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Voltage Drop

What it's for

A long run loses voltage in the wire. Enter the run, the load and the wire you plan to pull, and Voltage Drop tells you the drop in volts and percent, whether that is inside the limit you set, and the smallest listed size from yours upward that clears the limit and carries the amps. The limit is yours to set. It is a check on one run, with no material list.

Step by step

The example is the screen as it opens: a 75' 0" run, 20 A at 120 V, on 12 AWG copper, held to 3.0%.

Voltage Drop inputs: the answer bar reads 8 AWG, a green 8 AWG within 3.0% max drop and 12 AWG struck through at 4.94% drop (5.93 V) — over the 3.0% limit. Then ONE-WAY RUN 75' 0", SYSTEM VOLTAGE 120 V, AMPS 20 A, CONTINUOUS LOAD (×1.25) off, PHASE Single (Three beside it), WIRE SIZE 12 AWG, CONDUCTOR Copper (Aluminum beside it) and MAX DROP 3.0% with a stepper. RESULTS starts at the bottom edge: 12 AWG struck through, 8 AWG: 1.95% drop (2.34 V) — 75' 0" one way · 20 A · 120 V, and the green line. 1 2 3 4 5 6 7
Steps 1–7, the whole form on one screen. Step 3 includes CONTINUOUS LOAD (×1.25) under AMPS. Step 7, MAX DROP, is the last input row.
  1. ONE-WAY RUN — the length of cable from the panel to the load, one way, along the route the cable really takes. Do not double it. The calculator counts out and back for you. The example is 75' 0".
  2. SYSTEM VOLTAGE — 120, 208, 240, 277 or 480 V. The example is 120 V.
  3. AMPS — the current the load draws as it runs, from its nameplate, not the breaker size. Tap it and type the number. The example is 20 A. Under it is CONTINUOUS LOAD (×1.25): turn it on for a load that runs at full current for 3 hours or more, such as a storage water heater or an EV charger. The NEC wants the wire for a continuous load to carry 125% of it: 210.19(A)(1) for a branch circuit, 215.2(A)(1) for a feeder, 422.13 for a storage water heater, 625.41 and 625.42 for an EV charger. The toggle multiplies AMPS by 1.25 for the ampacity check only. The drop is still figured on the real AMPS. A 4000 W water heater at 240 V draws 16.67 A; with the toggle on, 125% is 20.8 A, over the 20 A that 12 AWG copper carries, so the answer is 10 AWG. The info button beside AMPS says the same. A current that is not a whole number shows to a tenth with ≈, such as ≈ 16.7 A, everywhere it appears; the math uses what you typed.
  4. PHASE — Single or Three. Single is a 2-wire circuit, out and back: drop = 2 × K × I × L ÷ circular mils. Three is a balanced three-phase circuit, and the line-to-line drop is √3 × K × I × L ÷ circular mils, about 13% less on the same wire. The example is Single.
  5. WIRE SIZE — the size you plan to pull, from 14 AWG up to 4/0 AWG. The example is 12 AWG.
  6. CONDUCTOR — Copper or Aluminum.
  7. MAX DROP — the limit you are holding the run to, in steps of 0.5%. The example is 3.0%.
  8. Read RESULTS. The headline is the smallest size within the limit: 8 AWG. Under it, your own size is struck through with its drop, 12 AWG 4.94% drop (5.93 V) — over the 3.0% limit, then the size within the limit with its drop, 8 AWG: 1.95% drop (2.34 V).
  9. Tap BASIS & WHAT WAS ASSUMED to see what the smallest size was governed by, the formula, and the NEC edition the sections come from (2023).
  10. Scroll down to the run drawing and its ladder of sizes.

Reading the results

Voltage Drop results and drawing: the answer bar keeps 8 AWG and the green check. RESULTS reads 12 AWG struck through at 4.94% drop (5.93 V) — over the 3.0% limit, 8 AWG: 1.95% drop (2.34 V) — 75' 0" one way · 20 A · 120 V and the green 8 AWG within 3.0% max drop, then the BASIS & WHAT WAS ASSUMED row and the link Check before you cut · Measure twice. The drawing runs from PANEL 20 A to LOAD showing 120 V and 114.1 V over 75' 0", with a ladder of sizes 12, 10, 8, 6, 4 under 4.9%, 3.1%, 2.0%, 1.2%, 0.8%, the 12 and 10 struck through and 8 filled in, an inset comparing #12 at 6530 and #8 at 16510 CIRCULAR MILS, and notes including DROP 5.93 V = 4.94% · OVER LIMIT 3.0% (STRUCK) in red, over the title strip VOLTAGE DROP RUN, Vd = 2·k·I·L / CM, NOT TO SCALE. A dimmed SHARE PDF and a lit SAVE end the screen. 8 9 10
Steps 8–10: the smallest size within the limit, the basis row, and the ladder of sizes.

The results card reads one of five ways.

One more red line can appear: 14 AWG NOT LISTED IN ALUMINUM — SMALLEST AL SIZE IS 12. Your 14 AWG is struck through with — not listed in aluminum after its drop, even when the drop itself is within the limit. Pick a larger WIRE SIZE or switch CONDUCTOR to Copper.

BASIS & WHAT WAS ASSUMED opens to the formula and a reminder that the limit is your own setting. When a size is within the limit, a line above it says whether that size was set by the drop limit or had to go bigger to carry the current, and names the table it used. The next line says which current the ampacity check used: AMPS as entered, or, with CONTINUOUS LOAD (×1.25) on, 125% of it, with the drop still on the real current. The smallest size always has to carry that current as well as stay within the drop limit. Another line says what the table assumes: a 30 °C (86 °F) room and no more than three current-carrying conductors together. The app applies no correction, so wire run through a hot attic or bundled with other cables needs a smaller rating than the table gives (NEC 310.15(B), 310.15(C)(1), 334.80).

The drawing is a one-line schematic, not to scale: panel to load and back, the voltage at each end on the wire you entered, and the one-way run. Under it is a ladder of sizes with the percent drop for each. One limit line strikes every size that misses the limit — here 12 and 10 — and stops where the answer starts. Your own size is the dashed cell and the passing size is filled in. The boxed inset draws your conductor and the one you have to go up to at their true relative area, in circular mils; when your size already passes it draws yours alone. The notes repeat the working: the out and back length, the wire you entered (ENTERED #12 Cu · 6530 CM · 20 A · k = 12.9), the drop — in red when it is over the limit — the upsize, then the circuit type and the ampacity table it read. With CONTINUOUS LOAD (×1.25) on, that note gives the current the table was read at, 1-PHASE 2-WIRE · T310.16 60°C AT 125%: ≈ 20.8 A for the water heater above. The last note on the drawing reads 3%/5% IS AN NEC INFORMATIONAL NOTE, NOT A VIOLATION. Pinch to zoom, drag to move around, double-tap to reset, or tap the expand button at its top right for the whole screen. See Reading the drawing.

There is no MATERIALS list. SHARE PDF stays dimmed until you change an input, then shares the drawing, the result and your inputs, with Continuous Load and Phase rows. The drop row names the size it belongs to, such as Drop On 12 AWG, so it never reads as the drop of the size that passes. When the answer is a different size, a second row, such as Drop On 10 AWG, gives that size's drop. History shows the same rows. SAVE puts the result in History. The PDF footer reads Estimate — verify dimensions and local code before cutting/ordering. Your local code and your inspector govern the conductor you install.

Voltage Drop is a Pro calculator. The PDF and History are Pro too. Pro is a one-time purchase. See Free and Pro.

Common mistakes