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Voltage Drop
Single-phase or three-phase voltage drop over length.
Example
You enter
- Phase single
- Material copper
- AWG 10
- Length one-way (ft) 150
- Current (A) 20
- Source voltage (V) 240
You get
- Voltage drop 7.45 V
- Percent drop 3.11 %
Checked against published worked examples from: Mike Holt Enterprises.
Details, formula, and sources
Formula or tableVD = 2 x K x I x L / cmil single-phase; sqrt(3) replaces 2 for three-phase; K = 12.9 (Cu) / 21.2 (Al) ohm-cmil/ft, the 75 C stranded basis of NEC Chapter 9 Table 8. DC resistance only: no reactance, no power factor (use voltage-drop-reactance for that).
Source and editionNEC 2023 (NFPA 70). Chapter 9 Tables 8 and 9; IEEE 141 by name.
Free accessFree read-only access at nfpa.org/freeaccess. IEEE 141 is licensed; principles free at university OCW and IEEE-USA outreach.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesphase, material, awg, length_ft, current_A, source_voltage_V, drop_V, percent
- Conductor temperature 75°C (fixed in K)NEC 310.15(B)(1)
- Power factor not modeled; resistive drop onlyengineering practice for general lighting / receptacle loads
- Voltage-drop target 3% branch + 5% feeder = 5% totalNEC informational notes 210.19(A)(1) FPN 4 / 215.2(A)(1) FPN 2