Voltage Drop Explained

One Ohm's-law multiplication, one code table, one rule of thumb

Voltage drop is the most arithmetic-light calculation in residential electrical work. The voltage drop calculator multiplies four numbers — current, length, resistance per length, and a phase factor — and divides by source voltage to get a percentage. Everything else is wrapped around three pieces of context: where the resistance number comes from (NEC Chapter 9, Table 8), why the phase factor is 2 on single-phase and √3 on three-phase, and why almost every textbook and inspector treats 3 % as the working ceiling.

This article is the long version of what the calculator does in one step. It walks the formulas for single-phase, DC, and three-phase, explains the NEC table values it pulls from, sets out where the 3 % guideline comes from and why it is not enforceable, and covers the practical wire-sizing moves you make when a run comes in too high. Copper vs aluminum, temperature derating, and the Table 8 vs Table 9 question all live at the end.

The formula, in one line per system

Voltage drop is current times the conductor path resistance. The difference between systems is what counts as the path:

Single-phase or DC: V_drop = 2 × L × I × R
Three-phase balanced: V_drop = √3 × L × I × R

L is the one-way run length (panel to load, not the round trip), I is the load current in amps, and R is the conductor resistance per unit length. The calculator uses NEC Chapter 9 Table 8 DC resistance in ohms per 1000 feet at 75 °C — the standard reference for branch-circuit sizing against the 75 °C ampacity column. Divide the resulting voltage drop by source voltage to get the percentage, the number the 3 % guideline applies to.

The 2 on single-phase is the round trip — current flows out on the hot and back on the neutral, so the conductor path is twice the one-way length. The √3 on three-phase comes from the line-to-line voltage relationship in a balanced system; the three phase currents sum to zero in a balanced load, there is no return current on a neutral, and the phasor geometry produces the √3 ≈ 1.732 multiplier. Net effect: a balanced three-phase circuit drops about 86.6 % of what the equivalent single-phase circuit would, for the same per-conductor resistance and current.

Where the resistance number comes from

Conductor resistance is a material property scaled by geometry — longer conductors and thinner conductors have more resistance. Tables bake in the geometry and let you look up an ohms-per-foot or ohms-per-kilometre figure by AWG size and material. The values the calculator uses come from NEC Chapter 9 Table 8, “Conductor Properties,” for uncoated stranded conductors at 75 °C.

A handful of the most common entries, in ohms per 1000 feet:

• 14 AWG copper: 3.07 Ω/kft (aluminum not rated at 14 AWG)
• 12 AWG copper: 1.93 Ω/kft, aluminum 3.18 Ω/kft
• 10 AWG copper: 1.21 Ω/kft, aluminum 2.00 Ω/kft
• 8 AWG copper: 0.764 Ω/kft, aluminum 1.26 Ω/kft
• 6 AWG copper: 0.491 Ω/kft, aluminum 0.808 Ω/kft
• 4 AWG copper: 0.308 Ω/kft, aluminum 0.508 Ω/kft
• 2 AWG copper: 0.194 Ω/kft, aluminum 0.319 Ω/kft
• 1/0 AWG copper: 0.122 Ω/kft, aluminum 0.201 Ω/kft

Two patterns are worth noticing. First, doubling the cross-sectional area (three AWG sizes lower in number) roughly halves the resistance — that is the linear relationship between area and resistance for a cylindrical conductor. Second, aluminum is consistently about 1.65× the resistance of copper at the same AWG; aluminum's conductivity is about 61 % of copper's, which is the inverse of that ratio. That is why ampacity tables list aluminum two AWG sizes larger than copper for the same current — you need more cross-section to carry the same current with the lower-conductivity material.

Worked example: a 20 A branch circuit on 120 V

A continuous 20 A load on a 12 AWG copper branch circuit, 50 ft one-way from the panel, on 120 V single-phase — the everyday residential or small-commercial case. Plug into the formula:

V_drop = 2 × L × I × R
V_drop = 2 × 50 ft × 20 A × (1.93 Ω / 1000 ft)
V_drop = 3.86 V

As a percentage of 120 V: 3.86 / 120 = 3.22 %. That is just over the NEC's recommended 3 % branch-circuit ceiling, which means a designer or inspector working to the Informational Note would size up. Move to 10 AWG copper at the same length and current:

V_drop = 2 × 50 × 20 × (1.21 / 1000) = 2.42 V → 2.02 %

Comfortably inside the 3 % limit. Drop the same 12 AWG circuit onto 240 V single-phase instead — a hard-wired water heater, say — and the absolute drop in volts stays 3.86, but the percentage halves to 1.61 %, because the same drop is now measured against a doubled reference. That is the most common reason long, low-current runs end up on 240 V even where 120 V would carry the load: percentage drop falls by the voltage ratio. Run the same calculation in the calculator with the system voltage swapped and the figures fall out the same way.

Where the 3 % rule comes from

The NEC does not impose an enforceable hard limit on voltage drop. The relevant text lives in Informational Note 4 to NEC 210.19(A) for branch circuits and a parallel note to 215.2(A)(1) for feeders, and both Informational Notes recommend the same numbers: ≤ 3 % drop on a branch circuit, ≤ 5 % combined for feeder plus branch circuit, “for reasonable efficiency of operation.” Informational Notes are explicitly non-enforceable text in the NEC — Article 90 is clear on that — so an inspector cannot fail a job because a branch circuit drops 3.5 %.

In practice the figures function as the working ceiling for almost every design. Equipment is specified to operate within a voltage tolerance — typically ±10 % of nameplate — and the 3 / 5 split keeps the worst-case load voltage inside that band even when the source is at the low end of utility tolerance. Specialty installations sometimes tighten the target: 2 % is common on motor branch circuits (to protect torque and starting current), on long PV string runs (where the power lost to drop is daily yield lost), and on sensitive electronics. Some local jurisdictions also adopt the Informational Note language as an enforceable requirement via state amendment, at which point an inspector can in fact reject a job for failing it — California Title 24 is a notable example for non-residential lighting feeders.

How to fix a circuit that exceeds the guideline

When the calculator hands back a percentage above your target, three levers exist. They are not interchangeable, but they are usually tried in this order.

Go up a wire size

Almost always the first move. Moving from 12 AWG to 10 AWG copper drops resistance from 1.93 Ω/kft to 1.21 Ω/kft — a 37 % reduction in voltage drop for the same current and length. Each subsequent thicker AWG step cuts another ~20 %. The cost is conductor and sometimes conduit fill; the upside is permanent and immediate. Use the wire gauge calculator to size directly to a target percentage rather than iterating with the drop calculator.

Shorten the run

Voltage drop scales linearly with length — half the run, half the drop. Available only at the design stage when panel locations are still negotiable, and rarely the cheapest solution because moving a panel implies new feeders, but it is the right move when the circuit is one of many long runs and a sub-panel closer to the load cluster would solve the percentage problem for the whole zone at once.

Raise the system voltage

For the same load power, doubling the source voltage halves the current and halves the percentage drop. A 240 V branch on a hard-wired appliance drops half the percentage of a 120 V branch carrying the same wattage. A three-phase 480 V feeder drops roughly 19 % of the percentage of a 208 V feeder at the same power. This is why long industrial distribution runs at 480 V three-phase rather than 208 V three-phase: the same conductors carry the same power with one-fifth the voltage-drop overhead. The catch is that the equipment downstream needs to be voltage-matched, so this lever applies mostly at the design or major-renovation stage.

Temperature, conduit, and ambient effects

Conductor resistance rises with temperature. The Table 8 values are DC resistance at 75 °C, which is the right reference when the conductor is sized against the 75 °C ampacity column — under steady full load it warms toward that figure anyway. At 25 °C copper resistance is roughly 16 % lower than the Table 8 figure; at 90 °C it is roughly 6 % higher. The copper temperature coefficient is about 0.393 % per °C, aluminum about 0.403 % per °C.

For most everyday calculations the 75 °C value is close enough — the cost of being precise about ambient on a residential branch circuit is small compared with the cost of stepping up a wire size. Where the precision matters is at the edges: cold-room refrigeration feeds where the conductor genuinely runs at 0–5 °C, outdoor work in northern winters, or motor branch circuits in mechanical rooms at 50 °C ambient. In those cases apply the temperature correction explicitly rather than assuming the 75 °C table value.

Copper vs aluminum, in one paragraph

Copper for branch circuits, aluminum for feeders and service entrances at higher gauges. Copper has roughly 60 % more conductance per unit cross-section, accepts standard terminations, and does not require antioxidant compound at the lug. Aluminum is cheaper per amp of ampacity at heavy gauges, but it needs AL- or CU/AL-rated terminations, antioxidant compound on the lug, and the two-AWG-size step-up that the resistance ratio implies. The notorious 1960s and 70s solid aluminum branch wiring used the wrong terminations for the material and produced the loose-connection fires that gave aluminum its reputation; modern stranded AA-8000-series aluminum on properly rated lugs is reliable. The voltage drop math is the same for both materials — the calculator just looks up a different R value when you switch the dropdown.

NEC Table 8 vs Table 9: when to switch

Table 8 is DC resistance. Table 9 is effective Z — the AC impedance that combines resistance and conductor reactance, broken out by conduit material (PVC, aluminum, steel). For ordinary 60 Hz branch circuits and short feeders the inductive reactance is small enough relative to resistance that the DC figure is within a percent or two of the true AC drop. For long, high-current feeders at 2/0 AWG and larger, reactance becomes a noticeable contributor and the Table 8 figure underestimates the drop. The crossover is rough — usage guides like Mike Holt's rule of thumb call it at around 100 ft of run at 100 A and 2/0 AWG or larger — and the difference rarely exceeds 10 % of the Table 8 answer until conductors are in the kcmil range. For everything the calculator covers (14 AWG through 4/0 AWG, branch-circuit and small-feeder territory), Table 8 is the standard choice and is what published industry tools use by default.

Common mistakes

Plugging in round-trip length instead of one-way

The 2 in the single-phase formula is the round trip — it is built into the formula, not something you add by doubling the input. The calculator's “run length” field is the panel-to-load distance, the same distance a tape measure would give. Doubling it manually produces an answer twice the correct drop and pushes designs to one wire size larger than needed.

Treating the 3 % limit as code

It is not. Designers and AHJs use it as a working ceiling because equipment expects to operate within it, and some local jurisdictions adopt it as enforceable text via state amendment, but the bare NEC Informational Note is non-enforceable. The cases where it gets enforced are listed in the local amendments to Article 90; check those rather than assuming the 3 % figure carries the same legal weight as an ampacity table.

Sizing for voltage drop on an undersized ampacity choice

Voltage drop sizing supplements ampacity sizing — it does not replace it. A conductor that meets the 3 % drop guideline but carries more current than its 75 °C ampacity allows is still a code violation. Always pick the conductor by ampacity first, then check voltage drop, then step up if the drop figure says you need to.

Ignoring termination temperature ratings

110.14(C) requires the conductor ampacity to be derated to the lowest temperature rating in the assembly — typically 75 °C for equipment over 100 A and 60 °C for equipment 100 A and under. A 90 °C-insulated conductor terminated on a 60 °C-rated lug uses the 60 °C ampacity column. The voltage-drop calculation does not care about insulation rating, only about R at the operating temperature, so the calculator's 75 °C value remains the right reference even when ampacity is being derated to 60 °C.

When to seek professional advice

Pulling resistance off a table and multiplying is bookkeeping, not electrical engineering. The cases worth a real conversation with a licensed electrician or PE:

• Long parallel feeder runs at kcmil sizes — the Table 9 reactance numbers, conduit-material derating, and parallel-impedance balance become genuinely complex.
• Motor branch circuits at long distances, where voltage drop interacts with starting current, contactor pickup voltage, and torque available at start.
• PV and battery DC string design, where the cost of accepting a larger drop is daily yield lost forever and the right target is well below the NEC 3 %.
• Anything involving service-entrance or utility-side conductors — the rules around utility-owned vs customer-owned wiring and the required clearances are jurisdiction-specific.

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