AWG and metric cable sizes look easy to convert until voltage drop, ampacity, terminals, and inspection language enter the same drawing. A 10 AWG copper conductor has a calculated area of about 5.26 mm2, so many people call it “roughly 6 mm2.” That shortcut is fine for a first conversation, but it is not enough for a submittal, a panel schedule, or a long feeder where 0.4 V per conductor can decide whether the equipment starts cleanly. The safe workflow is to compare resistance for voltage drop, then check ampacity under the code system that governs the job.

TL;DR

Convert AWG to mm2 for communication, but calculate voltage drop from resistance.
Do not mix NEC ampacity tables with IEC installation assumptions.
10 AWG is 5.26 mm2; 6 mm2 is slightly larger and drops less voltage.
Use 3% branch and 5% feeder-plus-branch as common NEC design targets.
Aluminum needs a resistance check, not just a cross-sectional-area check.

American Wire Gauge is a logarithmic conductor-size system used mainly in North America. A square millimeter cable size is a metric cross-sectional area designation used in many IEC markets. Voltage drop is the voltage lost when load current flows through conductor impedance. Those three definitions matter because each one answers a different question: “what is the size called,” “how much metal is in the conductor,” and “how much voltage reaches the load.”

For background terminology, see American Wire Gauge and the International Electrotechnical Commission . In the field, the practical references are NEC 110.14(C) for terminal temperature limits, NEC 310.16 for conductor ampacity, the voltage-drop informational notes around NEC 210.19 and 215.2, and IEC 60364-5-52 for cable selection, installation method, correction factors, and voltage drop in IEC-style work.

“The conversion table is only the start. For a 120 V circuit, a 0.6 ohm per 1000 ft difference can be the difference between 2.8% and 3.6% voltage drop at a 16 A load.”
— Hommer Zhao, Technical Director

Start With Resistance, Not the Name on the Cable

Voltage drop is controlled by current, path length, circuit type, conductor material, temperature, and impedance. The printed cable size is only a shortcut to the resistance value. If two cables have similar names but different resistance, the lower-resistance cable will deliver more voltage to the load. That is why a metric crosswalk should be used as a decision aid, not as a blind substitution chart.

A field example makes the point. Suppose a small workshop has a 120 V, 16 A receptacle load 85 ft from the panel, measured one way along the actual route. With 10 AWG copper at about 0.999 ohms per 1000 ft, the round-trip resistance is roughly 0.170 ohms, and the voltage drop is about 2.72 V, or 2.27%. If someone substitutes a smaller conductor because a catalog translation looked “close enough,” the same circuit can move above the 3% branch-circuit target before anyone notices.

On an IEC project, the same discipline applies with meters and ohms per kilometer. A 230 V, 20 A single-phase load 45 m from a distribution board sees the round-trip length of 90 m. A 4 mm2 copper cable may be serviceable for ampacity in one installation method, but a 6 mm2 cable often gives the cleaner voltage-drop result once grouping, ambient temperature, and future load are included. The label “roughly 10 AWG” does not decide that; the installed resistance and code context do.

Field scenario

In a 2026 pump-panel review, we compared a 230 V, 18 A load at 52 m one way. The installer proposed 4 mm2 because the ampacity looked acceptable. The voltage-drop check came back near 4.1% after route length was corrected from drawing distance to cable path. Moving to 6 mm2 brought the calculated drop below 2.8% and avoided a nuisance low-voltage complaint at motor start.

Practical AWG and Metric Crosswalk

The table below is not an ampacity permission slip. It is a voltage-drop planning crosswalk that helps you choose which sizes to compare in the calculator. For final design, use the governing code, listed cable type, insulation temperature, terminals, raceway fill, grouping, and ambient correction. NEC Table 310.16 and IEC 60364-5-52 can produce different allowable currents for conductors with similar copper area because the installation assumptions are different.

AWG/kcmil	Area mm2	Common metric size to compare	120 V 16 A at 100 ft	230 V 20 A at 50 m	Use note
14 AWG	2.08	2.5 mm2	4.2% with 14 AWG	Usually compare 2.5/4 mm2	Small lighting and receptacle checks
12 AWG	3.31	4 mm2	3.3% with 12 AWG	Often close for short final circuits	Common 20 A branch-circuit starting point
10 AWG	5.26	6 mm2	2.1% with 10 AWG	Good comparison for longer 20 A runs	Watch box fill and terminals
8 AWG	8.37	10 mm2	1.3% with 8 AWG	Useful for equipment circuits	Check 60 C/75 C termination limits
6 AWG	13.3	16 mm2	0.8% with 6 AWG	Feeder and charger comparisons	Do not ignore conduit fill
4 AWG	21.2	25 mm2	0.5% with 4 AWG	Long feeder candidate	Aluminum requires separate resistance check

“On mixed-standard jobs I document two separate checks: voltage drop by resistance, then ampacity by the legal installation method. Combining them into one shortcut is where mistakes start.”
— Hommer Zhao, Technical Director

Worked Example 1: 120 V Branch Circuit, AWG to Metric

A DIY workshop circuit uses a 20 A breaker with a realistic 16 A load because continuous or heavy-use receptacle loading should not be sized from wishful thinking. The farthest outlet is 95 ft from the panel by cable route. With 12 AWG copper, the voltage drop is about 5.1 V at 16 A, or roughly 4.2% on a 120 V circuit. That exceeds the common 3% branch-circuit design target. With 10 AWG copper, the drop falls to about 3.2 V, or 2.7%.

If the cable discussion uses metric sizes, compare 4 mm2 and 6 mm2. The 4 mm2 conductor is larger than 12 AWG by area, but the exact voltage drop depends on the listed cable resistance. The 6 mm2 conductor is the more natural comparison to 10 AWG for voltage-drop performance. Under NEC work, however, you still need wiring methods and conductors recognized for the installation. Under IEC work, you still need the installation method, grouping factor, and ambient correction from the applicable national rules.

Use the wire size calculator to compare candidate conductors, the wire resistance calculator when a metric cable datasheet gives ohms per kilometer, and the maximum circuit length calculator when you need to know the distance limit before buying cable.

Worked Example 2: 400 V Three-Phase Feeder in mm2

Now take a 400 V three-phase load at 63 A, 80 m from the distribution board. The design team is comparing 16 mm2 and 25 mm2 copper. A three-phase voltage-drop calculation uses the line current, one-way length, conductor resistance, and the square-root-of-three factor, with reactance added when the circuit is long or the power factor is low. If the 16 mm2 result lands around 3.0% and the specification allows only 3% for that feeder, 25 mm2 may be the cleaner choice because it creates margin for conductor temperature, terminals, and future load.

For a North American engineer reviewing the same equipment package, 16 mm2 is often compared near 6 AWG, and 25 mm2 near 4 AWG. That comparison is useful for intuition, but it should not override the actual cable datasheet. Multi-core cable, single conductors in conduit, tray cable, and buried cable can have different thermal behavior and different allowable ampacity even when the copper area is similar.

Design habit

Record the exact resistance source in your calculation notes. If you used a manufacturer table at 20 C, say so. If you corrected for 75 C operating temperature, say so. That note prevents confusion when a plan reviewer, inspector, or commissioning technician checks the numbers months later.

Copper, Aluminum, and the Area Trap

Copper and aluminum cannot be compared by area alone. Aluminum is lighter and often cheaper for feeders, but its conductivity is lower than copper. That means a 25 mm2 aluminum conductor does not behave like a 25 mm2 copper conductor for voltage drop. NEC work also brings termination compounds where required, connector ratings, and conductor material markings into the inspection path. IEC projects have the same engineering issue even when the paperwork looks different.

A practical rule is to calculate aluminum from its own resistance table, then compare the next two larger sizes before making a cost decision. For a long feeder, the cheapest installed option is not always the smallest allowed conductor. Labor, conduit size, lugs, voltage loss, heat, and future load all belong in the decision. On residential DIY work, aluminum branch-circuit conductors also bring extra termination concerns that should not be treated casually.

“When aluminum is on the table, I ask for the resistance value before I ask for the area. Voltage drop follows ohms, not the marketing name of the cable.”
— Hommer Zhao, Technical Director

Calculator Workflow That Holds Up

Use actual load current. For receptacles, motors, EVSE, pumps, and heaters, distinguish design load from breaker size. A 20 A breaker does not mean every voltage-drop check should use 20 A.
Use one-way route length. The calculator handles circuit type, but your input length must be the real cable path, including vertical drops and panel offsets.
Compare the nearest sizes. If a chart says 10 AWG is near 6 mm2, compare both the smaller and larger practical metric options before choosing.
Finish with code checks. Verify NEC ampacity, terminal ratings, box fill, conduit fill, or IEC installation method after the voltage-drop result looks good.

FAQ

Is 10 AWG the same as 6 mm2 cable?

Not exactly. 10 AWG copper is about 5.26 mm2, while a 6 mm2 metric conductor is larger. For voltage drop, the 6 mm2 cable has lower resistance, but ampacity still depends on NEC 310.16 or IEC 60364 installation conditions.

Can I use mm2 area directly in a voltage drop calculator?

Yes if the calculator accepts metric cable area or conductor resistance. For this site, choose the nearest AWG/kcmil size or use the wire resistance calculator to compare ohms per 1000 ft against ohms per kilometer before sizing.

Which voltage drop target should I use with mixed NEC and IEC projects?

Use the project specification first. In NEC work, the common design target is 3% for a branch circuit and 5% combined feeder plus branch circuit. IEC 60364-5-52 commonly evaluates final circuits and distribution circuits by the allowed voltage drop in the national implementation.

Does a larger metric cable automatically pass NEC ampacity?

No. NEC ampacity depends on conductor material, insulation temperature, terminal temperature under NEC 110.14(C), ambient correction, and adjustment factors. A 16 mm2 cable may look larger than 6 AWG by area, but the listed cable type and termination rating still control.

How close is 2.5 mm2 to 14 AWG?

2.5 mm2 is about 14% larger in copper area than 14 AWG, which is 2.08 mm2. That helps voltage drop, but it does not make the cable a substitute for a North American listed 15 A branch-circuit wiring method unless the installation rules permit it.

Should I convert aluminum cable by area or resistance?

Use resistance for voltage drop. Aluminum has about 61% of copper conductivity, so the same mm2 area drops more voltage. For example, 25 mm2 aluminum behaves closer to a much smaller copper conductor for voltage drop than its area alone suggests.

Run the Cross-Check Before You Pull Cable

Before you order cable or approve a substitution, run the load current, length, conductor material, and candidate AWG or mm2 sizes through the calculator. A five-minute comparison is cheaper than replacing a long feeder after equipment arrives on site.

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AWG to mm2 Voltage Drop Cable Sizing: NEC and IEC Crosswalk with Worked Examples