Can someone please explain to me why we use the 75 degree column to size wire? Thhn is what we typically use and it is rated for 90 degrees c. I understand that almost all terminal s have 75 degree rating but how does that affect the ampacity of the wire itself? After all isn't it the ampacity that would cause heat? So what's the ampacity of the 75 degree terminal on a 20 amp receptacle ? Probably that terminal is rated for 20 amps even at 75 degrees. Could anyone please clear this up for me. Thanks
90 degree or 75 degree
- Thread starter Rjv0858
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Let's take #8 AWG as an example, so as to avoid the "small conductors" rule of 240.4(D). A #8 THHN can handle 55 amps without risk that the released heat would damage its insulation system. However, that same 55 amps that is run through a termination that itself is only rated for 75C carries a risk that the heat generated as current passes through the termination can cause damage to the termination. If you have terminations that are rated for 90C (as yet, not an industry standard product line), then you can run 55 amps through the circuit.
That is not a scientific explanation. But does it answer your question?
That is not a scientific explanation. But does it answer your question?
Yes that is what I'm trying to get at , most lugs have a maximum size wire , so wouldnt a 75 degree lug rated for #12 to let's say #6 wouldnt the 55 amps have no affect on the termination. Considering a # 6 wire at 75 degree can handle 55 amps no problem.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I'm guessing that it's assumed that a #12 conductor protected at 20 amps will generate a similar amount of heat that the #6 conductor protected at 60 amps will generate with the threshold of 75° C being the limit of which the conductor and equipment will still be protected.
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
The ampacity of a wire with a given gauge is based on the current it would take to produce a specified temperature in the conductor (e.g, 60°C, 75°C, 90°C, etc.) under standardized test conditions. These temperatures were selected to reflect the ability of different insulation types to handle various temperatures. Because electric conductors such as copper and aluminum are also excellent conductors of heat, a wire will conduct heat into the terminal of a device until it reaches a temperature equilibrium. This equilibrium temperature at the termination may be close to that of the rest of the wire itself because the heat dissipation rate (e.g., BTU/hr, cal/hr, etc.) from the device terminal will often be small compared to the attached conductor. Therefore the temperature rating of the device terminal will set a limit on the ampacity of a connected wire if the terminal temperature rating is less than the maximum rating of the wire itself. For example, if the terminal is rated at 75°C then the allowed conductor ampacity can be no higher than its 75°C value, even when its insulation is capable of higher temperatures.
- Location
- Mission Viejo, CA
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- Professional Electrical Engineer
Ampacity (see the definition in Article 100) refers to "condition of use". Terminals (not just wires) are conductors also and their temperature rating cannot be exceeded by the current flow either. The maximum ampacity of the circuit is limited by the "conditions of use" of each component of the circuit at the point of evaluation. This is why the 90C rating may be used for other locations in the circuit [Section 110.14(C)] for "... ampacity adjustment, correction or both."
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- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
This is why the THHN is not required to be evaluated a 75C throughout a circuit.
This right here makes the most sense to me , I'm a journeyman electrician and I cannot even begin to explain how big this argument gets with other electrician s I work with. I see no reason why the temperature rating of the terminals directly affects the sizing of our wire. Since we size off of ampacity. I will continue to use the 90 degree column to size my thhn.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
You can use 90 degree column to size the wire but the final conductor ampacity cannot be more than 75C column as your breakers will be limited to 75C
So let me get this straight say I have 80 amp breaker I can't size my wire with the 90 degree column for thhn even tho thhn is rated for 90 degrees? The 75 degree terminals that are on the breaker are rated for 80 amps. That's where I think people get confused how can something that's rated for the same amperage as the wire cause an issue?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
If you sized the conductors for 90° C they would cause excessive heat at the terminal and the equipment because the smaller size conductor (90° C ampacity) would generate more heat then a (larger) conductor sized at 75° C.
Is it simply because the heat caused by 80 amps of the wire sized
Thats the best explanation I've got I think I get it , it has nothing to do with ampacity but heat . I take it back I will now use the 75 degree column , thanks
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
On a related note, the reason we multiply the ampacity of a circuit by 125% for a continuous load is that makes the wire used larger, the larger wire acts as a heat sink, to pull the heat out of the breaker, to keep it from tripping from the thermal element.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
You can use the 90 deg C column for the starting point for conductor bundling and ambient temp correction. After you correct for that, if the conductor ampacity is greater or equal to the 75 deg C you are ok. If under the 75 deg C value then go up a size or look at the bundling.
Mike Holt explains this really well - first take a break!
Conductor size based on terminal rating
"you have to take a lot of different pieces and put them together"
Mike Holt explains this really well - first take a break!
Conductor size based on terminal rating
"you have to take a lot of different pieces and put them together"
Carultch
Senior Member
- Location
- Massachusetts
We simply need 90d C terminals and be rid of all the confusion.
JAP>
90C terminals do exist, but equipment built to allow you to take credit for this rating is uncommon. And it's not just the terminals that need to be 90C rated, but the equipment as a whole. You may very well see lugs on your equipment marked AL9CU, which would appear to indicate this, but that isn't enough. Not only that, you'd have to have 90C rated equipment on the other side as well.
One example where you do get to use 90C for terminal ratings, is if you have separately-installed connectors, in an enclosure that otherwise empty. Such as split bolts, Polaris blocks, insulation piercing connectors. It could be a way to recover from an error, or a value-engineering decision, to locally use 75C sizing at your equipment, splice to 90C sizing in an adjacent enclosure, and use 90C sizing for the majority of the length.
Another example is Mike Holt's illustrations show this used when separate lugs are bolted to busbars. I would like to know what kind of equipment allows this rating? Is it just equipment that is nothing but busbar, that would allow this? Switchboard sections that are only busbar within the section? Many transformers have busbars with separately-installed connectors, but does the fact that it is in a transformer restrict you to 75C?
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