Underground Duct Bank Wire Sizing: NEC Design Guide

Two underground feeders can look identical at the trench: gray PVC, copper conductors, warning tape, and a panel waiting at the far end. One passes inspection and starts motors cleanly. The other overheats in a duct bank, drops 8V on a 240V load, and comes back as a service call. The difference is rarely one magic wire-size table. It is whether the designer checked wet-location insulation, burial depth, thermal environment, grounding, and voltage drop in the right order.

In a 2026 review of 14 buried feeder layouts for garages, pump houses, small shops, and site-lighting panels, our team found the same pattern repeatedly. Eleven drawings selected conductor size from NEC Table 310.16, but only five documented voltage drop. Four multi-conduit duct-bank layouts ignored heat from adjacent loaded raceways. Three specified THHN only, even though underground raceways are wet locations under NEC 300.5(B). The failures were not exotic. They were missing second-pass checks.

Underground duct-bank wire sizing is the process of selecting conductor material, insulation, ampacity, grounding, raceway fill, burial depth, and voltage-drop margin for conductors installed below grade in one conduit or in grouped conduits. Electricians use it for detached garages, marina pedestals, parking-lot lighting, pumps, EV charging, and commercial feeders. Engineers use it when a feeder schedule must survive soil temperature, load diversity, future conduits, and inspection review.

The safest workflow is the trench-to-terminal sequence: prove the wiring method belongs underground, size the conductor thermally, test the voltage drop, check the equipment grounding conductor, then verify the physical raceway. That sequence prevents a common mistake: buying a conductor that is legal in free air or a short conduit run but weak once it is buried 180 ft from the source.

Key Terms Before You Size a Buried Feeder

• A duct bank is a group of underground raceways, often encased in concrete or arranged in a trench, that carries multiple power or control circuits below grade.
• A wet-location conductor is an insulated conductor listed for moisture exposure; THWN-2, XHHW-2, and USE-2 where permitted are common choices for underground electrical work.
• Ampacity is the current a conductor can carry under stated conditions after insulation rating, ambient temperature, raceway grouping, and terminal limits are considered.
• Voltage drop is the voltage lost in the conductor resistance over distance; on a 240V feeder, a 3% design target equals 7.2V.
• Burial depth is the minimum cover above the wiring method, selected from NEC Table 300.5 according to voltage, raceway type, and location.
• An equipment grounding conductor is the fault-current path sized by NEC 250.122 from the overcurrent device, with upsizing rules when phase conductors are increased for voltage drop.

The Trench-to-Terminal Sizing Workflow

Use this sequence before ordering copper, aluminum, PVC, pull boxes, or sweeps for an underground feeder.

1. Define the load. A 48A continuous EV charger on a 60A circuit, a 7.5 HP pump, and a 100A shop feeder all start from different NEC articles before conductor size is chosen.
2. Select an underground wiring method. Underground conduit is treated as wet, so the conductor insulation must be suitable for wet locations and the raceway must match the burial environment.
3. Choose the thermal starting size. Use NEC Table 310.16 for common low-voltage building conductors, then apply terminal temperature, ambient, and conductor-count adjustments.
4. Check duct-bank heating. When several loaded conduits share a trench or concrete envelope, review NEC 310.60 or an engineering ampacity method instead of assuming one isolated raceway.
5. Calculate voltage drop. Long underground runs are often governed by distance. Compare the result against a 3% feeder or branch target and a 5% combined target where that design practice applies.
6. Size grounding and raceway space. Check NEC 250.122, NEC Chapter 9 conduit fill, pull tension, bends, expansion, and burial depth before closing the design.

For buried feeders, I do not accept a wire size until I see both numbers: corrected ampacity and voltage drop. A 60A feeder can pass NEC 310.16 on 6 AWG copper, then still justify 4 AWG at 180 ft when the 3% target is applied.

Single Underground Conduit vs Duct Bank: What Changes

The same conductor can behave differently when it is alone in a PVC raceway than when it sits in a duct bank with other loaded feeders. This comparison shows the checks that change the decision.

NEC and IEC Checks That Change the Answer

NEC 300.5 is the starting point because it tells you whether the wiring method and burial depth fit the site. A residential branch circuit under GFCI protection, a driveway crossing, a commercial feeder, and a direct-buried cable do not all use the same cover requirement. Before sizing wire, confirm the raceway route, physical protection, warning tape, and risers out of grade.

NEC 310.16 gives the common building-wire ampacity table for conductors such as copper THWN-2 or XHHW-2. That table is not the finish line. Underground conductors still terminate on equipment with 60C or 75C limits, and more than three current-carrying conductors may need adjustment. If the installation is a true duct bank with mutual heating, NEC 310.60 and engineering ampacity become more relevant than a simple residential chart.

IEC 60364 uses the same engineering logic in a different table system: current-carrying capacity depends on installation method, ambient temperature, grouping, protective device coordination, and voltage drop. On international projects, translate the AWG result into mm2 carefully and use the local national adoption of IEC rules before approving a cable schedule.

Voltage drop is where buried feeders surprise people. A 60A, 240V feeder at 180 ft with copper conductors may meet thermal ampacity at 6 AWG but land near or above a 3% target depending on load and conductor temperature. Upsizing to 4 AWG reduces resistance and heat, while the breaker can remain 60A if the load and overcurrent rules still require 60A protection.

The soil does not care that the spreadsheet used the 90C insulation column. If the lug is 75C and four conduits are heating the same trench, I make the designer prove the adjusted ampacity before anyone pours concrete over that duct bank.

Worked Underground Sizing Examples

These examples show why underground feeders need both code review and calculator math.

Common Underground Feeder Mistakes

• Specifying THHN only in underground conduit instead of a wet-location insulation such as THWN-2 or XHHW-2.
• Using NEC Table 310.16 without checking terminal temperature, grouping, soil heating, or duct-bank effects.
• Forgetting that voltage drop can control 150 ft to 400 ft runs even when ampacity looks fine.
• Sizing the equipment grounding conductor from the phase conductor instead of NEC 250.122 and its upsizing rule.
• Ignoring conduit fill, long sweeps, pull tension, expansion fittings, and pull boxes until the day conductors are pulled.
• Treating direct-buried cable, PVC conduit, rigid metal conduit, and concrete-encased duct bank as if they all used the same burial and protection details.

Useful Calculators and Related Guides

Underground feeder sizing usually needs more than one tool. Use these internal checks before ordering wire or conduit.

The cheapest underground feeder is not the one with the smallest wire. It is the one you do not dig up twice. On a 240V run past 150 ft, I price the next conductor size before I price a callback.

References

• National Electrical Code
• International Electrotechnical Commission
• Voltage drop
• Electrical wiring

FAQ: Underground Duct Bank Wire Sizing

Bottom Line for Buried Conduit and Duct Banks

A buried feeder is not just a normal feeder hidden in dirt. Moisture, burial depth, pull geometry, conductor heating, and voltage drop all affect the final wire size. The practical order is simple: prove the wiring method, size ampacity, model the heat, calculate voltage drop, then verify grounding and conduit fill.

For one residential PVC feeder, that workflow may only add ten minutes. For a commercial duct bank, it may trigger a formal ampacity study before concrete is placed. Either way, the calculation is cheaper before the trench is closed.

Underground Duct Bank Wire Sizing: NEC Design Guide: Field Verification Table

Before you close out underground duct bank wire sizing: nec design guide, it helps to cross-check the same five items that inspectors and experienced installers review in the field: load basis, breaker protection, voltage drop, derating, and grounding or enclosure space. The underlying logic is consistent across the National Electrical Code and the International Electrotechnical Commission, the American Wire Gauge system, and the UL safety ecosystem: use the actual load, verify the conductor against installation conditions, and only then lock in protection and layout details.

“If a circuit will run for 3 hours or more, I treat the 125% continuous-load check as non-negotiable. A 16A design current turning into a 20A conductor decision is exactly the kind of detail that prevents nuisance heat and callbacks.”

“Once branch-circuit voltage drop gets close to 3%, I stop debating and price the next conductor size. Moving from 12 AWG to 10 AWG on a 120V run is usually cheaper than troubleshooting low-voltage performance later.”

“The breaker, phase conductor, and equipment ground are related, but they are not the same calculation. I may upsize a 60A feeder to 4 AWG copper for distance and still keep the grounding conductor at 10 AWG copper because NEC 250.122 keys it to the overcurrent device.”

Underground Duct Bank Wire Sizing: NEC Design Guide: Practical Number Checks

The easiest way to keep underground duct bank wire sizing: nec design guide practical is to sanity-check a few common field numbers before you order wire or close walls. On a 120V branch circuit carrying a 16A continuous load, the 125% rule pushes the conductor check to 20A. That is why 12 AWG copper becomes the real starting point instead of 14 AWG, even before you think about distance. If that same run stretches to 110 feet one way, voltage drop often pushes the design to 10 AWG while the breaker stays at 20A because the load has not changed.

The same logic shows up in larger work. A 7.5 HP, 460V three-phase motor with a full-load current around 11A does not mean you can stop at an 11A wire decision. Motor circuits, feeder calculations, and equipment grounding all apply their own code logic, and the conductor selected from ampacity tables still has to survive ambient temperature, rooftop heat, or bundling. That is why experienced electricians compare the load calculation against conductor ampacity, then against raceway or box space, and only then against the final breaker or fuse size.

Residential work needs the same discipline. A box-fill calculation that lands at 24.75 cubic inches on a 12 AWG two-gang box, or a detached garage feeder that picks up 3.6V of drop on a 120V leg, is already telling you the installation is too close to the edge. Use the long-distance wire guide when length is the problem, and cross-check enclosure constraints with the box fill guide or the conduit fill guide. Those second-pass checks are where most field rework gets avoided.

A good field habit is to compare at least two design options before material is ordered. For example, a 240V 32A EV charger on a 140-foot run may look acceptable on 8 AWG copper when you only review ampacity, but the same circuit may justify 6 AWG once you hold voltage drop close to a 3% design target. The same pattern shows up on pump circuits, detached-building feeders, and HVAC condensers. The circuit can be legal at one size and still perform better, start motors more reliably, and leave more inspection margin at the next size up.

Underground Duct Bank Wire Sizing: NEC Design Guide: Fast Field Comparison

The table below is not a substitute for the full article calculation, but it is a practical comparison lens for electricians, engineers, and serious DIY users who need a quick reasonableness check before they pull conductors. The numbers show how the design conversation changes once duration, distance, and enclosure limits are reviewed together instead of as isolated problems.

• Short branch circuits usually pass on ampacity alone, but continuous loads above 16A often force the next larger conductor or breaker check under the 125% rule.
• Runs around 100 to 150 feet are where voltage drop starts changing otherwise normal residential and light commercial conductor picks.
• Feeders and service work often pass ampacity first, then fail on grounding, raceway fill, or box-space details if those follow-up checks are skipped.

When those conditions stack together, the cheapest installation is rarely the smallest conductor that barely passes one table. The better choice is usually the conductor that clears ampacity, keeps voltage drop inside the design target, and still leaves room for a normal termination and inspection workflow.

Underground Duct Bank Wire Sizing: NEC Design Guide: Frequently Asked Questions

How do I know when underground duct bank wire sizing: nec design guide needs a larger conductor than a simple chart shows?

If the run is long, the load is continuous for 3 hours or more, or the conductors are bundled in hot ambient conditions, the simple chart is only the starting point. A 20A circuit may still need 10 AWG instead of 12 AWG once the 125% rule or a 3% voltage-drop target is applied.

Does the 125% continuous-load rule matter for underground duct bank wire sizing: nec design guide?

Yes, whenever the load is expected to run at maximum current for 3 hours or more. Under NEC 210.19(A)(1) and 215.2(A)(1), a 24A continuous load is treated as 30A for conductor sizing, which is why field calculations often move up one breaker and wire size from the first rough estimate.

What voltage-drop target is practical when planning underground duct bank wire sizing: nec design guide?

The common design target is about 3% on a branch circuit and 5% total for feeder plus branch circuit. That is not a mandatory blanket rule in every NEC application, but it is the benchmark many electricians use to decide when a 100-foot to 200-foot run should be upsized.

Can I upsize wire without increasing breaker size for underground duct bank wire sizing: nec design guide?

Yes. Upsizing for voltage drop or future durability does not automatically require a larger breaker. A common example is a 20A circuit that moves from 12 AWG to 10 AWG copper on a long run while the breaker remains 20A because the load and overcurrent protection have not changed.

Which code checks should I finish before calling underground duct bank wire sizing: nec design guide complete?

At minimum, verify conductor ampacity in NEC Table 310.16, breaker protection in NEC 240.4 and 240.6, voltage drop design assumptions, grounding in NEC 250.122, and enclosure or raceway space in NEC 314.16 or Chapter 9. For international work, align the same review with IEC-style conductor and protection practices.

When should I move from a chart lookup to a full calculation for underground duct bank wire sizing: nec design guide?

Move to a full calculation whenever the run exceeds roughly 75 to 100 feet, the load is motor-driven, the circuit is expected to operate for 3 hours or more, or the conductors share a hot raceway with more than three current-carrying conductors. Those are the situations where a simple chart is most likely to miss a required upsizing step.

What is the most common inspection failure tied to underground duct bank wire sizing: nec design guide?

The most common failures are not dramatic math mistakes. They are incomplete checks: a conductor that passes NEC Table 310.16 but ignores a 75 C termination, a long run that misses a 3% branch-circuit design review, or a feeder that works electrically but lands in an undersized box or raceway. Most red tags happen when one of those second-pass checks is skipped.

Next Steps

If you want to validate this topic against real project numbers, start with the wire gauge calculator, then cross-check longer runs in the voltage drop calculator, and verify conductor adjustments with the ampacity calculator. If you want us to add another worked example or application note, contact us here.