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Voltage Drop Calculator

Wire gauge, length and load to voltage drop and percent drop on a circuit.

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Voltage drop

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3% is a guideline, not a code minimum

The National Electrical Code doesn't set a hard voltage drop limit for most branch circuits, but it recommends staying under 3% on the branch circuit and 5% total including the feeder, since a longer run at the same gauge visibly dims lights and slows motors.

Voltage drop happens because every wire has some resistance, and resistance times current equals a voltage loss along the wire. This calculator uses standard AWG resistance-per-1000-feet tables for copper and aluminum, doubles your one-way distance to account for both the outgoing and return conductor, and multiplies by your load's current to get the total voltage lost.

Percent drop (voltage drop divided by system voltage) is the number most electricians actually compare against the 3% guideline, since 2 V dropped on a 120 V circuit matters far less than 2 V dropped on a 12 V circuit.

Long runs to a detached garage, a well pump, or outdoor lighting are the most common places voltage drop becomes a real problem — often the fix is simply moving up one or two wire gauge sizes rather than changing anything else about the circuit.

Where these numbers come from

These are planning estimates only. Electrical work must meet the National Electrical Code and your local code, and should be installed or inspected by a licensed electrician. This is not a substitute for a permit or a licensed electrician's judgment.

Frequently asked questions

What causes voltage drop?

Every wire has some electrical resistance based on its material and gauge. That resistance, combined with the current flowing through it and the total length of wire (both directions), causes some voltage to be lost as heat before it reaches the load.

Is 3% a hard code requirement?

No. The National Electrical Code recommends staying under roughly 3% on a branch circuit and 5% total as good design practice, not as an enforced maximum for most residential circuits. Some jurisdictions or specific equipment may have their own requirements.

How do I fix excessive voltage drop?

The two levers are wire gauge and length. Moving to a heavier (lower AWG number) wire reduces resistance and drop; shortening the run or relocating the panel or load closer together has the same effect. This is not a substitute for consulting a licensed electrician on your specific circuit.

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