Duct Banks & Conduit

Concrete-encased duct banks, multi-way conduit, and the vaults and pull boxes that make up the underground pathway for primary distribution and communications.

This work is performed under our Arizona ROC KA Dual Engineering license (#312437). See the full A– General Engineering scope it carries, including A-17 Electrical and Transmission Lines for right-of-way and utility-side work.

What We Provide

Duct Banks & Conduit — in detail

Concrete-encased duct banks, multi-way conduit runs, manholes, and vaults — here is how we build the pathways that primary distribution rides on.

Duct Banks & Conduit

  • Concrete-encased duct banks — For primary feeders and critical runs we build concrete-encased duct banks — racked, spaced, and tied conduit encased in concrete to protect and identify the run.
  • Multi-way conduit runs — We install single and multi-way conduit in PVC, fiberglass, or steel, sized and arranged for the conductors plus spare capacity for the future.
  • Mandrel and proofing — Every duct is mandreled and proofed so conductors pull clean — no surprises when it is time to wire.

Vaults, Manholes & Pads

  • Pull boxes and hand holes — We set the pull boxes and hand holes that let conductors be pulled and spliced along a long run.
  • Manholes and vaults — We set precast manholes and vaults for primary distribution, with the racking, grounding, and sumps they require.
  • Transformer and switchgear pads — We form and pour the pads and stub up the conduit for pad-mounted transformers, switchgear, and equipment.

Coordination & Records

  • Stub-ups and terminations — We stub conduit into buildings, gear, and equipment at the right location and elevation, ready for the electrical crews to land.
  • Utility and trade coordination — We coordinate duct-bank routing with the utility, wet utilities, and other trades so crossings and code separations are right the first time.
  • As-built documentation — We record the installed routing and depths so the owner knows exactly what is in the ground.

Duct Bank 101

What a duct bank is — and when you need one

The term shows up on every campus, industrial, and utility project. Here is what the concrete is actually doing for you — and when direct burial simply is not enough.

A duct bank is underground raceway built as a structure. Multiple conduits — PVC, fiberglass, or steel — are racked on interlocking spacers at fixed separations, tied down against flotation, and on most primary and critical runs encased in structural concrete. The result behaves like a single buried structure with many bores through it: conductors are pulled through the ducts from vault to vault, and they can be pulled back out and replaced decades later without a shovel ever touching dirt.

Plenty of site circuits are fine direct-buried, and we install miles of them. But a duct bank earns its concrete in specific situations: crossings under roadways, drive aisles, and loading areas, where a future repair would otherwise mean sawcutting pavement and flagging traffic; corridors shared with water, sewer, and gas, where crossings and separations from other underground utilities have to be held exactly; runs carrying multiple feeders, where one engineered trench beats a half-dozen parallel ones; and medium-voltage primary, where the serving utility's standards call for encased duct, period. Encasement also buys protection the NEC recognizes directly — Table 300.5 permits reduced cover for concrete-encased raceways, and the underground rules for circuits over 1,000 volts lean on encasement the same way.

There is also a reason duct banks are engineered rather than sketched: heat. Every energized conductor warms the ducts around it, and the soil can only carry that heat away so fast. The NEC's underground ampacity figures for duct banks (the NEC 310.60 configurations) cut each circuit's allowable load as the bank gets bigger and deeper — which is why spacing, arrangement, burial depth, and soil thermal resistivity all feed the Neher-McGrath calculation behind a properly sized bank. Our in-house engineering runs that math during design; in a climate where 110°-plus summers keep the earth itself warm, guessing is how feeders end up derated into uselessness or replaced early.

Building duct banks in the Tucson basin adds desert-specific problems we plan for on day one. Caliche — the cemented calcium-carbonate layer under much of Pima County — excavates more like weak concrete than soil, and it is the single biggest schedule variable on an underground job; our owned Cat fleet and trenching crews are equipped for it rather than surprised by it. Monsoon season stacks the deck further: an open trench and a July storm cell are a bad combination, so we sequence excavation, pour, and backfill tightly and keep earthwork dust control compliant with Pima County PDEQ requirements through the dry months. And because most of our work sits in Tucson Electric Power or Trico territory, primary duct goes in to the serving utility's construction standards from the first stick of conduit — with Arizona 811 locates called in before any bucket touches the ground.

How We Build It

From open trench to proofed raceway

A duct bank is only as good as its weakest coupling, and every step in the sequence exists to protect the cable pull at the end of it. Here is the order of operations our crews run.

1. Survey, locate & trench

The run is staked from gear pad to gear pad, existing utilities are located and potholed, and the trench is cut to line and grade — benched, sloped, or shielded per OSHA 29 CFR 1926 Subpart P. Depth is set by code cover requirements, utility standards, and the crossings the route has to duck under, not by what the excavator felt like digging.

2. Spacers, rebar & conduit

Interlocking base and intermediate spacers hold every duct at its designed separation — the same spacing the ampacity calculation assumed. Reinforcing steel goes in where the structural design calls for it, typically at roadway crossings and where the bank bridges other utilities. Conduit is racked with long-radius sweeps, joints staggered and solvent-welded, and the whole assembly is anchored so it cannot float when the concrete arrives.

3. Concrete encasement

The encasement pour is controlled so the ducts stay put and the specified cover is held on all sides. Where the drawings call for it, the mix is dyed red — the universal warning that the next excavator has found electrical duct, not a footing. Stub-ups and bell ends are formed at vaults and equipment pads, and the bank cures before backfill goes over it.

4. Proof, string & record

After backfill, every raceway — spares included — is cleaned, swabbed, and proofed with a mandrel to verify it is round, unobstructed, and pullable end to end. A pull string goes into each way, and the as-builts record routing and depths so the owner knows exactly what is in the ground and where.

Manholes, vaults & pulling tension

Manhole and vault locations are physics, not preference. Cable pulls are limited by allowable pulling tension and by sidewall pressure at every bend, so pulling points go where the route changes direction and at intervals that keep tension on long runs inside the cable manufacturer's limits — on straight primary runs, that typically means a structure every few hundred feet. We set precast manholes and vaults with the racking, grounding, sumps, and traffic-rated covers each location requires, and we lay the duct out so no segment asks a cable to do the impossible.

Primary duct in TEP & Trico territory

On utility primary, the serving utility writes the rules: conduit type and size, sweep radius, separation, burial depth, encasement, and vault and pad placement all come from the utility's construction standards, and the utility inspects the trench and duct before it is backfilled or energized. We build to those standards from the start and fold the inspections into the schedule — our TEP standards hub collects the references we work from. The same discipline applies in the public right-of-way: the streetscape conduit behind our Historic Fourth Avenue kiosk project went in under one of Tucson's busiest corridors, where there are no second chances at the pathway.

Where It Applies

Markets we serve

Duct bank work takes our Tucson crews to campuses, industrial plants, and municipal and utility projects across Arizona.

  • Primary distribution
  • Campus and site power
  • Communications backbone
  • Utility coordination
  • Industrial and commercial

The Design-Build Advantage

Designed, self-performed, and accountable

A duct bank is excavation, conduit, concrete, and survey tolerance all at once. Self-performing all of it means the bank lines up with the gear pads — and the cable pull goes smoothly the first time.

Back to Underground
  • Encased, proofed, and documented duct banks
  • Conduit installed by the same firm that pulls the wire
  • Coordinated crossings and code separations
  • Owned crews keep the underground on schedule

FAQ

Duct bank questions, answered

Scoping underground work? Size your raceways with our free conduit fill calculator, or put a code question to our NEC code assistant.

What is a duct bank, and when is it better than direct burial?

A duct bank is a group of conduits installed in a common trench — racked on spacers at set separations and, on most primary and critical runs, encased in concrete — so feeders and communications share one protected pathway. Direct-buried cable is cheaper on day one, but it cannot be replaced without excavating, and it leaves no room for growth. A duct bank protects the conductors, lets them be pulled out and replaced from the surface, and puts spare ways in the ground for circuits that do not exist yet. For primary distribution, roadway crossings, and any feeder an owner cannot afford to dig up twice, the duct bank wins.

When is concrete encasement required?

Typically where ducts cross under roadways, drive aisles, and other traffic loading, where the serving utility's construction standards require it on primary runs, and where the design takes advantage of the reduced burial depths the NEC allows for concrete-encased raceways. Many engineers also specify encasement purely for protection and identification — a mass of red-dyed concrete is an unmistakable warning to the next excavator. Where encasement is not required, we still install conduit with proper bedding, cover, and warning tape.

Why does the number of conduits in a duct bank matter?

Heat. Every energized conductor warms its neighbors, and the NEC's underground ampacity provisions for duct banks (the NEC 310.60 configurations) reduce each circuit's allowable load as the bank grows and deepens. That is why duct banks are engineered — spacing, arrangement, burial depth, and soil thermal resistivity all feed the Neher-McGrath calculation behind a properly sized bank, and in Tucson's hot desert soils that math is less forgiving than the tables assume.

Does TEP or Trico require duct banks for primary service?

In most cases the customer's contractor installs the trench, primary conduit, and equipment pads to the utility's construction standards — conduit type, sweep radius, separation, depth, encasement, and vault placement are all specified — and the utility inspects the work before it is backfilled or energized. We build to Tucson Electric Power and Trico standards from the first stick of conduit and sequence the utility inspections into the schedule, so the project is never waiting on a re-dig.

What is mandrel testing, and do you proof every duct?

A mandrel is a sized cylinder pulled through a finished raceway to prove it is round, clean, and free of debris, ridges, or crushed sections before anyone commits a cable to it. We mandrel and proof every raceway we install — spares included — and leave a pull string in each way, so the ducts are ready the day the conductors show up, whether that is next month or in ten years.

How much does a duct bank cost?

It is driven by the dirt and the concrete: trench length and depth, the number of ways in the bank, soil conditions — Tucson caliche and rock change production rates fast — traffic control and pavement replacement, dewatering, and how many manholes or vaults the run needs. Rather than quote a number that ignores your site, send the one-line and the routing through our request-a-bid form and we will come back with a duct bank route, a section, and a real price.

More Underground

Related capabilities

Explore the rest of our underground scope, or see the full underground overview.

Have a project that needs duct banks and conduit?

Send the one-line and the site plan. We'll come back with a duct bank route, a section, and a schedule.

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