For a standard 120V, 20A branch circuit running 100 feet, you must upgrade from the minimum 12 AWG to 10 AWG copper to maintain a voltage drop under the recommended 3% (3.6V). If you are wiring a 240V, 30A double-pole circuit for a welder or EV charger at 120 feet, 8 AWG copper is your required minimum to prevent terminal overheating and equipment malfunction.
Wire sizing is not just about preventing breakers from tripping; it is about delivering adequate voltage to the load. The National Electrical Code (NEC) recommends a maximum 3% voltage drop for branch circuits and 5% total for feeder and branch combined. Below is the definitive reference to get your AWG sizing right the first time.
How to Read This Voltage Drop Calculation Table
Before pulling wire, you must know which column applies to your specific installation. The resistance of a conductor changes based on its insulation temperature rating and the termination limits of your breakers.
- 60°C Column: Use this for NM-B (Romex) cable and any terminations on devices rated only for 60°C. This is the default for most residential 15A and 20A receptacles and switches.
- 75°C Column: Use this for THHN/THWN-2 wire in conduit, provided your breakers and lugs are explicitly rated for 75°C (standard for most modern residential panels, subpanels, and 30A+ breakers).
- 90°C Column: Never use the 90°C ampacity for final wire sizing. It is strictly used as a starting point for calculating derating factors before applying the 75°C or 60°C termination limits per NEC 110.14(C).
The Master Voltage Drop Calculation Table
This table is derived from NEC Chapter 9, Table 8 conductor properties, pre-calculated for a maximum 3% voltage drop at standard nominal voltages. Bookmark the quick-jump rows below for the most common residential and workshop circuits.
| AWG Size | Material | Resistance (Ω/kFT @ 75°C) | Max Dist: 15A @ 120V | Max Dist: 20A @ 120V | Max Dist: 30A @ 240V | Max Dist: 40A @ 240V |
|---|---|---|---|---|---|---|
| 14 AWG | Copper | 3.14 | 38 ft | N/A (20A req 12AWG) | N/A | N/A |
| 12 AWG | Copper | 1.98 | 60 ft | 45 ft | N/A | N/A |
| 10 AWG | Copper | 1.24 | 96 ft | 72 ft | 96 ft | N/A |
| 8 AWG | Copper | 0.778 | 154 ft | 115 ft | 154 ft | 115 ft |
| 6 AWG | Copper | 0.491 | 244 ft | 183 ft | 244 ft | 183 ft |
| 4 AWG | Copper | 0.308 | 389 ft | 292 ft | 389 ft | 292 ft |
| 10 AWG | Aluminum | 2.00 | 60 ft | 45 ft | 60 ft | N/A |
| 8 AWG | Aluminum | 1.26 | 95 ft | 71 ft | 95 ft | 71 ft |
| 6 AWG | Aluminum | 0.797 | 150 ft | 113 ft | 150 ft | 113 ft |
Derating and Environmental Modifiers
The base values in the table above assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. When real-world conditions change, derating rows modify your base ampacity, which indirectly affects your voltage drop by forcing you to upsize the wire.
According to NEC Table 310.15(B)(16) and 310.15(C)(1), apply these modifiers:
- 4 to 6 current-carrying conductors in one conduit: Multiply base ampacity by 80%. (e.g., A 10 AWG THHN wire rated 35A at 90°C drops to 28A. It can still protect a 20A breaker, but if you add multiple neutrals, watch your fill limits).
- 7 to 9 current-carrying conductors: Multiply base ampacity by 70%.
- Ambient temp 41°C to 45°C (105°F - 113°F): Multiply the 75°C column ampacity by 0.82. This is common for wires running through hot attics in southern climates.
If derating drops your wire's adjusted ampacity below the breaker size, you must upsize the AWG. Upsizing the wire inherently reduces the resistance (Ω/kFT), which improves your voltage drop profile. For exact math on complex runs with parallel feeds, cross-reference with Cerrowire's official voltage drop calculator.
Decision Path: Sizing Your Next Circuit
Use this decision tree to terminate your planning phase with a concrete wire pick. Do not guess; follow the logic path based on your specific load and distance.
| Scenario Conditions | Decision Logic | Concrete Pick |
|---|---|---|
| 120V, 15A, standard bedroom outlet, < 50 ft from panel | Ampacity requires 14 AWG. Distance is under 38 ft limit for 15A on 14 AWG, but 50 ft pushes drop to ~3.8%. | 12 AWG Copper (NM-B) |
| 120V, 20A, garage workshop receptacle, 80 ft from panel | Ampacity requires 12 AWG. 80 ft on 12 AWG yields 4.8% drop (fails 3% rule). Must upsize. | 10 AWG Copper (THHN in conduit) |
| 240V, 30A, EV charger or dryer, 110 ft from panel | Ampacity requires 10 AWG. 110 ft on 10 AWG yields 3.4% drop (fails 3% rule). Must upsize. | 8 AWG Copper (THHN) |
| 240V, 50A, subpanel feeder, 150 ft from main | Ampacity requires 6 AWG. 150 ft on 6 AWG yields 2.0% drop (passes). 4 AWG yields 1.2%. | 6 AWG Copper or 4 AWG Aluminum (SER cable) |
What This Table Cannot Tell You
While this voltage drop calculation table handles the physics of resistance and distance, it does not replace a complete NEC review. Here is what the table omits and where you must apply additional rules:
- Continuous Load Sizing: If your load runs for 3 hours or more (like a hardwired space heater, server rack, or EV charger), NEC 210.20(A) requires you to multiply the load by 125% before sizing the breaker and wire. A 30A continuous load requires a 40A breaker and 8 AWG wire minimum, regardless of the distance shown in the table.
- Conduit Fill Limits: You cannot simply pull five 6 AWG wires through a 3/4-inch PVC conduit. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires to prevent heat buildup and physical jamming. Always check physical fill capacity before buying wire.
- Aluminum Termination Prep: If you choose aluminum wire for long feeder runs to save money, remember that aluminum oxidizes rapidly. You must use an antioxidant compound (like Noalox) and torque lugs to the manufacturer's exact inch-pound specification to prevent high-resistance joints that mimic voltage drop symptoms.
- Local AHJ Amendments: Some municipalities mandate a strict 2% maximum voltage drop for branch circuits rather than the NEC's recommended 3%. As always, consult Mike Holt Enterprises or your local inspector for jurisdiction-specific amendments.
By anchoring your wire size to the 75°C resistance values, applying the correct derating factors, and following the decision path above, you will eliminate dimming lights, overheated terminals, and failed inspections on your next pull.
