Voltage Drop Formula & Calculator

Free voltage drop calculator with the instant voltage drop formula for DC / 1φ / 3φ—then screen NEC AWG or IEC mm² on the same page.

DC / 1φ: VD = 2 × I × L × R (NEC: (2 × K × I × L) ÷ CM)
3φ voltage drop formula: VD = √3 × I × L × R (NEC: (√3 × K × I × L) ÷ CM)
AC with power factor: VD = factor × I × L × (R cos θ + X sin θ). At PF 1.00 the reactance term is zero.
%: (VD ÷ V_nom) × 100 — often ≤3% branch / ≤5% feeder+branch

Need R·X / NEC 3%/5% worked proofs? voltage drop calculation guide. After % is OK → cable size · breaker size.

Quick presets (IEC / mm²)

Load metric examples, then refine.

Calculator inputs

Code / units mode NEC / AWG (ft, K-method) IEC / mm² (m, ρ·L/A)

Solve for Choose the answer you need: the drop on a known run, the smallest conductor that still meets your % limit, or how far a known conductor can run.

Design current (A) Steady-state line current used for the voltage drop check.

One-way conductor length (ft) NEC K-method uses one-way feet; the 2 or √3 factor accounts for the circuit.

Wire gauge (AWG / kcmil)

Planning limit (%) Pick Custom limit… to enter any target drop — 2.5% is common for sensitive electronics, 10% for short DC runs.

Advanced calculator settings

Set conductor material, circuit type, nominal voltage, power factor, and raceway. Power factor 1.00 with PVC or aluminum raceway keeps the resistive screen. A lower power factor or steel conduit adds a screening reactance. This is not a full NEC Chapter 9 Table 9 lookup.

Material

System

Nominal voltage for % (V) Use L-N for typical 1φ / DC descriptions or L-L for 3φ (e.g. 480 V). Stay consistent with your design basis.

Power factor 1.00 keeps the resistive screen. Below 1.00 uses cos θ on resistance and sin θ on reactance. Not used for DC.

Raceway Screening reactance: 0.051 ohm/1000 ft nonmagnetic, 0.068 ohm/1000 ft steel. Not a size-by-size Chapter 9 Table 9.

Conductor temperature (°C) Resistance rises with temperature: R(T) = R(base) × (T + 234.5) / (base + 234.5) for copper (228.1 for aluminum). NEC K values are tabulated at 75 °C; IEC resistivity at 20 °C.

Conductors per phase (parallel sets) Identical size and length per set: circular mils (NEC) or mm² (IEC) multiply by N, so resistance divides by N. Verify terminations and ampacity rules with your AHJ.

About this calculator

This free voltage drop calculator is the live landing for voltage drop calculator and 3 phase voltage drop calculation formula screens—steady-state resistive drop for copper or aluminum runs with NEC AWG / K-method or IEC mm². Pick ampacity first on cable size, then verify % here. Method depth: 3-phase voltage drop formula guide. Hub: protection calculators.

Parallel conductors and temperature: Set conductors per phase to N (identical size and length) and the calculator divides resistance by N — no manual current splitting. Set conductor temperature to 60 / 75 / 90 °C and resistance is corrected from the 75 °C NEC K base or the 20 °C IEC resistivity base. A full NEC Chapter 9 Table 9 reactance row is still not modeled; steel versus PVC uses one screening reactance each.

Results

Results (NEC / AWG)

Voltage drop: ~7.9 V (example: 20 A · 100 ft · 12 AWG Cu · 120 V)
Drop (%): ~6.6% — compare to your 3% / 5% planning limit.
Model: NEC K-method screening (K=12.9 Cu). Switch modes or edit inputs for live results.

NEC planning check (5%): Above the selected planning limit — upsize AWG or shorten the run.

Live results update when you change inputs above. Formula: DC / 1φ / 3φ voltage drop formula.

Continue Your Calculation

If drop exceeds your planning target, return with the same load and one-way length to upsize AWG/mm². Then recheck breaker and OCPD coordination.

Cable / AWG sizing →

Engineering disclaimer

Power factor and a single screening reactance for PVC or steel are included. Parallel runs, conductor temperature, harmonics, and size-by-size Chapter 9 Table 9 rows are not. This is not a substitute for NEC Chapter 9 / IEC 60364-5-52 tables or engineer-stamped calculations.

Voltage drop % quick-reference table (NEC Cu, K-method)

Screening look-up for copper, single-phase / DC (2×K·I·L÷CM). Values are approximate percent of nominal voltage. Low-voltage DC (12 V) shows much higher % for the same AWG and feet—use the 12 V DC presets above to verify.

AWG	20 A · 100 ft · 120 V	40 A · 100 ft · 240 V	20 A · 200 ft · 120 V	20 A · 25 ft · 12 V DC
14 AWG	10.5%	10.5%	20.9%	26.2%
12 AWG	6.6%	6.6%	13.2%	16.5%
10 AWG	4.1%	4.1%	8.3%	10.4%
8 AWG	2.6%	2.6%	5.2%	6.5%
6 AWG	1.6%	1.6%	3.3%	4.1%

Note: 40 A · 100 ft · 240 V matches 20 A · 100 ft · 120 V % for the same AWG because both Vd and V_nom scale together in this resistive screen. Planning screens often target ≤3% branch / ≤5% feeder+branch. Confirm ampacity on the wire gauge / cable size calculator.

Voltage drop formula (NEC AWG & IEC)

3 phase voltage drop calculation formula

Live screen on this page: VD = (√3 × K × I × L) ÷ CM (NEC) or VD = √3 × I × L × R (IEC resistivity). Worked: 100 A · 200 ft · 3 AWG Cu → ≈ 8.5 V ≈ 1.8% @ 480 V — use preset 100 A · 200 ft · 3 AWG · 480 V 3φ. Full R·X derivation: guide Quick answer.

NEC / AWG (K-method)

Single-phase / DC: VD = (2 × K × I × L) ÷ CM

Three-phase: VD = (√3 × K × I × L) ÷ CM

K ≈ 12.9 (Cu) or 21.2 (Al); I = amps; L = one-way feet; CM = circular mils (NEC Chapter 9 Table 8 style). Percent = (VD ÷ V_nom) × 100. Planning discussions often cite ≤3% branch and ≤5% combined feeder+branch (informational notes).

AWG → circular mils (CM) screening values
AWG	CM	AWG	CM
14	4110	4	41740
12	6530	2	66360
10	10380	1/0	105600
8	16510	2/0	133100
6	26240	4/0	211600

DC voltage drop (explicit)

Choose DC (2-wire) in the calculator. NEC mode uses the same 2 × K × I × L ÷ CM screen as single-phase AC; IEC mode uses 2×R. For 12 V DC voltage drop / battery runs, percent loss rises quickly—use the 12 V presets, then confirm ampacity on the wire gauge calculator.

IEC / mm² (resistive)

Resistance one way: R = ρ · L / A. Copper ρ ≈ 0.017241; aluminum ≈ 0.028265 (Ω·mm²/m at 20 °C).

Single-phase / DC: V ≈ I · 2R. Three-phase: V ≈ √3 · I · R.

Deep dive: NEC 3%/5% voltage drop limits & worked examples.

Worked examples

NEC AC: 20 A, 100 ft one-way, 12 AWG Cu (6530 CM), 120 V 1φ → VD = (2 × 12.9 × 20 × 100) / 6530 ≈ 7.9 V ≈ 6.6%. Use the 120 V preset above.

12 V DC voltage drop: 20 A, 25 ft, 10 AWG Cu → ≈ 1.24 V ≈ 10.4% of 12 V (often too high for battery/inverter loads). Same run on 6 AWG → ≈ 4.1%. Use the 12 V DC presets.

Long feeder: 20 A, 250 ft, 10 AWG @ 120 V → ≈ 10.4%—upsize AWG or shorten the route. Use the long-run preset.

IEC: 16 A, 25 m, 2.5 mm² Cu, 230 V 1φ → ≈ 5.5 V / 2.4%. Use the metric presets.

Guides

Frequently asked questions

What is the NEC voltage drop formula (AWG)?

Screening form: 1φ/DC VD = (2 × K × I × L) ÷ CM; 3φ VD = (√3 × K × I × L) ÷ CM. K ≈ 12.9 Cu / 21.2 Al; L is one-way feet; CM from wire gauge. Select NEC / AWG mode above. These are planning checks—not a substitute for Chapter 9 impedance designs.

What is the 3 phase voltage drop calculation formula?

3 phase voltage drop calculation formula (resistive screen): VD = √3 × I × L × R, or NEC AWG VD = (√3 × K × I × L) ÷ CM. Example: 100 A · 200 ft · 3 AWG Cu · K 12.9 → ≈ 8.5 V; at 480 V ≈ 1.8%. Enter the same load above—full R·X form and NEC 3%/5% proofs: voltage drop calculation guide. Then confirm ampacity on cable size.

What is the voltage drop formula for 3 phase?

Voltage drop formula 3 phase (K-method): VD = (√3 × K × I × L) ÷ CM with one-way feet. With reactance: VD = √3 × I × (R cos θ + X sin θ) × L. Set System = 3-phase in this voltage drop calculator for an instant % screen.

Is this a voltage drop calculator?

Yes. Enter current, one-way length, and AWG or mm² for volts and percent on 1φ / 3φ / DC feeders. After % looks acceptable, size/confirm the conductor on the cable size calculator.

What are the NEC 3% and 5% voltage drop limits?

Informational notes discuss about 3% for branch (or feeder) and 5% combined feeder+branch. They are recommendations, not always enforceable rules—confirm with your AHJ. Use the planning-limit control in NEC mode.

How do you calculate voltage drop in a cable?

Use NEC AWG K-method (feet + circular mils) or IEC mm² resistivity (ρ·L/A). Select NEC or IEC mode above for 1φ, 3φ, or DC resistive screening—then confirm ampacity on the wire / cable size calculator.

What is the voltage drop per 100 ft?

It depends on amps, AWG, Cu/Al, phase, and voltage. Example screen: 20 A · 100 ft · 12 AWG Cu · 120 V 1φ ≈ 7.9 V ≈ 6.6%. Use the % quick-reference table or enter 100 ft in NEC mode for your exact load.

What is a volt drop calc vs voltage drop calculator?

Volt drop calc is the same screening job as a voltage drop calculator. Enter current, length, and conductor size (AWG or mm²) for volts and percent—then confirm ampacity on the cable size & wire gauge calculator.

Can I use this for DC voltage drop or DC voltage loss?

Yes. Choose DC (2-wire) in System. NEC mode uses the 2×K formula; IEC mode uses 2×R. It estimates ohmic loss, not converter or battery internal impedance. For 12 V battery / inverter runs, try the 12 V DC presets—percent drop is much higher than on 120 V for the same AWG and length.

Where is a voltage drop calculation table?

Use the % drop quick-reference table above for 20 A copper screening by AWG and length (including a 12 V DC column). For your exact current and gauge, enter values in the calculator—the table is a planning look-up, not a full Chapter 9 reprint.

Is this a voltage drop across a resistor calculator?

No. This page is for cable / feeder voltage drop in electrical distribution. Electronics V = IR across a discrete resistor is a different problem.

Why is my percent drop different from NEC examples?

At power factor 1.00 and a nonmagnetic raceway, this page matches the resistive K-method or 20 °C resistivity screen. A lower power factor or steel conduit adds a screening reactance (0.051 or 0.068 ohm/1000 ft), not a full Chapter 9 Table 9 row. Temperature correction is not applied.

Should I use line-to-line or line-to-neutral voltage for %?

Use the same reference your team uses for the circuit being studied. Be explicit in reports; mixing references changes the percentage even if the physical drop in volts is unchanged.

What if I need a smaller drop?

Increase conductor size (next AWG/mm²), shorten the route, reduce current, or raise system voltage. Confirm ampacity and protection independently on the wire gauge / cable size tool.

Does aluminum behave the same way in the tool?

Same formulas with higher K (21.2) or resistivity. Terminations, oxidation control, and ampacity tables still require manufacturer and code guidance.

How do you calculate volt drop in 240v?

Use the same screen with nominal voltage = 240 V: VD = (2 × K × I × L) ÷ CM for single-phase, then % = VD ÷ 240 × 100. At 40 A over 100 ft, 12 AWG copper lands near 6.6%, 10 AWG near 4.1%, and 8 AWG near 2.6%. Doubling the voltage from 120 V to 240 V halves the percent for the same load, length, and gauge.

What is the rule of thumb for voltage drop?

Keep the total at 5% or less from the source to the furthest outlet, and aim for 3% or less on any single branch or feeder. As a field estimate for copper at 120 V, budget roughly one AWG size per 100 ft of run: 12 AWG at 20 A drops about 6.6% over 100 ft, 10 AWG about 4.1%, and 8 AWG about 2.6%. Doubling the voltage to 240 V halves the percent for the same load, length, and gauge. If your spec is tighter than the NEC note — 2.5% is common for sensitive electronics — set the planning limit to Custom limit… and enter it.

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