Engineers always detail duct bank with encased concrete. What should I detail instead?

[LifeLongLearner]

The firms I've been at always detail a duct bank with encased concrete for the service no matter the situation. The system I'm working on is 480V and has a 600A secondary service for an office building, and a 600A service for a PV system over the parking lot. I'm thinking of detailing (2) 3" Sch. 40 PVC to be buried with back fill (no concrete). Depths will be detailed per Table 300.5(A). The runs are ~100'. When would fiberglass conduit come into play?

 

[tom baker]

Are you installing service raceway per poco specs and do they allow FG?
I often see one sack concrete dyed red.FG is great for pulling but is much slower to install.
PVC with is fine but spec use mule tape for pulling conductors

 

[ron]

What do your Neher McGrath calculations show for ampacity? Or are you using Annex B for ampacities?

 

[LifeLongLearner]

Are you installing service raceway per poco specs and do they allow FG?
I often see one sack concrete dyed red.FG is great for pulling but is much slower to install.
PVC with is fine but spec use mule tape for pulling conductors

Good point. I don't see fiberglass mentioned in their specs. They only mention PVC.

 

[LifeLongLearner]

What do your Neher McGrath calculations show for ampacity? Or are you using Annex B for ampacities?

I used Table 310.16. I'm not using more than 3 current-carrying conductors in a raceway, if that's what you mean.

 

[ron]

I used Table 310.16. I'm not using more than 3 current-carrying conductors in a raceway, if that's what you mean.

Underground ductbanks are usually derated due to the thermal resistivity of soil makes the heat transfer limited, so look at the NEC Annex B

 

[charlie b]

I used Table 310.16. I'm not using more than 3 current-carrying conductors in a raceway, if that's what you mean.

There is a point of view that says you can't use table 310.16 because the conditions shown at the top of that table are not met by underground installations. Decades ago, I got a ruling from the Washington state LNI, stating that that table could be used. I vaguely recall that this ruling only applied when the the load current was calculated per NEC 220. The point was that the conservatisms built into that article offset any non-conservatisms associated with the fact that underground installations don't release heat as well as conduits run inside buildings.

 

[electrofelon]

The firms I've been at always detail a duct bank with encased concrete for the service no matter the situation.

Sorry, that sounds dumb and a waste of your clients' money.

 

[LifeLongLearner]

There is a point of view that says you can't use table 310.16 because the conditions shown at the top of that table are not met by underground installations. Decades ago, I got a ruling from the Washington state LNI, stating that that table could be used. I vaguely recall that this ruling only applied when the the load current was calculated per NEC 220. The point was that the conservatisms built into that article offset any non-conservatisms associated with the fact that underground installations don't release heat as well as conduits run inside buildings.

That's interesting. It's good to know. The references I have only use Table 310.16, mainly "Code Calculations" by the Electrical Training Alliance.

 

[LifeLongLearner]

Sorry, that sounds dumb and a waste of your clients' money.

Exactly. I'm looking to make the client happy and not provide unnecessary measures.