Direct Buried Duct Banks - without Spacers

[Shujinko]

We are designing a direct buried duct bank 2x3 with 3" PVC conduits. This is for a utility transformer secondary service lateral to a chiller plant. The owner/contractor doesn't want to install spacers in the duct bank in order to save money on labor. Are there any code issues or installation issues with this? I usually see the spacers used for this application.

 

[charlie b]

Without maintaining spacing, you will have no way of knowing what the ampacity of the cable will be. The definition of ampacity includes "under the conditions of use." Spacing is a relevant condition of use. But to give you something to show your client, let me invite your attention to 310.15(B)(3)(b).

 

[xformer]

Without maintaining spacing, you will have no way of knowing what the ampacity of the cable will be. The definition of ampacity includes "under the conditions of use." Spacing is a relevant condition of use. But to give you something to show your client, let me invite your attention to 310.15(B)(3)(b).

Couldn't this be a phased conduit installation?

 

[tkb]

I would only use spacers for a concrete encased duct bank.
I have seen the conduits installed with spacers and then it just gets buried without concrete.
This causes spacers to collapse and makes the conduit like a roller coaster between the spacers that don’t collapse.

The way to install the conduit without spacers is to install the conduits in lifts during the backfilling. This is more labor.

If it is concrete encased, then I would use the spacers.

 

[charlie b]

I understand your point, tkb. But what do you think about 310.15(B)(3)(b)? It seems to me that you don't have that option.

 

[tkb]

I understand your point, tkb. But what do you think about 310.15(B)(3)(b)? It seems to me that you don't have that option.

I wasn’t clear, I would lay the first layer of conduits with space between them and bury with a few inches of sand, then add the next layer. The sand will provide the spacing.

This also helps with the soil compaction, you can’t compact the soil if the backfill doesn’t get between all the conduits when using the spacers.

Spacers should only be used where you are encasing in concrete or flowable fill.

https://www.nrmca.org/aboutconcrete/cips/17p.pdf

 

[Cow]

We are designing a direct buried duct bank 2x3 with 3" PVC conduits. This is for a utility transformer secondary service lateral to a chiller plant. The owner/contractor doesn't want to install spacers in the duct bank in order to save money on labor. Are there any code issues or installation issues with this? I usually see the spacers used for this application.

Can you go six wide instead of 2x3?

The spacers are a necessity when you stack them, as is flowable fill or CDF to get the compaction you need around and between the conduits. If they don't like the cost of spacers, they're really not going to like the added cost of flowable fill....

Again, I'd go six wide instead and use native backfill.

 

[Shujinko]

Can you go six wide instead of 2x3?

The spacers are a necessity when you stack them, as is flowable fill or CDF to get the compaction you need around and between the conduits. If they don't like the cost of spacers, they're really not going to like the added cost of flowable fill....

Again, I'd go six wide instead and use native backfill.

We have to stack the conduits. We are running these conduits between a utility transformer and building foundation footer so we have limited space horizontally and can't go six wide.