When makers, solar installers, and electricians search for an ohm's law table, they are rarely looking for abstract physics formulas. They are trying to solve a practical voltage drop problem ($V = I \times R$) or size a wire for a specific load without tripping a breaker or melting insulation. The definitive reference for this is a synthesis of NEC Chapter 9, Table 8 (conductor DC resistance) and NEC 310.16 (allowable ampacity).
For a quick baseline: 12 AWG copper wire has a DC resistance of 1.93 ohms per 1,000 feet at 75°C. If you push 15A through a 100-foot one-way run of 12 AWG, your voltage drop is $15A \times (1.93\Omega \times 0.2) \times 2 = 5.79V$. Below is the complete decision-forward reference to calculate this for any standard installation.
How to Read the Ohm's Law & Wire Resistance Table
To use this table effectively, you must understand how the columns interact, particularly the temperature rating column, which is the most common point of failure for DIY wiring.
- AWG / kcmil: The physical size of the wire. Remember that in AWG, a smaller number means a thicker wire with lower resistance.
- DC Resistance (Ω/kft): Sourced directly from NEC Chapter 9, Table 8. This is the one-way resistance per 1,000 feet of solid/stranded copper at 75°C. To find the total loop resistance for a single-phase circuit, multiply this value by 2 (for the hot and neutral/ground return path) and by your one-way distance in thousands of feet.
- Temperature Rating Columns (60°C vs 75°C vs 90°C): While modern THHN/THWN-2 wire is manufactured with 90°C insulation, you must default to the 75°C column for almost all residential and commercial terminations. NEC 110.14(C) dictates that your wire ampacity is limited by the lowest temperature rating of any connected component (breaker, receptacle, or lug). Since most standard breakers and receptacles are rated for 75°C, the 90°C column is only used as a starting point for applying ambient temperature derating factors.
- Max Standard OCPD: The maximum standard breaker size allowed by NEC 240.4(D) for small conductors, overriding the raw 75°C ampacity to prevent fire hazards.
The Master Ohm's Law Wire Table (Copper, 75°C)
This table combines the resistance values needed for Ohm's Law voltage drop calculations with the thermal limits required for safe breaker sizing. Source Standards: NEC 2023 Chapter 9 Table 8 (Resistance) and NEC 310.16 (Ampacity).
| AWG Size | DC Resistance (Ω/kft) @ 75°C | 75°C Ampacity (Cu) | Max Standard OCPD (Breaker) | V-Drop per 100ft @ Max OCPD (120V Nominal) |
|---|---|---|---|---|
| 14 AWG | 3.14 Ω | 20A | 15A | 4.71V (3.9%) |
| 12 AWG | 1.93 Ω | 25A | 20A | 3.86V (3.2%) |
| 10 AWG | 1.24 Ω | 35A | 30A | 3.72V (3.1%) |
| 8 AWG | 0.778 Ω | 50A | 40A | 3.11V (2.6%) |
| 6 AWG | 0.491 Ω | 65A | 60A | 2.94V (2.4%) |
| 4 AWG | 0.308 Ω | 85A | 80A (Standard) | 2.46V (2.0%) |
| 2 AWG | 0.194 Ω | 115A | 110A (Standard) | 2.13V (1.7%) |
| 1/0 AWG | 0.122 Ω | 150A | 150A | 1.83V (1.5%) |
| 2/0 AWG | 0.0967 Ω | 175A | 175A | 1.69V (1.4%) |
| 4/0 AWG | 0.0608 Ω | 230A | 225A (Standard) | 1.36V (1.1%) |
Note: Voltage drop calculations above assume a single-phase 120V circuit (multiplied by 2 for the return loop) at the maximum listed OCPD. For 240V circuits, divide the V-Drop percentage by 2.
Decision Path: Sizing Your Wire and Breaker
Use this decision tree to terminate your wire sizing process with one concrete pick. Do not guess; follow the sequence.
| Condition / Variable | Action / Modification | Concrete Result |
|---|---|---|
| Is the load continuous (on for 3+ hours)? | Multiply the continuous load amperage by 1.25. | Use this adjusted amperage for all subsequent steps. |
| Which temperature column applies? | Check breaker/receptacle markings. If unmarked or standard residential, use 75°C. | Lock your baseline ampacity to the 75°C column above. |
| Is ambient temperature > 86°F (30°C)? | Multiply the 90°C ampacity by the NEC 310.15(B)(1) derating factor (e.g., 0.88 for 104°F). | If derated 90°C ampacity < your load, step up one AWG size. |
| Are there >3 current-carrying conductors in the conduit? | Multiply by the NEC 310.15(C)(1) adjustment factor (e.g., 0.80 for 4-6 conductors). | If adjusted ampacity < your load, step up one AWG size. |
| Calculate Voltage Drop: $2 \times I \times R \times (L/1000)$ | If result > 3% of nominal voltage (3.6V for 120V, 7.2V for 240V). | Final Pick: Step up one AWG size to reduce resistance ($R$) and recalculate. |
What This Table Cannot Tell You
While this ohm's law table covers 95% of residential, solar, and maker applications, it has three blind spots that require deeper engineering analysis:
- AC Skin Effect and Reactance: For feeds larger than 1/0 AWG running in steel conduit, AC current pushes to the outer 'skin' of the wire, effectively increasing resistance. The DC resistance in NEC Chapter 9 Table 8 becomes inaccurate. For large industrial or heavy solar feeds, you must use NEC Chapter 9, Table 9, which provides AC impedance ($Z$) accounting for reactance ($X_L$) and power factor.
- Terminal Temperature Limits: The table assumes your terminations can handle the heat generated at 75°C. If you are wiring a cheap, unlisted import relay or a legacy 60°C-rated fuse block, you must artificially restrict your wire to the 60°C ampacity column, regardless of the wire's 90°C THHN insulation rating.
- Short-Circuit Let-Through Current: Ohm's law dictates that a dead short will pull hundreds of amps. This table tells you what the wire can handle continuously, but it does not tell you if the wire will survive a short circuit before the breaker's magnetic trip clears the fault in 10 milliseconds. For that, you must calculate the available fault current and verify the breaker's AIC (Ampere Interrupting Capacity) rating.
Quick-Jump Reference: Most Queried Scenarios
Bookmark these rows for the most common bench and jobsite calculations.
Scenario 1: 20A Breaker, 12 AWG Wire, 80-Foot Run to a Receptacle
- Load: 16A continuous (e.g., a space heater or server rack).
- Adjusted Load: $16A \times 1.25 = 20A$.
- Wire Pick: 12 AWG is rated 25A @ 75°C. It handles the 20A adjusted load.
- Voltage Drop: $2 \times 16A \times 1.93\Omega \times 0.08 = 4.94V$.
- Verdict: 4.94V is 4.1% of 120V. This exceeds the 3% NEC recommendation. Fix: Step up to 10 AWG wire to drop the voltage loss to 3.17V (2.6%), but terminate it on the 12 AWG pigtails or use a 20A breaker with 10 AWG pigtails at the panel.
Scenario 2: 50A EV Charger, 240V, 60-Foot Run
- Load: 40A continuous (chargers are continuous loads; $50A \times 0.8 = 40A$). Adjusted load = $40A \times 1.25 = 50A$.
- Wire Pick: 6 AWG copper is rated 65A @ 75°C. It handles the 50A breaker.
- Voltage Drop: $2 \times 40A \times 0.491\Omega \times 0.06 = 2.35V$.
- Verdict: 2.35V on a 240V circuit is a 0.98% drop. Final Pick: 6 AWG THHN copper is perfect. No upsizing required.
Scenario 3: 12V DC Solar Battery Bank to Inverter (2000W)
- Load: $2000W / 12V = 166A$ (plus inverter inefficiency, assume 180A peak).
- Wire Pick: 2/0 AWG is rated 175A @ 75°C. (Note: For chassis wiring / battery interconnects, NEC 310.16 doesn't strictly apply, and SAE J1128 allows higher ampacity, but we use NEC for safety margins).
- Voltage Drop: At 10 feet, $2 \times 180A \times 0.0967\Omega \times 0.01 = 0.34V$.
- Verdict: A 0.34V drop on a 12V system is 2.8%. Final Pick: 2/0 AWG pure copper welding cable is the correct choice. Do not use CCA (Copper Clad Aluminum), as its resistance is roughly 40% higher and will push the drop past the 3% threshold, causing low-voltage inverter shutdowns.
For deeper theory on how resistance changes with temperature, refer to the foundational physics at All About Circuits, and for field measurement techniques using a multimeter, consult the Fluke Electrical Learning Center.
