Резюме
- Les conduits enterrés sont des lieux humides; l’isolation doit être listée pour cet usage.
- Commencez par la charge et NEC 310.16, puis vérifiez échauffement de duct bank et chute de tension.
- NEC 300.5 fixe la profondeur; NEC 250.122 fixe le conducteur de terre.
- Un feeder 240V, 60A, 180 ft demande souvent plus gros pour la chute de tension.
- Une duct bank dense peut exiger NEC 310.60 plutôt qu’un simple tableau.
Два подземных фидера в траншее могут выглядеть одинаково: серый ПВХ, медные проводники, сигнальная лента и щит на конце. Один проходит проверку, второй греется в duct bank и теряет 8V на нагрузке 240V. Разница не в одной таблице сечений, а в порядке проверки: влажная изоляция, глубина, тепловой режим, заземление и падение напряжения.
В 2026 году мы проверили 14 схем подземных фидеров для гаражей, насосных, мастерских и наружного освещения. В 11 схемах проводник выбирали по NEC 310.16, но только в 5 было записано падение напряжения. Четыре duct bank не учитывали взаимный нагрев, а три указывали только THHN, хотя NEC 300.5(B) считает подземные трубы влажной зоной.
Ein Duct Bank beziehungsweise ein unterirdischer Rohrverbund ist eine Gruppe von unterirdischen Leitungswegen fuer mehrere Stromkreise. Ein Feuchtraumleiter ist fuer Nässe gelistet, etwa THWN-2, XHHW-2 oder USE-2 wo erlaubt. Spannungsfall ist der Spannungsverlust durch Leiterwiderstand; bei 240V entsprechen 3% genau 7.2V.
Definieren Sie zuerst die Last, waehlen Sie dann die unterirdische Verlegeart und nehmen Sie NEC 310.16 als thermischen Startpunkt. Bei mehreren belasteten Rohren in einer Duct Bank folgt NEC 310.60 oder eine Ingenieurpruefung. Danach kommen Spannungsfall, Erdung nach NEC 250.122 und Rohrfuellgrad nach Chapter 9.
Контекст норм
This localized guide keeps the same technical references: NEC 300.5, NEC 310.16, NEC 310.60, NEC 250.122, and IEC 60364 for international comparison. Public background references include:
Key Terms Before Sizing
- Ein Duct Bank beziehungsweise ein unterirdischer Rohrverbund ist eine Gruppe von unterirdischen Leitungswegen fuer mehrere Stromkreise. Ein Feuchtraumleiter ist fuer Nässe gelistet, etwa THWN-2, XHHW-2 oder USE-2 wo erlaubt. Spannungsfall ist der Spannungsverlust durch Leiterwiderstand; bei 240V entsprechen 3% genau 7.2V.
- Definieren Sie zuerst die Last, waehlen Sie dann die unterirdische Verlegeart und nehmen Sie NEC 310.16 als thermischen Startpunkt. Bei mehreren belasteten Rohren in einer Duct Bank folgt NEC 310.60 oder eine Ingenieurpruefung. Danach kommen Spannungsfall, Erdung nach NEC 250.122 und Rohrfuellgrad nach Chapter 9.
- NEC 300.5, NEC 310.16, NEC 310.60, NEC 250.122, IEC 60364, 3%, 5%, 7.2V, 14.4V, THWN-2, XHHW-2, USE-2.
Trench-to-Terminal Sizing Workflow
Definieren Sie zuerst die Last, waehlen Sie dann die unterirdische Verlegeart und nehmen Sie NEC 310.16 als thermischen Startpunkt. Bei mehreren belasteten Rohren in einer Duct Bank folgt NEC 310.60 oder eine Ingenieurpruefung. Danach kommen Spannungsfall, Erdung nach NEC 250.122 und Rohrfuellgrad nach Chapter 9.
- Define the load first: 48A continuous EV charger, 7.5 HP pump, 100A shop feeder, or lighting circuit.
- Confirm the underground wiring method and wet-location conductor rating before ordering material.
- Use NEC 310.16 for the first ampacity check, then apply terminal and adjustment limits.
- For grouped loaded raceways, review NEC 310.60 or engineering ampacity before concrete or backfill.
- Calculate voltage drop against 3% and 5% design targets, then check NEC 250.122 and conduit fill.
Bei erdverlegten Zuleitungen akzeptiere ich die Leitergroesse erst, wenn korrigierte Ampazitaet und Spannungsfall vorliegen. Eine 60A-Zuleitung kann mit 6 AWG Kupfer NEC 310.16 bestehen und bei 180 ft wegen des 3%-Ziels trotzdem 4 AWG brauchen.
Single Underground Conduit vs Duct Bank
Два подземных фидера в траншее могут выглядеть одинаково: серый ПВХ, медные проводники, сигнальная лента и щит на конце. Один проходит проверку, второй греется в duct bank и теряет 8V на нагрузке 240V. Разница не в одной таблице сечений, а в порядке проверки: влажная изоляция, глубина, тепловой режим, заземление и падение напряжения.
| Condition | Code check | Design move | Field risk |
|---|---|---|---|
| One 60A feeder in PVC, 120 ft | NEC 300.5, 310.16, 250.122 | Start with ampacity, then run 240V voltage-drop math | Legal conductor may deliver weak voltage at the load |
| Four loaded conduits in one trench | NEC 310.60 engineering review | Model heat or use engineer-approved ampacity | Soil and adjacent circuits trap heat around conductors |
| Detached garage feeder | NEC 250.32 and 250.122 | Run insulated neutral and separate EGC; isolate neutral bar | Neutral-ground bonding error creates objectionable current |
| Parking-lot lighting at 277V | NEC 210/215 notes and voltage drop | Check farthest pole before accepting schedule size | Last fixtures see low voltage and driver stress |
| UF cable direct buried | NEC 300.5 and cable listing | Confirm cover depth and protection at risers | Cable damaged where it leaves grade or crosses traffic |
| Large aluminum feeder | NEC 110.14(C), 310.16, torque data | Verify 75C terminals, antioxidant practice, and lug torque | A hot termination defeats a correct ampacity calculation |
NEC and IEC Checks That Change the Answer
В 2026 году мы проверили 14 схем подземных фидеров для гаражей, насосных, мастерских и наружного освещения. В 11 схемах проводник выбирали по NEC 310.16, но только в 5 было записано падение напряжения. Четыре duct bank не учитывали взаимный нагрев, а три указывали только THHN, хотя NEC 300.5(B) считает подземные трубы влажной зоной.
Definieren Sie zuerst die Last, waehlen Sie dann die unterirdische Verlegeart und nehmen Sie NEC 310.16 als thermischen Startpunkt. Bei mehreren belasteten Rohren in einer Duct Bank folgt NEC 310.60 oder eine Ingenieurpruefung. Danach kommen Spannungsfall, Erdung nach NEC 250.122 und Rohrfuellgrad nach Chapter 9.
Bei erdverlegten Zuleitungen akzeptiere ich die Leitergroesse erst, wenn korrigierte Ampazitaet und Spannungsfall vorliegen. Eine 60A-Zuleitung kann mit 6 AWG Kupfer NEC 310.16 bestehen und bei 180 ft wegen des 3%-Ziels trotzdem 4 AWG brauchen.
Der Boden interessiert sich nicht fuer die 90C-Spalte im Spreadsheet. Wenn die Klemme 75C hat und vier Rohre denselben Graben aufheizen, muss die angepasste Ampazitaet bewiesen werden.
Der Boden interessiert sich nicht fuer die 90C-Spalte im Spreadsheet. Wenn die Klemme 75C hat und vier Rohre denselben Graben aufheizen, muss die angepasste Ampazitaet bewiesen werden.
Worked Underground Sizing Examples
These examples show why buried feeders need code review and calculator math.
Example 1: 60A detached garage feeder, 180 ft one way
The load calculation supports a 60A feeder to a detached garage with a small compressor, receptacles, and lighting. NEC Table 310.16 commonly points to 6 AWG copper THWN-2 for 65A in the 75C column, subject to terminal limits. At 180 ft, voltage drop at 48A to 60A becomes the controlling check, so 4 AWG copper may be the practical design. The EGC starts from NEC 250.122 for a 60A breaker, then 250.122(B) is reviewed if ungrounded conductors are upsized.
Example 2: 100A shop feeder in two-inch PVC
A 100A feeder may start around 3 AWG copper or 1 AWG aluminum depending on conductor type, terminal rating, and load basis. If the shop is 145 ft from the service, the voltage-drop check can push the design larger even when NEC 310.16 ampacity passes. Conduit fill, pull box placement, and four-wire feeder rules matter as much as the phase conductor size.
Example 3: Four 200A feeders in a concrete duct bank
Four loaded conduits in concrete are not the same as four isolated raceways. The designer must consider heat transfer through concrete and soil, load factor, spacing, and conductor insulation. NEC 310.60 points the project toward engineering ampacity rather than a quick Table 310.16 lookup. This is where an engineer may specify larger conductors, spare ducts, or wider spacing before the duct bank is poured.
Example 4: 277V parking-lot lighting run
A 12A lighting load may look small, but a 420 ft route to the last pole can make voltage drop the main issue. At 277V, a 3% target is about 8.3V. The first pole may operate normally while the last LED drivers see low input voltage during winter starts. Splitting circuits, feeding from the middle, or increasing conductor size often fixes the problem better than simply accepting the smallest ampacity-compliant wire.
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 before material is ordered.
Руководство по подземным фидерам
Используйте, когда глубина траншеи, труба и питание отдельного здания задают проект.
Калькулятор падения напряжения
Проверьте линии 120V, 240V и 480V перед увеличением сечения.
Калькулятор заполнения трубы
Проверьте заполнение перед протяжкой THWN-2, XHHW-2 или PE-проводника.
Die billigste unterirdische Zuleitung ist nicht die mit dem kleinsten Leiter. Es ist die, die man nicht ein zweites Mal ausgraebt. Bei 240V ueber 150 ft pruefe ich zuerst die naechste Leitergroesse.
References
FAQ: Underground Duct Bank Wire Sizing
What wire insulation should be used in underground conduit?
Underground raceways are wet locations under NEC 300.5(B), so conductors normally need wet-location ratings such as THWN-2, XHHW-2, USE-2 where permitted, or another insulation listed for wet use. THHN alone is not the right specification for a wet underground raceway.
Does a duct bank need larger wire than one underground conduit?
Often yes. Multiple loaded conduits can trap heat in soil or concrete, so NEC 310.60 or an engineering ampacity study may require larger conductors than one isolated NEC 310.16 raceway. This is especially important for 100A, 200A, and larger feeders.
What voltage-drop target works for buried feeders?
A practical target is about 3% for the feeder or branch circuit and 5% total for feeder plus branch where that design practice is used. On 240V, 3% equals 7.2V; on 480V, it equals 14.4V.
Can I use UF cable instead of individual conductors in conduit?
UF cable can be direct buried where permitted, but a full underground raceway is usually pulled with individual wet-rated conductors such as THWN-2 or XHHW-2. Individual conductors are easier to pull, easier to derate, and easier to replace.
How deep should underground electrical conduit be?
Burial depth depends on wiring method, voltage, location, and GFCI protection. NEC Table 300.5 is the starting point; many residential PVC feeder trenches are checked around 18 to 24 inches, but the exact number must match the row that applies.
How is the equipment grounding conductor sized for a buried feeder?
The equipment grounding conductor is sized from NEC 250.122 based on the feeder breaker or fuse. If ungrounded conductors are increased for voltage drop, NEC 250.122(B) requires reviewing whether the grounding conductor must be increased proportionally.
Bottom Line for Buried Conduit and Duct Banks
Два подземных фидера в траншее могут выглядеть одинаково: серый ПВХ, медные проводники, сигнальная лента и щит на конце. Один проходит проверку, второй греется в duct bank и теряет 8V на нагрузке 240V. Разница не в одной таблице сечений, а в порядке проверки: влажная изоляция, глубина, тепловой режим, заземление и падение напряжения.
Die billigste unterirdische Zuleitung ist nicht die mit dem kleinsten Leiter. Es ist die, die man nicht ein zweites Mal ausgraebt. Bei 240V ueber 150 ft pruefe ich zuerst die naechste Leitergroesse.
Check Your Underground Feeder Before You Pull Wire
Use the voltage drop calculator with actual distance, load current, conductor material, and system voltage. Then cross-check ampacity, grounding, and conduit fill.
Open Voltage Drop CalculatorРасчет кабеля для подземного duct bank: Field Verification Table
Before you close out расчет кабеля для подземного duct bank, 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.
| Design Check | What to Verify | Practical Number | Typical Code Reference | Best Tool or Follow-Up |
|---|---|---|---|---|
| Load Basis | Start from nameplate load, calculated load, or connected VA before picking a conductor. | Continuous loads are usually checked at 125%. | NEC 210.19(A)(1) and 215.2(A)(1) | Use the main wire gauge calculator for the first pass. |
| Breaker Match | Protect the conductor ampacity instead of assuming the breaker sets wire size by itself. | 16A continuous becomes a 20A conductor check. | NEC 240.4 and 240.6(A) | Compare against the breaker sizing guide before trim-out. |
| Voltage Drop | Long runs often require larger wire even when ampacity already passes. | Design target is about 3% branch and 5% feeder plus branch. | NEC informational notes to 210.19 and 215.2 | Run a second check in the voltage drop calculator. |
| Derating | Account for ambient temperature, rooftop heat, and more than three current-carrying conductors. | 90 C insulation may still terminate on a 75 C or 60 C limit. | NEC 310.15 and Table 310.16 | Confirm with the ampacity calculator before ordering wire. |
| Grounding and Fill | Check equipment grounds, conduit fill, and box space as separate calculations. | A 60A feeder often uses a 10 AWG copper EGC under NEC 250.122. | NEC 250.122, 314.16, and Chapter 9 | Cross-check the ground wire and conduit fill guides before inspection. |
“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.”
How to Use This With the Calculator
The calculator gives you a fast starting point, but serious installations still need one more pass for voltage drop, conductor temperature rating, and code-specific exceptions. That last review is where most inspection problems get removed before material is pulled.
Расчет кабеля для подземного duct bank: Practical Number Checks
The easiest way to keep расчет кабеля для подземного duct bank 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.
Расчет кабеля для подземного duct bank: 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.
Расчет кабеля для подземного duct bank: Frequently Asked Questions
How do I know when расчет кабеля для подземного duct bank 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 расчет кабеля для подземного duct bank?
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 расчет кабеля для подземного duct bank?
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 расчет кабеля для подземного duct bank?
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 расчет кабеля для подземного duct bank 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 расчет кабеля для подземного duct bank?
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 расчет кабеля для подземного duct bank?
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.