Concrete Trenches

[manuel bonilla]

I am an electrical engineer working in San Jose, Costa Rica. We try to apply NEC in Costa Rica as much as local conditions permit.

It is standard prectice here to do concrete trenches within electrical rooms (in the floor) and then do feeders between panelboards using those trenches. Some sort of cable tray will usually be installed inside the trench and the cable would be laid on top of the trench.

Would this be and acceptable practice per NEC?
What would be the limitations on cable gauge that could be installed there?
What would be the limitations on the cross section?

I thank you for your help.

 

[Smart $]

I am an electrical engineer working in San Jose, Costa Rica. We try to apply NEC in Costa Rica as much as local conditions permit.

It is standard prectice here to do concrete trenches within electrical rooms (in the floor) and then do feeders between panelboards using those trenches. Some sort of cable tray will usually be installed inside the trench and the cable would be laid on top of the trench.

Would this be and acceptable practice per NEC?
What would be the limitations on cable gauge that could be installed there?
What would be the limitations on the cross section?

I thank you for your help.

Yes, it is acceptable. Read Article 390: Underfloor Raceways and Article 392: Cable Trays...

...but I hope you meant the cable would be laid on the cable tray in the trench

 

[manuel bonilla]

So, in your opinion, would it be limited by cable type and cable fill as stated in article 390 or as stated in article 392?

(Yes, cable is in the trench, on top of the cable tray. Trench has a metal cover. "Walls" of trench would be exposed concrete)

 

[Smart $]

So, in your opinion, would it be limited by cable type and cable fill as stated in article 390 or as stated in article 392?

As long as the cable tray along with its associated wires/cables do not continue beyond the confines of the trench, IMO you are bound only by 390, as long as all the additional requirements of 390 are met or exceeded... but I'd still bond the tray as required in 392.

 

[petersonra]

why not use UF cable and forget the cable tray?

 

[don_resqcapt19]

I don't see how cable tray in a trench is an Article 390 application. You do not have a raceway. The use of trenches is a gray area in the code. There is no direct reference to this type of installation. It is a common industrial installation and often without any cable tray, just using the trench to run the cable in. The closest code rules are the ones in Article 392.
Don

 

[petersonra]

why not use UF cable and forget the cable tray?

When I was in Mexico, I recall that it was the norm to run conductors in a sand filled, concrete covered trench. Same way they did it in China, now that I think about it.

No raceway - conduit or cabletray - was used except where the conductors passed through the concrete.

 

[Smart $]

I don't see how cable tray in a trench is an Article 390 application. You do not have a raceway.

How does the trench not meet the criteria for being a raceway?

 

[don_resqcapt19]

Smart,

How does the trench not meet the criteria for being a raceway?

It is not "designed expressly" for the purpose.

Raceway. An enclosed channel of metal or nonmetallic materials designed expressly for holding wires, cables, or busbars, with additional functions as permitted in this Code. Raceways include, but are not limited to, rigid metal conduit, rigid nonmetallic conduit, intermediate metal conduit, liquidtight flexible conduit, flexible metallic tubing, flexible metal conduit, electrical nonmetallic tubing, electrical metallic tubing, underfloor raceways, cellular concrete floor raceways, cellular metal floor raceways, surface raceways, wireways, and busways.

Don

 

[petersonra]

Smart,

It is not "designed expressly" for the purpose.

Raceway. An enclosed channel of metal or nonmetallic materials designed expressly for holding wires, cables, or busbars, with additional functions as permitted in this Code. Raceways include, but are not limited to, rigid metal conduit, rigid nonmetallic conduit, intermediate metal conduit, liquidtight flexible conduit, flexible metallic tubing, flexible metal conduit, electrical nonmetallic tubing, electrical metallic tubing, underfloor raceways, cellular concrete floor raceways, cellular metal floor raceways, surface raceways, wireways, and busways.

Don

There is a difference between "designed expressly for" and "designed exclusively for".

What if I were to deisgn a trench expressly to be a raceway?

 

[Smart $]

It is not "designed expressly" for the purpose...

Sorry to say, I don't agree with you...

How can you say it is not "designed expressly for holding wires, cables, etc."? Did you design it? I'll bet the person who did design it will say otherwise. Through remarks of the OP'er, the trench w/cable tray is designed "expressly" for the purpose of holding wires and/or cables. That is, it will be used for nothing else. And as the second sentence of the definition states, "Raceways include, but are not limited to..." If by chance the trench was used to hold water, hydraulic, pneumatic, or such processing lines in addition to wires, cables, etc., then I'd agree the trench is not a[n electrical] raceway.

 

[manuel bonilla]

Under the conditions that I have in a particular case, I have 1/0 cable mixed with #10 cable.

The trench is being used only for the purpose of running electrical wires. In that sense it was designed expressly and exclusively for holding wires.

If I accept this to be a 390 application, then I think I do not have an argument to reject this installation. I do not see in the definition of raceway how this would not comply as such.

Art. 390.4 does not seem to be very specific on the type of cables accepted.

If I see it as a 392 application, then obviously the #10 cable cannot be there.

At this point I think I agree with it being a 390 application. But let me tell you that the #10 wire does not look good to the side of the larger cable gauge wires. Would there be any other code article prohibiting that? (They are all the same system voltage).

 

[petersonra]

It is not a 390 application because that is refering to a specific type of manufactured raceway, not something you are making yourself.


Nor is it a 392 application.

392.6(G) Through Partitions and Walls. Cable trays shall be
permitted to extend transversely through partitions and
walls or vertically through platforms and floors in wet or
dry locations where the installations, complete with installed
cables, are made in accordance with the requirements
of 300.21.
(H) Exposed and Accessible. Cable trays shall be exposed
and accessible except as permitted by 392.6(G).

Your proposed application would not meet these requirements.

The best argument against just laying the cables/conductors in the trench is that method is not one of the methods recognized in chapter 3.

I think you should run some PVC conduit in the trench under the floor and be done with it.

 

[steve66]

Since there is a cable tray in the trench, it seems obvious that the trench wasn't "designed expressly for holding wires, cables, etc", otherwise the cable tray would not be used. In addition, a trench is not an" enclosed channel".

If you read 390, it seems obvious that it is meant to apply to raceways burried in the concrete pour, or installed in the pour flush with the top of the concrete. If you are going to say 390 applies, how do you comply with 390.4, or 390.9, or 390.14. And now all the conductors have to be derated per 310.15

Even if you still think 390 applies, that does not throw 392 out the window. That still has to apply if the wiring is in a cable tray. So the #10 should be some type of cable or in a conduit.

 

[steve66]

Nor is it a 392 application.

Bob:

I don't see anything in 392.6(G) that says the rest of 392 does not apply. If you are going to use 392.6(G), you still have to comply with all the other rules in Article 392 and 300.21.


Steve

 

[petersonra]

Bob:

I don't see anything in 392.6(G) that says the rest of 392 does not apply. If you are going to use 392.6(G), you still have to comply with all the other rules in Article 392 and 300.21.


Steve

I was trying to point out that once under the concrete floor it would not be accessable thus would not be a legal 392 application. 392.6H would prohibit this installation, at least if it was under concrete.

 

[steve66]

Sorry I misunderstood. All the trench like this that I have seen have removable covers. Sometimes they are metal, sometimes removable concrete slabs. (One sidewalk had a trench under it, and the concrete slabs of sidewalk were removable.)

Steve

 

[Smart $]

Steve (et al),

The OP'er mentioned in his second post there is a metal cover plate. In most situations as such that I have witnessed first hand, the cover has been 1/4" or thicker tread plate (usually of a size that required a crane to place and remove) and the edges of the trench have slightly recessed angle iron rails embedded in the concrete. In my mind, this is a far safer design than most any trench-type underfloor raceway considered for the intent of Article 390, and most certainly safer than exposed cable in cable tray. We did place cable tray(s) our trenches and used 392 for requirements. However, the cable tray and associated cables almost always continued beyond the confines of the trench... at least on one end.

The situation under discussion here has the cable tray completely within the confines of the trench (and cover plate, but specs unknown). In my mind, the only purpose for the cable tray to even be in the trench is to aid in the arrangement of cables and wires, and also to provide better ventilation to the same (that is provided it is ladder-type tray).

There are only two ways to look at the cable tray in the trench: 1) Is it a design part of the trench, or 2) is it your basic all-purpose cable tray installation? I lean towards the former. In defense of that, say we were to use solid-bottom (and sides, of course) cable tray, embedding it in concrete no less than 4 inches thick to the sides, bottom, and each end (under to be installed equipment), leaving the top recessed enough to put a cover on it where not covered by the to be installed equipment. Would this be a cable tray or a trench-type underfloor raceway installation? What if you were to do the same using metal "trough" of the same size that is listed for the application? Does simply being listed for the application change the rationale of implementation? I was always told the NEC is the "minimum" requirement.

Anyway, I'm getting awfully close (if not already there) to "a place" I don't want to go here on this forum...

 

[peter d]

I have worked in one of these trenches and it was quite scary. Just ask Iwire/Bob.