Skip to main content
ilovecalcs logoilovecalcs.

Other · Live

Voltage drop, calculated precisely.

Calculate the voltage loss in any electrical circuit based on wire gauge, material, distance, and load. Check NEC compliance in real time. Results update as you type.

How it worksReal-time

Inputs

Circuit parameters

V
A

Voltage drop

Marginal — 3–5% range
5.926V

end-of-run voltage: 114.07 V

% drop
4.94%
3–5% total limit
End voltage
114.07 V
from 120 V source
Wire resistance
0.1975 Ω
one-way conductor

NEC compliance

Threshold check

4.94% drop
Branch circuit — NEC 210.19limit: 3%

+1.94% over the 3% limit

Total circuit — NEC 215.2limit: 5%

NEC informational notes recommend ≤ 3% for branch circuits and ≤ 5% total (feeder + branch combined). Excessive drop degrades equipment performance and increases energy consumption.

Calculation detail

Step by step

Source voltage
120 V
Load current
15.00 A
One-way distance
100 ft (30.48 m)
Phase factor
×2 (single phase, out + return)
Material
Copper (K = 12.9 Ω·CM/ft)
Wire size
12 AWG (6,530 CM)
One-way resistance
0.19755 Ω
Voltage drop
5.9265 V
Drop percentage
4.939%
End-of-run voltage
114.0735 V

Field guide

How the voltage drop formula works.

Voltage drop is the reduction in electrical potential along a conductor as current flows through it. Every wire has resistance, and by Ohm's Law that resistance turns some of the source voltage into heat before it reaches the load. The longer the wire, the smaller the cross-section, and the higher the current — the worse the drop.

The NEC method (AWG, US)

The National Electrical Code chapter 9 uses circular mils (CM) to express conductor area. For a single-phase circuit, the voltage drop is:

Vd = (2 × K × I × L) / CM
  • K: resistivity constant: 12.9 for copper, 21.2 for aluminum (Ω·CM/ft)
  • I: one-way current in amperes
  • L: one-way distance in feet
  • CM: conductor cross-section in circular mils (e.g., 6,530 for 12 AWG)

The factor of 2 accounts for the current traveling out to the load and returning through the neutral/return conductor — both conductors contribute resistance.

Three-phase circuits

For three-phase systems, the geometry of the three conductors means the effective round-trip multiplier is √3 ≈ 1.7321 instead of 2:

Vd = (√3 × K × I × L) / CM

This assumes a balanced three-phase load. Unbalanced loads require per-phase analysis.

The metric method (mm²)

IEC-based systems use cross-section area in mm² and resistivity in Ω·mm²/m. The one-way resistance is:

R = ρ × L / A

Then voltage drop is:

Vd = phaseFactor × R × I
  • ρ: resistivity: 0.01724 for copper, 0.0282 for aluminum (Ω·mm²/m)
  • L: one-way distance in metres
  • A: conductor cross-section in mm²

NEC compliance limits

The NEC does not make voltage drop a code violation, but it provides two widely enforced informational notes:

  • NEC 210.19 (branch circuits): maximum 3% drop from panel to outlet.
  • NEC 215.2 (feeders): maximum 3% for the feeder alone; maximum 5% for the combined feeder + branch circuit.

Exceeding 5% total causes motors to overheat, lights to dim, and sensitive electronics to malfunction or fail prematurely. Many utilities, AHJs, and equipment manufacturers enforce these limits contractually even when not legally required.

Copper vs. aluminum

Aluminum has about 64% of copper's conductivity, which is why its K factor (21.2) is higher than copper's (12.9). In practice, aluminum wiring requires upsizing by one or two gauges to carry the same current with equivalent voltage drop. Aluminum is common for large feeders and service entrances where its lower cost per pound outweighs the gauge penalty.

How to reduce voltage drop

  • Upsize the conductor: moving from 12 AWG to 10 AWG roughly halves the voltage drop at the same current and distance.
  • Shorten the run: move the sub-panel closer to the load center.
  • Raise the supply voltage: a 240 V circuit at 4% drop loses 9.6 V; the same 4% on a 120 V circuit loses only 4.8 V, but equipment suffers more on a 120 V base.
  • Split the load: distribute high-current loads across multiple circuits.

Worked example

120 V source, 15 A load, 100 ft one-way run, single phase, copper, 12 AWG (6,530 CM):

Vd = (2 × 12.9 × 15 × 100) / 6,530 ≈ 5.94 V (4.95%)

This exceeds the 3% branch-circuit limit. Upsize to 8 AWG (16,510 CM):

Vd = (2 × 12.9 × 15 × 100) / 16,510 ≈ 2.34 V (1.95%)

Now safely under 3%. The calculator lets you toggle wire sizes instantly to find the minimum gauge that keeps you compliant.

Disclaimer

This calculator implements the NEC chapter 9 DC resistance method for copper and aluminum conductors. It does not account for AC reactance, temperature correction factors, conduit fill, or conductor bundling derating. Always verify with a licensed electrician and the applicable code edition for your jurisdiction.