TrueDrop – Voltage Drop

509 Electric

About this app

Voltage drop, done right. TrueDrop is a fast, accurate voltage-drop calculator for working electricians, apprentices, estimators, and inspectors. Enter your conductor, circuit, and load, and get the answer in seconds — with the math shown.

WHAT IT DOES
Three calculation modes:
- Voltage Drop — percent VD and voltage at the load for a given conductor size and run length.
- Min Wire — the smallest standard conductor that keeps VD within your target percentage.
- Max Length — the longest run that stays within your target VD.

Two calculation methods:
- NEC Chapter 9, Table 9 (default) — AC resistance, reactance, and power factor for the most accurate result.
- K-Factor — a quick resistive estimate using the standard K constant (12.9 copper, 21.2 aluminum), clearly labeled as an estimate.

Full input coverage:
- Copper and aluminum conductors, 14 AWG through 2000 kcmil.
- Single-phase and three-phase.
- PVC, aluminum, and steel conduit (drives the Table 9 R/X lookup).
- Parallel conductor sets.
- Adjustable power factor (0.70 to 1.00).

Ampacity and breaker cross-check:
Every result is checked against NEC 310.16 conductor ampacity and NEC 240.6 standard overcurrent-device sizes, including the 240.4(D) small-conductor rules. If a conductor is undersized for the load or the pairing is nonstandard, you get a plain-language warning. TrueDrop warns — it never blocks the calculation or hides the numbers.

Show your work:
Every result expands to reveal R, X, power factor, effective impedance (Zeff), and the exact formula used. Nothing is a black box.

Honest thresholds:
The 3% and 5% voltage-drop figures are NEC Informational Notes (210.19, 215.2), not code requirements. TrueDrop labels them recommended, never a violation.

Built for the field:
- Save calculations on-device and reload them later.
- Light and dark mode.
- Works fully offline — no login, no account, no setup.

WHO IT'S FOR
Electricians pulling feeders and branch circuits, apprentices checking their work, estimators sizing runs during takeoff, and inspectors verifying a design.

ACCURACY
NEC data (Chapter 9 Table 9, Table 8, 310.16, and 240.6) is transcribed from published engineering references. TrueDrop is a field aid for qualified professionals, not a substitute for the NEC, engineering judgment, or field verification — always verify against the NEC edition adopted in your jurisdiction.

PRICE
TrueDrop is completely free — no ads, no in-app purchases, no subscription, and no account.

PRIVACY
Every calculation runs on your device. TrueDrop collects no personal data, requires no account, and works fully offline. Saved calculations stay in local storage on your phone and are never transmitted.

Built by 509 Electric LLC, a licensed Washington electrical contractor (EL01) in Yakima — field tools made by people who wire buildings for a living. Part of the True suite of NEC field calculators for electricians.

What’s new

Initial closed testing release. Voltage drop calculator with NEC Table 9 math, wire sizing, and ampacity checks.