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: conductors
Here is a glossary of wire terms. It is not as good as some of my old Southwire and Royal catalogs but it is the best I have found on the web.
Here is a glossary of wire terms. It is not as good as some of my old Southwire and Royal catalogs but it is the best I have found on the web.
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