How to Build a Duct Bank

1. Read the section detail before you dig

The engineered drawings should give you the duct bank section: conduit count and sizes, rows and columns, spacing, concrete cover on each face, reinforcement if any, and depth. Build exactly that geometry — the spacing isn't aesthetic, it's thermal. Conductor ampacity in a duct bank is calculated from the physical arrangement (the Neher-McGrath method behind the Annex B tables), and squeezing nine ducts into the window drawn for six quietly de-rates every feeder in the bank.

If you're designing the bank yourself on a small job, keep the standard pattern: ducts on 7.5 in. centers (3 in. clear between 4 in. ducts is a common spec), 3 in. of concrete cover minimum all around, power and communications in separate banks or separated per the utility's rules. Put spares in now — a 25% spare count is cheap while the trench is open and astronomical after the pour.

Confirm the elevations at both ends first. A duct bank has to arrive at the manhole knockout, the transformer pad window, or the building entry at a specific depth, and the trench profile between them has to hold consistent slope — ideally draining toward the manhole, never bellying in the middle where water will sit.

2. Set the ducts on spacers and lock the bank down

Assemble the bank on manufactured interlocking spacers — base spacers on the trench bottom, intermediate spacers stacked per the manufacturer's spacing chart, typically every 5 to 8 ft depending on duct size. Spacers hold row-and-column geometry and guarantee concrete flows fully around every duct. Blocks of wood and rocks do neither, and voids in the encasement are thermal hot spots and water paths.

Stagger the couplings between adjacent ducts so joints don't line up in one weak plane, and solvent-weld every joint fully — a joint that weeps concrete paste into the duct will stop a mandrel cold. As you assemble, keep checking alignment: sight down the bank; a straight duct run pulls dramatically easier than one that wanders an inch per stick.

Then anchor against flotation. Empty PVC in wet concrete floats hard, and a bank that racks or rises during the pour is scrap. Tie the bank down to duckbill anchors or rebar pins driven in the trench bottom at regular intervals, and don't trust the spacers alone to resist buoyancy.

3. Pour the encasement without wrecking the geometry

Concrete encasement does three jobs: physical protection, heat transfer to surrounding earth, and a hard warning to future excavators. Pour in lifts and vibrate gently or rod the concrete so it flows under and between ducts — the void under the bottom row is the classic defect. Don't drop concrete from height onto the bank or let the chute push the assembly sideways; place it alongside and let it flow.

Where the spec calls for identification, add the red dye to the mix or use red-topped encasement — a widely used utility convention that says 'electric' to the next backhoe operator. Some specs call for rebar only at crossings and structure entries; follow the structural detail, because reinforcement changes how the bank behaves at joints.

Concrete-encased duct picks up a code break: Table 300.5(A) allows raceways in not less than 2 in. of concrete envelope at reduced burial depth compared to unencased conduit. Between encasement, cover, and the utility's own standards for their portions, run every segment against its governing rule — the utility side of a service lateral follows their spec book, not the NEC table.

4. Understand the derating — and what the crew controls

Two different heat problems get confused in the field. Table 310.15(C)(1) adjustment factors apply to more than three current-carrying conductors in a single raceway or cable — that's per-duct math, same as above grade, and it's why each feeder typically gets its own duct with only its three phases and neutral. Mutual heating between ducts in the bank is the second problem, and it's handled by engineering calculation — the Neher-McGrath method and the Annex B ampacity tables for underground electrical ducts — not by a simple table lookup in the field.

What the crew controls: the geometry the engineer assumed. Spacing per the detail, encasement full and void-free, depth per the profile, and the load arrangement per the schedule — heavily loaded feeders to the outside and corner positions where the drawings say so, because center ducts run hottest. Build it as drawn and the ampacity math holds; improvise and it doesn't.

If you're asked to add circuits to spare ducts years later, flag the ampacity question to the engineer instead of just pulling wire. A bank calculated for six loaded ducts doesn't automatically support nine — the added heat de-rates its neighbors.

5. Build the transitions: risers, manholes, and building entries

Every transition out of the bank is a detail worth doing slowly. Risers turn up in long-radius sweeps — steel elbows under equipment pads where the pull tension or spec demands — arriving inside the pad window or floor opening at the drawn coordinates. Set them off the equipment template, plumb, braced so the pour can't move them, and double-check against the actual equipment submittal, not just the pad drawing.

At manholes and vaults, enter through the knockout with bell ends or terminators flush with the inside wall face, grouted smooth so pulling ropes don't saw on a sharp edge. Slope duct runs to drain toward structures where the design allows, and leave the specified clearances between your bank entry and the utility's — their inspectors check their spec book, and TEP's requirements for their portions are non-negotiable.

At building entries, seal the ducts against water and gas migration — conduit is a pipe, and an unsealed duct bank will happily deliver storm water into your main electrical room. Duct sealant or mechanical plugs around the conductors after the pull, and spare ducts plugged hard.

6. Proof, string, and record before turnover

After the pour cures, mandrel every duct — a mandrel close to duct ID proves round, aligned, and clear; follow with a swab to pull out water and grit. Do it now, while the crew and equipment are on site. A failed mandrel after paving is a directional-bore repair; before, it's a dig and re-glue.

Blow mule tape into every duct, spares included, and plug every end. Label both ends of every duct with its destination — duct 3 of 9 at the manhole should say what it is at the switchgear too. Your labeling scheme should match the drawings' duct numbering so the pull crew and the as-built agree.

Record the as-built before backfill and again after: bank alignment ties to permanent structures, depths, duct assignments, spare locations, photos with scale. Duct banks outlive the people who build them; the record is the only memory that survives.

Does a duct bank have to be concrete-encased?

Not always — direct-buried duct arrangements on spacers with select backfill are common where the spec allows. Encasement buys physical protection, better heat dissipation, and shallower permitted cover per Table 300.5(A). Service laterals and anything under traffic usually get concrete; follow the drawings and the utility standard.

Why does duct spacing affect ampacity?

Each loaded feeder is a heat source warming its neighbors through the concrete and soil. The engineer's ampacity calculation (Neher-McGrath / Annex B) assumes the drawn geometry — spacing, depth, soil thermal resistivity, load arrangement. Change the geometry and the assumed ampacity no longer holds.

What is the red concrete about?

Red-dyed encasement is a utility-world convention marking electrical duct — a visual warning to excavators before their bucket reaches the ducts. Many municipal and utility specs require it; the NEC doesn't, but it's cheap and it works.

How many spare ducts should a bank carry?

Common practice is around 25% spares, minimum one per bank. Spares cost trench space and a few sticks of PVC now; adding capacity to a buried, paved-over bank later costs a directional bore or a second trench. Owners never regret spares.

Can communications share the power duct bank?

Not in the same ducts, and generally not in the same bank without the separation the codes and serving utilities require. Standard practice is a separate comms bank or a defined separation in a joint trench — check the low-voltage spec and the utility rules before layout.

What size mandrel do I pull?

Per the spec — commonly about 1/4 in. under duct ID and a duct-diameter long. It should pass with hand tension. If the spec is silent, the utility standard for that duct size is a good default; the point is proving a full-size pull head and conductors will make it.