Voltage drop grows with current, distance, and resistance — which is why a short run can use a thinner wire than a long one carrying the same current. The formulas are:
Vd = 2 × K × I × L ÷ CM
K = 12.9 (copper) / 21.2 (aluminum)
I = current (A)
L = one‑way length (ft)
CM = circular mils
Vd = √3 × K × I × L ÷ CM
Same variables as single‑phase, with √3 (1.732)
for balanced three‑phase systems.
Branch circuit: 3% or less
Feeder + branch: 5% or less
Exceed these limits and motors run hot,
lighting dims, equipment misbehaves.
A 7‑amp load at 120 V over a 200‑foot run on 14 AWG copper drops about 8.8 volts — more than double the 3.6‑volt limit. Step up to 10 AWG and the drop falls to roughly 3.5 volts, inside the limit. Distance is why a "correct" wire for a short run can be wrong for a long one.
Motor Feeder Sizing (Continuous Load ×1.25): Circuits that run for three hours or more count as continuous loads and must be sized at 125% of the load current under NEC 210.20(A). A motor drawing 40 amps requires a conductor sized for 50 amps of ampacity, not 40. The wire size calculator applies this factor in motor‑feeder mode so you do not undersize a feeder that will run all day.
A short run can use a thinner wire than a long one carrying the same current. Always check voltage drop for runs over 50 ft.
Branch circuits: ≤3% drop. Feeders plus branch: ≤5% total. Exceeding these limits leads to performance issues.
Continuous loads (≥3 hours) must be sized at 125% of the load current under NEC 210.20(A) — a motor drawing 40 A needs a 50 A conductor.
Copper: K = 12.9. Aluminum: K = 21.2. Aluminum conductors must be larger to achieve the same voltage drop performance.
Correct wire sizing follows a simple rule: calculate for ampacity and for voltage drop, then use the larger result. Ampacity is the maximum current a conductor can carry safely, set by its insulation temperature rating. Voltage drop is the reduction in voltage between the source and the load, driven by conductor resistance over distance. Both matter, and they rarely agree on the same gauge.
The calculator checks both ampacity and voltage drop, then returns the larger gauge that satisfies both.
Continuous motor loads (≥3 hours) require 125% of the load current per NEC 210.20(A). The calculator applies this factor before checking ampacity and voltage drop.
American Wire Gauge (AWG) sizes conductors by cross‑sectional area, and — counterintuitively — a smaller AWG number means a thicker wire. The ampacity table below lists allowable current for insulated copper conductors at 30°C ambient with no more than three conductors in a raceway.
| Wire Size | 60°C | 75°C | 90°C |
|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A |
| 12 AWG | 20 A | 25 A | 30 A |
| 10 AWG | 30 A | 35 A | 40 A |
| 8 AWG | 40 A | 50 A | 55 A |
| 6 AWG | 55 A | 65 A | 75 A |
| 4 AWG | 70 A | 85 A | 95 A |
| 2 AWG | 95 A | 115 A | 130 A |
| 1/0 AWG | 125 A | 150 A | 170 A |
| 3/0 AWG | 165 A | 200 A | 225 A |
| 250 kcmil | 215 A | 255 A | 290 A |
Industrial systems add a layer the residential charts do not cover. A generator feeder carries continuous load and often a large motor-starting surge, so both the 125% continuous factor and the voltage-drop check matter more. The conductor between a generator and its switchgear is no place to guess a gauge.
Continuous loads and motor-starting surges define the minimum conductor size. The 125% continuous load factor (NEC 210.20(A)) is non-negotiable for feeders running three hours or more.
A conductor sized for ampacity alone can still drop unacceptable voltage during motor starts. Voltage drop must be calculated at starting current to avoid nuisance tripping and undervoltage.
The conductor between a generator and its switchgear is no place to guess. It must be sized for the total connected load, the starting surge, and the distance from the generator.
Once the conductors are right, the next question is whether the generator itself is correctly sized. The feeder size and generator size must be coordinated to handle the full system demand.
Shandong Huali Electromechanical Co., Ltd. manufactures diesel generator sets, dust-suppression equipment, and industrial automation systems, with over 25 years of experience and exports to more than 20 countries. We publish free electrical calculators to help engineers and buyers specify power systems correctly.
A: At 75°C, a 12 AWG copper conductor carries 25 amps and a 14 AWG carries 20 amps — so 12 AWG is the standard choice for a 20-amp circuit, with 14 AWG marginal. Confirm with the voltage-drop check for long runs, and use this wire size calculator for the exact result.
A: Size for both ampacity and voltage drop, then use the larger gauge. Check ampacity against a table such as NEC 310.16, then check voltage drop using Vd = 2 × K × I × L ÷ CM for single-phase (×√3 for three-phase). Step up a gauge if the drop exceeds 3%.
A: AWG stands for American Wire Gauge, a standard that sizes wire by cross-sectional area. A smaller AWG number indicates a thicker conductor. Sizes above 4/0 are expressed in thousands of circular mils (kcmil).
A: Use Vd = 2 × K × I × L ÷ CM for single-phase, or Vd = √3 × K × I × L ÷ CM for three-phase. K is 12.9 for copper and 21.2 for aluminum. Divide the result by the source voltage and multiply by 100 to get the percentage.
A: Both measure conductor cross-sectional area. AWG is the North American standard; mm² is the metric (IEC) standard used in most of the world. A 10 AWG wire equals roughly 5.26 mm². This wire size calculator returns both.
A: Aluminum has higher resistivity than copper, so it requires a larger conductor for the same current and distance. Aluminum is lighter and cheaper, which is why it is common in large feeders, while copper dominates branch circuits. Use K = 21.2 for aluminum instead of 12.9 for copper.
