Voltage Drop Calculator
This free voltage drop calculator defaults to NEC / AWG (feet, circular mils, K-method, 3%/5% check), with IEC / mm² resistive screening as an alternate mode. Covers 1φ, 3φ, and DC ohmic loss—not a discrete-resistor electronics calculator.
Next step: confirm ampacity with the Cable Size & Wire Gauge Calculator (NEC) and protection with the breaker size calculator.
Quick Answer
NEC informational notes commonly discuss ≤3% branch and ≤5% feeder+branch planning targets. Use NEC / AWG for US wire-gauge jobs (including 12 V DC voltage drop / loss and long feeders), or IEC / mm² for metric runs. Both modes are PF≈1 resistive screens—not full Chapter 9 impedance designs. Scan the % drop quick-reference table for a fast look-up.
Quick presets (IEC / mm²)
Load metric examples, then refine.
Quick presets (NEC / AWG)
US AC feeders plus 12 V DC and long-run screens—then refine AWG/length.
Calculator inputs
About this calculator
This voltage drop calculator screens steady-state resistive drop for copper or aluminum runs—NEC AWG / K-method first, then IEC mm²—so you can compare feeders before full reactance and code-table work. Part of the protection calculators hub.
Parallel conductors & temperature (Southwire-style notes): If you run N parallel conductors per phase, divide the per-conductor current by N before reading % drop (or multiply ampacity capacity). Ambient above 30 °C and steel conduit AC reactance are not modeled here—use OEM/Southwire tables for final compliance. This page remains a resistive screening tool.
Results
Results (NEC / AWG)
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.
Apply suggested ~10 AWG on cable / wire gauge →
Live results update when you change inputs above. Formula: voltage drop formula.
Engineering disclaimer
Reactance, power factor, parallel runs, conduit temperature, and harmonic currents are not modeled. 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 Vnom 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)
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 ÷ Vnom) × 100. Planning discussions often cite ≤3% branch and ≤5% combined feeder+branch (informational notes).
| 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: voltage drop formula (NEC 3%/5%, 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 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?
Full NEC examples often include AC resistance/reactance, power factor, and temperature. This page’s NEC mode is a K-method resistive screen; IEC mode uses 20 °C DC resistivity. Use code tables for binding designs.
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.
Related tools and hub
Use this result with: Cable Size Calculator, Breaker Size Calculator, and kVA to Amps Calculator.