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Calculator · Electrical · NEC + AS/NZS 3008

Voltage drop calculator

Calculate voltage drop for DC, single-phase and 3-phase circuits. Built into NEC 2023 and AS/NZS 3008.1.1 limits, with copper and aluminium conductors. Free, no signup, math fully explained. Reviewed by a licensed PE.

Use the calculator

Enter your load current, conductor length, wire size and system voltage. Result updates live with NEC compliance check.

CALC.001 Voltage Drop · Cu/Al · DC + 1Φ + 3Φ
A
m
V
°C
Voltage drop · Vdrop
— V
3%
5%
0510%
Drop ratio
—
V at load
—
Power loss
—
ρ used (T-corrected)
—
FORMULA · V = 2 · L · I · ρ / A NEC 3% / 5% RULE
SOURCE 240 V nominal supply LOAD 228 V delivered length L area A resistivity p I (current) voltage drop along the run 12 V (5%) NEC limit: 3% branch / 5% total
Figure 1 — Voltage drop along a 240 V wire run: 12 V drop (5% threshold), 228 V delivered to load

What is voltage drop?

When current flows through a conductor, the conductor's own resistance causes a small portion of the source voltage to be lost. That lost voltage is the voltage drop. The longer the wire, the higher the current, or the smaller the conductor, the more voltage is lost — and the less reaches your load.

Voltage drop in a DC circuit A battery, copper conductor of length L = 30 metres carrying 20 amperes, and a resistive load. The voltage drop across the wire is approximately 3.99 volts (33% of source). + − 12 V load L = 30 m I = 20 A V drop 3.99 V
Voltage drop in a 12 V DC circuit · 30 m of 5.26 mm² (10 AWG) copper · 20 A load · ρ_Cu = 0.0175 Ω·mm²/m. Result: 3.99 V drop (33% of source — far above NEC 5% limit).

In the circuit above, a 12 V source pushes 20 A through 30 m of 10 AWG (5.26 mm²) copper wire to a load. The wire's resistance dissipates 3.99 V (33% of source) before the load — meaning the load only sees about 8.0 V. For a 12 V DC system, this is a serious problem: motors stall, LEDs dim, electronics malfunction. Use the online calculator above to find the minimum wire size that keeps drop within acceptable limits.

The voltage drop formula

Eq. 01 — DC and single-phase AC SI · NEC Table 9 (resistance) · IEC 60287 (general)
Vdrop=2⋅L⋅I⋅ρAV_{drop} = \frac{2 \cdot L \cdot I \cdot \rho}{A}
V
voltage drop, V
L
one-way conductor length, m
I
current, A
ρ
resistivity (material), Ω·mm²/m
A
cross-sectional area, mm²
Eq. 02 — Three-phase balanced SI
Vdrop=3⋅L⋅I⋅Z/1000V_{drop} = \sqrt{3} \cdot L \cdot I \cdot Z / 1000
V
line-to-line drop, V
L
length, m
I
line current, A
Z
impedance, mΩ/m

Resistivity values to use:

Materialρ (Ω·mm²/m)Where used
Copper (Cu)0.0175Branch circuits, panels, residential
Aluminium (Al)0.0280Service entrance, large feeders

How to calculate voltage drop, step by step

  1. Identify the load current. From nameplate, or compute from VA / V for AC equipment. Use full-load amps (FLA), not rated amps.
  2. Measure the one-way conductor length. From source (panel/breaker) to the farthest load. Do not double — the formula already accounts for the return path.
  3. Choose conductor material and size. Copper or aluminium; pick from standard wire sizes (AWG / mm²). Material affects resistivity ρ.
  4. Apply the formula. Vdrop = 2 · L · I · ρ / A. Use ρ_Cu = 0.0175 Ω·mm²/m, ρ_Al = 0.028 Ω·mm²/m.
  5. Check against the standard. NEC: ≤3% branch / ≤5% total. AS/NZS 3008: ≤5% from main switchboard.
  6. If exceeded — increase wire size. Step up one or two sizes; recalculate. Or shorten the run, or use a higher system voltage.