Understanding Voltage Drop
Voltage drop occurs when electrical current flows through a conductor. The wire's resistance causes some voltage to be lost as heat, reducing the voltage available at the load. Excessive voltage drop can cause equipment to malfunction or operate inefficiently.
Voltage Drop Formula
- Single Phase: VD = 2 × L × R × I / 1000
- Three Phase: VD = √3 × L × R × I / 1000
Where L = one-way length (ft), R = wire resistance (Ω/1000ft), I = current (amps).
NEC Voltage Drop Limits
- Branch circuits: 3% maximum recommended
- Feeder + branch combined: 5% maximum recommended
- These are recommendations, not code requirements, but are considered best practice
How to Reduce Voltage Drop
- Use a larger wire gauge: Going from 14 AWG to 12 AWG cuts resistance by ~37%. Each step up in wire size roughly halves the voltage drop
- Shorten the wire run: Move the panel closer or use a sub-panel near the load
- Use copper instead of aluminum: Copper has ~60% of aluminum's resistance per foot. Aluminum requires going 1-2 gauge sizes larger to match copper's performance
- Increase source voltage: 240V circuits have half the current of 120V for the same wattage, dramatically reducing voltage drop
Common Wire Gauge Reference
- 14 AWG: 15-amp circuits, lighting, most bedroom outlets
- 12 AWG: 20-amp circuits, kitchen, bathroom, garage outlets
- 10 AWG: 30-amp circuits, dryers, large window AC units
- 8 AWG: 40-amp circuits, electric ranges, large appliances
- 6 AWG: 50-60 amp circuits, hot tubs, sub-panels, EV chargers
For long runs (over 50 feet), you'll often need to go 1-2 gauge sizes larger than the minimum to stay within the 3% voltage drop recommendation. This is especially important for outbuildings, detached garages, and well pumps.
When Voltage Drop Matters Most
- LED lighting: Even small voltage drops can cause flickering or dimming
- Motors and compressors: Low voltage causes overheating, reduced efficiency, and shortened lifespan
- EV chargers: Long runs to a garage or carport need careful wire sizing for the 40-50 amp draw
- Well pumps: Often 200+ feet from the panel, making voltage drop a primary concern
Copper vs. Aluminum Wire
| Property | Copper | Aluminum |
|---|---|---|
| Resistivity | 1.68 × 10⁻⁸ Ω·m | 2.82 × 10⁻⁸ Ω·m |
| Weight | Heavier | ~30% lighter |
| Cost | Higher | ~40% less |
| Typical use | Branch circuits, interiors | Service entrances, feeders |
Aluminum requires anti-oxidant compound at connections and rated connectors (marked AL/CU). Using copper-only connectors with aluminum wire causes oxidation, loose connections, and fire risk. Most modern residential branch circuits use copper; aluminum is common for 100+ amp feeders and service entrance cables where cost savings are significant.
Wire Gauge and Resistance
American Wire Gauge (AWG) uses a counterintuitive numbering system — smaller numbers mean larger wire. Each step down by 3 gauge sizes (e.g., 14 to 11 AWG) roughly doubles the cross-sectional area and halves resistance. For long runs, upgrading wire gauge is the most practical way to reduce voltage drop. The cost of slightly larger wire is almost always less than the ongoing energy loss from an undersized conductor.