Voltage drop calculator.
Enter the wire, the load and the length. Get voltage drop in volts and percent, the voltage that actually arrives at the load, a PASS/FAIL against the 3% branch / 5% total guidance — and the smallest conductor that brings the run back in spec.
| What we computed | Value | Reference |
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How the math works
Voltage drop is a resistance calculation using the standard K-constant method that estimators rely on. Every value comes from published NEC data:
- Single phase: VD = 2 × K × I × L ÷ CM — the 2 accounts for current down and back.
- Three phase: VD = 1.732 × K × I × L ÷ CM — the √3 factor for a balanced three-phase load.
- K — resistivity constant: 12.9 for copper, 21.2 for aluminum (ohm-cmil/ft).
- CM — conductor circular mils from NEC Chapter 9, Table 8.
- I = load current (amps), L = one-way length (feet).
- VD % = VD ÷ system voltage × 100. Voltage at load = system voltage − VD.
- Wire size needed — the calculator steps up through standard sizes and reports the first one that lands under your target percentage.
Worked example: 12 AWG copper, single phase, 120 V, 16 A, 60 ft. CM for 12 AWG = 6,530. VD = 2 × 12.9 × 16 × 60 ÷ 6,530 = 3.79 V = 3.16%. That’s just over the 3% target, so the tool flags it and suggests 10 AWG.
This is the easy 20%. The other 80% is on your drawings.
One voltage-drop check is quick. The work that drains an electrical bid is doing it for every long homerun and feeder on the set — then counting the conductor and conduit footage that goes with each, across a one-line, a panel schedule and a floor plan that never sit on the same sheet. That’s the part that leaks money — and it’s exactly what Pilars AI takeoff does from your actual plans: it follows the runs, measures the lengths, and returns wire and conduit by size and type. $100 per trade, per plan.
Questions electricians actually ask
What is the voltage drop formula?
Single phase: VD = 2 × K × I × L ÷ CM. Three phase: VD = 1.732 × K × I × L ÷ CM. K = 12.9 (copper) or 21.2 (aluminum), I = amps, L = one-way feet, CM = circular mils from Chapter 9 Table 8.
What’s the maximum allowable voltage drop?
The NEC doesn’t mandate one, but its informational notes recommend ≤ 3% on a branch or feeder and ≤ 5% total (feeder + branch). The tool flags both.
Why is three-phase voltage drop lower?
Single phase uses a factor of 2; balanced three phase uses 1.732 (√3). For the same current and conductor, a three-phase run drops about 13% less.
How do I fix excessive voltage drop?
Increase the conductor size, shorten the run, raise the voltage, or reduce the current. The tool shows the smallest standard conductor that brings the run within your target.
Is this accurate enough for design?
It uses the standard K-constant method with Chapter 9 Table 8 circular mils — what most estimators use. It ignores reactance and power factor, so it’s slightly conservative on long high-PF runs. It’s a reference tool; verify against your adopted code cycle.
Let Pilars take off your whole set.
Upload your plans. Pilars follows every run, measures the lengths, and returns wire and conduit by size and type. $100 per trade.