Voltage Drop Calculator

Voltage Drop: The Silent Performance Killer

You've run 12-gauge wire 150 feet from the panel to a detached garage. A 20-amp circuit. Seems reasonable. But when the customer flips the breaker, the lights dim the second the heater kicks on. Classic voltage drop problem.

Voltage drop is the voltage loss that happens as current travels through a wire. The longer the run, the smaller the wire, the higher the current—the worse it gets. A 120V circuit that drops 5% is delivering only 114V to the load. Motors start struggling. Lights get dim. Equipment runs hot. And your callback list grows.

The NEC gives us limits: 3% for branch circuits, 5% combined. Don't ignore them. Inspectors sure won't.

NEC Voltage Drop Limits—Know Them Cold

The National Electrical Code doesn't call it a requirement (Article 210.19(A) is technically a recommendation), but every inspector and engineer treats it like one. Here's what you need:

• Single branch circuit: 3% max. On a 120V circuit, that's 3.6 volts. Stays below that, you're good.
• Feeder + branch combined: 5% max. Use this when you're running a feeder to a sub-panel and branching from there.
• Feeder only: 3% max if there's nothing else after it.

At 480V, 3% = 14.4 volts. Bigger number, but same rules apply. Stay under the limits and your equipment operates at rated performance. Blow past them and you're heading for a callback—or worse, a liability issue.

The Formula—Don't Panic

Stay with us—this formula looks worse than it is:

• Single Phase: VD = (2 × K × I × D) / CM
• Three Phase: VD = (1.732 × K × I × D) / CM

That's it. K is the conductor constant (12.9 for copper, 21.2 for aluminum—memorize these). I is your load current in amps. D is the one-way distance in feet. CM is the wire's circular mil area (use any reference table). The 2 in single-phase? Current flows out and back, so you count the distance twice. The 1.732 in three-phase? That's just √3, accounting for the phase angle. Plug the numbers in and you've got voltage drop in volts.

Practical Wire Sizing

So you've done the math and your circuit's pushing limits. What's the fix?

• Upsize the wire: Jump from 12 to 10 AWG? You cut the voltage drop in half. Simple physics—bigger conductor, less resistance.
• Shorten the run: Move the sub-panel closer. If you can cut the distance in half, voltage drop drops in half. This is usually the cheapest fix long-term.
• Go three-phase: If it's available, three-phase gives you roughly 1.73x better voltage drop than single-phase. Industrial jobs love this.
• Use copper: Copper's resistance is about 60% lower than aluminum (K=12.9 vs. 21.2). But it costs more. Sometimes it's worth it, sometimes not.
• Plan ahead: Size your wire for future load growth, not just today's draw. Better to pull a heavier wire now than tear it all out in five years.

Real Jobs That Bite You

Running a 20A kitchen circuit 150 feet in 12 AWG copper? At full load you're dropping about 11.8 volts (nearly 10%). That's hosed. You need 10 AWG minimum, and honestly, 8 AWG is better. Do the math before you pull the wire, not after.

Running a 100A feeder 500 feet to a motor on 480V three-phase using 1/0 copper? You're only dropping about 8.2V (1.7%). Compliant. Three-phase saves you from needing massive wire.

Doing a solar installation? DC systems don't follow NEC voltage drop percentages, but the physics is the same. A 2 AWG aluminum run carrying 80A for 150 feet drops 19.3V. At 400V input, that's 4.8%. Acceptable for solar, but you feel the efficiency loss. Step up to 1/0 and you're golden.

What Happens When You Ignore It

• Motors: Run at 95V instead of 120V? Motor pulls extra current trying to maintain torque. It heats up. Insulation breaks down. You're looking at a burned-out motor in a year, not ten.
• Lights: Dim kitchen lights the second you turn on the microwave. Every time. Your customer notices. Every. Time.
• Breakers: Motor inrush current climbs higher than the breaker can tolerate at low voltage. Nuisance trips at 2 AM. On a Saturday. You're getting the call.
• Heating loads: Water heater, baseboard heat—they need voltage to do their job. Low voltage means they draw more current to compensate. Your nice 20A circuit becomes a fire hazard.
• Data equipment: Servers, networking gear—they shut down if voltage drops below 95%. You just took a business offline because you weren't paying attention to the spec sheet.
• Performance: Excessive drop leads to poor operation, callbacks, and avoidable troubleshooting. Catch it before the wire is pulled.

Using This Calculator

Pick your voltage and phase type. Select copper or aluminum (copper every time if budget allows). Choose your wire size. Enter the one-way distance and load current. The calculator spits out your voltage drop in volts and percentage, plus NEC compliance status. Green = good. Red = you need thicker wire or a shorter run. Do yourself a favor and get it right on paper before you cut into the wall.

Wire Reference Table