comparison · voltage-drop / validation
Southwire Voltage Drop Calculator Alternative: 21,000 Circuits Tested
Tested on 21,000 circuits against Southwire’s voltage drop calculator: all 10,500 DC results match to the last digit. Every input and result is downloadable.
Same circuits, both calculators, every result published.
- Circuits run through Southwire and ZapSketch
- 21,000 Inputs fixed before the first request; none dropped afterwards
- NEC 2023 reference cases
- 86,580 All matched on voltage drop, max run and ampacity
DC: all 10,500 circuits match Southwire to the last digit it displays. AC: on the 9,096 circuits at or under 3% drop, the largest gap is 0.09 percentage points. Every one of the 10,500 AC circuits lands within 5% of Southwire’s result.
DC is compared to Southwire’s two-decimal display (0.005 percentage points). AC is compared within 5% of Southwire’s drop plus that rounding, at 0.9 power factor in non-steel raceway or cable.
DC: identical to the last digit.
Southwire shows voltage drop to two decimal places. On all 10,500 DC circuits, ZapSketch’s result rounds to the same number: 19 copper and 16 aluminum sizes, 12 V to 480 V, 5 A to 100 A, 10 ft to 1,000 ft.
| Conductor | Southwire (%) | ZapSketch (%) | Gap (pp) |
|---|---|---|---|
| 12 AWG copper | 6.60 | 6.6000 | 0.0000 |
| 10 AWG copper | 4.13 | 4.1333 | 0.0033 |
| 8 AWG copper | 2.59 | 2.5933 | 0.0033 |
| 8 AWG aluminum | 4.27 | 4.2667 | 0.0033 |
ZapSketch is shown to four decimals so you can see the rounding. Southwire results captured Oct 22, 2026 (Pacific).
Try the #12 copper exampleSee all 19 copper sizes at the same inputs
| Size | Southwire (%) | ZapSketch (%) | Gap (pp) |
|---|---|---|---|
| 14 AWG | 10.47 | 10.4667 | 0.0033 |
| 12 AWG | 6.60 | 6.6000 | 0.0000 |
| 10 AWG | 4.13 | 4.1333 | 0.0033 |
| 8 AWG | 2.59 | 2.5933 | 0.0033 |
| 6 AWG | 1.64 | 1.6367 | 0.0033 |
| 4 AWG | 1.03 | 1.0267 | 0.0033 |
| 3 AWG | 0.82 | 0.8167 | 0.0033 |
| 2 AWG | 0.65 | 0.6467 | 0.0033 |
| 1 AWG | 0.51 | 0.5133 | 0.0033 |
| 1/0 AWG | 0.41 | 0.4067 | 0.0033 |
| 2/0 AWG | 0.32 | 0.3223 | 0.0023 |
| 3/0 AWG | 0.26 | 0.2553 | 0.0047 |
| 4/0 AWG | 0.20 | 0.2027 | 0.0027 |
| 250 kcmil | 0.17 | 0.1717 | 0.0017 |
| 300 kcmil | 0.14 | 0.1430 | 0.0030 |
| 350 kcmil | 0.12 | 0.1223 | 0.0023 |
| 400 kcmil | 0.11 | 0.1070 | 0.0030 |
| 500 kcmil | 0.09 | 0.0860 | 0.0040 |
| 600 kcmil | 0.07 | 0.0713 | 0.0013 |
AC: within a tenth of a point where it counts.
Most branch circuits and feeders are designed to 3% drop or less. On the 9,096 AC circuits in that range, the largest gap between the two calculators is 0.09 percentage points and the typical gap is 0.004. Single and three phase, copper and aluminum, 120 V to 480 V.
| Circuit / conductor | Southwire (%) | ZapSketch (%) | Gap (pp) |
|---|---|---|---|
| 1φ · 10 AWG copper | 1.78 | 1.7216 | 0.0584 |
| 3φ · 8 AWG copper | 0.98 | 0.9806 | 0.0006 |
| 1φ · 8 AWG aluminum | 1.84 | 1.8635 | 0.0235 |
| 3φ · 1 AWG aluminum | 0.34 | 0.3316 | 0.0084 |
Southwire AC results captured Oct 27, 2026 (Pacific).
Comparison method and data
The 21,000 circuits are 10,500 DC and 10,500 AC. The AC set is every combination of 120, 208, 240, 277 and 480 V; 10, 15, 25, 50 and 100 A; 50, 75, 100, 150, 200 and 300 ft; single and three phase; and all 35 conductor sizes both calculators offer. We fixed the inputs and the pass band before sending the first request, and kept every result. The set includes all 2,100 circuits from an earlier, smaller run, and Southwire returned the identical result for every one.
The AC band is 5% of Southwire’s drop plus 0.005 percentage points for its rounding, so a 3% Southwire result allows a 0.155-point gap. DC uses the 0.005-point rounding alone. ZapSketch calculates AC from the AC resistance and reactance in NEC 2023 Table 9, using the effective-impedance method in its Note 2. Other power factors and steel conduit were not part of this comparison.
Beyond Southwire: 86,580 NEC 2023 reference cases.
Southwire’s calculator only accepts so many inputs. To cover every combination ZapSketch supports, each case also runs through a separate reference calculation built directly from NEC 2023 Chapter 9 Tables 8 and 9, and the two must agree on voltage drop, maximum run and ampacity. All 86,580 matched.
| Circuit | Distinct cases | Matched | Reference basis |
|---|---|---|---|
| Single-phase AC | 39,960 | 39,960 | Table 9 |
| Three-phase AC | 39,960 | 39,960 | Table 9 |
| Two-wire DC | 6,660 | 6,660 | Table 8 |
Copper and aluminum · 14 AWG–600 kcmil where supported · 5–100 A · 10–1,000 ft · six voltages per circuit type · EMT and PVC Schedule 40 for AC · power factors 0.5, 0.9 and 1.
Method and test coverage
Each reference case compares voltage loss, percent drop, maximum run length and ampacity flags, and the sweep also checks that the smallest fitting conductor is chosen. AC uses Table 9 at 75°C and 60 Hz with lagging power factor, and balanced loads for three phase; DC uses Table 8. Reference inputs use 75°C terminals, 30°C ambient and a 3% target. Source-table fixtures and hand-worked examples are tested separately.
Southwire comparisons use its Commercial 75°C mode and one conductor set. The grid deliberately includes overloaded and high-drop circuits to exercise the math, not because anyone should install them. Every returned input was checked against what was sent.
Captures are dated snapshots of Southwire’s public calculator; its results may change after these dates.
Why we tested against Southwire
If you have checked a voltage drop on a job site, there is a good chance you used Southwire’s voltage drop calculator. It is free, it has been around for years, and a lot of electricians treat its answer as the one to beat. That made it the fairest test we could set: if ZapSketch disagreed with it, you would want to know why.
So we sent both calculators the same circuits. We chose the full grid of inputs before the first request, saved every response with its timestamp, and kept every result, including the ones that don’t match exactly. On DC there are none. On AC, at the voltage drops you would actually design to, the two are within a tenth of a percentage point.
Don’t take our word for any of it. Every input, both results and the captured Southwire responses are in the downloads above.
Where ZapSketch goes further
Southwire’s calculator answers one circuit at a time. The ZapSketch voltage drop calculator runs the same math and shows every conductor size side by side, with the maximum run each one allows at your target drop. The wire size calculator adds ampacity from NEC 2023 Table 310.16 and picks the smallest conductor that passes both.
In the ZapSketch workspace, the length comes from the home run you draw on the plan. Reroute the run and the voltage drop updates with it, so the number on your drawing never goes stale.
Scope
Checked against NEC 2023. Follow all applicable codes and regulations.
Southwire® is a registered trademark of Southwire Company, LLC. ZapSketch is not affiliated with, sponsored by, or endorsed by Southwire.