If you are running a standard 120V, 15A branch circuit, 14 AWG copper is legally permitted by ampacity tables for up to 50 feet. However, at 100 feet, resistance causes the voltage at the receptacle to sag below acceptable limits. To maintain a strict 3% maximum voltage drop on a 120V circuit at 100 feet, you must upsize to 10 AWG or 8 AWG copper. Sizing wire for distance is entirely about managing resistance, not just preventing the wire from melting.
Below is the definitive wire size chart by distance, engineered for single-phase residential and light commercial branch circuits. We will cover how to read the temperature columns, apply derating factors, and use a decision tree to lock in your exact AWG pick.
The Master Wire Size Chart by Distance (NEC Guidelines)
This table provides the minimum copper wire gauge required to keep voltage drop at or below 3% for branch circuits, aligning with the recommendations in NEC 210.19(A) Informational Note and calculated using conductor resistance values from NEC Chapter 9, Table 8. All values assume solid or stranded copper wire, a single-phase circuit, and an ambient temperature of 30°C (86°F).
| Circuit Amps | Nominal Voltage | One-Way Distance (ft) | Max Allowable Drop (3%) | Recommended Copper AWG | Actual Voltage Drop % |
|---|---|---|---|---|---|
| 15A | 120V | 50 | 3.6V | 14 AWG | 1.8% |
| 15A | 120V | 100 | 3.6V | 10 AWG | 3.0% |
| 15A | 120V | 150 | 3.6V | 8 AWG | 2.9% |
| 20A | 120V | 50 | 3.6V | 12 AWG | 2.4% |
| 20A | 120V | 100 | 3.6V | 8 AWG | 2.5% |
| 20A | 120V | 150 | 3.6V | 6 AWG | 2.9% |
| 30A | 240V | 50 | 7.2V | 10 AWG | 1.5% |
| 30A | 240V | 100 | 7.2V | 10 AWG | 3.0% |
| 30A | 240V | 150 | 7.2V | 8 AWG | 2.9% |
| 50A | 240V | 50 | 7.2V | 8 AWG | 1.9% |
| 50A | 240V | 100 | 7.2V | 6 AWG | 1.9% |
| 50A | 240V | 200 | 7.2V | 3 AWG | 2.5% |
How to Read the Chart and Apply Derating Factors
Before you buy 500 feet of THHN, you need to understand which column applies to your installation and how real-world conditions modify these base values. The AWG recommendations above are derived from the 75°C (167°F) ampacity column of NEC Table 310.16. Even if you are using 90°C THHN wire, NEC 110.14(C) mandates that you must size the overcurrent protection and base your voltage drop calculations on the lowest temperature rating of any connected terminal, which is almost always 75°C in modern breakers and receptacles.
How Derating Modifies the Base Value: The chart above assumes a single circuit (two or three current-carrying conductors) in a raceway at 30°C ambient. If you pull more than three current-carrying conductors through the same conduit, NEC 310.15(C)(1) requires you to derate the wire's ampacity. For 4-6 conductors, you multiply the base ampacity by 80%. If your derated ampacity falls below your breaker size, you must upsize the wire again, regardless of what the distance chart says. Furthermore, if your attic or conduit runs in an environment exceeding 86°F (30°C), apply the ambient temperature correction factors from Table 310.15(B)(1).
For a deeper dive into the math behind these calculations and how the National Electrical Code treats voltage drop as a recommendation rather than a strict enforceable rule for most branch circuits, refer to this mastering NEC voltage drop calculations guide by EC&M.
Decision Tree: Pick Your Exact Wire Gauge
Stop guessing and follow this decision path to terminate on a concrete wire size for your specific run. Start at Step 1 and follow the logic down.
| Step | Condition / Question | Action / Next Step |
|---|---|---|
| 1 | What is the continuous load (Amps)? | Multiply continuous loads (>3 hours) by 1.25. Use this new number as your 'Circuit Amps'. |
| 2 | Is the one-way distance under 50 feet? | YES: Use standard ampacity sizing (14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A). Stop here. NO: Proceed to Step 3. |
| 3 | Locate your Voltage, Amps, and Distance in the Master Chart above. | Note the Recommended Copper AWG. Proceed to Step 4. |
| 4 | Are there 4 or more current-carrying conductors in the conduit? | YES: Upsize the wire by one AWG step (e.g., from 10 AWG to 8 AWG) to compensate for the 80% derating factor. Proceed to Step 5. NO: Proceed to Step 5. |
| 5 | Are you using aluminum wire instead of copper? | YES: Upsize by two AWG steps (e.g., 8 AWG Copper becomes 4 AWG Aluminum) due to higher resistance. NO: Your final pick is the AWG from Step 3/4. |
Concrete Pick Example: You are wiring a detached garage subpanel. The load is 40A continuous at 240V, and the trench is 120 feet long.
Step 1: 40A x 1.25 = 50A Circuit Amps.
Step 2: Distance > 50ft.
Step 3: Chart says 50A / 240V / 100ft (closest) is 6 AWG Copper.
Step 4: Only 2 current-carrying conductors (plus ground).
Step 5: Using copper.
Final Pick: 6 AWG Copper THHN. (If using direct burial UF-B, verify 60°C column limits, which would require upsizing to 4 AWG).
What This Chart Cannot Tell You
Voltage drop charts are excellent for ensuring your tools and appliances receive adequate voltage, but they are blind to three critical physical and code limitations:
- Conduit Fill Limits: Upsizing from 12 AWG to 6 AWG drastically increases the cross-sectional area of the wire. A 3/4-inch EMT conduit can hold sixteen 12 AWG THHN wires, but only six 6 AWG THHN wires. Always check NEC Chapter 9, Table 1 for conduit fill percentages before pulling.
- Terminal Physical Acceptance: A standard 20A duplex receptacle is physically designed to accept a maximum of 12 AWG or 10 AWG wire. If your distance calculation demands 8 AWG or 6 AWG to prevent voltage drop, you cannot terminate that thick wire directly on the receptacle. You must pigtail it down to 12 AWG inside a deep junction box using a wire nut or Wago connector.
- Short Circuit Withstand Rating: While a larger wire handles voltage drop beautifully, it also allows massive fault currents to flow during a dead short. Ensure your breaker's AIC (Ampere Interrupting Capacity) rating and the wire's insulation rating are matched to the available fault current at the panel.
Quick-Jump Reference for the Most Queried Runs
Bookmark this section for the most common long-run scenarios encountered in residential and light commercial retrofits.
120V, 20A Receptacle Circuit at 100 Feet (Garage or Workshop)
Pick: 8 AWG Copper. Do not use 10 AWG; at 20A, 10 AWG will yield a 4.0% drop (4.8V), which will cause noticeable dimming in LED lighting and sluggish performance in high-draw miter saws or air compressors.
240V, 30A Dryer or RV Outlet at 150 Feet
Pick: 8 AWG Copper. While 10 AWG is the standard ampacity minimum for 30A, pushing it to 150 feet results in a 4.5% voltage drop. Upsizing to 8 AWG keeps the drop at a healthy 2.9%, ensuring the RV AC unit or dryer heating element operates efficiently.
240V, 50A Hot Tub or Welder Circuit at 200 Feet
Pick: 3 AWG Copper (or 1 AWG Aluminum). A 50A load at 200 feet is highly susceptible to voltage sag. Using the standard 6 AWG wire here would result in a catastrophic 7.7% drop. 3 AWG Copper brings this down to a safe 2.5%.
When your specific distance falls between the rows in the master chart, or your calculated load sits exactly on the boundary of two wire sizes, the rule is absolute: always pick the larger wire (the smaller AWG number). The upfront cost of an extra spool of 6 AWG is negligible compared to the cost of tearing out drywall to replace a melted terminal lug or troubleshooting a compressor that refuses to start on a hot summer day.
