Is there a rule on how many pvc conduits you can stack on top of each other in a trench. The inspector says they have to be seperated because of heat. They are feeder conduits from the power co to the main switch gear.. There is about 8 stack on top of eachother in one trench. Any code sections you can help with????
pvc's in a trench
- Thread starter wire monk
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chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
Not unless they are over 2000V then see 310.60.
- Location
- Massachusetts
If you have to do it just get some conduit spacers.
http://www.carlon.com/Master Catalog/Snap-N-Stac Spacers_7F78.pdf
http://www.carlon.com/Master Catalog/Snap-N-Stac Spacers_7F78.pdf
tryinghard
Senior Member
- Location
- California
It?s best to use spacers and avoid compaction failure issues, but I don?t think heat has anything to do with spacing in premises wiring. I also don?t believe this is an NEC item but most contracts tie the back-fill and compaction responsibility to us regarding underground conduit. Notice 352.10(G) & 300.5 for installation.
- Location
- Logan, Utah
Agreed, I have used these in the past and they work pretty good. This was in the job specifications.
Chris
Fulthrotl
~Autocorrect is My Worst Enema.~
- Occupation
- E
never heard of heat being an issue in low voltage. soil compaction is
the primary issue, and standard spacing is 2" between conduits to
allow for backfill to encircle each conduit, leaving no voids.
let's say for example, that you have a bank 2 conduits deep by 8
conduits wide... the heat issue is going to be the same if you have
a bank 2 conduits wide, by 8 conduits deep, assuming the spacing
is the same.
i've seen duct banks entering underground substructures for POCO
distribution that all have MV (5kv and 34.5kv cables) without problems,
that contain over 30 conduits in arrays like 6x5....
- Location
- Illinois
- Occupation
- retired electrician
Actually there is more heat build up in the vertical stack as the heat rises from the lower ducts. The biggest reason we don't hear much about heat damage in underground duct banks is the fact that the load calculations in Article 220 are vastly inflated. There are a number of documented cases where underground duct banks have been damaged by excessive heat where they were actually loaded to the NEC ampacities of the conductors. The information in Annex B shows the reduction in actual ampacity for underground duct banks. Engineering firms often use software to calculate the required wire sizes for duct bank installations. They need to do this when the real world load is close to the conductor ampacity as shown in Article 310.
beanland
Senior Member
- Location
- Vancouver, WA
Ampacity Basis
Ampacity Basis
Remember that the 310 ampacity tables are based on a specific spacing between conduits. If they are installed closer than the basis, the ampacity tables cannot be used. If they are installed closer, the ampacity will go down. You may be in violation of loading unless you comply with the intended spacing or have engineering supervision.
Ampacity Basis
Remember that the 310 ampacity tables are based on a specific spacing between conduits. If they are installed closer than the basis, the ampacity tables cannot be used. If they are installed closer, the ampacity will go down. You may be in violation of loading unless you comply with the intended spacing or have engineering supervision.
- Location
- Massachusetts
I sure would like to see a reference on that.
beanland
Senior Member
- Location
- Vancouver, WA
MV vs LV
MV vs LV
I stand partially corrected. 310.60 defines the conduit spacing used for calculation of ampacity of MV cables. However, Table 310.16 states "based on ambient temperature of 30C". Sincde this applies to one or more conduits or groups of conductors, I would think this would be the ambient conditions for the individual conduit or conductors. If too many conductors are placed in too close of proximity, the 30C ambient will no longer apply. Adjustments could be made using correction factors but knowing the specific ambient would require engineering supervision. Thus, the design conditions on which Table 310.16 was predicated may not apply to large duct banks.
I have modeled and studies 100's of underground cable installations with multiple cables. Even conduits within 5 of 6 feet of each other will influence the "ambient" conditions. Construction of a large duct bank, whether LV or MV should be done using engineering design, not tables.
MV vs LV
I stand partially corrected. 310.60 defines the conduit spacing used for calculation of ampacity of MV cables. However, Table 310.16 states "based on ambient temperature of 30C". Sincde this applies to one or more conduits or groups of conductors, I would think this would be the ambient conditions for the individual conduit or conductors. If too many conductors are placed in too close of proximity, the 30C ambient will no longer apply. Adjustments could be made using correction factors but knowing the specific ambient would require engineering supervision. Thus, the design conditions on which Table 310.16 was predicated may not apply to large duct banks.
I have modeled and studies 100's of underground cable installations with multiple cables. Even conduits within 5 of 6 feet of each other will influence the "ambient" conditions. Construction of a large duct bank, whether LV or MV should be done using engineering design, not tables.
Heat
Heat
Yes, heat can be an issue but something the engineer should have addressed in the design phase of the project to insure that the cables will be capable of carrying the current and if any future anticpated load there is a lot of items that need to be know also, is it encased in concrete, what is the fill, if you are running under a road for a significant distance there will be additional concrete, what is the RHO of the soil / concrete / asphalt, and the depth is important too as the deeper you go the harder it is to disipate the heat, the IEEE Brown Book also has some good information on this regarding cable sizes, duct bank configuration and load factor percentages all this comes into account when designing a large underground duct bank with multiple circuits. Hope this helps.
Another question for everyone if you install MV cable in a duct bank with sched.40 pvc the pvc is only rated for 90degree so what do you utilize when installing MV cables with 105degree operating temperature and a max of 250degree for an emergency rating?What type of conduit do you install?
Heat
Yes, heat can be an issue but something the engineer should have addressed in the design phase of the project to insure that the cables will be capable of carrying the current and if any future anticpated load there is a lot of items that need to be know also, is it encased in concrete, what is the fill, if you are running under a road for a significant distance there will be additional concrete, what is the RHO of the soil / concrete / asphalt, and the depth is important too as the deeper you go the harder it is to disipate the heat, the IEEE Brown Book also has some good information on this regarding cable sizes, duct bank configuration and load factor percentages all this comes into account when designing a large underground duct bank with multiple circuits. Hope this helps.
Another question for everyone if you install MV cable in a duct bank with sched.40 pvc the pvc is only rated for 90degree so what do you utilize when installing MV cables with 105degree operating temperature and a max of 250degree for an emergency rating?What type of conduit do you install?
Fulthrotl
~Autocorrect is My Worst Enema.~
- Occupation
- E
orangberg? some of that nice asbestos conduit, soaked in creosote....
it's best installed on a nice, hot, summer afternoon, with no shade.
it' really nice.... you can breath the asbestos dust when sawing it,
and have the creosote all over your forearms as well..... life is good.
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