TradeToolsElectrical · in the TradeTools app
Voltage Drop Calculator
AC voltage drop on a branch circuit or feeder from the Table 9 conductor impedance, the voltage left at the load, and the minimum conductor size that keeps the drop within the recommended 3 %.
NEC Ch. 9 Table 9
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Worked example
200 ft to the shop on 10 AWG at 20 A — how much do you lose?
| Voltage / phase | 240 V, single-phase |
|---|---|
| Conductor | 10 AWG copper |
| Raceway | PVC |
| One-way length | 200 ft |
| Load | 20 A |
| Power factor | 0.85 |
| Voltage drop | 8.37 V |
|---|---|
| Percent | 3.49 % — above the recommended 3 % |
| Voltage at the load | 231.6 V |
| Effective impedance | 1.046 Ω/kft (NEC Ch. 9 Table 9) |
| Smallest size within 3 % | 8 AWG |
Impedance is computed from Table 9's resistance and reactance for uncoated copper in PVC at 0.85 power factor: 1.2 × 0.85 + 0.050 × 0.527 = 1.046 Ω per 1000 ft. Two conductors, 200 ft, 20 A: 2 × 200 × 20 × 1.046 / 1000 = 8.37 V, or 3.49 % of 240 V. 8 AWG brings it under 3 %.
How it's calculated
The table behind the number
The app takes the conductor's AC resistance and inductive reactance from NEC Chapter 9 Table 9 for the raceway material, combines them at the power factor you enter (R·cos θ + X·sin θ, Table 9 Note 2), and computes the drop for single-phase (2 × length) or three-phase (√3 × length) circuits. It reports the drop in volts and percent, the voltage at the load, and the smallest conductor that stays under the 3 % that 210.19(A) and 215.2(A) recommend in their informational notes.
- NEC 2023 Chapter 9, Table 9AC resistance and reactance for 600 V conductors, by raceway material
- NEC 2023 210.19(A) Informational Note No. 43 % branch-circuit drop — a recommendation, not a requirement (90.5(C))
- NEC 2023 215.2(A) Informational Note No. 25 % combined feeder and branch-circuit drop — recommendation
Questions
Before you trust the number
Is 3 % voltage drop a code requirement?
No. The 3 % and 5 % figures live in informational notes, which 90.5(C) defines as explanatory material. Your AHJ or the job specification may make them mandatory; the NEC itself does not.
Why does the printed 0.85 PF column give a different answer?
Table 9 also prints an 'effective Z at 0.85 PF' column rounded to two digits; the app computes the impedance from the R and X columns for the power factor you enter, which is what the table's Note 2 describes. The conclusion — over 3 %, 8 AWG gets under — is the same.