Voltage Drop Calculator – Cable Volt Drop & % Loss (NEC)

Voltage drop is the reduction in electrical potential (voltage) along a conductor as current flows through it. NEC - the US codes for electrical wiring installations - limits voltage drop to 3% of the supply voltage on branch circuits to protect equipment performance and efficiency.

Excessive voltage drop causes motors to run hot and stall, LED drivers to flicker, and UPS systems to malfunction. The NEC 3% limit applies to branch circuits; NEC also recommend 3–5% depending on circuit type. Always calculate from the supply point - not from the sub-distribution board - to capture the cumulative drop in long feeders.

A 3-phase distribution board feeds a 30A load through a 45m copper cable (one-way length) at 415V. Using a 10mm² XLPE copper cable (ρ = 0.0206 Ω·mm²/m):

Vd = (√3 × L × I × ρ) / A = (1.732 × 45 × 30 × 0.0206) / 10 ≈ 4.82V

As a percentage: 4.82 / 415 × 100 = 1.16% - well within the NEC 3% limit (12.45V max). If the cable were run at 25mm² instead, the drop would fall to about 1.93V (0.46%), while dropping to 6mm² would push the drop to 8.03V (1.93%) - still compliant but leaving less margin for future load growth.

How the Voltage Drop Calculator Works

Every conductor has resistance, so some voltage is inevitably lost between the panel and the load. This calculator quantifies that loss using the same circular-mil method taught in electrical engineering and referenced throughout the National Electrical Code (NEC). You enter the system voltage, load current, one-way run length and conductor size; it returns the volts dropped, the receiving-end voltage, and the percentage drop — then compares that percentage against the NEC targets of 3% for branch circuits and 5% total (feeder plus branch). If a run is too long for the chosen conductor, the tool shows you exactly how far over the limit you are so you can upsize before the design leaves your desk.

The Voltage Drop Formula

The industry-standard approximation uses the conductor's resistance expressed in circular-mil-ohms per foot (K):

• Single-phase: VD = (2 × K × I × L) ÷ CM
• Three-phase: VD = (√3 × K × I × L) ÷ CM
• Percentage drop: %VD = (VD ÷ V_source) × 100

where K ≈ 12.9 for copper and 21.2 for aluminum (ohm-circular-mils per foot at 75°C), I is the load current in amperes, L is the one-way length in feet, and CM is the conductor cross-section in circular mils. For power-factor-sensitive work the exact form VD = I × (R·cosθ + X·sinθ) per conductor adds the reactance term X, which matters on large feeders and long three-phase runs.

Variable & Unit Reference

Unit handling: in imperial mode you enter length in feet and the calculator uses circular mils and K-factors directly, which is how US conductor tables are published. In SI mode it switches to meters, mm² and ρ = 0.0175 Ω·mm²/m for copper. One foot = 0.3048 m and 1 kcmil = 0.5067 mm², so the two systems agree to within rounding.

Worked Example 1 — Single-Phase Branch Circuit (120 V)

A 120 V lighting circuit draws 16 A and runs 150 ft to the first fixture using 12 AWG copper (6,530 circular mils).

1. Voltage drop: VD = (2 × 12.9 × 16 × 150) ÷ 6530 = 9.48 V.
2. Percentage: 9.48 ÷ 120 = 7.9% — far above the 3% branch limit.
3. Upsize to 8 AWG (16,510 cmil): VD = (2 × 12.9 × 16 × 150) ÷ 16510 = 3.75 V = 3.1% — still just over.
4. Upsize to 6 AWG (26,240 cmil): VD = 2.36 V = 2.0% ✓.

Answer: although 12 AWG easily carries 16 A, voltage drop forces 6 AWG copper on this long run. This is the classic case where voltage drop — not ampacity — governs the conductor size, and why every long circuit must be checked.

Worked Example 2 — Three-Phase Feeder (480 V)

A 480 V, three-phase feeder carries 200 A a distance of 300 ft to a distribution panel using 3/0 AWG copper (167,800 cmil).

1. Voltage drop: VD = (√3 × 12.9 × 200 × 300) ÷ 167800 = 7.99 V.
2. Percentage: 7.99 ÷ 480 = 1.66% ✓ — well under the 3% feeder allowance, leaving headroom for the downstream branch circuits within the 5% total.
3. Check the total budget: if branch circuits add another 2.5%, the combined 4.16% still satisfies the 5% end-to-end recommendation.

Answer: 3/0 copper is comfortably sized for both ampacity and voltage drop; no upsize needed. Reserving part of the 5% budget for downstream branches is the discipline that separates a robust feeder design from one that just barely passes.

Standards & Code References

• NEC 210.19(A), Informational Note 4 — recommends branch-circuit voltage drop not exceed 3%.
• NEC 215.2(A), Informational Note 2 — recommends feeder drop not exceed 3%, with combined feeder + branch ≤ 5%.
• NEC Chapter 9, Table 8 — conductor DC resistance and circular-mil area; Table 9 — AC resistance and reactance for the exact method.
• IEEE 141 (Red Book) — recommended practice for power distribution voltage-drop analysis in industrial plants.
• NEC 695.7 — the specific voltage-drop limits for fire-pump feeders (≤ 15% at start, ≤ 5% running).
• Secondary / international: IEC 60364-5-52 and IS 732 recommend ≤ 3% for lighting and ≤ 5% for power, using mm² conductors — the same physics with different units.

Key Facts to Remember

• NEC voltage-drop limits are recommendations, not mandates — but nearly every project specification makes 3%/5% a contractual requirement, and fire-pump and sensitive-electronics limits are enforceable.
• Voltage drop scales linearly with length and current and inversely with conductor area — double the run or the load and you double the drop.
• Doubling the circular-mil area (going up ~3 AWG sizes) roughly halves the voltage drop.
• Low-voltage systems suffer most: a 5 V drop is 4.2% on a 120 V circuit but only 1.0% on a 480 V circuit.
• Excessive drop causes dim lighting, nuisance motor tripping, contactor drop-out and premature equipment failure.
• Aluminum drops about 65% more voltage than copper of the same size because K is 21.2 versus 12.9.
• On motor and transformer feeders, include reactance (Table 9) — ignoring X under-predicts the real drop.
• Always design to the continuous running current, but verify motor circuits also start acceptably (inrush can be 6× FLC).

Copper Conductor Reference Table (the "money table")

Use circular mils in the voltage-drop formula, or resistance × length × current × 2 (or √3) if you prefer the ohmic form. Both give the same answer.

Real-World Applications

• Long branch circuits in warehouses, parking structures and outdoor lighting where runs exceed 100 ft.
• Motor feeders where excessive drop reduces starting torque and causes nuisance trips.
• Fire-pump circuits, which have their own strict NEC 695.7 voltage-drop limits.
• Elevator and escalator feeders with high inrush and long vertical risers.
• EV charging stations, often located far from the service with sustained high current.
• Agricultural and site power with very long feeders to remote pumps and buildings.
• Solar PV and battery systems, where DC-side drop directly reduces harvested energy.
• Data-center and sensitive-electronics power, where tight voltage tolerances are specified.

Common Mistakes

• Sizing only for ampacity and never checking voltage drop on long runs.
• Using round-trip instead of one-way length (or vice-versa) — the factor of 2 / √3 already accounts for the return path.
• Applying the single-phase factor to a three-phase circuit, over-predicting the drop by ~15%.
• Ignoring reactance on large or long feeders, under-predicting the true drop.
• Designing to average rather than peak or starting current.
• Forgetting the aluminum penalty and treating AL and Cu as interchangeable.
• Blowing the whole 5% budget on the feeder, leaving nothing for branch circuits.
• Neglecting temperature — hot conductors have higher resistance and drop more voltage.

Step-by-Step: Checking Voltage Drop

A complete voltage-drop check takes only a few minutes when you follow a consistent procedure:

1. Confirm the load current. Use the running current for steady-state drop and the starting current for motor inrush checks — they can differ by a factor of six.
2. Measure the one-way length from the source (panel or transformer) to the load. Do not double it; the formula's multiplier already accounts for the return path.
3. Look up the conductor's circular-mil area (or its ohms per 1000 ft) for the size you intend to use.
4. Apply the correct formula — factor of 2 for single-phase, √3 for three-phase — and compute the volts dropped.
5. Divide by the source voltage and multiply by 100 to get the percentage.
6. Compare against the budget: 3% for the branch or feeder alone, 5% combined. If it fails, increase the conductor area (a jump of three AWG sizes roughly halves the drop) and recompute.

Because voltage drop is inversely proportional to conductor area, the fix is always a bigger conductor, a higher system voltage, or a shorter run. On very long circuits, raising the distribution voltage (for example feeding a remote building at 480 V and stepping down locally) is far more economical than continually upsizing copper.

The Exact Method: Including Reactance

The circular-mil formula captures conductor resistance, which dominates on small conductors and high-power-factor loads. On large feeders, long three-phase runs and inductive loads, the conductor's reactance becomes significant and the resistance-only estimate under-predicts the real drop. The exact per-conductor expression is:

VD = I × (R·cosθ + X·sinθ)

where R and X are the effective AC resistance and reactance in ohms (from NEC Chapter 9, Table 9), cosθ is the power factor and sinθ its sine. At a power factor of 0.85 the reactive term adds meaningfully; at 0.70 it can dominate. For a 500 kcmil conductor, R ≈ 0.027 Ω and X ≈ 0.043 Ω per 1000 ft — the reactance is actually larger than the resistance, so ignoring it on a long 480 V motor feeder can understate the drop by 30% or more. Use the approximate method for branch circuits and the exact method for large or reactive feeders.

Conductor Temperature & Resistance

Copper and aluminum resistance rise about 0.4% per °C, so a conductor running hot under full load drops more voltage than the same conductor at room temperature. The NEC Chapter 9 tables give values at 75°C, which is a reasonable design assumption for a fully loaded conductor. If your circuit runs lightly loaded and cool, the actual drop will be slightly less; if it runs hot in a warm ambient, slightly more. For precision work, correct the resistance to the expected operating temperature: R_T = R₇₅ × [1 + 0.00323 × (T − 75)] for copper. In most designs the 75°C value provides an appropriately conservative result.

Maximum One-Way Run for 3% Drop (Copper, the "distance table")

A quick way to sanity-check a design is to know how far a given conductor can carry a given current before hitting 3% drop. The values below are for single-phase 120 V (multiply distance by 2 for 240 V, and by about 4 for 480 V three-phase at the same current):

Read this as "the longest one-way distance before the 3% limit is reached." Note how quickly the limit tightens as current rises — a 40 A load on 10 AWG is exhausted at just 36 ft, which is why higher-current branch circuits so often need upsizing for distance.

Design Tips from the Field

• Budget the whole path. Allocate the 5% total deliberately — for example 2% to the feeder and 3% to the branch — rather than letting the feeder consume it all.
• Raise the voltage before you raise the copper. For long runs, higher distribution voltage is almost always cheaper than progressively larger conductors.
• Check motor starting, not just running. A feeder that is fine at running current may drop enough at locked-rotor to prevent the motor from starting or to drop out contactors elsewhere.
• Respect the special cases. Fire pumps (NEC 695.7), elevators and sensitive electronic equipment have their own, enforceable limits — do not apply the generic 3% blindly.
• Remember the aluminum penalty. Aluminum drops about 65% more voltage than copper of the same size; upsize accordingly on long AL feeders.
• Document the calculation on the drawings so the drop budget is transparent to reviewers and future designers.

Voltage Drop in DC & Solar Systems

Direct-current and low-voltage systems are where voltage drop bites hardest, because the system voltage is low and the currents are often high. In a 12 V or 24 V battery, RV or off-grid circuit, even a fraction of a volt lost is a large percentage, so conductor sizing is dominated almost entirely by voltage drop rather than ampacity. Solar PV designers apply a tight budget — typically 1–2% on the DC string conductors and another 1–2% on the AC output — because every percent of drop is harvested energy permanently lost to conductor heating. The formula is the pure resistive case (VD = 2 × I × R × L, no √3 and no reactance for DC), but the low voltage means conductors are frequently upsized two or three steps beyond their ampacity requirement. When you size a solar or battery circuit, treat the voltage-drop limit, not ampacity, as the governing constraint.

Motor Starting Voltage Dip

A running motor draws its full-load current, but at the instant of starting it draws locked-rotor current of roughly six times FLC for a second or two. That inrush causes a momentary voltage dip along the feeder that is six times larger than the steady-state drop. If the dip is too deep, the motor may not develop enough starting torque, its own contactor can drop out, and — more insidiously — the dip can dim lights and trip other equipment on the same bus. Good practice keeps the starting voltage dip under about 10–15% at the motor terminals. This is why a feeder that comfortably passes the 3% running-current check can still be inadequate: you must also verify the circuit at locked-rotor current, especially for large across-the-line-started motors. Soft starters and VFDs limit inrush and largely eliminate the concern, which is one of their major system-level benefits.

Three-Phase vs Single-Phase Drop

For the same power delivered, a three-phase circuit drops less voltage than a single-phase circuit, which is one reason three-phase distribution is preferred for larger loads. The three-phase formula uses the √3 (1.732) multiplier while single-phase uses 2, and three-phase also delivers the same power at lower current per conductor. In practice, converting a marginal single-phase feeder to three-phase — where the utility supply allows it — can solve a voltage-drop problem without adding any copper. When you compare options in this calculator, notice how the same conductor and length yield a smaller percentage drop in three-phase mode; that difference compounds on long runs and is often the deciding factor in how a large load is served.

Economic & Code Perspective

Beyond comfort and equipment protection, voltage drop is an energy-efficiency issue: the voltage "lost" is dissipated as heat in the conductor, so a chronically high-drop circuit wastes energy continuously for the life of the installation. The 2020 and later NEC cycles reference voltage drop in the context of energy management, and green-building standards such as ASHRAE 90.1 and LEED credit tighter drop limits. Upsizing a heavily used feeder one size to cut its drop from 4% to 2% often pays for the extra conductor in energy savings within a few years, the same economic-conductor logic used for ampacity. The takeaway: treat the 3%/5% figures as a floor for compliance, but consider going tighter on circuits that run continuously or serve efficiency-sensitive loads.

Practical Field Rules for Managing Voltage Drop

Experienced designers carry a handful of shortcuts that keep voltage drop under control without a calculation on every circuit. The most useful is the "one size per 50 feet" heuristic for branch circuits: beyond about 100 ft on a 120 V circuit, plan to go up one conductor size, and up another size past roughly 150 ft. A second rule is to keep the full 5% budget visible on the one-line diagram so nobody unknowingly spends it twice — a note such as "feeder 2% max, branch 3% max" prevents the common failure where a long feeder and a long branch each individually pass but combine to 6%. Third, on any run over about 200 ft, ask whether the load can be served at a higher voltage; stepping a remote building up to 480 V and transforming down locally almost always beats pulling oversized 120/240 V copper the whole distance.

For motor circuits, remember to verify two operating points — running and starting — because the six-times inrush current produces six times the momentary drop. For lighting, tighter drop (2% or less) is worth targeting because LED drivers and dimming performance are sensitive to supply voltage, and because lighting circuits are often the longest branch runs in a building. For sub-metered and tenant spaces, excessive drop is effectively energy the landlord pays for as conductor heat, so the economics favor generous conductors on continuously loaded feeders. Finally, when a circuit is close to the limit, prefer to solve it by increasing conductor size rather than shortening the run in a way that complicates the installation — copper is cheap relative to the labor and disruption of rerouting. Applying these rules alongside the exact calculation this tool performs will keep virtually every circuit comfortably within code and specification, and will flag the rare cases where a fundamentally different distribution approach is the right answer.

Key Takeaway

Voltage drop is the quiet constraint that ampacity tables ignore: a conductor can be perfectly safe from an overheating standpoint yet still deliver too little voltage to the load at the end of a long run. Keep branch circuits under 3% and the total path under 5%, verify motor circuits at both running and starting current, remember that low-voltage and DC systems are dominated by drop rather than ampacity, and treat the limits as tighter on continuously loaded and efficiency-sensitive circuits. Whenever a run approaches the limit, the fix is a larger conductor, a higher distribution voltage, or a shorter path — and this calculator tells you exactly which conductor size clears the budget for your specific current, length and voltage.

Limitations & Disclaimer

This tool uses the standard circular-mil approximation at 75°C and typical power factors. It does not automatically include conductor reactance for every case, harmonic effects on nonlinear loads, unbalanced loading, or the exact temperature of the installed conductor. Voltage-drop limits in the NEC are recommendations; the enforceable limits are those in your project specification and the specific cases the NEC does mandate (e.g., fire pumps, sensitive electronic equipment). Verify critical circuits with the exact AC-resistance method and have final designs reviewed by a licensed Professional Engineer against the NEC edition adopted by your Authority Having Jurisdiction.