What is an acceptable voltage drop?

The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for individual branch circuits, and a total maximum of 5% for both the feeder and the branch circuit combined to ensure equipment functions safely.

How do you fix voltage drop?

To reduce voltage drop over a long distance, you must increase the thickness (gauge) of the wire. A thicker wire has less electrical resistance, which allows the voltage to travel further without degrading.

How is voltage drop calculated?

The standard single-phase formula is: Voltage Drop = (2 × K × I × D) / CM. K is the resistivity of the material (12.9 for Copper), I is the current in Amps, D is the one-way distance in feet, and CM is the circular mils (thickness) of the wire.

⚡ Voltage Drop Calculator

Calculate electrical voltage drop across copper or aluminum wires over long distances.

Wire Material

Wire Gauge (AWG)

System Voltage

Load Current (Amps)

One-Way Cable Distance

Voltage Drop Percentage

NEC recommends maximum 3% drop

⚠ Warning: The voltage drop exceeds the NEC recommended 3%. You should use a thicker wire gauge (lower AWG number).

Voltage Dropped

0 V

Voltage at Equipment

0 V

What Causes Voltage Drop?

No electrical wire is a perfect conductor. All wires have electrical resistance. When electricity travels through a wire, some of the energy is lost to heat due to this resistance.

The longer the wire, the more resistance the electricity has to push through. Eventually, the voltage at the end of the wire drops so low that the electrical equipment cannot operate correctly. Motors will overheat, lights will dim, and electronics may fail.

How Do You Fix It?

If your voltage drop is too high, you have two options:

Increase the wire size: A thicker wire (like stepping up from 12 AWG to 10 AWG) acts like a wider highway, reducing resistance and allowing the voltage to travel further without dropping.
Reduce the load: If you pull fewer Amps across the wire, the voltage will not drop as severely.

Note: Aluminum wire is a worse conductor than copper. If you switch from Copper to Aluminum, you generally have to increase the wire thickness by one full size to compensate.

Frequently Asked Questions

What is the maximum allowed voltage drop? +

The NEC recommends a maximum 3% voltage drop for any single branch circuit. If you have a 120V circuit, a 3% drop means you lose 3.6V, leaving you with 116.4V at the outlet.

Does this formula apply to 3-phase power? +

No, this calculator uses the single-phase formula: VD = (2 × K × I × D) / CM. If you are calculating a 3-phase commercial system, the multiplier changes from 2 to 1.732 (the square root of 3).

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