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Copper to Aluminum Wire Size Calculator (Aluminium Equivalent, AWG and mm²)

The smallest aluminum conductor with no more resistance than a given copper one, in AWG and kcmil or mm², and how close the size below it comes.

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The copper size you want an aluminum equal for: AWG and kcmil, or mm².

North American sizes are AWG up to 4/0 and kcmil above it; British, European and Australian sizes are square millimetres. The answer comes back in the same family: aluminum AWG or kcmil for a copper AWG size, aluminium mm² for a copper mm² size.

Smallest aluminium (aluminum) size with no more resistance, in kcmil or mm²

250 kcmil

High confidence

The smallest aluminum size with no more resistance than the copper, on the 75 °C (167 °F) K constants of the US voltage-drop method. The size below it carries a few percent more resistance than the copper, as the row shows; it is often the size an ampacity table pairs with this copper, which is a different basis. Minnesota's 2023 NEC checklist pairs 2/0 AWG copper with 4/0 AWG aluminum for a 200 A dwelling service (NEC 310.12) — an ampacity pairing, not an equal-resistance one.

Copper conductor area (kcmil)
133.1
Aluminum area for equal resistance (kcmil)
218.74
The same area in mm²
110.84 mm²
This aluminum size's resistance against the copper's (%)
87.5
The size below it, 4/0 AWG: area (kcmil)
211.6
The size below it: resistance against the copper's (%)
103.37
Aluminum's resistance per circular mil against copper's, 21.2 ÷ 12.9
1.64
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • IAEI Magazine, "Voltage Drop Formulas" (March/April 2017): K constants at 75 °C (167 °F) read off NEC Chapter 9 Table 8 — 12.9 for uncoated copper (1,000 kcmil at 0.0129 Ω per 1,000 ft, 0.0423 Ω/km) and 21.2 for aluminum (0.0212 Ω per 1,000 ft, 0.0696 Ω/km) — with every size in the table close to them
  • Bayerische Kabelwerke AG (Bayka), Conductor resistance at 20 °C (68 °F) acc. to EN 60228: plain copper and aluminium, class 1 and 2 — 16 mm² copper 1.15 Ω/km (0.351 Ω per 1,000 ft) against 25 mm² aluminium 1.20 (0.366) and 35 mm² aluminium 0.868 (0.265)
  • NBS Handbook 109, Aluminum Wire Tables: EC-H19 aluminum at 61.0% IACS, 17.002 Ω·cmil/ft (0.028264 Ω·mm²/m) at 20 °C (68 °F), against the International Annealed Copper Standard's 10.371 (0.017241) — a ratio of 1.639
  • Minnesota Department of Labor and Industry, Electrical Inspection Checklist for One-Family Dwellings based on the 2023 NEC (revised 2023-05-06): dwelling service conductors under NEC 310.12 — 4 AWG copper or 2 AWG aluminum at 100 A, 1 AWG or 2/0 at 150 A, 2/0 or 4/0 at 200 A, 400 or 600 kcmil at 400 A; and dissimilar metals not intermixed unless the device is listed for it (NEC 110.14)

Inputs used

Copper conductor
2/0 AWG copper

Intermediate steps

Copper conductor area (kcmil)
133.1
Aluminum area for equal resistance (kcmil)
218.74
The same area in mm²
110.84 mm²
This aluminum size's resistance against the copper's (%)
87.5
The size below it, 4/0 AWG: area (kcmil)
211.6
The size below it: resistance against the copper's (%)
103.37
Aluminum's resistance per circular mil against copper's, 21.2 ÷ 12.9
1.64
Final result250 kcmil

Confidence note: The smallest aluminum size with no more resistance than the copper, on the 75 °C (167 °F) K constants of the US voltage-drop method. The size below it carries a few percent more resistance than the copper, as the row shows; it is often the size an ampacity table pairs with this copper, which is a different basis. Minnesota's 2023 NEC checklist pairs 2/0 AWG copper with 4/0 AWG aluminum for a 200 A dwelling service (NEC 310.12) — an ampacity pairing, not an equal-resistance one.

What this calculation does not cover

  • The basis is resistance, not ampacity. The aluminium size given drops no more voltage than the copper and wastes no more power in the run; whether it may carry the circuit's current is a separate lookup in the ampacity tables of the code in force, at the temperature rating of the terminations, which this page does not make and does not reproduce.
  • Nothing here sizes a circuit. A substitution is the designer's decision, made with the overcurrent device, the terminations and the run's voltage drop in view, and in most places it is notifiable or licensed work.
  • Aluminium needs terminations, lugs and connectors listed or rated for it, and the code does not let the two metals be joined except in a device listed for the purpose. The larger aluminium conductor may also need a larger conduit, gland or lug than the copper it replaces.
  • The AWG and kcmil comparison uses one constant per metal, the figure the US circular-mil method uses; the size-by-size resistances of NEC Chapter 9 Table 8 differ from it by a percent or so either way, and the table is not reproduced here. The mm² comparison uses IEC 60228's maximum resistances at 20 °C (68 °F) for each size.
  • Above 1,000 kcmil there is no single aluminium conductor in this list; feeders that large are a design question, not a substitution.

Add the equipment this sizes

This result is a specification — 250 kcmil — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-10-05 · v1.0.0

Regulatory standards & verification citations4
  1. IAEI Magazine, "Voltage Drop Formulas" (March/April 2017): K constants at 75 °C (167 °F) read off NEC Chapter 9 Table 8 — 12.9 for uncoated copper (1,000 kcmil at 0.0129 Ω per 1,000 ft, 0.0423 Ω/km) and 21.2 for aluminum (0.0212 Ω per 1,000 ft, 0.0696 Ω/km) — with every size in the table close to them
  2. Bayerische Kabelwerke AG (Bayka), Conductor resistance at 20 °C (68 °F) acc. to EN 60228: plain copper and aluminium, class 1 and 2 — 16 mm² copper 1.15 Ω/km (0.351 Ω per 1,000 ft) against 25 mm² aluminium 1.20 (0.366) and 35 mm² aluminium 0.868 (0.265)
  3. NBS Handbook 109, Aluminum Wire Tables: EC-H19 aluminum at 61.0% IACS, 17.002 Ω·cmil/ft (0.028264 Ω·mm²/m) at 20 °C (68 °F), against the International Annealed Copper Standard's 10.371 (0.017241) — a ratio of 1.639
  4. Minnesota Department of Labor and Industry, Electrical Inspection Checklist for One-Family Dwellings based on the 2023 NEC (revised 2023-05-06): dwelling service conductors under NEC 310.12 — 4 AWG copper or 2 AWG aluminum at 100 A, 1 AWG or 2/0 at 150 A, 2/0 or 4/0 at 200 A, 400 or 600 kcmil at 400 A; and dissimilar metals not intermixed unless the device is listed for it (NEC 110.14)

Which documents these citations point at

  • National Electrical Code (NFPA 70) — Chapter 9, 310.12, 110.14 (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.

A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.

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The data behind it: Copper conductor sizes and ampacity

How to calculate copper to aluminum wire size (aluminium equivalent, AWG and mm²) in 2 steps

  1. Copper conductorThe copper size you want an aluminum equal for: AWG and kcmil, or mm².
  2. Smallest aluminium (aluminum) size with no more resistance, in kcmil or mm²The tool computes the smallest aluminium (aluminum) size with no more resistance, in kcmil or mm² from those figures and shows the formula, its sources, and a confidence rating alongside it.

Frequently asked questions

Why does the page say 250 kcmil when a 200 A service uses 4/0 aluminum with 2/0 copper?
Because the two answers stand on different bases. The pairing of 2/0 copper with 4/0 aluminum for a 200 A dwelling service is the code's ampacity rule for services and main feeders, as Minnesota's 2023 NEC checklist reproduces it. This page matches resistance: 4/0 aluminum has about 3% more resistance than 2/0 copper, so on the same run it drops about 3% more voltage, and 250 kcmil is the first size that does not. Both facts are true at once; the code decides what may be installed.
How much bigger does aluminum have to be?
About 1.64 times the area of the copper for the same resistance — 21.2 against 12.9 in the US constants, and 1.639 from first principles in the National Bureau of Standards' aluminum wire tables. In AWG that is a little more than two gauge steps, which is why two steps up usually comes out a few percent short and the page names both sizes.
Can I use this to swap copper for aluminum on my circuit?
Not on its own. It answers one question — which aluminum size has no more resistance — and a substitution also has to satisfy the ampacity tables at the termination temperature rating, the overcurrent device, terminations listed for aluminum and the run's voltage drop. That is the electrician's design to make, under the code in force.
Why are mm² answers sometimes equal and sometimes a size over?
Because the comparison is of IEC 60228's tabulated maximum resistances, size by size, and the two metals' series do not line up evenly. Copper at 6 mm² and aluminium at 10 mm² are both listed at 3.08 Ω/km (0.939 Ω per 1,000 ft), so they are equal; 150 mm² copper is 0.124 and 240 mm² aluminium 0.125 (0.0378 and 0.0381 Ω per 1,000 ft), so the larger 300 mm² is the first that is not over, though 240 is within 1%.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.