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Wire size calculator / Aluminum vs copper

Aluminum vs copper wire size

Same label, different table cells. Resistance and ampacity are read from the stored NEC rows, one size at a time.

Copper and aluminum rows that share an AWG label do not share an ampacity or a resistance. Aluminum’s stranded resistance is higher and its table ampacity is lower where both are listed. This page divides those two resistance cells so you can see the ratio move. The quotient is a description of that row. It is not the illustrative K factor this project refuses to use for sizing, and it is not a conversion you can apply to the next size.

Nothing in the ratio establishes equipment compatibility. Follow the conductor-material marking and the torque instruction. A lug rated for a hotter temperature does not, by itself, approve aluminum. If the aluminum ampacity cell is a dash, stop. Do not borrow the copper cell.

75°C Table 310.16 ampacity and 75°C Table 8 stranded resistance for 14 AWG through 4/0. The ratio is aluminum resistance divided by copper resistance on that row, shown to two decimals. A dash means no listing. The ratio is not a sizing constant.
Size Cu 75°C (A) Al 75°C (A) Cu Ω/kft Al Ω/kft Al/Cu resistance
14 AWG 20 — 3.14 5.17 1.65
12 AWG 25 20 1.98 3.25 1.64
10 AWG 35 30 1.24 2.04 1.65
8 AWG 50 40 0.778 1.28 1.65
6 AWG 65 50 0.491 0.808 1.65
4 AWG 85 65 0.308 0.508 1.65
3 AWG 100 75 0.245 0.403 1.64
2 AWG 115 90 0.194 0.319 1.64
1 AWG 130 100 0.154 0.253 1.64
1/0 AWG 150 120 0.122 0.201 1.65
2/0 AWG 175 135 0.0967 0.159 1.64
3/0 AWG 200 155 0.0766 0.126 1.64
4/0 AWG 230 180 0.0608 0.1 1.64

One worked drop, two metals

The home page’s 120 V, 20 A, 100 ft illustration uses Table 8 at 75°C. 12 AWG stranded copper (1.98 Ω/kft) drops 7.92 V (6.60%). 6 AWG stranded aluminum (0.808 Ω/kft) drops 3.23 V (2.69%). These are two conductors under one hypothetical load, not a statement that they are interchangeable. Reactance is omitted.

Planning estimate only. Consult local code and a qualified electrician before buying wire or energizing a circuit. Presets are hypothetical loads; this tool does not design motor, PV, battery, appliance, or EV installations.

Sources for these cells: existing Table 310.16 numerical cells in NFPA’s public development record, pp. 142–143 (proposed changes are not treated as adopted code) and NFPA 70 (2017), Chapter 9 Table 8, printed p. 689, in a third-party-hosted copy. The same publication supplies 110.14 (terminals and aluminum), 210.19/215.2 (continuous loads), 240.4(D) (small conductors), 250.24/250.32 (grounding), and 625.41/625.42 (EV loads). Schneider Electric’s voltage-drop formulas support the arithmetic; Table 8 AC results here omit reactance and assume unity power factor. Resistance and geometry are construction- and temperature-specific. Adopted editions and local amendments can differ.

Related guides

Open the wire size calculator · Ampacity chart · Voltage drop calculator · Printable chart

Compare metals on a 120 V example

The form starts from this page’s example. The address stays on this guide until you edit a field. After an edit, a shared link includes those settings in the URL, and opening that link sends the URL to the host.

Circuit

Headline metal

Both metals are compared. Verify equipment compatibility; some equipment permits copper only.

System

General NEC notes suggest 3% on a branch or feeder and 5% combined. Special equipment and local codes may require other limits; upstream drop is not included.

Estimated size

Enter the load current and the one-way distance. The size appears here, with the limit that set it.

Temperature and bundling

These conditions affect the table ampacity used for the comparison.

Conductor insulation

Use the rating marked for wet or dry conditions. THHN is 90°C dry; XHHW is 90°C dry or 75°C wet. For NM-B use 90°C insulation and a 60°C cap below.

Terminal / cable ampacity cap

Use the lowest applicable rating at either end, including cable limits. The general unmarked default is 60°C for equipment rated ≤100 A or marked for 14–1 AWG. Confirm equipment markings before selecting 75°C; NM-B remains capped at 60°C.

Equipment grounding conductors are excluded. Verify neutrals, harmonic loads, and all other current-carrying conductors in the raceway or cable; the automatic count does not check these.

AC impedance (Chapter 9 Table 9)

At power factor 1, the reactance term is zero, but Table 9 AC resistance still applies. Unlisted sizes have no Table 9 result.

Planning estimate only. Consult local code and a qualified electrician before buying wire or energizing a circuit. Presets are hypothetical loads; this tool does not design motor, PV, battery, appliance, or EV installations.

Questions

Is aluminum always two sizes larger than copper?

No. That shortcut is not a rule in this tool. Read the 75°C cells and the stranded resistances on the row you actually have. The ratio of those two resistance cells changes from size to size. It is not a constant, and it is not a way to assign ampacity to a metric cable.

Can I replace copper with aluminum if the ampacity cell is high enough?

Not from this table alone. The equipment has to list the metal, and the terminals have to be tightened as instructed. Some equipment permits copper only. A higher resistance also changes voltage drop even when the ampacity cell clears the load. The calculator’s aluminum column is a comparison, not approval.

Why does 14 AWG aluminum show a dash?

Table 310.16, as stored here, has no 14 AWG aluminum ampacity. The dash is not zero. Stranded aluminum resistance can still be printed for that row. Resistance without an ampacity listing is not permission to use the conductor.