pvc conduit separation

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jjavier

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Are there any restrictions in the separation of pvc conduits inside the trenches, if ,for example, with ampcal software we calculate that we have enough separation with 3 inches between pipes?
 
I've done several commerial jobs with trenches full of pvc pipes just stacked several layers deep (i.e. no spacing). They always passed inspection.

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tkb said:
So, how much space would be required?
0.001" is a space.
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So get some 0.001 material and stick it in between. :p

In all seriousness, my point was simply that I worked on a job where the inspector dinged us for stacking conduits on top of one another like Coppersmith described. So if it's never happened to you then you've just gotten lucky that the inspector doesn't know of that code section or chooses not to enforce it.
 
jaggedben said:
In my opinion you've gotten lucky. See 310.15(B)(3)(b).
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All it says is "spacing between conduit, tubing, or raceways shall be maintained". I don't see where it says how much space.

One of the jobs I did was for a roller coaster. One trench from the control building had 40 4" PVC conduits stacked 10 wide and 4 high. (Not my company's job. I was working for another contractor at the time.)
 
Coppersmith said:
All it says is "spacing between conduit, tubing, or raceways shall be maintained". I don't see where it says how much space.
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Sure, but if they are stacked one right on top of the other then you don't have any spacing. I am neither defending it as a particularly sensible rule, nor am I saying that every inspector will raise this issue. I'm just saying you've been luckier than me if you done that many jobs and no one has ever brought it up, since the language is there in the code.
 
jaggedben said:
Sure, but if they are stacked one right on top of the other then you don't have any spacing. I am neither defending it as a particularly sensible rule, nor am I saying that every inspector will raise this issue. I'm just saying you've been luckier than me if you done that many jobs and no one has ever brought it up, since the language is there in the code.
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Well in a sense, you still do, because it is physically impossible for any two objects to truly touch. But that's splitting hairs on what "touching" actually means.

I think the intent of this rule, is so that no conduit in the trench is structurally loaded by another conduit. You maintain enough space that you can see with the naked eye between each conduit, and you are all set to meeting this rule. Either install plastic supports or backfill a layer of sand for the next layer vertically.
 
Carultch said:
Well in a sense, you still do, because it is physically impossible for any two objects to truly touch. But that's splitting hairs on what "touching" actually means.

I think the intent of this rule, is so that no conduit in the trench is structurally loaded by another conduit. You maintain enough space that you can see with the naked eye between each conduit, and you are all set to meeting this rule. Either install plastic supports or backfill a layer of sand for the next layer vertically.
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Given that the rule is in a section on adjusting for temperature, I assumed the intent is to prevent transmission of heat from one raceway to another, or build-up of heat if adjacent raceways are both heating. How this section exactly achieves preventing that is a bit beyond me though.
 
When we have small ditches and a lot of conduits, we'll more often than not cut 12" pieces of 1/2" emt and drive them between the conduits every 10' to maintain separation. For duct banks, we typically use T&B chairs for spacing.
 
jaggedben said:
Given that the rule is in a section on adjusting for temperature, I assumed the intent is to prevent transmission of heat from one raceway to another, or build-up of heat if adjacent raceways are both heating. How this section exactly achieves preventing that is a bit beyond me though.
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If that is the case, I would expect some standard for how to know how much that spacing should have to be. Either in inches, or based on the conduit size or OD.
 
Cow said:
When we have small ditches and a lot of conduits, we'll more often than not cut 12" pieces of 1/2" emt and drive them between the conduits every 10' to maintain separation. For duct banks, we typically use T&B chairs for spacing.
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See, I find this ironic. If the purpose of the rule is to prevent direct heat transfer between raceways then it seems to me that using metal spacers with a low specific heat is going to go a long way towards defeating that purpose. :roll: Of course the rule applies equally to multiple metal raceways hung on same piece of metal strut. So whatever. Use metal if you like, it's not against the rule. Again, I'm not defending this rule, just pointing out that it's there.
 
jaggedben said:
See, I find this ironic. If the purpose of the rule is to prevent direct heat transfer between raceways then it seems to me that using metal spacers with a low specific heat is going to go a long way towards defeating that purpose. :roll: Of course the rule applies equally to multiple metal raceways hung on same piece of metal strut. So whatever. Use metal if you like, it's not against the rule. Again, I'm not defending this rule, just pointing out that it's there.
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The spacers aren't continuous, and probably occupy only 5% of the length. 5% = 6 inches per stick, which would be 3" long spacers at two per stick. The majority of the length is spaced by backfill, which would insulate it. Even if metal, the heat transfer path through the metal spacers is not a controlling conductance.
 
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