Every electrical project that involves a long run starts with the same nagging question: is the wire big enough? The lights work fine on the bench but dim every time the compressor cycles. The motor runs but draws more amps than the nameplate says it should. The control panel resets randomly under load. The diagnosis, nine times out of ten, is voltage drop — the voltage lost between the panel and the load because the conductor is too small for the distance.
Voltage drop is governed by Ohm's Law and the resistance of the wire. The math is well-defined, the National Electrical Code gives clear recommendations, and a 60-second calculation tells you whether to use 12 AWG or step up to 10 AWG before the conduit goes in. This guide walks through the NEC-compliant formula, the wire size chart, the difference between copper and aluminum, three worked examples for residential, commercial and industrial circuits, and the mistakes that cause undersized installations.
Single-phase: VD = (2 × K × L × I) ÷ CM. K = 12.9 for copper, 21.2 for aluminum. L is one-way length in feet. I is current in amps. CM is circular mils from NEC Chapter 9, Table 8. NEC recommends max 3% drop on branch circuits and 5% combined feeder + branch. Step up one or two AWG sizes on any run over 100 ft.
The Quick Answer
Three facts run all of voltage drop math, and if you only remember these you can size any conductor:
- NEC formula (single-phase): VD = (2 × K × L × I) ÷ CM
- K constants: 12.9 for copper, 21.2 for aluminum (at 75°C operating temperature)
- NEC recommendation: max 3% on branch circuits, max 5% combined feeder + branch
The 2 in front of K accounts for the round-trip path (hot + neutral). For three-phase, swap the 2 for √3 (about 1.732). Everything else stays the same. The whole calculation takes longer to type than to do.
What Voltage Drop Is and Why It Matters
Voltage drop is the voltage lost as current flows through the resistance of a conductor. Every wire has some resistance, and Ohm's Law (V = I × R) says that current flowing through that resistance produces a voltage. That voltage is "dropped" along the length of the wire, leaving less voltage available at the load than was applied at the source.
A perfect conductor would have zero resistance and zero voltage drop. Real copper and aluminum have measurable resistance, and over long distances or at high currents the drop adds up fast. On a 100-foot run carrying 20 amps through 12 AWG copper, you lose about 4 volts before the load ever sees the circuit. That is roughly 3.3% of a 120 V system — right at the NEC recommended limit.
Why it matters in practice:
- Motors lose torque. Motor torque is proportional to the square of the voltage, so a 5% drop reduces starting torque by about 10%. Motors run hotter, draw more current to compensate, and fail earlier.
- Lighting dims and flickers. Incandescent lamps lose about 3% of their light output for each 1% of voltage drop. LED drivers usually compensate but lose efficiency.
- Electronics misbehave. Switching power supplies have undervoltage shutdown points. Computers, controllers and instrumentation can reset or behave erratically.
- Heating elements run cool. Resistive loads lose energy proportional to V². A 5% voltage drop means about 10% less heat from a water heater or oven.
- Wires run hot. The energy lost as voltage drop is dissipated as heat in the conductor itself. Excessive drop means excessive heating.
The NEC Voltage Drop Formula
The standard NEC voltage drop formula, used by every electrician and engineer in North America:
About the K constant
K is the resistance of one circular mil of conductor, one foot long, at 75°C operating temperature. The standard values:
- K = 12.9 for copper conductors
- K = 21.2 for aluminum conductors
The ratio (21.2 ÷ 12.9 = 1.64) tells you that aluminum has 64% more resistance than copper for the same wire size — which is why aluminum runs typically need to be one or two AWG sizes larger to match copper's performance.
About the L distance
L is the one-way distance from the source to the load. The factor of 2 in the single-phase formula already accounts for the return path through the neutral. Do not double the distance — the formula does that for you.
NEC Code Requirements
The National Electrical Code does not mandate a maximum voltage drop in any enforceable section. Voltage drop is officially a recommendation, not a code requirement. But the recommendation is universal and every inspector, engineer and electrician treats it as a hard target:
- NEC Article 210.19(A) Informational Note No. 4: Branch-circuit conductors that supply continuous loads should be sized for a maximum voltage drop of 3 percent at the farthest outlet.
- NEC Article 215.2(A)(1) Informational Note No. 2: Feeder conductors plus branch circuits combined should be sized for a maximum voltage drop of 5 percent.
The math: on a 120 V system, 3% equals 3.6 V; on 240 V, 3% equals 7.2 V; on 480 V, 3% equals 14.4 V. Higher voltage systems tolerate the same percentage drop with much larger absolute voltage allowances — one reason long runs are done at 480 V wherever possible.
If voltage drop on the branch alone exceeds 3%, upsize the branch wire. If feeder + branch combined exceeds 5%, upsize the feeder. Both targets are recommendations — local codes, manufacturer specs and engineering practice can require stricter limits.
Step-by-Step Voltage Drop Calculation
The four steps to size any conductor for voltage drop:
- List your inputs. One-way length (L, feet), current (I, amps), system voltage (120, 208, 240, 480), single- or three-phase, conductor material (copper or aluminum).
- Pick a starting wire size from NEC Table 310.16 based on ampacity. This is the minimum size that can carry the current. Voltage drop may push you larger.
- Look up the circular mils for that AWG from NEC Chapter 9, Table 8 (chart below).
- Plug into the formula and check the percentage. If under 3% (branch) or 5% (combined), you are good. If over, step up one AWG and recalculate.
Start at 12 AWG: CM = 6530
VD = (2 × 12.9 × 50 × 20) ÷ 6530 = 3.95 V
Percentage: 3.95 ÷ 120 × 100 = 3.3%
Verdict: Slightly over 3% NEC recommendation. Step up to 10 AWG:
VD = (2 × 12.9 × 50 × 20) ÷ 10380 = 2.49 V = 2.07% ✓
AWG Wire Size Chart with Circular Mils and Ampacity
The numbers you need for every voltage drop calculation. Circular mils from NEC Chapter 9, Table 8. Ampacity from NEC Table 310.16 (copper, 75°C, three current-carrying conductors in raceway).
| AWG / kcmil | Circular Mils (CM) | Ampacity (Copper 75°C) | Ampacity (Aluminum 75°C) |
|---|---|---|---|
| 14 AWG | 4,110 | 20 A | — |
| 12 AWG | 6,530 | 25 A | 20 A |
| 10 AWG | 10,380 | 35 A | 30 A |
| 8 AWG | 16,510 | 50 A | 40 A |
| 6 AWG | 26,240 | 65 A | 50 A |
| 4 AWG | 41,740 | 85 A | 65 A |
| 2 AWG | 66,360 | 115 A | 90 A |
| 1 AWG | 83,690 | 130 A | 100 A |
| 1/0 AWG | 105,600 | 150 A | 120 A |
| 2/0 AWG | 133,100 | 175 A | 135 A |
| 3/0 AWG | 167,800 | 200 A | 155 A |
| 4/0 AWG | 211,600 | 230 A | 180 A |
| 250 kcmil | 250,000 | 255 A | 205 A |
| 500 kcmil | 500,000 | 380 A | 310 A |
The ampacity column tells you the minimum legal size for the current. The circular mils column drives the voltage drop calculation. Use ampacity to find your starting size, then check voltage drop to see if you need to go larger. For the full wire gauge reference — every size, both metals, and wire size by amperage — see our AWG Wire Gauge & Ampacity Chart.
Skip the manual lookup
Enter length, current and voltage — the calculator returns voltage drop, recommended wire size and NEC compliance status. Free, no sign-up.
Copper vs Aluminum Wire
The choice between copper and aluminum matters for voltage drop because aluminum has about 64% more resistance per circular mil. Practical implications:
- Aluminum needs to be 1–2 AWG sizes larger to match copper's voltage drop for the same length and current. A 4 AWG copper run typically becomes 2 AWG aluminum.
- Aluminum is cheaper per foot, often by 30–50%. On long feeders and service entrances, the cost savings outweigh the size penalty.
- Aluminum requires anti-oxidation paste (Penetrox, Noalox, or equivalent) at every termination, and listed AL/CU rated connectors. Old aluminum branch circuits in homes are a known fire hazard; modern aluminum feeders properly terminated are not.
- Copper is standard for branch circuits, especially anything 8 AWG or smaller. Aluminum is common for service entrances, feeders to subpanels, and large industrial runs.
- Aluminum expands and contracts more than copper with temperature, so torqued connections can loosen over time. Listed compression lugs and proper installation are critical.
Old aluminum branch circuit wiring (typically 12 AWG) installed in homes before 1972 used standard wire connectors not rated for aluminum. The thermal expansion mismatch loosens connections over time, creating arcing and fire risk. If your home has aluminum branch wiring, have a qualified electrician install COPALUM crimps or AlumiConn connectors at every outlet, switch and junction. Modern aluminum feeders (large gauges with listed connectors) do not share this issue.
Three Worked Examples
Example 1 — Residential garage subpanel
Scenario: 100 ft from main panel to detached garage, 60 A subpanel, 240 V single-phase, copper conductors.
VD = (2 × 12.9 × 100 × 60) ÷ 26,240 = 5.90 V
Percentage: 5.90 ÷ 240 × 100 = 2.46% ✓ Under 3%
Verdict: 6 AWG copper works for the branch. Use 6 AWG.
Example 2 — Commercial HVAC compressor
Scenario: 175 ft from MDP to rooftop unit, 50 A continuous load, 480 V three-phase, aluminum conductors.
VD = (√3 × 21.2 × 175 × 50) ÷ 26,240 = 12.25 V
Percentage: 12.25 ÷ 480 × 100 = 2.55% ✓
Verdict: 6 AWG aluminum stays under 3%. Confirm 50 A ampacity per Table 310.16 (50 A allowed). Use 6 AWG aluminum.
Example 3 — Industrial motor feeder
Scenario: 300 ft from switchgear to 100 HP motor, full-load amps 124 A, 480 V three-phase, copper conductors. Use 125% of FLA for the conductor size per NEC 430.22 = 155 A design load.
VD = (√3 × 12.9 × 300 × 124) ÷ 133,100 = 6.24 V
Percentage: 6.24 ÷ 480 × 100 = 1.30% ✓
Verdict: 2/0 copper has comfortable margin on both ampacity and voltage drop. Use 2/0 copper.
Higher system voltage = lower percentage drop for the same physical setup. Three-phase = lower drop than single-phase. Copper = smaller required wire size than aluminum. Long runs always punish small conductors. Plan for these tradeoffs before the conduit is in the wall.
Common Voltage Drop Mistakes
The errors that lead to undersized installations and field problems:
- Using round-trip distance instead of one-way. The 2 in the single-phase formula already accounts for the neutral return. Doubling L again gives you twice the drop and pushes you to unnecessarily large wire.
- Sizing only for ampacity, ignoring voltage drop. Code-minimum ampacity (Table 310.16) gets you a wire that will not melt; it does not guarantee acceptable voltage at the load. Always check voltage drop on any run over 50 feet.
- Mixing up K-values. Copper K = 12.9, aluminum K = 21.2. Swap them by accident and your calculation is off by 64%.
- Using motor starting current instead of running current. Motors draw 6–8× FLA at startup, but startup is measured in seconds. Voltage drop is a steady-state concern — size for running current, not locked-rotor current.
- Ignoring derating for ambient temperature or conduit fill. NEC Table 310.16 ampacity assumes 30°C ambient and ≤3 current-carrying conductors. Hot environments, MC cable with multiple circuits, or conduit fill above 3 conductors all require derating per NEC 310.15(B).
- Forgetting to check both feeder and branch. The 5% combined limit catches feeders that look fine alone but combine with a long branch to exceed the recommendation.
- Using nominal voltage instead of actual voltage. If the panel is already running low (say 117 V instead of 120 V), the voltage available at the load is even lower than the calculation suggests. On critical loads, measure the actual panel voltage.
- Skipping voltage drop on motor circuits. Motor torque is V², so a 5% drop is a 10% loss of torque. Branch circuits to motors should target 3% or less, even when the formula technically allows more.
"Ampacity keeps the wire from melting. Voltage drop keeps the load from suffering. You have to check both."— The two-step rule every conductor sizing follows
Once you know length, current and voltage, the Voltage Drop Calculator returns drop and percentage for a wire you have in mind — or the Wire Size Calculator works the other way, giving the minimum AWG from your load and distance. For wire-fill and conduit sizing, NEC Chapter 9 Tables remain the standard.
Key Takeaways
- Single-phase voltage drop: VD = (2 × K × L × I) ÷ CM. Three-phase: replace 2 with √3.
- K = 12.9 for copper, 21.2 for aluminum. L is one-way length in feet. CM from NEC Chapter 9, Table 8.
- NEC recommends 3% maximum on branch circuits, 5% combined feeder + branch (Article 210.19 / 215.2 Informational Notes).
- Always check both ampacity (Table 310.16) and voltage drop. Ampacity is the minimum; voltage drop usually controls on long runs.
- Aluminum has 64% more resistance than copper for the same wire size — plan for 1–2 AWG larger on aluminum runs.
- Higher system voltage tolerates longer runs at the same percentage. Use 480 V three-phase wherever practical on industrial circuits.
- Motor branch circuits are voltage-drop-sensitive (torque is V²). Target under 3% even when the formula allows more.
- Step up one AWG and recalculate when you hit the percentage limit. The cost increase is usually marginal compared to the cost of redoing the install.
