Consensus on Cable Trench Ampacity

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kingpb

kingpb

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Trenwa style UG concrete trench with cover, open bottom(partially anyway) ampacity.

I searched the topics on Mike Holt but found no concrete (pun intended) resolution to the correct ampacity tables for this type of install.

Leaning towards following cable tray ampacity, and calling it good, but don't want to leave a bunch of cable on the table, so to speak.

Lets not turn this into a 12-page dissertation, short answers appreciated.

Cheers~
 
I would be interested to this answer also. Like you I could not find anything about Trenwa vs. capacity.
 
kingpb said:
Trenwa style UG concrete trench with cover, open bottom(partially anyway) ampacity.

I searched the topics on Mike Holt but found no concrete (pun intended) resolution to the correct ampacity tables for this type of install.

Leaning towards following cable tray ampacity, and calling it good, but don't want to leave a bunch of cable on the table, so to speak.

Lets not turn this into a 12-page dissertation, short answers appreciated.

Cheers~
Click to expand...

Cable tray....? That has a higher ampacity.....I would think more along the line of metal trough ampacities....and no derating until 30 CCC....
 
UPDATE -

I contacted Trenwa, and although they won't specifically say what table to use, they were nice enough to send me some studies that were done by ComEd and OG&E. The long and short of it is that you can use the in air ratings, as long as you maintain the one-cable, cable separation. The tests indicate that with that separation the air space in the trench around the cables in sufficient to assume open air installation. The ability of the earth below and concrete structure on the sides actually helps by acting as a heat sink, thus providing an additional 5-10% increase in current capability, if needed.

So, for multi-conductor, 3-core with 1 cable diameter spacing, I'm going to use Table 310.15(B)(2)(3).

For single conductor in triangular with 2.15OD spacing, I will use Table 310.15(B)(20)
 
I know trench type installations are often used in industrial applications and have been for a long long time, but what code section covers that type of installation?
 
kingpb said:
UPDATE -

I contacted Trenwa, and although they won't specifically say what table to use, they were nice enough to send me some studies that were done by ComEd and OG&E. The long and short of it is that you can use the in air ratings, as long as you maintain the one-cable, cable separation. The tests indicate that with that separation the air space in the trench around the cables in sufficient to assume open air installation. The ability of the earth below and concrete structure on the sides actually helps by acting as a heat sink, thus providing an additional 5-10% increase in current capability, if needed.

So, for multi-conductor, 3-core with 1 cable diameter spacing, I'm going to use Table 310.15(B)(2)(3).

For single conductor in triangular with 2.15OD spacing, I will use Table 310.15(B)(20)
Click to expand...

I would not go with what Trenwa said....I would contact your AHJ, and go with that. Inspector may not agree with what the Rep. told you.....
 
don_resqcapt19 said:
I know trench type installations are often used in industrial applications and have been for a long long time, but what code section covers that type of installation?
Click to expand...

None I know of. But that is not really odd. Most anything out of the ordinary is generally not well covered by the NEC Design Guide.

On the few I've seen, It was MC-HL multiconductor cable, ampacity per 310.15.B.16.

king's method (calling the mfg) is excellent

ice
 
Canton said:
I would not go with what Trenwa said....I would contact your AHJ, and go with that. Inspector may not agree with what the Rep. told you.....
Click to expand...

With all due respect:
Will all the AHJ reps that are comfortable with a 310.15.C calculation, can budget the time, and have the inclination, please raise their hand? ................

This is not the AHJ's job. I would hope she would say, "Go hire competent engineering. Submit a stamped calculation."

ice
 
iceworm said:
With all due respect:
Will all the AHJ reps that are comfortable with a 310.15.C calculation, can budget the time, and have the inclination, please raise their hand? ................

This is not the AHJ's job. I would hope she would say, "Go hire competent engineering. Submit a stamped calculation."

ice
Click to expand...

No one said it was. The call to the AHJ was to clarify what table would be acceptable to use since it is not a recognized wiring method in the NEC. If he has started the job and has a permit i would think he would already have engineered and stamped drawings, if not then maybe none needed. The OP did not say what size wire or what application this is for.....could be some MV cables or #12 wire for a receptacle in his garage....with all due respect.
 
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