Voltage Drop Explained

Why voltage drops

Every conductor has some resistance. Push current through it over a distance and you lose a little voltage as heat along the way. The longer the run and the smaller the wire, the more you lose. By the time it reaches the far end, the load may be starved.

You cannot see it with your eyes, but the motor at the end of a long run feels it, and it will run hot and fail early on low voltage.

The recommended limits

The code addresses voltage drop as a recommendation rather than a hard mandate in most cases, but the guidance is well known and inspectors expect you to honor it. Aim to keep the drop on a branch circuit under three percent, and the total from service to the farthest load under five percent.

Treat these as design targets. A install that technically passes but browns out equipment is a callback you brought on yourself.

How to estimate it

The drop grows with current and one-way distance and shrinks with a bigger conductor. The practical formula uses the conductor's resistance per distance, the current, and twice the one-way length, since current has to travel out and back.

• More amps: more drop.
• Longer run: more drop.
• Bigger wire: less drop.

Run the numbers before you pull. It is far cheaper to upsize on paper than after the whip is in the wall.

The fix is usually bigger wire

When a run is too long, the cure is to go up a conductor size or two to lower the resistance. This comes up constantly on long feeders, well pumps, detached buildings, and increasingly EV chargers in a far garage.

Size for ampacity first to satisfy the code minimum, then check voltage drop and upsize if the distance demands it. The two checks together give you a conductor that is both legal and actually works well.