The DC resistance of uncoated 4/0 AWG aluminum wire is 0.194 ohms per 1000 ft at 75°C (167°F), and 0.156 ohms per 1000 ft at 20°C (68°F), per NEC Chapter 9, Table 8. For standard AC circuits routed in PVC conduit, the effective AC resistance is 0.19 ohms per 1000 ft at 75°C (Table 9). These values are the baseline for calculating voltage drop on heavy feeders and 200A residential service entrance conductors.
The Master Reference Table: 4/0 AWG Aluminum Resistance Data
Before pulling wire or calculating voltage drop, you need the exact impedance values. The table below consolidates the most queried resistance and reactance figures for 4/0 AWG (211,600 cmil) uncoated aluminum from NEC Chapter 9, Tables 8 and 9.
| Measurement Type (NEC Source) | Temp Rating | Conduit / Environment | Resistance (Ω/kft) | Reactance (Ω/kft) |
|---|---|---|---|---|
| DC (Table 8) | 20°C (68°F) | Free Air / Bench | 0.156 | N/A |
| DC (Table 8) | 75°C (167°F) | Free Air / Bench | 0.194 | N/A |
| AC (Table 9) | 75°C (167°F) | PVC Conduit | 0.19 | 0.039 |
| AC (Table 9) | 75°C (167°F) | Steel Conduit | 0.20 | 0.044 |
| AC (Table 9) | 75°C (167°F) | Aluminum Conduit | 0.19 | 0.041 |
Which Column Applies to Your Installation?
Choosing the wrong column is the most common reason DIY voltage drop calculations fail to match real-world multimeter readings in the field. Here is how to select the correct row for your specific 4/0 aluminum run.
20°C vs. 75°C Temperature Columns
The 20°C (68°F) column represents the physical resistance of the aluminum at room temperature. You will only use this value if you are bench-testing a spool of wire with an ohmmeter before installation.
The 75°C (167°F) column is the operational baseline. Under a full 200A load, the conductor will heat up. Because aluminum has a positive temperature coefficient, its resistance increases as it gets hotter. Furthermore, NEC 110.14(C) mandates that termination temperature limits for equipment rated over 100A default to the 75°C column. Therefore, for all load-bearing voltage drop calculations, use the 75°C values.
DC vs. AC and Conduit Material
Table 8 provides DC resistance, which is perfectly uniform regardless of what surrounds the wire. Table 9 provides AC resistance, which introduces skin effect (current pushing to the outer edge of the conductor) and proximity effect.
Notice that routing 4/0 aluminum in steel conduit bumps the AC resistance from 0.19 to 0.20 Ω/kft. This happens because the alternating magnetic field induces eddy currents and hysteresis losses in the ferrous steel pipe, effectively adding resistance to the circuit. If you are pulling 4/0 aluminum through rigid metal conduit (RMC) or intermediate metal conduit (IMC), you must use the steel conduit row.
How Derating and Temperature Modify the Base Value
A frequent point of confusion on the jobsite is how NEC derating factors affect the base resistance of the wire. Let's clear this up: NEC 310.15(B)(3)(a) derating rows do not change the physical resistance of the aluminum. They modify the allowable ampacity to prevent the wire from overheating.
Always use the 75°C base resistance (0.194 Ω/kft DC or 0.19 Ω/kft AC) for your math, but ensure your actual installed current does not exceed the derated ampacity limit. If your load requires 200A and derating reduces your 4/0 wire's capacity to 184A (80% of 230A), you must step up to 250 kcmil aluminum to maintain both safety and the expected resistance profile.
What the Table Cannot Tell You (Field Variables)
NEC Chapter 9 tables assume a perfect, pristine conductor. In real-world installations, three field variables will alter your effective resistance and voltage drop.
1. Termination Oxidation and Contact Resistance
Aluminum rapidly forms a layer of aluminum oxide when exposed to air. Unlike copper oxide, which is somewhat conductive, aluminum oxide is a highly effective electrical insulator. If you land 4/0 aluminum on a 200A main breaker lug without abrading the wire and applying an oxidation inhibitor (like Noalox or Penetrox), the contact resistance at the termination can easily exceed the resistance of 100 feet of the wire itself. This causes localized heating at the breaker lug, which thermal cameras easily pick up during inspections.
2. Harmonic Distortion and Skin Effect
Table 9 assumes a clean 60Hz sine wave. If your 4/0 aluminum feeder is supplying a subpanel loaded with variable frequency drives (VFDs), LED drivers, or solar inverters, you will have high-frequency harmonic currents. Harmonics drastically exacerbate the skin effect, forcing current into a thinner cross-section of the aluminum strand and effectively raising the AC resistance well above the 0.19 Ω/kft baseline. In high-harmonic environments, oversizing the neutral and phase conductors by one step is standard engineering practice.
3. Real-World Voltage Drop Verification
Let's run the math on a standard 200A residential service using the table values. Assume a 100-foot underground run in PVC conduit from the utility transformer to the main panel, carrying a continuous 160A load.
- Formula: Voltage Drop = 2 × Length × Current × (AC Resistance / 1000)
- Calculation: 2 × 100 ft × 160A × (0.19 / 1000) = 6.08 Volts
- Percentage: (6.08V / 240V) × 100 = 2.53%
This falls comfortably within the NEC's recommended 3% maximum for feeder voltage drop. However, if you measure this in the field and read a 4% drop, do not blame the NEC table. Check your lug torque (using an inch-pound torque screwdriver or wrench per NEC 110.14(D)) and verify that your utility transformer isn't already delivering low secondary voltage under neighborhood peak load. For further reading on aluminum conductor properties and resistivity physics, the Georgia State University HyperPhysics database provides excellent baseline material science data that aligns with NEC standards.
