Voltage Drop Calculator
This free voltage drop calculator answers the question behind every long wire run: how much of your voltage is lost in the wire before it reaches the load? Enter the circuit type, voltage, load current, conductor material and gauge, and the one-way run length, and it returns the drop in volts and percent, the voltage your equipment actually receives, and the longest run that wire can serve while staying inside the 3% recommendation.
Voltage drop is mostly a long-run problem — the circuits to a well pump, a detached garage or shed, a gate operator, or a hot tub at the far corner of the yard. Motors are the loads that suffer most: a pump or compressor starved of voltage runs hot, starts hard, and dies early. On runs past 75–100 feet, drop — not ampacity — is usually what forces the wire a size or two larger.
The math is the standard K-factor method derived from NEC Chapter 9 Tables 8 and 9 (K = 12.9 for copper, 21.2 for aluminum), and the calculator is explicit about a widely misquoted point: the 3% and 5% figures are Informational Note recommendations in NEC 210.19 and 215.2(A), not enforceable code. Sizing reference only — always verify your circuit with a licensed electrician and your local AHJ. Free, no signup.
Voltage Drop Calculator
How much voltage do you lose over a wire run? Enter your circuit, wire size, and one-way distance to get the drop in volts and percent — with the NEC 3% recommendation check. Free, no signup.
Circuit
Conductor & run
Measure the actual cable path (up walls, across ceilings), not the straight-line distance. The out-and-back length is handled by the formula — enter one-way only.
Calculation Formulas
Out-and-back circuit: K is the conductor resistivity constant (ohms·circular-mil per foot at 75°C), I is load current in amps, L is the ONE-WAY run length in feet, CM is the conductor cross-section in circular mils. The factor 2 accounts for the round trip.
Example:
120 V, 15 A, 100 ft one-way on 12 AWG copper: Vd = 2 × 12.9 × 15 × 100 ÷ 6,530 = 5.93 V (4.9%).
Balanced three-phase circuits replace the round-trip factor 2 with √3 (1.732) because the return path is shared across phases.
Example:
Same run on a 3-phase circuit drops 1.732/2 = 86.6% of the single-phase value.
The drop expressed against the system voltage — the number the NEC Informational Notes reference.
Example:
5.93 V on a 120 V circuit = 4.9%.
What the equipment actually receives. Motors, compressors, and well pumps are the loads most affected by low voltage.
Example:
120 V − 5.93 V = 114.1 V at the load.
The same formula solved for length: the longest one-way run this gauge can serve at this load while staying within the 3% branch-circuit recommendation.
Example:
12 AWG copper at 15 A on 120 V: L_max = 0.03 × 120 × 6,530 ÷ (2 × 12.9 × 15) ≈ 60 ft.
The mathematical definition of the American Wire Gauge — diameter in mils, squared. Not a code table; it is how AWG sizes are defined.
Example:
14 AWG → ~4,110 CM; 12 AWG → ~6,530 CM.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| K (copper, 75°C) | 12.9 Ω·CM/ft | Resistivity constant for copper conductors — derived from NEC Chapter 9 Table 8/9 conductor properties. |
| K (aluminum, 75°C) | 21.2 Ω·CM/ft | Resistivity constant for aluminum conductors — aluminum has roughly 61% of copper's conductivity. |
| Three-phase multiplier | 1.732 (√3) | Replaces the single-phase round-trip factor of 2 for balanced three-phase circuits. |
| Branch-circuit recommendation | 3% | From the Informational Note to NEC 210.19 — a recommendation, NOT an enforceable code limit (NEC 90.5(C)). |
| Combined feeder + branch recommendation | 5% | From the Informational Note to NEC 215.2(A) — also a recommendation, not enforceable code. |
| 12 AWG copper | 6,530 CM | The most common 20 A branch-circuit conductor in US homes. |
| 14 AWG copper | 4,110 CM | The most common 15 A branch-circuit conductor in US homes. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
NEC Chapter 9, Tables 8 & 9(NFPA 70 (2023) Ch. 9)
View StandardConductor properties: DC resistance (Table 8) and AC resistance/reactance (Table 9). The K-factor method used here is the standard field derivation from these tables.
Key Requirements:
- •Conductor resistance values by size and material
- •Basis for the K = 12.9 (Cu) / 21.2 (Al) constants at 75°C
NEC 210.19 Informational Note(NFPA 70 (2023) 210.19 IN)
View StandardRecommends limiting branch-circuit voltage drop to 3% (and 5% combined with the feeder) for reasonable efficiency of operation. Informational Notes are explanatory, not enforceable.
Key Requirements:
- •3% branch-circuit drop recommendation
- •5% combined feeder + branch recommendation
NEC 215.2(A) Informational Note(NFPA 70 (2023) 215.2(A) IN)
View StandardThe feeder-side counterpart of the 210.19 note — same 3%/5% guidance for feeders.
Key Requirements:
- •Feeder voltage-drop guidance mirroring 210.19
NEC 90.5(C) — Explanatory material(NFPA 70 (2023) 90.5(C))
View StandardStates that Informational Notes are explanatory material and are not enforceable as requirements of the Code — the reason this calculator labels 3%/5% as recommendations.
Key Requirements:
- •Informational Notes are non-mandatory
American Wire Gauge (AWG)(ASTM B258)
View StandardThe geometric definition of wire gauge sizes. Circular-mil areas used here follow directly from the AWG definition.
Key Requirements:
- •d = 0.005 × 92^((36−n)/39) inches
- •CM = (diameter in mils)²
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Homeowner electrical permits
Owner-occupied single-family homes
Many states let an owner-occupant pull their own electrical permit for their primary single-family residence, with required inspection. Rules exclude rentals and multifamily almost everywhere.
Regional Examples:
Restrictive jurisdictions
Licensed-electrician states and cities
Some jurisdictions effectively require a licensed electrician for nearly all wiring work.
Regional Examples:
NEC edition adoption
2020 / 2023 / 2026 editions
States adopt NEC editions on different schedules, typically lagging 2–4 years. The K-factor math is unchanged across editions, but the table numbering and some requirements differ.
Regional Examples:
Long-run rural situations
Wells, gates, outbuildings
Voltage drop is mostly a rural and outbuilding problem — long runs to well pumps, detached garages, and gate operators are where the 3% recommendation actually bites.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
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How to Use This Calculator
- Pick your circuit type — single-phase AC for almost every home circuit, DC for solar/battery runs, or three-phase.
- Select the system voltage (120 V or 240 V for household circuits) and enter the load current in amps.
- Choose the conductor material (copper or aluminum) and the wire size you are running or considering.
- Enter the ONE-WAY run length in feet — measure the actual cable path, not the straight-line distance.
- Click Calculate to get the voltage drop in volts and percent, the voltage at the load, and the maximum one-way run this wire supports at the 3% recommendation.
Why the 3% Figure Is a Recommendation, Not Code
The 3% branch-circuit and 5% combined figures most electricians quote come from Informational Notes to NEC 210.19 and 215.2(A) — and NEC 90.5(C) states that Informational Notes are explanatory material, not enforceable requirements. That does not make them ignorable: excessive drop wastes energy as heat in the wire and starves motors, which is why the recommendation exists. But it does mean an inspector cannot fail an installation on voltage drop alone in most jurisdictions (a few local codes do adopt hard limits — check yours). This calculator flags the recommendation honestly, shows the actual voltage reaching your load, and leaves the conductor sizing decision where it belongs: verified against ampacity and confirmed with a licensed electrician.
Frequently Asked Questions
How do you calculate voltage drop?
The standard field method is the K-factor formula: voltage drop = 2 × K × I × L ÷ CM for single-phase and DC circuits (1.732 instead of 2 for three-phase), where K is the conductor constant (12.9 for copper, 21.2 for aluminum at 75°C, derived from NEC Chapter 9 Tables 8–9), I is the load in amps, L is the one-way run length in feet, and CM is the wire's cross-section in circular mils. Example: 15 A over 100 ft of 12 AWG copper on a 120 V circuit drops 2 × 12.9 × 15 × 100 ÷ 6,530 = 5.9 V, or about 4.9%. This calculator runs that math for any AWG size from 14 to 4/0.
Is the 3% voltage drop rule actually code?
No — and this is one of the most misquoted points in residential wiring. The 3% branch-circuit and 5% combined figures come from Informational Notes to NEC 210.19 and 215.2(A), and NEC 90.5(C) explicitly says Informational Notes are explanatory material, not enforceable requirements. An inspector generally cannot fail an installation on voltage drop alone (though a few local amendments do adopt hard limits — check your jurisdiction). It is still a good recommendation: excessive drop wastes energy as heat and shortens the life of motors.
Do I enter the one-way distance or the round trip?
Enter the ONE-WAY distance from the panel to the load — the formula's factor of 2 accounts for the round trip out and back on both conductors. Measure the actual path the cable takes (up the wall, across the ceiling, around obstructions), not the straight-line distance; the real path is often 20–30% longer than it looks on paper, and length is a direct multiplier in the drop.
Why does voltage drop matter on long runs?
Two reasons: wasted energy and unhappy motors. Every volt dropped in the wire becomes heat in the wire instead of work at the load. Resistive loads like heaters just run slightly weaker, but motors — well pumps, compressors, garage door openers, hot tub pumps — draw MORE current at lower voltage, run hotter, start harder, and fail earlier. That's why voltage drop is mostly a long-run problem: circuits to wells, detached garages, sheds, and gates are where the 3% recommendation actually changes what wire you pull.
Does this calculator tell me what size wire I need?
Not by itself, on purpose. This tool answers one question — how much voltage a given wire loses over a given run — and shows the longest run each gauge supports at the 3% recommendation. Choosing a conductor for a circuit also requires an ampacity check (NEC Table 310.16 with its correction factors) and the breaker rules of NEC 240.4(D), which are a separate calculation. Use this for the run-length side of the decision, and verify the full circuit design with a licensed electrician.
Can I do my own electrical work as a homeowner?
It depends on where you live. Many states let an owner-occupant pull a permit and do wiring in their own single-family home with a required inspection — Washington, Minnesota, and Michigan, for example, have explicit homeowner provisions. Others are far more restrictive: Massachusetts effectively requires licensed electricians for installation work, New York City requires a licensed master electrician, and Hawaii generally requires a contractor even for homeowners. Rentals and multifamily are excluded almost everywhere. Always confirm with your local Authority Having Jurisdiction before starting — a permit and inspection are required even where DIY is allowed.