Aluminum vs copper conductors on solar

Aluminum is not a lesser conductor, it is a different one. Used where it belongs it saves real money on a large service; used where it does not it causes the failure mode inspectors look for first.

Where each belongs on a PV job

The three rules that decide it

1. Ampacity. Aluminum carries less per size. Size from Table 310.16's aluminum columns, then apply the same temperature and fill corrections you would for copper. Do not convert with a rule of thumb.

2. Terminations. This is where installations fail. Terminals must be listed for aluminum — look for AL/CU or CU/AL marking. Torque to the manufacturer's specification with a calibrated wrench; 110.14(D) makes torque a code requirement, not best practice. Where an antioxidant is specified by the listing, use it.

3. Termination temperature. 110.14(C) still caps you at the terminal's rating, typically 75 °C. A 90 °C insulation does not raise the terminal.

Why aluminum terminations fail

Two mechanisms, both slow:

• Oxidation. Aluminum forms an oxide layer immediately, and that oxide is an insulator. A properly made connection excludes air from the contact area; a loose one lets the oxide grow and resistance climb.
• Creep. Aluminum relaxes under sustained pressure more than copper. An under-torqued lug loosens over years, and a loose high-current joint gets hot.

Both are why aluminum has a reputation earned mostly by 1960s-era branch wiring with terminals never listed for it. Modern AA-8000 series alloy on listed terminations, correctly torqued, is a code-compliant and reliable choice.

Voltage drop cuts both ways

Aluminum's higher resistivity means more drop per size — but you are usually installing a larger size, which offsets it. Compute rather than assume, especially on a long ground-mount run. See voltage drop in solar circuits.

Do not mix at a joint

Dissimilar metals in contact with moisture corrode. Where copper meets aluminum, use a connector listed for the transition — do not just twist them together under a lug rated for one of them.

On the drawing

State material explicitly on every run: "4/0 AL" or "2/0 Cu". A conductor size with no material is ambiguous, and ampacity depends on it. OneLine Studio's conductor sizing carries material through the calculation and prints it on the schedule, so the ampacity shown is the ampacity for the metal actually specified.

See also: sizing solar conductors and breakers and solar conductor types.

> Design aid — a licensed electrician or PE must review and stamp before filing.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.