Voltage Drop Calculator for Flickering Lights
Calculate voltage drop in your electrical circuit to diagnose flickering lights. A voltage drop greater than 3% (NEC recommendation) or 5% (maximum allowable) often causes flickering, dimming, or premature bulb failure.
Total amperage drawn by the light fixture(s)
Distance from panel/source to the fixture (one way)
Formulas Used
Wire Resistance:
R = (ρ × Ltotal) / CM
- ρ = Resistivity at 75°C: Copper = 12.9 Ω·cmil/ft, Aluminum = 21.2 Ω·cmil/ft
- Ltotal = 2 × one-way length (single-phase) or √3 × one-way length (three-phase)
- CM = Circular mils of the selected wire gauge
Voltage Drop:
VD = I × R
Voltage Drop Percentage:
VD% = (VD / Vsource) × 100
Required Circular Mils (for ≤ 3% drop):
CMrequired = (ρ × Ltotal × I) / (0.03 × Vsource)
Assumptions & References
- Wire resistivity values are based on 75°C conductor temperature per NEC Table 9 (standard for most residential/commercial wiring calculations).
- The NEC (NFPA 70) recommends ≤ 3% voltage drop on branch circuits and ≤ 5% total (feeder + branch). Exceeding these thresholds is a common cause of flickering lights.
- Single-phase calculations use 2× the one-way length to account for both the hot and neutral conductors.
- Three-phase calculations use √3 × one-way length (line-to-neutral factor for balanced loads).
- Circular mil values are from standard AWG tables (ASTM B3 / NEC Chapter 9, Table 8).
- This calculator assumes solid or stranded copper/aluminum conductors at normal operating temperature. Actual resistance may vary with temperature, conduit fill, and conductor bundling.
- Flickering can also be caused by loose connections, load switching, utility supply issues, or incompatible dimmers — voltage drop is one contributing factor.
- Reference: NFPA 70 National Electrical Code, Article 210 & 215; IEEE Std 1100 (Emerald Book).