Voltage Drop Calculator - Wire Size, NEC 3% & 5% (Free)

Every metre of wire quietly steals a little voltage. As current pushes through a conductor's resistance, some voltage is lost as heat before it ever reaches the load — that's voltage drop. Too much of it and lights dim, motors overheat, and sensitive electronics misbehave. This free Voltage Drop Calculator tells you exactly how much voltage you'll lose for a given current, wire size, length and material — for single-phase, three-phase or DC — and checks it against the NEC 3% and 5% recommendations so you know whether to upsize your conductor.

Table of Contents

1. The Voltage Drop Calculator
2. What Is Voltage Drop?
3. The Voltage Drop Formula
4. NEC 3% & 5% Limits
5. Single-Phase vs Three-Phase
6. Copper vs Aluminium
7. How to Reduce Voltage Drop
8. Worked Examples
9. Common Mistakes
10. FAQ

The Voltage Drop Calculator

Choose imperial (AWG) or metric (mm²), enter the circuit details, and get the voltage drop in volts and percent with an instant NEC pass/fail check.

⚡ Voltage Drop Calculator

Single-phase · three-phase · DC · with NEC 3% / 5% check

Imperial (AWG, ft)

Metric (mm², m)

Circuit

System voltage (V)

Load current (A)

System type Single-phase / DC (×2) Three-phase (×√3)

Conductor material Copper Aluminium

Conductor

Wire size (AWG) 14 AWG 12 AWG 10 AWG 8 AWG 6 AWG 4 AWG 2 AWG 1 AWG 1/0 AWG 2/0 AWG 3/0 AWG 4/0 AWG

Cross-section (mm²)

One-way length (ft)

Max allowed drop (%)

—

voltage drop

—

% drop

—

voltage at load (V)

—

power lost (W)

Imperial: VD = factor × K × I × D / CM (K = 12.9 Cu, 21.2 Al; CM = circular mils; D = one-way ft). Metric: VD = factor × I × ρL/A (ρ = 1.724×10⁻⁸ Cu, 2.65×10⁻⁸ Al Ω·m). factor = 2 (single-phase/DC) or √3 (three-phase). NEC recommends ≤3% branch, ≤5% total.

Validation note: the calculator uses the standard NEC design method. A 20 A load on 12 AWG copper over 100 ft (single-phase) drops 7.9 V = 6.6% on a 120 V circuit — over the 3% target. Upsizing to 8 AWG brings it to 3.1 V = 2.6% (pass). A 100 A three-phase feeder on 2 AWG copper over 200 ft drops ~1.4% — all matching published references.

What Is Voltage Drop?

Voltage drop is the loss of voltage as current flows through a conductor's resistance. It follows directly from Ohm's Law (V = I·R): the wire has resistance R, and the current I through it produces a drop V_drop = I·R that's "used up" before the load. Longer runs and thinner wires mean more resistance — and more drop.

Why it matters: the load only gets source voltage minus the drop. Excessive drop makes lights dim, causes motors to draw more current and overheat (shortening their life), and can make electronics unreliable. Controlling it is a core part of safe, efficient wiring design.

The Voltage Drop Formula

The common NEC design formula (imperial, circular mils) is:

VD = (factor × K × I × D) / CM

• factor = 2 for single-phase/DC (go + return), √3 ≈ 1.732 for three-phase
• K = resistivity constant: 12.9 (copper), 21.2 (aluminium) at ~75°C
• I = load current (A); D = one-way length (ft); CM = conductor area (circular mils)

In metric, the equivalent uses resistivity ρ and area A:

VD = factor × I × (ρ L / A)

with ρ = 1.724×10⁻⁸ Ω·m (copper), 2.65×10⁻⁸ Ω·m (aluminium), L one-way in metres, A in m².

NEC 3% & 5% Limits

Circuit	Recommended max drop
Branch circuit	3%
Feeder + branch (total)	5%
Sensitive equipment (computers, motors)	≤ 2%

Important nuance: the 3% / 5% figures live in NEC Informational Notes (210.19 / 215.2) — they're recommendations, not strictly enforceable code in most jurisdictions. But designers and inspectors treat them as targets, and exceeding them causes real equipment problems. Always follow your local code and the actual load requirements.

At 120 V, 3% = 3.6 V; at 240 V, 3% = 7.2 V.

Single-Phase vs Three-Phase

The only difference in the formula is the phase factor:

• Single-phase / DC → ×2: current flows out on one conductor and back on another, so both add resistance.
• Three-phase → ×√3 (1.732): the three conductors share the load geometry, giving less drop for the same current and wire.

That's one reason big loads are supplied at three-phase — it's more efficient over distance.

Copper vs Aluminium

Copper conducts ~50–60% better than aluminium, so it drops less voltage for the same size. Aluminium is lighter and cheaper — common for large feeders and long outdoor runs — but usually needs upsizing by one or two gauges to match copper's drop. The calculator switches between them so you can compare directly.

How to Reduce Voltage Drop

• Use a bigger conductor. Up one wire size ≈ ~⅓ less resistance — the most direct fix.
• Switch to copper. Lower resistivity than aluminium for the same size.
• Shorten the run where the layout allows.
• Raise the voltage. The biggest lever: same power at 240 V instead of 120 V halves the current — and halves the drop.
• Use three-phase for large loads.

Worked Examples

1. A 20 A branch circuit

20 A, 12 AWG copper (CM = 6530), 100 ft one-way, single-phase, 120 V:

• VD = (2 × 12.9 × 20 × 100) / 6530 = 7.9 V
• % = 7.9 / 120 = 6.6% — over the 3% target.
• Upsize to 8 AWG (CM = 16510): VD = 3.1 V = 2.6% ✓

2. A three-phase feeder

100 A, 2 AWG copper (CM = 66360), 200 ft, three-phase, 480 V:

• VD = (1.732 × 12.9 × 100 × 200) / 66360 = 6.7 V
• % = 6.7 / 480 = 1.4% — well within limits ✓

Common Mistakes

• Using round-trip length. Enter one-way distance — the factor of 2 already accounts for the return.
• Forgetting the phase factor. 2 for single-phase/DC, √3 for three-phase.
• Ignoring material. Aluminium drops ~60% more than copper of the same size.
• Only checking ampacity. A wire can be thick enough to carry the current safely yet still drop too much voltage over a long run.
• Treating 3% as hard law. It's an NEC recommendation — but a good design target.
• Mixing units. Keep AWG with feet, or mm² with metres — don't mix.

Authoritative external references: NFPA 70 (National Electrical Code), and the Electronics Tutorials — Voltage Drop reference.

Related calculators on Free CFD Tutorial: start with the fundamentals in the Ohm's Law Calculator, size backup power with the Battery Storage Calculator, and see the same "driving force over resistance" idea for fluids in the Pressure Drop & Head Loss Calculator and the Pump Power Calculator.

Frequently Asked Questions

What is voltage drop?

The reduction in voltage as current flows through a wire's resistance. Longer runs and smaller wires drop more, so the load receives less than the source voltage.

What is the voltage drop formula?

VD = factor × K × I × D / CM (imperial). factor = 2 (single-phase/DC) or √3 (three-phase); K = 12.9 copper, 21.2 aluminium; CM = circular mils.

What is an acceptable voltage drop?

NEC recommends ≤3% on a branch circuit and ≤5% total (feeder + branch). These are Informational-Note recommendations; sensitive gear is often kept under 2%.

How do I reduce voltage drop?

Bigger conductor, copper instead of aluminium, shorter run, higher voltage (halving current halves the drop), or three-phase distribution.

What is the difference between single-phase and three-phase voltage drop?

Only the phase factor: 2 for single-phase/DC (go + return), √3 (1.732) for three-phase, which gives less drop for the same current and wire.

Does copper or aluminium have less voltage drop?

Copper — it conducts ~50–60% better, so it drops less for the same size. Aluminium is cheaper/lighter but usually needs upsizing.

Conclusion

Voltage drop is the hidden tax every conductor charges on the current it carries. Keep it in check — ≤3% on branch circuits, ≤5% total — and your equipment runs cooler, safer and more efficiently. Enter your current, wire size, length and material in the calculator above to see the drop in volts and percent, check NEC compliance instantly, and find out whether you need to upsize the conductor.

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