Derating question

M

MattG0311

Senior Member
Location
Illinois
Occupation
Apprentice Wireman
Say I have 4 current carrying conductors in a raceway. They have been derated in accordance with 310.15(C)(1) to 80 percent of their ampacity. The conductors then enter an enclosure and go through a nipple that is less than 24in long into switchgear. I know derating does not apply inside boxes or nipples shorter than 24in. With that said, is it compliant to reduce the conductor size upon entering the enclosure?

In this scenario, I essentially have 5 parallel runs of 600 Kcmil conductors, 3 hots and neutral per raceway. Neutral is being counted as current carrying per engineering. OCPD is set at 1800 amps.
My derating is as follows:

600 Kcmil CU @ 90 degrees- 475A
475A x .80 = 380A x 5 runs = 1900A

I want to reduce my conductor size upon entering my enclosure to 500Kcmil CU class K stranded THHN due to extremely limited space in the switchgear. (Existing installation, just trying to make it work)
500 Kcmil CU @ 75 degrees- 380 A
380A x 5 sets = 1900 A

Does anyone see issues with this, and if you do, please point me towards a code reference. Thank you.
 
MattG0311 said:
Say I have 4 current carrying conductors in a raceway. They have been derated in accordance with 310.15(C)(1) to 80 percent of their ampacity. The conductors then enter an enclosure and go through a nipple that is less than 24in long into switchgear. I know derating does not apply inside boxes or nipples shorter than 24in. With that said, is it compliant to reduce the conductor size upon entering the enclosure?

In this scenario, I essentially have 5 parallel runs of 600 Kcmil conductors, 3 hots and neutral per raceway. Neutral is being counted as current carrying per engineering. OCPD is set at 1800 amps.
My derating is as follows:

600 Kcmil CU @ 90 degrees- 475A
475A x .80 = 380A x 5 runs = 1900A

I want to reduce my conductor size upon entering my enclosure to 500Kcmil CU class K stranded THHN due to extremely limited space in the switchgear. (Existing installation, just trying to make it work)
500 Kcmil CU @ 75 degrees- 380 A
380A x 5 sets = 1900 A

Does anyone see issues with this, and if you do, please point me towards a code reference. Thank you.
Click to expand...
Other than making sure the splices and terminations are suitable for use with Class K stranding, I don't see any issues. Standard terminals and splicing devices are only suitable for Class B and C stranding.
 
don_resqcapt19 said:
Other than making sure the splices and terminations are suitable for use with Class K stranding, I don't see any issues. Standard terminals and splicing devices are only suitable for Class B and C stranding.
Click to expand...
absolutely. We looked at DLO cable but to get my required ampacity it is a massive outside diameter and requires a very expensive "pin" style adapter sleeve. I am hoping to use a class I or K stranded THHN or THHW jacket that i can simply terminate with a copper wrap sleeve.
 
MattG0311 said:
absolutely. We looked at DLO cable but to get my required ampacity it is a massive outside diameter and requires a very expensive "pin" style adapter sleeve. I am hoping to use a class I or K stranded THHN or THHW jacket that i can simply terminate with a copper wrap sleeve.
Click to expand...
Why can't you use the same copper wrap sleeve on DLO? Yes, the jacket of the DLO will make the diameter much greater than that of THHN.
As far as I know only one terminal manufacturer has approved the use of the copper wrap.
 
don_resqcapt19 said:
Why can't you use the same copper wrap sleeve on DLO? Yes, the jacket of the DLO will make the diameter much greater than that of THHN.
As far as I know only one terminal manufacturer has approved the use of the copper wrap.
Click to expand...
We are getting some pushback using copper sleeves in general. I am realizing now any flexible conductor I use will require the use of said adapters. I may have to rethink using flex stranded conductors at all.

Regardless, as long as my derating is correct I can at least drop one wire size into my switchboard.
 
MattG0311 said:
We are getting some pushback using copper sleeves in general. I am realizing now any flexible conductor I use will require the use of said adapters. I may have to rethink using flex stranded conductors at all.

Regardless, as long as my derating is correct I can at least drop one wire size into my switchboard.
Click to expand...
There is a problem with the adapters too :) ...most have solid pins and the terminals are only listed for use with Class B or C stranded conductors.
There is on brand that has a stranded pin.

My preference would be to remove the mechanical connections in the gear and replace them with crimp terminals that are listed for the stranding of the cable you are using.

I did that every time I could, even when using Class B or C stranded conductors. I just don't like the mechanical terminations. It was rare when I could not replace the supplied terminal with a crimp on one.
 
don_resqcapt19 said:
There is a problem with the adapters too :) ...most have solid pins and the terminals are only listed for use with Class B or C stranded conductors.
There is on brand that has a stranded pin.

My preference would be to remove the mechanical connections in the gear and replace them with crimp terminals that are listed for the stranding of the cable you are using.

I did that every time I could, even when using Class B or C stranded conductors. I just don't like the mechanical terminations. It was rare when I could not replace the supplied terminal with a crimp on one.
Click to expand...
That’s good to know, I think I can do exactly that. I imagine swapping for crimp on lugs will be significantly cheaper than the adapters
 
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