Caída de tensión en circuitos seccionales largos: sección, distancia y NEC/IEC

Long branch circuits are where voltage drop stops being a classroom formula and starts changing real installation decisions. A 20 amp breaker and 12 AWG copper may be legal for ampacity, but that does not mean the far receptacle, garage tool, mini workshop, outdoor load, or control transformer will receive healthy voltage. Once a run stretches past 50, 75, or 100 feet, the practical question is no longer “what is the smallest conductor allowed?” It becomes “what conductor keeps the load working correctly after the circuit is installed?”

The code framework is familiar. The National Electrical Code uses informational notes in NEC 210.19(A)(1) and NEC 215.2(A)(1) to recommend 3% maximum voltage drop on a branch circuit and 5% total on feeder plus branch circuit. International work commonly references IEC 60364-5-52, where installation method, conductor material, grouping, and final-circuit voltage drop all matter. The NEC notes are not written as mandatory branch-circuit rules, but they are the numbers many engineers, plan reviewers, and owners expect to see on a professional design.

“For a long 120 volt branch circuit, breaker size tells you only the safety limit. It does not tell you whether the equipment will see 120 volts, 116 volts, or 112 volts when the real load is running.”
— Hommer Zhao, Technical Director

Start With Load, Distance, and Circuit Type

A useful voltage-drop calculation starts with the actual load current, not only the breaker handle. For continuous loads in NEC work, the branch-circuit ampacity normally accounts for 125% of load, but voltage drop should be checked at the expected operating current. A 20 amp receptacle circuit feeding a 12 amp bench tool, a 16 amp continuous charger, and a 20 amp temporary load are three different calculations. The route length is also one-way physical length, while the current path is out and back on a single-phase circuit.

The simplified copper formula for single-phase circuits is:

Voltage drop = 2 x K x I x D / circular mil area

K is conductor resistivity, I is load current, and D is one-way distance in feet. For three-phase circuits, replace 2 with 1.732.

You do not need to hand-calculate every circuit, but understanding the variables prevents bad inputs. Use the voltage drop calculator for the math, then cross-check conductor ampacity with the wire size calculator and tables before ordering material.

Practical Example: 120 Volt Workshop Receptacle

Suppose a detached workshop has a 120 volt, 20 amp branch circuit with a 16 amp continuous tool load located 95 feet from the panel. With 12 AWG copper at about 1.588 ohms per 1,000 feet, the round-trip conductor path is 190 feet.

12 AWG copper at 16 A

Resistance = 1.588 ohms/kft x 0.190 kft = 0.302 ohms

Voltage drop = 16 A x 0.302 ohms = 4.83 V

Percent drop = 4.83 / 120 x 100 = 4.0%

The breaker and conductor ampacity may look ordinary, but the voltage drop is already beyond the 3% branch-circuit target. Upsizing to 10 AWG copper, at about 0.999 ohms per 1,000 feet, gives 3.04 volts drop, or 2.5%. The better design is a 20 amp breaker with 10 AWG copper for the long run, then devices and terminals selected for the conductor or properly pigtailed where needed.

Comparison Table: Long 120 V Branch Circuit Choices

This table uses 16 amps on a 120 volt single-phase circuit with copper conductors at common resistance values. It shows why distance often controls wire size before ampacity does.

One-way distance	12 AWG copper	10 AWG copper	8 AWG copper	Field decision
40 ft	1.7%	1.1%	0.7%	12 AWG usually acceptable
60 ft	2.5%	1.6%	1.0%	Check feeder drop first
80 ft	3.4%	2.1%	1.3%	10 AWG is the practical choice
100 ft	4.2%	2.7%	1.7%	10 AWG minimum for performance
150 ft	6.4%	4.0%	2.5%	Consider 8 AWG or subpanel strategy

NEC and IEC Checks That Still Apply

Voltage drop never replaces code-compliant ampacity, overcurrent protection, grounding, termination, box fill, or temperature correction. NEC Table 310.16 ampacity, NEC 110.14(C) terminal temperature limits, NEC 210 branch-circuit rules, and NEC 250 equipment grounding rules still control the installation. IEC designs face the same layered logic: choose a cable that satisfies current capacity, protective-device operation, shock protection, thermal constraints, and voltage drop.

Common mistake

Do not upsize conductors and forget the enclosure. A 10 AWG conductor has a larger NEC 314.16 box-fill allowance than 12 AWG, and some receptacles do not accept larger conductors directly. Pigtails, larger boxes, or different terminals may be required.

“Voltage drop is a performance check, but the fix has code consequences. If 10 AWG solves the math, you still need to check box volume, device terminals, grounding conductor sizing, and pulling space.”
— Hommer Zhao, Technical Director

When a Subpanel Beats a Giant Branch Circuit

At some distance, repeatedly upsizing individual branch circuits stops making sense. If a shed, detached garage, pump house, workshop, or remote equipment pad needs several loads, a feeder and small panel may reduce material cost and improve future flexibility. The voltage-drop budget can then be divided between feeder and branch circuits, keeping the combined total near the NEC 5% recommendation while avoiding a bundle of oversized home runs.

A 100 foot run for one 16 amp receptacle may be solved with 10 AWG copper. A 150 foot route feeding lighting, receptacles, a charger, and a small motor is a different design problem. In that case, evaluate a feeder with calculated load, grounding and bonding requirements, local disconnect rules, and a branch-circuit layout that keeps final runs short. The wire tables and NEC standards guide are useful companions when comparing those options.

“My trigger point is simple: when two or three long branch circuits all want upsized copper, stop and price a feeder. A 60 amp feeder at 150 feet may be cleaner than three separate 20 amp home runs.”
— Hommer Zhao, Technical Director

Field Workflow for Long Branch Circuits

Measure the real route. Include vertical drops, offsets, panel routing, and exterior detours. A guessed 75 foot run often becomes 95 feet after layout.
Use actual load current. Calculate voltage drop at the expected operating load, and run a second check at full circuit rating if the receptacle could be heavily used.
Keep the breaker honest. Upsized conductors do not justify a larger breaker unless the whole circuit, device rating, load calculation, and code rules allow it.
Check the complete installation. Confirm box fill, conduit fill, derating, terminal temperature, grounding, and local amendments before calling the design finished.

Design target

For ordinary branch circuits, 3% is a practical maximum. For motors, electronic power supplies, long outdoor circuits, or customer-facing equipment, a 2% branch-circuit target often buys reliability at a modest material premium.

FAQ

What voltage drop should I design for on a long branch circuit?

Use 3% as the normal branch-circuit target from NEC 210.19(A)(1) Informational Note No. 4, with 5% total feeder plus branch circuit from NEC 215.2(A)(1). For sensitive loads or motors, many designers hold the branch circuit closer to 2%.

How far can a 12 AWG 20 amp circuit run before voltage drop matters?

At 120 V and 16 A continuous load, 12 AWG copper reaches about 3% voltage drop around 58 feet one-way. A full 20 A load reaches 3% around 46 feet, so long garage, shed, and outdoor runs often need 10 AWG.

Does upsizing wire require a larger breaker?

No. Upsizing from 12 AWG to 10 AWG for voltage drop does not require changing a 20 A breaker. The breaker still protects the circuit ampacity; the larger conductor simply reduces resistance and voltage loss.

How does IEC 60364 handle voltage drop?

IEC 60364-5-52 treats voltage drop as a design-performance check tied to installation method, conductor material, grouping, and load type. Many IEC projects use 3% for lighting and 5% for other final circuits unless local rules specify a tighter value.

Should I use copper or aluminum for a long branch circuit?

Copper is usually preferred for branch circuits because terminations are compact and resistance is lower. Aluminum can work on larger feeders, but for the same voltage drop it often needs roughly two AWG sizes larger plus listed AL/CU terminations.

Can voltage drop cause nuisance problems without tripping the breaker?

Yes. A breaker responds to overcurrent, not poor delivered voltage. A 120 V circuit at 5% drop delivers about 114 V under load, which can make motors start harder, chargers reduce output, and lights dim even though the breaker never trips.

Calculate Before You Pull Wire

Long branch circuits are manageable when the numbers are visible early. Enter the voltage, phase, load current, conductor material, wire size, and one-way distance before you buy cable, then check the result against your 3% branch-circuit budget and 5% feeder-plus-branch budget.

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