Aluminum vs copper feeders: sizing, cost and terminations

For branch circuits, copper is the default and nobody argues about it. For services and feeders of 60 A and up, aluminum is the more common choice in new work, and for good reason: it costs much less for the same ampacity and weighs about half as much. The tradeoff is a larger conductor and more care at the terminations.

Modern aluminum is not the 1970s problem

The aluminum branch-circuit wiring that caused trouble in houses built in the 1960s and early 1970s was an older alloy on terminals not designed for it. Building wire made today uses AA-8000 series aluminum alloy, which NEC Article 310 requires for aluminum building wire (section 310.106(B) in the 2017 edition, renumbered in 2020). Combined with lugs listed for aluminum (marked AL/CU or CU/AL), it has a long service record on feeders.

Sizing: go up about two sizes

Aluminum has higher resistance than copper, so it needs a larger cross section for the same ampacity. Using the 75 °C column of NEC Table 310.16, which is what most breaker and panel terminals are rated for (110.14(C)):

Each row pairs a copper size with the aluminum size of about the same ampacity.

The 200 A service example

For a dwelling service or main feeder, NEC 310.12 lets you size the conductors at 83 percent of the service rating. For 200 A that is 166 A, so from the 75 °C column:

• Copper: 2/0 AWG (175 A)
• Aluminum: 4/0 AWG (180 A)

That is why 4/0 aluminum SER and URD are the common 200 A residential feeders. The 83 percent rule only applies to a service or feeder that carries the entire load of a single-family dwelling or dwelling unit; a feeder to a subpanel that serves part of the house is sized from Table 310.16 directly.

Termination practice

Most aluminum problems are termination problems. On every connection:

• Use lugs and breakers listed for aluminum (AL/CU).
• Wire-brush the stripped conductor and apply an oxide-inhibiting compound where the lug maker calls for it.
• Torque to the value on the equipment label with a calibrated torque tool. NEC 110.14(D) has required this since the 2017 edition.
• Do not nick the strands when stripping.

Cost and weight

Per ampere of capacity, aluminum feeder wire usually costs a fraction of copper, and the savings grow with length and size. It is also roughly half the weight for the same ampacity, which helps on long pulls and in overhead work. The larger diameter can mean a bigger conduit, so check fill with NEC Chapter 9 before you commit.

When copper still makes sense

• Branch circuits of 30 A and below, where copper is the norm and small aluminum is rare.
• Tight conduit or tight gutter space where the larger aluminum conductor will not fit.
• Equipment with terminals listed for copper only.
• Short runs where the cost difference is small.

Shop feeder wire

• 4/0 XHHW-2 aluminum, 2/0 XHHW-2 aluminum and 250 kcmil XHHW-2 aluminum
• 2/0 THHN/THWN-2 copper and 2/0 XHHW-2 copper
• 4/0-4/0-4/0 SEU and the rest of aluminum building wire

Verify with your local code. Service and feeder sizing depends on the load calculation, the NEC edition your jurisdiction uses and local amendments. This guide is general information, not a design for your installation.