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Aluminum Feeder Sizing

Aluminum feeders are usually larger than copper for the same ampacity and often still cheaper. The material swap is only finished when terminations, voltage drop and raceway fill still work.
Code basis
310.16, 110.14(C)
Also check
Ch. 9 Table 5A, drop
Usual tradeoff
Larger size, lower cost
Hard stop
Lugs not listed for Al

The Procedure

5 steps, in the order the code applies them.

  1. Step 1

    Size copper and aluminum side by side

    Run the same load, ambient and termination assumptions for both materials so the size jump is visible.

  2. Step 2

    Confirm terminations accept aluminum

    If the lugs are not listed for aluminum, the cheaper conductor is not available without listed adapters or different equipment.

  3. Step 3

    Re-check voltage drop at the aluminum size

    A larger aluminum conductor may still drop more than the copper alternative on long feeders. Enter the real length.

  4. Step 4

    Re-check raceway fill

    The larger OD can force a bigger conduit. Compact stranding helps only when the product is actually compact.

  5. Step 5

    Lock installation details

    Torque, antioxidant where required by the listing, and dissimilar-metal practices are part of accepting aluminum — not optional finish work.

Bigger, often cheaper

Aluminum usually needs a larger size than copper for the same ampacity, but feeder lengths and material cost often make it economical.

The substitution is not complete until the lugs, temperature rating and installation instructions allow aluminum.

Fill and drop both move

Larger aluminum conductors can change raceway fill. Compact stranding can offset some of that increase when the actual conductor is compact.

Voltage drop also increases at a given size, so long feeders need the drop check before a material decision is final.

Decision Guide

The choices that change the answer, and how to make them.

  • Do not substitute size for size

    Replacing copper with the same AWG aluminum almost always undersizes the feeder.

  • Compact is a product claim

    Use Table 5A areas only for conductors that are actually compact stranded. Guessing understates fill risk.

  • Branch circuits are rarely the aluminum use case

    Aluminum shines on services and longer feeders. Small branch circuits usually stay copper for termination simplicity.

FAQs

When should I use aluminum instead of copper for a feeder?
When the aluminum size clears ampacity, terminations are listed for aluminum, voltage drop is acceptable, and the material savings beat the larger raceway or lug work.
How much larger is aluminum than copper?
Often one to two standard sizes for the same load under common termination assumptions, but the calculator result for the actual conditions is the number to use.
Does aluminum always need antioxidant?
Follow the equipment and connector listing. Some modern listed terminations specify their own preparation; others require antioxidant. The listing controls.

Calculated against the NEC 2023 edition using 310.16, 110.14(C), Chapter 9 Table 5A. Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.

Not covered by this tool

  • Motor, air-conditioning and welder circuits, which are sized under their own articles.
  • Conductors in free air and photovoltaic circuits.
  • Parallel conductor sets, taps and busbars.
  • Local amendments, which can be stricter than the published code.

Data last verified