A voltage loss chart (more accurately called a voltage drop chart) calculates the expected voltage decrease over a specific wire length due to conductor resistance. When current flows through copper, it encounters friction. For standard uncoated copper wire, you lose roughly 1.5V to 9.5V per 100 feet depending on the AWG size and the current load. The National Electrical Code (NEC) recommends a maximum combined voltage drop of 5% (3% for the branch circuit, 2% for the feeder) to ensure efficient operation and prevent motor burnout or dimming lights.

Below is a combined reference chart that merges ampacity limits with real-world voltage loss calculations, saving you from doing the math on the jobsite.

How to Read This Voltage Loss Chart (and Which Column Applies)

Before sizing your wire, you must understand which temperature column governs your installation. This is the most common mistake DIYers and junior electricians make. Wire insulation (like THHN) might be rated for 90°C, but your terminations (breakers, receptacles, lugs) are usually only rated for 60°C or 75°C.

The 60°C vs. 75°C Rule (NEC 110.14(C)): For circuits rated 100 amps or less, you must use the 60°C ampacity column unless the equipment is explicitly marked for 75°C. Furthermore, NM-B (Romex) cable is strictly limited to the 60°C column for ampacity, regardless of the fact that the individual wires inside it have 90°C insulation.

The following data-dense table provides the baseline ampacities and the exact voltage loss you will experience per 100 feet of one-way run (which equals a 200-foot round-trip loop) on a standard 120V single-phase circuit.

Table 1: Copper Wire Ampacity & Voltage Loss Reference (120V Single-Phase)
AWG Size 60°C Ampacity (NM-B / Standard Terminations) 75°C Ampacity (THHN in Conduit / Marked Lugs) Resistance (Ω/1000 ft)
NEC Ch. 9, Table 8		 Voltage Loss per 100 ft @ Max 60°C Amps (120V 1Ø) % Drop @ Max 60°C Amps
14 AWG 15A 20A 3.140 9.42V 7.85% (Fails 3% rule)
12 AWG 20A 25A 1.980 7.92V 6.60% (Fails 3% rule)
10 AWG 30A 35A 1.240 7.44V 6.20% (Fails 3% rule)
8 AWG 40A 50A 0.778 6.22V 5.18% (Fails 3% rule)
6 AWG 55A 65A 0.491 5.40V 4.50% (Fails 3% rule)
4 AWG 70A 85A 0.308 4.31V 3.59% (Borderline)
2 AWG 95A 115A 0.194 3.69V 3.07% (Borderline)
1/0 AWG 125A 150A 0.154 3.85V 3.20% (Borderline)

Sources: Ampacities derived from NFPA 70 (NEC) Table 310.16. Resistance values from NEC Chapter 9, Table 8 (Uncoated Copper). Calculations assume a 200ft total loop length (100ft out, 100ft back).

Derating Factors and What This Chart Cannot Tell You

The chart above assumes a single cable in an ambient temperature of 86°F (30°C). On a real jobsite, conditions are rarely this perfect. You must apply derating factors from NEC Article 310.15 which will modify your base ampacity and force you to upsize the wire.

How Derating Modifies the Base Value

  • Conductor Bundling: If you pull 4 to 6 current-carrying conductors through the same conduit, you must multiply the 90°C ampacity by 80%. For 7 to 9 conductors, you multiply by 70%. This reduces the current the wire can safely carry, which in turn reduces the actual voltage loss (since V=IR, lower I means lower V-drop), but it means you must start with a thicker wire to handle your target load.
  • Ambient Temperature: If your conduit runs across a hot attic where temperatures reach 113°F (45°C), you must multiply the 90°C ampacity by 0.82.

What the Table Cannot Tell You

A standard DC-resistance voltage loss chart has blind spots. It will not account for:

  1. AC Reactance and Skin Effect: For large feeders (1/0 AWG and larger) in steel conduit, the alternating magnetic field creates inductive reactance. The actual AC voltage drop can be 10% to 20% higher than the DC resistance chart indicates, especially if the power factor is low (like with large induction motors).
  2. Utility Supply Sag: The chart assumes your panel is receiving exactly 120V or 240V. If the utility is delivering 114V during peak summer loads, your 3% drop starts from a lower baseline, pushing your end-device voltage below acceptable tolerances.
  3. Termination Resistance: A loose lug or a back-stabbed receptacle can introduce 0.5 ohms of resistance, causing a localized voltage loss and massive heat generation that no wire-sizing chart can predict.

Quick-Jump Scenarios: Sizing for 3% Drop at 120V and 240V

Rather than calculating the math for every run, here are the most queried real-world scenarios. Bookmark these quick-jump rows for your next rough-in.

Table 2: Quick-Jump Wire Sizing for Maximum 3% Voltage Drop
Application Scenario Load (Amps) Voltage / Phase One-Way Distance Minimum Copper AWG Required Actual Voltage Loss
Standard Receptacle (Garage/Shop) 16A (Continuous) 120V / 1Ø 100 ft 8 AWG 2.49V (2.0%)
EV Charger (Level 2 Hardwired) 40A 240V / 1Ø 75 ft 6 AWG 3.68V (1.5%)
Electric Dryer 30A 240V / 1Ø 50 ft 10 AWG 3.72V (1.5%)
Subpanel Feed (Detached Shed) 60A 240V / 1Ø 150 ft 2 AWG 5.53V (2.3%)
Well Pump (Deep Submersible) 10A 240V / 1Ø 250 ft 10 AWG 6.20V (2.5%)
Pro-Tip for Long Runs: When your voltage loss chart dictates a wire size larger than 6 AWG for a residential branch circuit, consider switching to Aluminum (XHHW-2 or SER). Aluminum is roughly 40% cheaper than copper by weight and volume. To match copper ampacity and voltage drop, simply upsize the aluminum by two AWG steps (e.g., use 4 AWG Aluminum instead of 6 AWG Copper). Just ensure you use aluminum-rated lugs and apply antioxidant paste (like Noalox) at the terminations.

Always verify your final voltage at the furthest receptacle with a true-RMS digital multimeter under a loaded condition. If your calculated voltage loss exceeds 3% on the branch or 5% total, upsize the conductor by one AWG step and recalculate. Local Authority Having Jurisdiction (AHJ) inspectors may not strictly enforce the 3% informational note in the NEC, but adhering to it ensures your power tools run at full torque, your LED drivers don't flicker, and your smart home hubs stay connected without browning out.