Voltage Drop Calculator
Voltage drop calculator. NEC-compliant AWG wire sizing for single + three phase circuits. % voltage drop, recommended conductor size, pass/fail verdict.
Voltage Drop Calculator
How to use the voltage drop calculator
Set phase + material
Single phase: residential 120/240 V circuits. Three phase: commercial / industrial 208/480 V circuits. Copper (default) vs aluminum (cheaper, larger size needed). The K factor differs: 12.9 Cu vs 21.2 Al — aluminum has higher resistance so needs upsized conductors.
Pick AWG / kcmil size
Standard branch circuits: 14 AWG (15A), 12 AWG (20A), 10 AWG (30A). Feeders go larger: 4/0 + kcmil sizes. Wire ampacity is NEC Table 310.16 — separate concern from voltage drop. Voltage drop becomes more important for long runs.
Enter voltage + amps + length
Source voltage at panel. Load current in amps. One-way length — the calculator handles the round-trip multiplier. For typical residential branch circuits 50-150 ft; long farm/industrial runs can be 500+ ft.
Check % drop against NEC
NEC Informational Note: ≤3% for branch circuits, ≤5% combined (branch + feeder). Industrial/commercial designs often target ≤2% for sensitive loads. Above 5%: equipment may malfunction, motors overheat, lighting flickers, breakers nuisance-trip.
Use recommended AWG if needed
If your selected AWG fails, the calculator shows the smallest acceptable size. Real-world consideration: upsize one step beyond minimum for margin (load growth + future-proofing). Cost vs reliability trade-off; commercial designs typically err conservatively.
Voltage drop — the engineering check every electrician runs
Voltage drop is the unavoidable loss of voltage along a conductor due to its resistance. The longer the run + the higher the current + the smaller the conductor → the larger the voltage drop. The National Electrical Code (NEC) Informational Note 215.2(A)(1) recommends voltage drop not exceed 3% on branch circuits and 5% on the combined branch + feeder run. Excessive voltage drop causes: motors to run hot + lose torque, incandescent lights to dim, electronics to malfunction, breakers to nuisance-trip, energy waste. Every commercial electrical design includes voltage-drop analysis; residential code-compliance work uses it for long branch circuits and dedicated equipment.
The formula
Single phase: VD = 2 × K × I × L / CM. Three phase: VD = √3 × K × I × L / CM. Where K = 12.9 Ω·CM/ft for copper or 21.2 for aluminum, I = amps, L = one-way length in feet, CM = conductor circular mils. The factor of 2 in single-phase reflects the round-trip (out + return); √3 in three-phase reflects the phase relationship in balanced 3-wire systems. This is the NEC-standard simplified formula assuming resistance-dominated drop. For very long runs or AC harmonics, reactance terms matter too; engineering practice uses fuller AC equations.
Voltage drop is invisible until your equipment misbehaves. A 5% drop on a motor circuit can reduce torque by 10% — exactly when the motor needs full power on a hot summer day. Engineers calculate this before construction; failure to do so causes intermittent problems for years.
Practical upsizing
Long runs are the typical culprit. A 20A branch circuit (12 AWG) over 100 ft has acceptable drop on 120 V (around 3%); same circuit at 500 ft would need upsizing to 6 AWG or larger. Commercial designs frequently upsize feeders one step beyond minimum for capacity headroom. Solar PV interconnections + EV charging stations are voltage-drop-sensitive due to long runs from inverter to utility connection. Industrial motor circuits especially benefit from low-drop conductor sizing — 2-3% voltage drop reduces motor torque ~5-8%, increasing failure rates over long-term operation.
ASEAN voltage standards
Voltage drop math is universal but voltage standards differ. Singapore + Malaysia + Indonesia: 230/400 V single/three phase, 50 Hz. Hong Kong: 220 V single, 380 V three phase. Vietnam: 220/380 V, 50 Hz. US/Canada: 120/240 V single (split phase), 208/480 V three phase, 60 Hz. The K factor in metric units is different but conceptually identical. ASEAN engineers use IEC 60364 standards (BS 7671 for Singapore equivalent of NEC); same fundamentals different code numbers.
10 Things to Know About Voltage Drop
NEC limit: ≤3% branch, ≤5% combined. Informational Note 215.2(A)(1).
Single phase: VD = 2×K×I×L/CM. Three phase: VD = √3×K×I×L/CM.
K factor: 12.9 copper, 21.2 aluminum Ω·CM/ft. Aluminum needs larger wire.
Longer runs = more drop. Doubling length = doubling drop.
Motors at 5% drop: ~10% torque loss. Significant for industrial loads.
Voltage drop is different from ampacity. NEC Table 310.16 covers ampacity; voltage drop is the additional check.
Solar PV + EV charging are commonly voltage-drop-sensitive due to long runs.
NEC voltage drop is informational, not mandatory — but adopted as best practice in most jurisdictions.
Conductor temperature affects K — hot conductors have higher resistance. Engineering formulas use 75°C standard.
Power factor + AC harmonics affect real-world drop. Simplified DC formula is conservative for most cases.
Frequently asked questions
Technically informational, not mandatory in the strict code-enforcement sense — but virtually all professional designs treat it as a requirement. Some jurisdictions adopt voltage drop as code mandate via local amendments. UL listings + manufacturer specifications often require it for compliance.
Copper: lower resistance, smaller size for same load, more expensive. Standard for branch circuits. Aluminum: higher resistance (needs larger conductor — typically 2 AWG sizes larger), cheaper, lighter. Common in service entrance, feeders, large utility runs. Cost vs labor + physical size trade-off — copper for short runs, aluminum for long heavy runs.
The single-phase formula works for DC since both involve only resistive drop. For DC PV systems (solar arrays), use the single-phase formula at array voltage. Battery + EV charging DC circuits: same math. The 3%/5% NEC limits are usually applied to DC PV circuits as well per NEC Article 690.
Multiple conductors in same conduit operate at higher temperature, reducing ampacity per NEC Table 310.15. This doesn\'t affect voltage drop directly but does affect what wire size is permissible. Use NEC tables for ampacity adjustment; the voltage drop formula assumes a single conductor at standard 75°C.
The simplified DC-resistance formula assumes 100% resistive drop. Actual AC circuits have a small reactance component (~5% additional drop on long industrial cables at 60 Hz). The conservative simplified formula errs on the side of larger conductor recommendations. Engineering software (ETAP, SKM) uses fuller AC formulas for precision.
No. All inputs stay in your browser.
For metric wire sizing (mm² cross-section): conductor resistance is in Ω/km values from IEC 60228. Formula: VD = 2 × R × I × L (single phase) or VD = √3 × R × I × L. The 3%/5% drop limit is internationally consistent (IEC 60364, BS 7671). For Singapore + Malaysia practice, use mm² + Ω/km from local supply company specifications.
Level 2 EV charging is typically 32A or 40A continuous (need to size for 125% per NEC 625.41, so 40A or 50A circuit). Voltage drop matters because charge sessions are long: a 5% drop reduces charging power 10%, extending charge time. For 40A @ 240V over 100 ft: 6 AWG copper works; over 150 ft, upsize to 4 AWG. Always verify with your installer.
For motor circuits, yes. Motor starting current can be 5-7× running current for a few seconds. Voltage drop at startup can be 15-25%, causing other equipment on the same panel to flicker / restart. Engineering practice for motor circuits often targets ≤3% steady-state drop knowing inrush will be worse. Soft-starters + VFDs reduce inrush dramatically.
NEC 2023 (NFPA 70) — full code text via nfpa.org. NEC Handbook with commentary. Mike Holt Enterprises (mikeholt.com) — practical electrical training. IEEE Std 141 (Red Book) for industrial power systems. ETAP + SKM Power Tools — professional voltage drop modeling software.
Related News
You may be interested in these recent stories from our newsroom.
No related news yet for this tool. Our editorial team publishes new pieces every week.
Browse all news →75 more free tools
Calculators, converters, security tools — no signup.