Voltage Drop Calculator
The K-method law — VD = 2KIL/CM — as a checker and as a solver: does the wire arrive, and what is the smallest size that makes it?
Voltage Drop Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Single-phase DC-equivalent law — no three-phase ×1.732 form.
- Table 8 sizes 14 AWG through 1/0 only.
- K at 75°C; extreme temperatures shift resistivity.
In short: 20 A on 12 AWG across 100 ft: 7.9021 V — 6.59% at 120 V, 3.29% at 240. Solve the same run to a 3% limit at 240 V and the law wants ≥ 7,166.67 circular mils → 10 AWG (2.07%). Stretch to 150 ft and 10 AWG fails at 3.107% — 8 AWG is the honest answer.
Formula
VD = 2 × K × I × L ÷ CM
K = 12.9 copper / 21.2 aluminum (75°C)
percent = VD ÷ system volts × 100
solver: CM ≥ 2KIL ÷ (V × limit%)
[('2', 'out and back — the round trip'), ('12.9 / 21.2', 'K for copper / aluminum'), ('6,530 / 10,380', 'CM of 12 / 10 AWG, Table 8'), ('3% / 5%', 'branch advisory / feeder+branch total')]
Worked Example
- Enter the load current and the ONE-WAY run.
- Pick the size and metal — K switches with it.
- Read volts and percent against the limit.
- Fail? Flip to solve mode for the minimum size.
- Mind motors: 90% voltage ≈ 80% starting torque.
12 AWG, 20 A, 100 ft: 7.9021 V = 6.59% @120 V (3.29% @240). Solver to 3% @240: ≥ 7,166.67 CM → 10 AWG at 2.07%. At 150 ft, 10 AWG fails (3.107%) → 8 AWG.
Strengths & Limits Of This Model
Where this engine is strong
- Checker AND solver in one law
- Both system voltages shown on the percent card
- The 150-ft failure case lives in the canonical answer
Where it stops
- Single-phase only
- No reactance at large sizes
Practical Use Cases
Long EV runs
Garage-to-panel distance checks.
Outbuildings
Shed and barn feeders.
Well pumps
Motor-starting sensitive loads.
Landscape lighting
Low-voltage, brutally distance-sensitive.
Feeder sizing
The 5% total advisory.
Methodology & Editorial Standards
VD = 2KIL/CM with K switched by conductor term (12.9/21.2) and CM from the Chapter 9 Table 8 ladder (14 AWG 4,110 → 1/0 105,600). Percent = VD ÷ system volts. Solve mode inverts: required CM = 2KIL ÷ (volts × limit%), then the smallest table size at or above it, with its own re-computed drop printed. Zero/negative current, length or voltage refused; sizes outside 14–1/0, limits outside 1-10%, unknown conductors refused; solutions beyond 1/0 refused with the parallel-conductor guidance.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Voltage Drop Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How much voltage drop is acceptable?
The working custom: 3% on a branch circuit, 5% feeder-plus-branch combined. Both are informational notes in the NEC, not violations — but motors and electronics feel every point of them.
What is K in the voltage drop formula?
The resistivity constant in ohms·cmil/ft: 12.9 for copper, 21.2 for aluminum at operating temperature. Aluminum's K is 64% worse — the arithmetic behind 'aluminum runs two sizes up'.
Why the factor of 2 in 2KIL/CM?
Current makes a round trip: out on the hot, back on the neutral (or the other hot). Both legs drop voltage, so the length counts twice.
Is voltage drop worse at 120 V or 240 V?
The volts lost are identical; the PERCENTAGE halves at 240 V. The same 7.9-V run is 6.59% at 120 V and 3.29% at 240 — the oldest reason to pull a 240-V circuit to a far load.
What size wire for 100 feet at 20 amps?
12 AWG fails the 3% advisory at 120 V (6.59%); at 240 V it just clears at 3.29%. For a hard 3% at 120 V, 10 AWG is the first size that holds.
Wire page or this page?
The Wire Size page enforces ampacity — what the metal survives. This page enforces delivery — what arrives at the load. Big wires on long runs need both answers to agree.
Does voltage drop waste electricity?
Yes — every dropped volt burns watts as heat in the conductor itself, paid for on your bill and delivered as warm copper. At 3% the loss is small; at 10% you are heating the wall with a tenth of the circuit’s energy.
Does 1/0 cover every long run?
No — past 1/0 (105,600 CM) this table ends. Longer or heavier runs move to parallel conductors or higher voltage, which is feeder engineering.