Underground Electrical Duct Banks
Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
Showing changes from Rev 1
to Rev 2
in Underground Electrical Duct Banks.
---
title: Underground Electrical Duct Banks
…20 unchanged lines
## Where a referenced standard and the NEC conflict, the more stringent requirement shall govern unless the Engineer directs otherwise in writing.
−## Where the serving utility has its own duct bank standard for the service-entrance pathway, the utility standard shall govern that portion of the work. {note}
+## Where the serving utility has its own duct bank standard for the service-entrance pathway, the utility standard shall govern that portion of the work.
# Referenced Standards {toc}
…106 unchanged lines
### Average soil across most of the United States has a thermal resistivity near 90 °C-cm/W, damp coastal soil near 60, and dry rocky or sandy soil near 120; the duct bank heating calculation is only as good as the rho it assumes. {note}
+### The Engineer shall establish the design soil thermal resistivity from the geotechnical investigation for any duct bank whose feeder ampacity depends on the burial configuration.
+
```datasheet
label: Design Soil Thermal Resistivity (rho)
…8 unchanged lines
```
−### The Engineer shall establish the design soil thermal resistivity from the geotechnical investigation for any duct bank whose feeder ampacity depends on the burial configuration.
−
### Where the native soil thermal resistivity is high or the duct bank is heavily loaded, the Engineer shall evaluate a thermal (controlled-resistivity) backfill or a corrective fill envelope to limit the cable operating temperature.
…32 unchanged lines
### A direct-buried conduit run is faster and less costly and is suitable for secondary feeders, site lighting, and communications where the additional protection and the fixed thermal geometry of concrete are not required. {note}
+### Power feeder duct banks under roadways and in traffic-bearing areas shall be concrete-encased.
+
```datasheet
label: Duct Bank Construction Type
…6 unchanged lines
```
−### Power feeder duct banks under roadways and in traffic-bearing areas shall be concrete-encased.
−
### Where a duct bank crosses beneath a roadway, railroad, or other traffic-bearing surface, it shall be concrete-encased through the crossing regardless of the construction type used elsewhere on the run.
…38 unchanged lines
### A spare allocation of one in four ducts (25 percent), and not fewer than one spare duct, is a common minimum for site feeder duct banks. {note}
+### The duct bank shall include spare ducts in the percentage selected, distributed across the configuration so that spares are available in each functional group.
+
```datasheet
label: Spare Duct Allocation
…6 unchanged lines
```
−### The duct bank shall include spare ducts in the percentage selected, distributed across the configuration so that spares are available in each functional group.
−
### Each spare duct shall be installed complete, proved with a mandrel, fitted with a pull rope, and capped at both ends.
…3 unchanged lines
## Rigid PVC and the heavier-wall PVC utility ducts are the standard duct bank raceway because they are corrosion-proof, nonconductive, and economical; metal conduit is used at specific points where mechanical strength or grounding continuity is required. {note}
−## The conduit type shall be selected for the construction type: encased-burial duct for concrete-encased banks, and Schedule 40, Schedule 80, or Type A duct for direct burial. {note}
+## The conduit type shall be selected for the construction type: encased-burial duct for concrete-encased banks, and Schedule 40, Schedule 80, or Type A duct for direct burial.
## PVC Duct {toc}
…2 unchanged lines
### For direct-buried runs, Schedule 40 PVC is the standard wall, Type A duct is used where its thicker wall suits direct burial, and Schedule 80 is used where the conduit is subject to greater load or emerges from the ground. {note}
+### The PVC duct type used within the concrete-encased duct bank shall be specified, selected for the encasement and loading conditions of the run.
+
```datasheet
label: PVC Duct Type — Concrete-Encased Bank
…5 unchanged lines
```
+### The PVC duct type used for the direct-buried run shall be specified, selected to suit the burial depth and load exposure of the run.
+
```datasheet
label: PVC Duct Type — Direct-Buried Run
…66 unchanged lines
### Coloring the encasement concrete red marks it unmistakably as an electrical duct bank to any future excavator, reducing the risk of a dig-in; the red layer is the buried equivalent of warning tape. {note}
+### Where red pigmented concrete is required, a red coloring admixture shall be added to the concrete at the dosage required to produce a distinct and permanent red color.
+
```datasheet
label: Red Concrete Pigment
…6 unchanged lines
```
−### Where red pigmented concrete is required, a red coloring admixture shall be added to the concrete at the dosage required to produce a distinct and permanent red color.
−
### Where the entire envelope is not pigmented, the top portion of the encasement over the conduits shall be placed as red concrete.
…3 unchanged lines
### To limit induction heating from heavy circuits, reinforcing steel is placed only in the outer concrete envelope and is not run between the ducts. {note}
+### Reinforcing bars shall conform to ASTM A615 and shall be detailed and placed in accordance with ACI 318 and the structural drawings.
+
+### Where reinforcement is provided, the reinforcing steel shall be placed in the outer concrete envelope only and shall not be run between the ducts.
+
+### Reinforced duct bank sections shall be provided where the duct bank crosses over another utility, spans soft or disturbed soil, or supports surface load, as detailed by the Structural Engineer.
+
```datasheet
label: Duct Bank Reinforcement
…6 unchanged lines
```
−### Reinforcing bars shall conform to ASTM A615 and shall be detailed and placed in accordance with ACI 318 and the structural drawings.
−
−### Where reinforcement is provided, the reinforcing steel shall be placed in the outer concrete envelope only and shall not be run between the ducts.
−
−### Reinforced duct bank sections shall be provided where the duct bank crosses over another utility, spans soft or disturbed soil, or supports surface load, as detailed by the Structural Engineer.
−
# Cover, Separation, and Routing {toc}
…60 unchanged lines
# Manholes, Handholes, and Building Entries {toc}
−## Manholes and handholes provide the access points where cables are pulled, spliced, and racked, and where the duct bank changes direction; building entries are where the duct bank transitions into the structure and must be sealed against water and gas. {note}
+## Manholes and handholes provide the access points where cables are pulled, spliced, and racked, and where the duct bank changes direction; building entries are where the duct bank transitions into the structure and must be sealed against water and gas.
## Manholes and Handholes {toc}
…79 unchanged lines
### Buried warning tape gives an excavator a visible warning above the duct bank before the dig reaches it, and detectable tape allows the duct bank to be located from the surface. {note}
+### A continuous warning tape marked for buried electric line shall be installed in the backfill above the duct bank at the depth required by NEC 300.5(D) and the contract drawings, generally about 12 in. above the duct bank.
+
```datasheet
label: Underground Warning Tape
…6 unchanged lines
```
−### A continuous warning tape marked for buried electric line shall be installed in the backfill above the duct bank at the depth required by NEC 300.5(D) and the contract drawings, generally about 12 in. above the duct bank.
−
### Where the duct bank is to be locatable from the surface, the warning tape shall be the detectable (metallized) type, or a separate tracer means shall be provided.
…8 unchanged lines
# Testing and Proofing {toc}
−## Every duct, occupied and spare, shall be proved clear and round before any cable is pulled and before the duct bank is accepted, because a duct that fails after cables are installed cannot be corrected without removing the cables. {note}
+## Every duct, occupied and spare, shall be proved clear and round before any cable is pulled and before the duct bank is accepted, because a duct that fails after cables are installed cannot be corrected without removing the cables.
## Mandrel and Proofing {toc}
…42 unchanged lines
default: "1 year from substantial completion"
```