75 C INSULATED CONDUCTORS

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JEFF MILLAR

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Are conductors rated 75 / 90 C the standard in daily. applications and the preference.
Are 75C insulated copper conductors still in common use, or is 75 / 90 C the standard.

And are conductor sizes 300 , 400, 600, 700, 800 and 900 kcmil really used.
I thought contractors never use 3 /O, and. only use 250, 350, 500, 750 and 1000 kcmil.

Thank you, I want to learn.
 
Dual rated insulation 75/90 degree is typical for my line of work. 90 degree rated conductors will be your friend when dealing ampacity adjustments/temp corrections.

I can’t speak for all those other conductor sizes other than 3/0. But yes for 3/0. Any 200amp service or feeder that’s non-residential will need that size if using copper.
 
Dsg319 said:
Dual rated insulation 75/90 degree is typical for my line of work. 90 degree rated conductors will be your friend when dealing ampacity adjustments/temp corrections.

I can’t speak for all those other conductor sizes other than 3/0. But yes for 3/0. Any 200amp service or feeder that’s non-residential will need that size if using copper.
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Practically all conductor is now are – 2 rated which means 90° wet or dry.

I have used 300 and 600, granted not often. Most of the conductor sizes you list are probably not stocked most places, but we often make custom conductor orders, say to get the colors we want, and they will have pretty much any size.
 
On large buildings we will regularly use 3/0, 250, 300, 400, 500, 600, 750 kcmil. Nothing ever larger than 750.
 
600 kcmil is common for 400 amp feeder taps and transformer secondary conductors where you cannot use the "round-up" provisions of 240.4(B).
 
Do you ever check the short circuit rating of your conductor sizes. Against the circuit breaker type and it's available short circuit rating ?
They say to calculate a tripping time for the breaker as 200 mili seconds. Does this make sense to you.
 
JEFF MILLAR said:
Do you ever check the short circuit rating of your conductor sizes. Against the circuit breaker type and it's available short circuit rating ?
They say to calculate a tripping time for the breaker as 200 mili seconds. Does this make sense to you.
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Never heard of checking SCCR “for” conductors, either above my pay grade or skill set lol, do you mean pulling a calculation to find available fault current from a source with your conductor size and length included? Maybe I’m not understanding your question correctly...
 
Difficult for me to imagine a 100 ft run of 3-1/2 GRS conduit with 750 kmil cu.
Nearly 900# of conduit and almost that weight in conductors (figuring 3) ...
Some really tough men doing those installs.
 
don_resqcapt19 said:
600 kcmil is common for 400 amp feeder taps and transformer secondary conductors where you cannot use the "round-up" provisions of 240.4(B).
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I think also common for where you can't round up for paralleled conductors for over 800 amps applications. Particularly 1200, 1600 and 2000 amps.
 
JEFF MILLAR said:
Are conductors rated 75 / 90 C the standard in daily. applications and the preference.
Are 75C insulated copper conductors still in common use, or is 75 / 90 C the standard.

And are conductor sizes 300 , 400, 600, 700, 800 and 900 kcmil really used.
I thought contractors never use 3 /O, and. only use 250, 350, 500, 750 and 1000 kcmil.

Thank you, I want to learn.
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Basically 90C insulation is about all you will find for up to 600 volts applications, and 75C is about all you will find for terminations. So you size ampacity based on 75 C minimum size, but can use 90C for adjustments when needed.

Then there is still NM and UF that must use 60C as the base but still can use 90C for adjustments if marked with -B suffix.
 
Also if you ever use XHHW, then you have to know if its being installed in a dry or damp location or a wet location. Dry or damp you use 90* and wet location you use 75*. Also most disconnect lugs are rated for 75* so that's what you have to size your cable for. I'm not exactly sure where I read this or heard it but, Switchboards, Panelboards and switchgear terminations are rated @ 90* BUT the main breaker is still rated at 75* so you'd still go off the 75* column to size the cable if it has a main breaker. Most equipment lugs are rated at 90*

Someone correct me if I'm wrong
 
THE NEC gives the MINIMUM size for power conductors and equipment ground conductors. That means not having to use 300, 400, 600, 900 etc.
Contractors can use then on hand inventory and still meet code. You can install larger conductors,
All power conductors have a short circuit with stand max. current limit, And THHN - 2 also has an insulation short circuit rating of 90 C TO 150 C.
That is the max temperature a short circuit can raise the insulation up to , after that if the breaker has not tripped BAM
Circuit breakers non thermal are calculated to trip in 200 mili seconds. You know all this.
 
If the conductor is THHN - 2. and installed in a wet location. does the code require ANY needed ampacity corrections be made. at 75 C.
Instead of 90C.
 
JEFF MILLAR said:
If the conductor is THHN - 2. and installed in a wet location. does the code require ANY needed ampacity corrections be made. at 75 C.
Instead of 90C.
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THHN-2 is a fictional designation. THWN-2 is the only designation in this family of insulation types, that has a -2 rating.

T = thermoplastic. Two H's (high heat) = 90C rated. Single H (heat) = 75C rated. W = wet location rated. "-2" = dual rated for both 75C and 90C

THHN if it existed on its own, would mean 90C rated, and only allowed in a dry location. I don't think it ever has existed on its own, without also having a THWN rating in some form or another.
THWN is rated 75C in both wet and dry locations.
THHN/THWN is rated 90C in dry locations and 75C in wet locations.
THWN-2 is rated 90C in both wet and dry locations. THWN-2 will carry the previous generation of designations by definition, and all three will be marked on the print legend (THHN/THWN/THWN-2).

There are other conditions where you are still limited to the 75C rating for THWN-2, such as conditions requiring the gasoline and oil resistance rating. But for general wet and dry, it is 90C rated.

You also still are limited to 75C terminations most of the time. The value in having 90C rated wire, is that it gives you a larger value to use as a starting point for your derate calculations.
 
Clint623 said:
Also if you ever use XHHW, then you have to know if its being installed in a dry or damp location or a wet location. Dry or damp you use 90* and wet location you use 75*. Also most disconnect lugs are rated for 75* so that's what you have to size your cable for. I'm not exactly sure where I read this or heard it but, Switchboards, Panelboards and switchgear terminations are rated @ 90* BUT the main breaker is still rated at 75* so you'd still go off the 75* column to size the cable if it has a main breaker. Most equipment lugs are rated at 90*

Someone correct me if I'm wrong
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Lug is often a stand alone design and maybe even made by different party than the switch/breaker manufacturer. That lug may even be marked for 90C, but when looking at the switch/breaker you have to look at overall listing of the entire assembly.
 
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