Every wire run loses some voltage between the source and the load, and if that loss gets too large, motors run hot, lights dim, and sensitive equipment misbehaves. A voltage drop calculator lets you check a cable run before you install it, so you can confirm the wire size you picked will actually deliver enough voltage at the far end instead of finding out after the fact.
How to use this voltage drop calculator
- Open the Wire Drop tab, which is the default view for checking a cable run.
- Choose a sizing system: AWG, mm², or Custom. AWG and mm² let you enter a standard wire size, while Custom lets you enter a known resistance value directly.
- If you selected AWG, enter the wire gauge. If you selected mm², enter the wire size in mm². If you selected Custom, enter the resistance in ohms per 1000 feet or per 1000 meters, using the unit toggle.
- Select the conductor material: copper or aluminum.
- Enter the length of the run (one-way, not round trip), in feet or meters using the unit toggle.
- Enter the current the circuit will carry, in amps.
- Enter the source voltage, in volts.
- Select the circuit type: DC/single-phase 2-wire, AC single-phase, or AC three-phase.
- Review the result: the calculator displays the voltage drop along with an NEC-style 3%/5% pass-fail check, plus a circuit diagram, a breakdown, and a step-by-step solution.
- Export the result as an image, PDF, or CSV if you need to keep a record, or use Clear All to reset the calculation history.
What this voltage drop calculator does
This tool is built around one core question: given a wire’s size, material, and length, how much voltage will be lost by the time current reaches the load, and does that loss stay within an acceptable limit. You supply the electrical details of the run, the calculator applies the resistance of the conductor to the current and length, and it returns the drop in volts along with a pass-fail check against the commonly used NEC guidance of 3% for a branch circuit or 5% for a feeder plus branch combined.
The same widget also includes a Resistor Divider tab, an Ohm’s Law tab, and a Min Gauge Finder tab. The Min Gauge Finder works in the opposite direction from the main voltage drop calculator: instead of checking a wire size you already chose, you enter the current, length, source voltage, and a maximum allowed drop percentage, and it works out the smallest gauge that keeps the drop under that limit.
Why wire gauge, length, and current all affect voltage drop
A conductor is not a perfect path for electricity. Every length of wire has some resistance, and that resistance depends on the material, the cross-sectional area, and the total length of the run. When current flows through that resistance, some voltage is used up just pushing the current through the wire itself, and that lost voltage never reaches the load.
Voltage drop depends on three things: the current flowing through the circuit, the resistance of the conductor, and the length of the run. More current, higher resistance, or a longer run all increase the voltage lost along the way.
Why gauge and material matter
A thicker wire has more cross-sectional area, which lowers its resistance per foot, so a heavier gauge generally loses less voltage over the same distance carrying the same current. Copper and aluminum conduct differently as well, which is why the calculator asks you to pick a material rather than assuming one. This is also why the Custom sizing option exists: if you already know the resistance of a specific cable or a nonstandard conductor, you can skip the AWG or mm² tables entirely and enter that value directly.
Why circuit type matters
How current and voltage behave differs between DC, AC single-phase, and AC three-phase circuits, so the calculator asks you to select the circuit type before it runs the math. Selecting the wrong type on a real installation would give you a drop figure that does not match how the circuit actually behaves, so it’s worth double-checking this setting against the panel or equipment nameplate before you calculate.
Reading the pass-fail check
| Setting |
What it controls |
| Sizing System |
Whether you enter AWG, mm², or a custom resistance value |
| Material |
Copper or aluminum, which affects conductor resistance |
| Circuit Type |
DC/single-phase 2-wire, AC single-phase, or AC three-phase |
| Max Allowed Drop |
Available on the Min Gauge Finder tab, used to find the smallest gauge that stays under your chosen percentage |
On the Wire Drop tab, the result compares your calculated drop against the commonly referenced NEC guidance of roughly 3% for a branch circuit and 5% for a combined feeder and branch. This gives you a quick sense of whether the run is likely to be acceptable, but it stays a general guideline rather than a substitute for the specific code your jurisdiction enforces.
A few things to keep in mind
This voltage drop calculator gives you an estimate based on the numbers you enter. It is a useful check before running conduit or ordering cable, but the page itself notes that results are estimates only and should be verified against local electrical code before you rely on them for an actual installation. Wire runs with unusual conditions, such as high ambient temperature or bundled conductors, may need adjustments beyond what a general voltage drop calculation covers.
Frequently asked questions
What counts as an acceptable voltage drop?
The calculator uses the commonly referenced NEC-style guidance of about 3% for a branch circuit and 5% for a combined feeder and branch as its pass-fail check. Your local code may set different limits, so treat this as a starting point rather than a final answer.
Should I enter the one-way length or the round-trip length?
Enter the one-way length of the run. The calculator’s Length field is labeled as one-way, so you should not double it to account for the return path yourself.
What is the Min Gauge Finder tab for?
It works backward from the main voltage drop calculator. Instead of checking a wire size you already picked, you enter the current, length, source voltage, and a maximum allowed drop percentage, and it returns the smallest gauge that stays within that limit.
Can I use a custom resistance value instead of a standard wire gauge?
Yes. The Sizing System dropdown includes a Custom option that lets you enter resistance in ohms per 1000 feet or per 1000 meters directly, which is useful if you’re working with a cable that isn’t a standard AWG or mm² size.
