Resistivity for aluminum is the intrinsic material property that quantifies how strongly the metal opposes electric current flow, measured at approximately 2.82 × 10-8 Ω·m (or 17.0 Ω·cmil/ft) at 20°C. In a real circuit or installation, this higher resistivity—which gives aluminum only about 61% of the conductivity of copper by volume—dictates that you must upsize your wire gauge by one or two AWG steps to carry the same ampacity and prevent excessive voltage drop on long feeder runs. People commonly confuse resistivity (a fixed material constant) with resistance (which changes based on the wire’s length and cross-sectional area), and they frequently mistake modern AA-8000 series aluminum alloys for the brittle, fire-prone AA-1350 branch circuit wire that was banned in the 1970s.
The Core Data: Aluminum vs. Copper Resistivity and Sizing
When sizing conductors, you are balancing the material’s inherent resistivity against its physical cross-section and thermal limits. Because aluminum is less dense and cheaper than copper, it dominates service entrance cables and subpanel feeders, provided you account for its electrical and mechanical differences. The table below contrasts the fundamental physics and the practical NEC Table 310.16 ampacity limits at the standard 75°C termination column.
| Property / Metric | Aluminum (AA-8000) | Copper (Annealed) | Practical Impact on Installation |
|---|---|---|---|
| Resistivity at 20°C | 2.82 × 10-8 Ω·m | 1.72 × 10-8 Ω·m | Al requires ~1.6x the cross-sectional area for equal DC resistance. |
| DC Resistance (1/0 AWG, 75°C) | 0.20 Ω / 1,000 ft | 0.12 Ω / 1,000 ft | Higher operating resistance increases I²R heating and voltage drop. |
| Thermal Expansion Coefficient | 23 × 10-6 /°C | 17 × 10-6 /°C | Al expands/contracts more, requiring precise torque to prevent lug loosening. |
| NEC Ampacity (1/0 AWG, 75°C Col) | 120 Amps | 150 Amps | For a 125A load, you need 1/0 Cu but must step up to 2/0 Al. |
| Weight per 1,000 ft (1/0 AWG) | ~101 lbs | ~318 lbs | Al is roughly 70% lighter, making long overhead service drops manageable. |
Worked Example: Calculating Voltage Drop on a 100A Feeder
Let’s look at how resistivity forces a sizing change in a real-world scenario. You are running a 240V, single-phase feeder to a 100-amp subpanel in a detached garage, located 150 feet from the main panel. Your target is to keep the voltage drop under the NEC-recommended 3% limit (7.2 volts) for optimal equipment performance.
Instead of relying on generic online calculators that use the 20°C K-factor (which underestimates voltage drop on a fully loaded wire), we will use the exact 75°C DC resistance values from NEC Chapter 9, Table 8. This reflects the actual operating temperature of the metal under load.
Testing 2 AWG Aluminum
From NEC Chapter 9, Table 8, the resistance of uncoated 2 AWG aluminum at 75°C is 0.319 Ω per 1,000 ft.
- Resistance per foot = 0.000319 Ω/ft
- Voltage Drop (VD) = 2 × Current × Resistance/ft × Distance
- VD = 2 × 100A × 0.000319 × 150 ft = 9.57 Volts
Result: 9.57V is a 3.98% drop. This fails our 3% design target, even though 2 AWG Al is rated for 90A (or 100A if using specific 75°C derived adjustments, though 90A is standard here). We must upsize.
Testing 1/0 AWG Aluminum
The resistance of uncoated 1/0 AWG aluminum at 75°C is 0.20 Ω per 1,000 ft.
- Resistance per foot = 0.00020 Ω/ft
- VD = 2 × 100A × 0.00020 × 150 ft = 6.0 Volts
Result: 6.0V is a 2.5% drop. This passes the 3% limit. Furthermore, 1/0 AWG Al has an ampacity of 120A at 75°C, safely clearing the 100A breaker requirement. If you had chosen copper, 2 AWG Cu (0.194 Ω/kft at 75°C) would yield a 5.82V drop, passing the test. This perfectly illustrates the field rule of thumb: step up one AWG size when switching from copper to aluminum for long runs. For exact calculations on your specific site, tools like the Southwire Voltage Drop Calculator can help verify your math before you pull wire.
Where You Meet This in Practice: Terminations and Code Rules
Understanding the math is only half the battle; the physical behavior of aluminum at the termination point is where installations fail or succeed. Because aluminum oxidizes rapidly and has a higher coefficient of thermal expansion than copper, you must follow strict physical protocols when landing these wires in a panel.
1. The Oxide Layer and Anti-Oxidant Paste
When bare aluminum is exposed to air, it instantly forms a layer of aluminum oxide. Unlike copper oxide, which is somewhat conductive, aluminum oxide is a hard electrical insulator. If this layer builds up inside a lug, resistance spikes, generating intense heat. For stranded aluminum feeder wire, you must apply an anti-oxidant compound (like Noalox or Ideal Noalox) to the conductor before termination. This paste contains zinc dust that breaks through the oxide layer under the mechanical pressure of the lug screw, sealing out oxygen. Note: Never apply this paste to copper wires or fine-stranded flexible cord; it is strictly for aluminum or AL/CU bimetallic lugs.
2. Torque Requirements and Thermal Creep
Aluminum undergoes “thermal creep”—it expands when heated by current flow and contracts when it cools. Over hundreds of load cycles, an under-torqued lug will slowly loosen, increasing contact resistance and eventually melting the terminal block. Per NEC 110.14(D), you must use a calibrated torque screwdriver or torque wrench set to the exact inch-pound value printed on the panelboard or breaker label. Do not guess, and do not use the “tight until it stops, then give it a quarter turn” method. A 1/0 AWG wire in a standard 200A main breaker lug typically requires between 250 and 350 in-lbs of torque; always verify the manufacturer’s spec sheet.
3. AL/CU and CO/ALR Ratings
You can only terminate aluminum wire on lugs explicitly marked AL, AL/CU, or CO/ALR. Standard copper-only lugs will suffer galvanic corrosion if paired with aluminum in the presence of ambient moisture, leading to catastrophic high-resistance failures.
Frequently Asked Questions (Field Troubleshooting)
Can I use aluminum wire for 15A or 20A branch circuits inside my walls?
Technically, the NEC allows modern AA-8000 series aluminum wire for branch circuits if you use CO/ALR rated receptacles and switches. Practically, almost no electricians do this. Small-gauge solid aluminum (12 or 10 AWG) is stiff, brittle, and prone to snapping if you score the surface during stripping. Furthermore, the cost savings at 12 AWG are negligible compared to the labor and liability risks. Stick to copper for branch circuits and reserve aluminum for 2 AWG and larger feeder/service entrance cables.
Why did my older aluminum service panel lug melt and scorch?
If your home was built or wired between 1965 and 1973, it may contain AA-1350 alloy aluminum wire. This older alloy had severe thermal expansion issues and high creep rates, leading to widespread connection failures and fires, which prompted the NEC to effectively ban it for small branch circuits. If you have AA-1350, the only permanent fix is a full rewire or the installation of specialized Alumiconn pigtail connectors to transition to copper before hitting modern devices. If your wire is modern AA-8000 (printed on the jacket), scorching is almost always due to improper torque or a missing anti-oxidant compound.
Does the insulation type (THHN vs. XHHW) change the resistivity?
No. Resistivity is a property of the bare metal conductor, not the plastic insulation. However, XHHW-2 insulation is thinner and more water-resistant than THHN/THWN-2, which means you can often fit more aluminum conductors into a given conduit size, and XHHW is generally preferred for underground SER or direct burial applications due to its superior moisture resistance.
