Run a long wire from a breaker panel to a distant outlet, and the voltage at the far end will be lower than at the source. This is voltage drop, and ignoring it can cause motors to fail, lights to dim, and equipment to malfunction.
What Is Voltage Drop?
Voltage drop is the reduction in voltage along a conductor due to its resistance. Every wire has some resistance — even copper — and when current flows through resistance, some voltage is "consumed" by the wire itself instead of reaching the load.
Voltage Drop (V) = Current (A) × Wire Resistance (Ω)
Why Voltage Drop Matters
- Motors: A motor rated for 120V receiving only 110V draws more current to produce the same power — leading to overheating and early failure
- Lighting: LED and incandescent lights will be noticeably dimmer
- Electronics: Voltage-sensitive equipment (chargers, power supplies) may malfunction
- Heating elements: Ovens and water heaters take longer to reach temperature
Acceptable Voltage Drop Limits
The NEC (National Electrical Code) recommends:
- Branch circuits: Maximum 3% voltage drop
- Feeders + branch circuit combined: Maximum 5% total
The Voltage Drop Formula
For single-phase AC or DC circuits:
VD = (2 × K × I × L) ÷ CM
Where:
- K = resistivity constant (12.9 for copper, 21.2 for aluminum)
- I = current in amps
- L = one-way length of wire in feet
- CM = wire cross-section in circular mils (from AWG tables)
Practical Example
You're running a 20A circuit 75 feet from the panel using 12 AWG copper wire. 12 AWG has 6,530 circular mils:
VD = (2 × 12.9 × 20 × 75) ÷ 6,530 = 5.94V
That's 5.94 ÷ 120 = 4.95% — just under the 5% combined limit but exceeding the 3% branch circuit recommendation. Solution: upsize to 10 AWG (10,380 CM):
VD = (2 × 12.9 × 20 × 75) ÷ 10,380 = 3.73V (3.1%)
How to Prevent Excessive Voltage Drop
- Upsize the wire gauge — most common solution
- Move the panel closer — for new construction, plan subpanels near heavy loads
- Install a subpanel — for outbuildings or workshops far from the main panel
- Reduce the load — split large loads across multiple circuits
- Use higher voltage — 240V circuits have half the current for the same power (P = V × I), dramatically reducing drop
Quick Wire Sizing for Long Runs
| Circuit | Up to 50 ft | 50–100 ft | 100–150 ft |
|---|---|---|---|
| 15A branch | 14 AWG | 12 AWG | 10 AWG |
| 20A branch | 12 AWG | 10 AWG | 8 AWG |
| 30A branch | 10 AWG | 8 AWG | 6 AWG |
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