Voltage Drop Guide: NEC Limits, Formula & Charts
What Is Voltage Drop?
Voltage drop is the reduction in electrical pressure as current flows through wire resistance. Every conductor has resistance — the longer and thinner the wire, the more voltage is lost. Excessive voltage drop causes:
- Dim lights and slow motors
- Overheating in motors (they draw more current to compensate)
- Electronics malfunction or shutoff
- Reduced efficiency (wasted energy as heat in the wire)
Voltage Drop Formula
Vdrop = (2 × I × L × R) / 1000
- I = current (amps)
- L = one-way wire length (ft)
- R = resistance per 1000 ft (from wire table)
- 2 = accounts for round trip (out and back)
Percentage: Vdrop% = (Vdrop / Vsource) × 100
NEC Voltage Drop Recommendations
| Circuit Type | Max Voltage Drop | NEC Reference |
|---|---|---|
| Branch circuit (feeder to panel) | 3% | NEC 215.2(A)(1) FPN |
| Branch circuit (panel to device) | 3% | NEC 210.19(A) FPN |
| Combined (feeder + branch) | 5% | NEC FPN recommendation |
| Fire alarm circuits | 5% | NEC 760.121 |
Note: These are FPN (Fine Print Notes) — recommendations, not enforceable code. However, they represent best practice and most jurisdictions enforce them.
Voltage Drop Chart (120V, copper wire)
| Wire AWG | Resistance Ω/1000ft | 50 ft | 100 ft | 150 ft | 200 ft | Max length @ 15A (3%) |
|---|---|---|---|---|---|---|
| 14 | 2.525 | 2.5% | 5.0% | 7.6% | 10.1% | 47 ft |
| 12 | 1.588 | 1.6% | 3.2% | 4.8% | 6.4% | 75 ft |
| 10 | 0.999 | 1.0% | 2.0% | 3.0% | 4.0% | 120 ft |
| 8 | 0.628 | 0.6% | 1.3% | 1.9% | 2.5% | 191 ft |
| 6 | 0.395 | 0.4% | 0.8% | 1.2% | 1.6% | 304 ft |
Values at 15A, 120V. For 20A: multiply percentage by 1.33. For 240V: divide percentage by 2.
Worked Examples
Example 1: 120V, 15A, 100 ft
14 AWG: Vdrop = (2 × 15 × 100 × 2.525) / 1000 = 7.58V = 6.3% ❌ (exceeds 3%)
12 AWG: Vdrop = (2 × 15 × 100 × 1.588) / 1000 = 4.76V = 4.0% ❌
10 AWG: Vdrop = (2 × 15 × 100 × 0.999) / 1000 = 3.00V = 2.5% ✅
Result: Use 10 AWG for this 100-ft, 15A, 120V circuit.
Example 2: 240V, 30A, 150 ft (AC unit)
10 AWG: Vdrop = (2 × 30 × 150 × 0.999) / 1000 = 8.99V = 3.7% ❌
8 AWG: Vdrop = (2 × 30 × 150 × 0.628) / 1000 = 5.65V = 2.4% ✅
Result: Use 8 AWG for this 240V, 30A AC circuit at 150 ft.
Common Voltage Drop Mistakes
- Forgetting the round trip: Current flows out AND back. Always multiply by 2.
- Using circuit breaker rating instead of actual load: Use the actual load current, not the breaker rating.
- Ignoring temperature: Copper resistance increases 0.39%/°C. At 75°C operating temp, resistance is 21% higher than at 20°C table values.
- Not considering future loads: If you might add loads later, size the wire for the potential future load.
Standards Reference
- NEC 210.19(A) FPN — Branch Circuit Voltage Drop
- NEC 215.2(A)(1) FPN — Feeder Voltage Drop
- NEC Table 8 — Conductor Properties
- IEEE Standard 141 — Power Distribution
Frequently Asked Questions
What is acceptable voltage drop?
NEC recommends less than 3% voltage drop for branch circuits and less than 5% total (feeder + branch combined). For a 120V circuit, 3% = 3.6V. For a 240V circuit, 3% = 7.2V. Fire alarm circuits have a 5% maximum.
How do I calculate voltage drop?
V_drop = (2 × I × L × R) / 1000, where I = current (amps), L = one-way wire length (ft), R = wire resistance per 1000 ft (from NEC Table 8 or wire manufacturer data). The factor of 2 accounts for the round-trip distance.
Does voltage drop affect 240V circuits?
Yes, but 240V circuits have half the percentage drop for the same absolute voltage loss. A 6V drop on 120V is 5%, but on 240V it's only 2.5%. This is one advantage of 240V for long runs and high-power loads.
When should I upsize wire for voltage drop?
When the calculated voltage drop exceeds 3% at the expected load current. Common scenarios: garage subpanels, outdoor outlets, shed wiring, landscape lighting, and any circuit with wire runs over 75 ft on 120V or 150 ft on 240V.
Does voltage drop waste electricity?
Yes. The voltage lost in the wire is dissipated as heat (P = I²R). A 15A circuit with 5% voltage drop wastes about 90W in the wire. Over a year, that's ~790 kWh or about $100. Proper wire sizing pays for itself in energy savings.