Motor Starting Voltage Dip
Voltage dip during motor start from locked-rotor current and conductor length.
Example
You enter
- Source voltage (V) 480
- One-way conductor length (ft) 250
- Conductor circular mils (cmils) 250000
- Locked-rotor current (A) 180
- Phase three
- Conductor material (K) 12.9
- Dip limit (%) 15
You get
- Voltage drop during start 5.1 V
- Dip (%) 1.0538
- Terminal voltage during start 474.9 V
Checked against published worked examples from: Ohm's-law voltage-drop method (first principles) + NEC Article 430 LRC.
Details, formula, and sources
ScopeWith terminal voltage and a pass/fail against the dip limit (contactor dropout risk).
Formula or tableV_drop = (2 for 1-phase, sqrt(3) for 3-phase) x LRC x L x (R cos phi + X sin phi), R = K / cmils per ft, X the conductor reactance and phi the locked-rotor power-factor angle (typ. pf 0.2-0.4); V_terminal = V_source - V_drop; %dip = V_drop / V_source x 100. K is the conductor constant (Cu ~12.9, Al ~21.2 ohm-cmil/ft) and LRC the motor locked-rotor current.
Source and editionImpedance voltage-drop method at the locked-rotor power factor (IEEE 141 by name); motor locked-rotor current per NEC Article 430 code-letter tables; contactor pickup/dropout ~85% nominal per NEMA ICS 2, by name.
Free accessNFPA 70 free read-only at nfpa.org/freeaccess; LRC user-supplied from the nameplate code letter or estimated as 6x FLA.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namessource_voltage_V, length_ft, cmils, lrc_A, phase, k_const, dip_limit_pct, v_drop_V, dip_pct, v_terminal_V
- Locked-rotor current user-supplied from the NEC 430 code-letter table, or estimated as 6x full-load ampsNEC Article 430
- Conductor constant K 12.9 ohm-cmil/ft copper, 21.2 aluminum (one-way)voltage-drop convention