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
The Informational Notes to NEC 210.19(A) (branch circuits) and 215.2(A) (feeders) put the figure at 3% to the farthest outlet and 5% for feeder plus branch together, as the level that gives "reasonable efficiency of operation". They are advisory (NEC 90.5). Beyond them motors run hotter and deliver less torque, and lamps and electronics see less than their rated voltage.
Use recommended AWG if needed
If your selected size fails, the calculator shows the smallest listed size that meets 3% at these inputs. Voltage drop is one of two checks: confirm the ampacity, the 240.4(D) breaker cap and any derating separately — the wire size calculator on this site does both together.
Voltage drop — the engineering check every electrician runs
Voltage drop is the voltage lost along a conductor to its own resistance: the longer the run, the higher the current and the smaller the conductor, the more is lost before the load. The National Electrical Code sets no mandatory limit. The Informational Notes to NEC 210.19(A) (branch circuits) and 215.2(A) (feeders) recommend no more than 3% to the farthest outlet and 5% for feeder and branch circuit together as the level that gives "reasonable efficiency of operation", and NEC 90.5 makes informational notes explanatory rather than enforceable. Local amendments sometimes adopt the figures as requirements. Motors are the load that suffers first — torque falls with the square of voltage — then lamps dim and electronics see less than their rated supply.
The formula
Single phase (and DC): VD = 2 × K × I × L / CM. Balanced three phase: VD = √3 × K × I × L / CM. I is the load current in amps, L the one-way length in feet and CM the conductor area in circular mils from NEC Chapter 9 Table 8. K is the resistance of one foot of conductor one circular mil in area at 75 °C, and it comes from the same table: 1 AWG stranded copper is 0.154 Ω per 1000 ft over 83,690 circular mils, which is 12.9 Ω·cmil/ft; the aluminium column gives 0.253 Ω per 1000 ft, or 21.2. The 2 is the out-and-back path of a two-wire circuit; √3 is the line-to-line relationship of a balanced three-wire system. This is the resistive drop only: reactance is ignored, which matters little at branch-circuit sizes and more for large conductors at lagging power factor, where the full phasor method of IEEE Std 141 applies.
Induction-motor torque follows the square of the voltage. A 5% drop leaves 0.95² — about 90% — of the torque, on the hot afternoon when the motor needs all of it. That is why 3% is a design number, not a fault threshold.
Practical upsizing
Long runs are the usual culprit, and the numbers turn quickly. A 20 A load on 12 AWG copper over 100 ft one-way drops 7.9 V by the formula above — 3.3% at 240 V, just outside the note, but 6.6% at 120 V, where the same run needs 8 AWG (2.6%). Stretch it to 500 ft at 120 V and nothing smaller than 1 AWG stays inside 3%; at 240 V it takes 4 AWG. That is why voltage drop rather than ampacity sets the size on PV string runs, far-end EV chargers and long feeders to outbuildings — and why a 3% drop already costs about 6% of motor torque (0.97² = 0.94).
ASEAN voltage standards
The formula is universal; the voltages and codes are not. Singapore, Malaysia and Indonesia run 230/400 V at 50 Hz, Hong Kong 220/380 V, Vietnam 220/380 V, and the US and Canada 120/240 V split-phase and 208/480 V three phase at 60 Hz — so the same volts of drop are a smaller percentage at 230 V than at 120 V. Metric practice works in mm² and Ω/km rather than circular mils and K, under the IEC 60364 / BS 7671 family of codes that Singapore's SS 638 and Malaysia's MS 1979 follow; BS 7671 Appendix 4 recommends 3% for lighting and 5% for other loads from a public supply. Same physics, different tables.
Why single-phase doubles, three-phase takes √3, and where 12.9 comes from
NEC 210.19(A) Informational Note: ≤3% at the farthest outlet, ≤5% feeder plus branch; 215.2(A) says the same for feeders. Advisory, not enforceable (NEC 90.5).
Single phase: VD = 2×K×I×L/CM. Three phase: VD = √3×K×I×L/CM.
K = 12.9 copper, 21.2 aluminium Ω·cmil/ft — Chapter 9 Table 8 at 75 °C: 1 AWG stranded is 0.154 and 0.253 Ω/kft over 83,690 cmil.
Longer runs = more drop. Doubling length = doubling drop.
Induction-motor torque follows V²: a 5% drop leaves about 90% of the torque (0.95² = 0.9025).
Voltage drop is different from ampacity. NEC Table 310.16 covers ampacity; voltage drop is the additional check.
Long runs, not big loads, are where drop governs: PV string runs and far-end EV chargers commonly need a size above what ampacity alone requires.
NEC 90.5: informational notes are explanatory, not enforceable. Local amendments can make 3%/5% mandatory — check the code of record.
Table 8 resistances are stated at 75 °C; its Note 2 corrects them with R₂ = R₁[1 + α(T₂ − 75)], α = 0.00323 copper, 0.00330 aluminium. A hotter conductor drops more.
Resistive drop only: reactance and power factor are ignored. Fine at branch-circuit sizes; on large conductors at lagging power factor the phasor method of IEEE Std 141 gives more drop, not less.
Frequently asked questions
No. NEC 90.5 makes informational notes explanatory only, so the 3% and 5% figures in 210.19(A) and 215.2(A) are recommendations for "reasonable efficiency of operation". Jurisdictions can adopt them through local amendments and project specifications often do, so check the code of record for the job. The mandatory parts of conductor sizing are ampacity and overcurrent protection.
Copper has the lower resistance, so it is the smaller conductor for a given drop and the standard choice for branch circuits. Aluminium is 61% IACS conductivity (Chapter 9 Table 8, Note 4), so its resistance is about 1.64 times copper's at the same size; each AWG step changes area by about 1.26×, so aluminium needs roughly two sizes up for the same drop. It is lighter and cheaper per ampere, which is why it is common on service entrances and long feeders.
Yes. The single-phase formula is the two-wire resistive drop, which is exactly the DC case: use it for PV strings at array voltage, battery circuits and DC charging runs. The NEC states its 3%/5% figures for AC branch circuits and feeders, not for DC; designers borrow them, and PV designers often work to tighter targets because a string run is long and the drop is energy lost all day.
Not in this formula. They change the conductor's ampacity — NEC Tables 310.15(B)(1)(1) and 310.15(C)(1) — which is a separate check, done in the wire size calculator. Temperature does change resistance, and this page's K is the 75 °C value from Chapter 9 Table 8, so a conductor running cooler than 75 °C drops slightly less than shown.
Two effects pull opposite ways. K is taken at 75 °C, the top of the common insulation rating; a conductor running cooler has lower resistance, so the page over-states drop there. Reactance is ignored; at branch-circuit sizes it is a small term, but for large conductors — roughly 1/0 AWG and up — at lagging power factor the reactive drop is significant and the page under-states it. Use the IEEE Std 141 phasor method for large feeders.
Because it applies more checks. This page sizes for voltage drop only. The wire size calculator also applies Table 310.16 ampacity with ambient and grouping corrections, the 110.14(C) termination clamp and the 240.4(D) small-conductor breaker cap, and takes the larger of the two answers. If it gives a bigger conductor, ampacity governed; if this page does, drop governed.
Work in metres, mm² and Ω/km. VD = 2 × I × L × R / 1000 for single phase or DC and √3 × I × L × R / 1000 for three phase, with L the one-way length in metres and R the conductor resistance in Ω/km (IEC 60228 gives the maximum at 20 °C by conductor class; correct it to operating temperature). Singapore's SS 638 and Malaysia's MS 1979 sit in the IEC 60364 / BS 7671 family, where BS 7671 Appendix 4 recommends 3% for lighting and 5% for other loads from a public supply.
NEC 625.42 treats EV supply equipment as a continuous load, so a 40 A charger is sized at 125%: 50 A conductors and breaker. For the drop itself use the actual 40 A. On 6 AWG copper at 240 V that is 1.6% at 100 ft one-way and 2.5% at 150 ft; it reaches 3% at about 180 ft, and beyond that 4 AWG. Enter your own length above rather than trusting a rule of thumb, and have a licensed installer confirm the ampacity side.
For motors, yes. Starting current is several times the running current for the first seconds, so the momentary drop is several times the steady-state figure this page reports, and other loads on the same panel see it as a flicker. Motor-circuit practice sizes conductors for the steady-state drop and checks starting voltage separately; soft-starters and variable-frequency drives cut the inrush at source.
NFPA 70, National Electrical Code, 2023 — the 210.19(A) and 215.2(A) Informational Notes, 90.5, and Chapter 9 Table 8 for conductor properties; nfpa.org offers free read-only access. IEEE Std 141-1993, Recommended Practice for Electric Power Distribution for Industrial Plants (the Red Book), Chapter 3, for the full AC voltage-drop method.
Method & sources
How it computes
Resistive voltage drop over a one-way run: VD = 2·K·I·L/CM for single phase or DC and √3·K·I·L/CM for balanced three phase, with I in amps, L in feet and CM from NEC Chapter 9 Table 8. K = 12.9 Ω·cmil/ft (copper) and 21.2 (aluminium) at 75 °C, i.e. Table 8's 1 AWG stranded resistances 0.154 and 0.253 Ω/kft × 83,690 cmil. Judged against the 3% (branch or feeder) / 5% (total) NEC 210.19(A) and 215.2(A) Informational Notes; recommends the smallest listed size meeting 3%.
What this tool implements
- NFPA 70 (NEC) 2023, 210.19(A) and 215.2(A) Informational Notes — 3% to the farthest outlet, 5% feeder plus branch circuit, for 'reasonable efficiency of operation'; advisory under NEC 90.5, not enforceable.
- NEC Chapter 9 Table 8 conductor properties: circular-mil areas (14 AWG 4,110 … 4/0 211,600; kcmil exact) and DC resistance at 75 °C; K is derived from that table rather than taken as a rule of thumb.
- Resistive drop only: reactance and power factor are ignored, which is close at branch-circuit sizes and under-states drop on large conductors at lagging power factor; K at 75 °C over-states drop for a conductor running cooler.
- Voltage drop uses the actual load current, not the 125% continuous-load design current — ampacity, derating and the 240.4(D) cap are not checked here (see the wire size calculator).
Sources
- NFPA 70, National Electrical Code, 2023 edition. Quincy, MA: National Fire Protection Association; 2022. 90.5; 210.19(A) and 215.2(A) Informational Notes (voltage drop); Chapter 9 Table 8, Conductor Properties. https://www.nfpa.org/codes-and-standards/nfpa-70-standard-develop…
- NEC Chapter 9 Table 8, Conductor Properties — reproduction with Notes 1–5 (75 °C basis, temperature-correction equation R₂ = R₁[1 + α(T₂ − 75)], α_Cu = 0.00323, α_Al = 0.00330; aluminium 61% IACS). NECA-IBEW Electricians. http://necaibewelectricians.com/wp-content/uploads/2013/11/Table_…
- ASTM B258-18. Standard Specification for Standard Nominal Diameters and Cross-Sectional Areas of AWG Sizes of Solid Round Wires Used as Electrical Conductors. West Conshohocken, PA: ASTM International; 2018. https://www.astm.org/b0258-18.html
What can make this go out of date
- NFPA 70 is revised on a three-year cycle (2023 edition current; next edition 2026). The 3%/5% notes and Table 8 have been stable across editions, but the section numbering of the notes has moved before.
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