How do I calculate voltage drop?

Voltage Drop = (2 × Length × Current × Wire Resistance) ÷ 1000. The factor of 2 accounts for the round trip (both conductors). For 3-phase circuits, use 1.732 instead of 2. Wire resistance varies by gauge — 12 AWG copper is 1.588 ohms per 1000 ft.

How do I fix excessive voltage drop?

Three solutions: 1) Use larger wire gauge (lower resistance). 2) Shorten the wire run. 3) Increase voltage (240V has half the voltage drop percentage of 120V for the same power). For long runs, upgrading one wire size often solves the problem.

Does voltage drop apply to DC circuits?

Yes, voltage drop is equally important for DC circuits (solar panels, batteries, 12V/24V systems). DC circuits often have longer runs and lower voltages, making voltage drop a bigger percentage issue. Solar string wiring and battery bank connections need careful sizing.

What is an acceptable voltage drop?

NEC recommends no more than 3% for branch circuits and 5% for the total circuit (feeder + branch). Exceeding these limits can cause equipment issues.

How does three-phase differ from single-phase?

Three-phase circuits use a multiplier of √3 (1.732) instead of 2 in the voltage drop formula, resulting in about 13% less voltage drop for the same wire size and distance.

Does voltage drop waste electricity?

Yes. The power lost to voltage drop (shown as Power Loss in watts) is converted to heat in the wire. This wasted energy shows up on your electricity bill.

How do I reduce voltage drop?

Use larger wire (lower AWG number), shorten the wire run, reduce the load, increase supply voltage, or switch from aluminum to copper conductor.

Why is voltage drop worse for 120V than 240V?

The same wire carrying the same watts at 120V draws twice the amperage as at 240V. Since voltage drop is proportional to current, 120V circuits have significantly more voltage drop for the same power delivered.

Voltage Drop Calculator

Calculate voltage drop for any electrical circuit. Verify that your wire gauge is adequate for the distance, amperage, and voltage of your installation.

Voltage Drop Calculator

Calculate voltage drop, power loss, and voltage at the load for any wire gauge, amperage, and circuit distance. Supports single-phase and three-phase systems.

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Quick Presets

Wire Gauge (AWG)

Amperage (A)

One-Way Distance

Voltage

Conductor

Phase

How This Calculator Works

Voltage Drop Formula

Single Phase: V_d = (2 × K × I × D) / CM

Three Phase: V_d = (√3 × K × I × D) / CM

K = Resistivity (10.4 copper, 17.0 aluminum)
I = Current in amps
D = One-way distance in feet
CM = Wire area in circular mils

Power loss is calculated as V_d × I, representing wasted energy as heat in the wire.

Key NEC Code References

NEC 310.16 — Ampacity tables for conductors rated 0–2000V
NEC 210.19 — Branch circuit conductor sizing requirements
NEC 215.2 — Feeder conductor sizing requirements
NEC 310.15(B)(7) — Special ampacity allowance for dwelling unit service and feeder conductors

Frequently Asked Questions

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Understanding Voltage Drop

Voltage drop is the reduction in voltage as electricity travels through wire. Every conductor has resistance, and that resistance converts some electrical energy to heat. The longer the wire run and the higher the current, the greater the voltage drop.

The Voltage Drop Formula

VD (single-phase) = 2 × L × I × R ÷ 1000
VD (three-phase) = 1.732 × L × I × R ÷ 1000
Where: L = one-way length (ft), I = current (amps), R = resistance per 1000 ft
VD% = (VD ÷ Source Voltage) × 100

NEC Voltage Drop Recommendations

Per NEC 210.19(A) Informational Note No. 4:

Branch circuits: Maximum 3% voltage drop recommended
Feeder circuits: Maximum 3% voltage drop recommended
Total (feeder + branch): Maximum 5% combined
These are recommendations, not hard requirements, but exceeding them causes problems

Wire Resistance by Gauge (Copper, per 1000 ft)

14 AWG: 2.525 ohms
12 AWG: 1.588 ohms
10 AWG: 0.999 ohms
8 AWG: 0.628 ohms
6 AWG: 0.395 ohms

Effects of Excessive Voltage Drop

Lighting: Dim or flickering lights
Motors: Overheating, reduced torque, shortened lifespan
Electronics: Malfunction, data errors, damage
Heating elements: Reduced output (power drops with voltage squared)

Frequently Asked Questions

What is acceptable voltage drop?

NEC recommends max 3% for branch circuits, 5% total. On 120V: 3% = 3.6V drop. On 240V: 3% = 7.2V.

How to calculate voltage drop?

VD = 2 × Length × Current × Resistance/1000. Use our calculator above for instant results.

How to fix excessive drop?

Use larger wire, shorten the run, or increase voltage (240V vs 120V).

Does it apply to DC?

Yes. DC systems (solar, batteries) are equally affected. Low-voltage DC systems are especially sensitive.

What causes voltage drop?

Wire resistance. All wire has resistance that increases with length and decreases with wire gauge.

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All electrical work must comply with NEC and local codes. Consult a licensed electrician for circuit design and installation. Calculations assume standard temperature conditions per NEC Chapter 9, Table 8.