Derating conductors running in seperate conduits in the same trench?

[Bad269]

Hello everyone,
I am a first time poster, but have visited the forum when doing research many times. I'd like some opinions on a situation I have with an engineer.
We have a couple trenches that has conduits laying flat on top of each other. Something that I have seen on any job with underground.
The engineer has a detail on the plans calling for spacers for the conduit. Like I have used in the past for utility duct banks, usually 4".
The engineer has stated that if we dont use the spacers he will have to do calculations requiring us to up size conductors. He referred me to N-M calculations. Each conduit has a,b,c phase, neutral and a ground. I have never heard of the need to derate conductors because they asre literally running parallel in a trench. One trench has 4 2-1/2" conduits 2 on bottom 2 on top. the other has 2 2-1/2" conduits on top of 6 1" conduits. Each have the amount of wires I stated before.
I do not see the need for the spacers or derating. He is stating the wires in each conduit are going to heat up adjacent wires in conduits.
Is there a need for derating in this situation?Thank you for any help or info.

 

[jaggedben]

I've seen spacers be required by an inspector but I'm not sure what code section was being invoked, if any. I do not believe that derating conductors in separate conduits (as opposed to cables) is required by the code in this situation.

 

[Smart $]

310.15(B)(3)(b) Raceway Spacing. Spacing between raceways shall be maintained.

 

[SparkyHC]

Neher-McGrath Conductor Ampacity

Neher-McGrath Conductor Ampacity

Find Informative Annex "B"... I expect this is what the engineer is using for derating.
It's an interesting read and sometimes worth some consideration.

Just for fun, consider (1) 4" PVC conduit with 3-#500 kCM THWN conductors for a 400 ampere
service. 310.15(B)(16) gives the ampacity as 380 amperes...
Now if we bury this conduit in average soil which is 90% of the USA and see Table B.310.15(B)(2)(7)
in the first column with 1 Electrical Duct and the column under RHO 90, Load Factor 100, it gives the ampacity as
427 amperes...tell that to the inspector!

But this is only for a single conduit, as you begin to add conduits the tables reduce the conductor ampacity, sometimes
to what seems to be excessive.

We all use Table 310.15(B)(3)(a), so is it surprising that we might consider multiple conduits in a trench in a similar way
as multiple conductors in a conduit?

 

[jaggedben]

310.15(B)(3)(b) Raceway Spacing. Spacing between raceways shall be maintained.

:lol: Thanks. I totally thought I read that whole page last night and somehow I missed it.

SparkyHC raises an important point to the original question. 310.15(A)(1) allows conductors to be sized according to engineering calculations or tables. If this project was designed with the engineer's own calcs then he would be more than justified in objecting to any changes to installation specifications he gave. So I take back what I said about the code in my first post.

 

[charlie b]

I am inclined to agree with the engineer. But I’ll start by saying that this is not a matter of derating the ampacity of the conductors. Rather, it is a matter of determining what the ampacity of the conductors is, under “the conditions of use” (a phrase taken from the definition of ampacity). Table 310.15(B)(16) gives us ampacity values for which the conditions of use include “not more than three current carrying conductors in raceway, cable, or earth (direct buried).” Your installation does not match that description. Four conduits that are in contact with each other will not be able to reject heat as well as conduits that are some distance from each other. The fact that they will be surrounded by dirt will make it even harder for them to reject heat. The temperature that any given conductor will reach, by virtue of the heat that it generates, is going to be affected by the presence of other nearby conductors that are also generating heat. I do not think it unreasonable to take this into account, when determining what the ampacity of the conductors will be under these conditions of use. How far apart do the conduits need to be, before the “mutual heating effect” becomes negligible? That I cannot say, without first performing a series of calculations that I have neither the time nor the expense account to be able to do. But if the engineer for your project is willing to accept 4” spacing, I would go with that.

 

[jaggedben]

... Table 310.15(B)(16) gives us ampacity values for which the conditions of use include “not more than three current carrying conductors in raceway, cable, or earth (direct buried).” Your installation does not match that description. ...

In my opinion it does match. (Well, we don't know if there are more than 3 conductors in each conduit, but there's another table for that.) However the point I made is that the NEC allows other ways besides that table to size the conductors. What we don't know is what method was used to size the conductors, and not knowing that we have no concrete basis to criticize the engineer's position.

 

[Carultch]

310.15(B)(3)(b) Raceway Spacing. Spacing between raceways shall be maintained.

The handbook text discusses the raceway termination fittings (locknuts, bushings, hubs, etc), as commentary to this vague rule.

So would that imply that as long as the raceways are separated enough to fit respective fittings, that they are separated enough to not think about derating?

 

[iwire]

The handbook text discusses the raceway termination fittings (locknuts, bushings, hubs, etc), as commentary to this vague rule.

For exposed work as a practical mater that is correct. The conduits will not be run closer than the fittings or hangers allow.

On the other hand if you just toss a bunch of PVC in a trench together it will flex when backfilled on so spacing will not be maintained.

 

[charlie b]

310.15(B)(3)(b) Raceway Spacing. Spacing between raceways shall be maintained.

I believe that this rule is not talking about establishing any minimum separation. Rather, it is saying that whatever separation distance you have, keep it consistent throughout the run. My best guess is that it has to do with the fact that any two conductors in close proximity will have an amount of mutual inductance and mutual capacitance. I believe that the idea is to maintain these mutual values the same throughout the run, so that whatever influence they have on insulation temperatures will not vary from point to point along the run.

So would that imply that as long as the raceways are separated enough to fit respective fittings, that they are separated enough to not think about derating?

Here again, it is not about a minimum distance. And please forgive my repeating this, but we are not talking about derating. We are talking about establishing the rating, meaning the ampacity, of the conductors. Taking a value from a table is one way. The Neher-McGrath calculation is another way. There are still others. But first we need to be aware of the conditions under which the conductors will be used.

 

[Smart $]

I believe that this rule is not talking about establishing any minimum separation. Rather, it is saying that whatever separation distance you have, keep it consistent throughout the run. My best guess is that it has to do with the fact that any two conductors in close proximity will have an amount of mutual inductance and mutual capacitance. I believe that the idea is to maintain these mutual values the same throughout the run, so that whatever influence they have on insulation temperatures will not vary from point to point along the run.
Here again, it is not about a minimum distance. And please forgive my repeating this, but we are not talking about derating. We are talking about establishing the rating, meaning the ampacity, of the conductors. Taking a value from a table is one way. The Neher-McGrath calculation is another way. There are still others. But first we need to be aware of the conditions under which the conductors will be used.

Perhaps. Can you present anything more than speculation?

 

[charlie b]

Can you present anything more than speculation?

Not much. The words in the rule say that spacing must be maintained. They don't include any specific minimum distance, nor are there any words that speak to distance in any way. The only way that makes sense to me is if I interpret as saying, "if you have two inches of spacing, then keep the spacing at two inches, and if you have four inches of spacing, then keep the spacing at four inches, but whatever spacing you start with, keep that spacing throughout." Then I tried to infer the reason for such a rule, using the physical properties of the installation, and I arrived at the basis that I described above. I have no references to cite that would back up my reasoning. That is why I used such terms as "I believe" and "my best guess." I would certainly welcome alternative interpretations and explanations.

 

[Smart $]

Not much. The words in the rule say that spacing must be maintained. They don't include any specific minimum distance, nor are there any words that speak to distance in any way. The only way that makes sense to me is if I interpret as saying, "if you have two inches of spacing, then keep the spacing at two inches, and if you have four inches of spacing, then keep the spacing at four inches, but whatever spacing you start with, keep that spacing throughout." Then I tried to infer the reason for such a rule, using the physical properties of the installation, and I arrived at the basis that I described above. I have no references to cite that would back up my reasoning. That is why I used such terms as "I believe" and "my best guess." I would certainly welcome alternative interpretations and explanations.

I'm good with your spacing analysis. But I'm also good with the reason for maintaining that spacing is to achieve, retain enough environmental "volume" for adequate heat dissipation.

 

[jaggedben]

Not much. The words in the rule say that spacing must be maintained. They don't include any specific minimum distance, nor are there any words that speak to distance in any way. The only way that makes sense to me is if I interpret as saying, "if you have two inches of spacing, then keep the spacing at two inches, and if you have four inches of spacing, then keep the spacing at four inches, but whatever spacing you start with, keep that spacing throughout." Then I tried to infer the reason for such a rule, using the physical properties of the installation, and I arrived at the basis that I described above. I have no references to cite that would back up my reasoning. That is why I used such terms as "I believe" and "my best guess." I would certainly welcome alternative interpretations and explanations.

If I take it at its most literal, I would simply say it means that raceways should not be touching each other.