Wire Size by Amperage: How to Choose the Right AWG

Wire size by amperage comes down to matching your circuit's current draw to the smallest AWG conductor rated to carry it safely using the ampacity figures published in NEC Table 310.16. The chart below covers common amp loads from 15A lighting circuits up to 150A+ feeders.

For the full methodology behind these numbers temperature ratings, derating and the separate voltage drop check see our wire size calculator guide; this page focuses on the quick reference lookup itself.

Key Takeaways

A single amp value doesn't always map to exactly one wire size which is why the chart below is worth reading alongside these three points.

One Amp Figure Can Have More Than One Correct Answer

The right wire size depends on which insulation temperature rating (60°C, 75°C or 90°C) applies to your conductor, not just the amp draw alone.

The Chart Assumes Standard Conditions

Figures below apply to a single conductor in free air or standard conduit fill at 30°C ambient bundling or heat exposure changes the result.

Voltage Drop Still Needs Its Own Check

A wire that's correctly sized for amperage can still lose too much voltage over a long run ampacity and voltage drop are separate calculations.

What Does Wire Size by Amperage Mean?

Wire size by amperage means selecting a conductor based on the amount of electrical current it needs to carry safely. In the United States wire sizes are commonly expressed using American Wire Gauge (AWG). As the AWG number becomes smaller the conductor becomes larger.

For example:

The larger conductor generally has greater ampacity and lower resistance. However there is no universal rule such as one wire gauge always equals one exact amp rating. Allowable ampacity depends on the conductor and installation conditions.

Choosing the correct wire size by amperage starts with understanding how AWG sizes relate to ampacity. For a broader reference, see our wire size chart for AWG sizes, ampacity, and conductor dimensions.

• 14 AWG is smaller than 12 AWG.
• 12 AWG is smaller than 10 AWG.
• 10 AWG is smaller than 8 AWG.

Wire Size by Amperage: The Full Chart

This is a general copper conductor reference and does not account for every installation condition, equipment specific requirement or special NEC rule.

The table values are based on the standard ampacity data for insulated conductors at 30°C ambient with no more than three current carrying conductors in the raceway, cable or earth.

Don't Default to the 90°C Column

The 90°C rating looks like the best deal for any given wire size but NEC 110.14(C) requires the ampacity to be coordinated with the temperature rating of the terminals and equipment it connects to.

Most breakers and terminals are only rated for 60°C or 75°C so the 90°C column is mainly useful for derating calculations rather than as the final allowable ampacity.

Wire Size for Common Household Circuits

These are typical starting points not universal answers always confirm against the full chart above and your specific installation conditions.

Common Amp Loads Explained

What Size Wire for 40 Amps?

8 AWG copper handles 40A across all three temperature ratings making it the standard choice for loads like electric water heaters and smaller subpanels.

What Size Wire for 25 Amps?

10 AWG copper covers 25A at both 60°C and 75°C though 90°C rated 10 AWG is limited to a lower practical rating once derating factors are applied.

Note that 12 AWG shows 25A in the 75°C column too but NEC 240.4(D) caps 12 AWG copper at a 20A overcurrent device regardless of its table ampacity which is why 10 AWG, not 12 AWG is the correct choice for a genuine 25A load.

What Size Wire for 100 Amps?

Depending on temperature rating 100A needs 1 AWG (60°C or 90°C) or 2 AWG (75°C) copper this is one of the clearest examples of why the insulation rating changes the answer.

Can 1.5mm² Cable Take 10 Amps?

Yes. 1.5mm² copper cable (roughly equivalent to 16 AWG) is commonly rated around 13 to 16A depending on the installation method, comfortably covering a 10A load.

When the Chart Isn't Enough

The chart above answers what size wire for this amperage under standard conditions but two situations still need a separate check. Bundling several current carrying conductors together in the same conduit reduces each one's usable ampacity below the table figure.

Long cable runs especially in 12V or 24V systems can also force a jump to a larger wire size than ampacity alone requires once voltage drop is factored in.

Ampacity alone does not determine the best conductor for a long circuit run. Use our wire voltage drop calculator to check whether conductor size and circuit length keep voltage drop within an appropriate range.

Conductor Count Derating Factors

Multiply the chart's ampacity by the matching factor to find the actual usable rating once conductors share a raceway.

Ambient Temperature Correction Factors

These factors apply on top of any conductor count derating so a hot attic run with several bundled conductors can stack both corrections together.

• Use the calculator above to check your specific circuit against both ampacity and voltage drop together.

A Quick Note on Breaker Size vs Wire Ampacity

These are two different numbers that get confused often: the breaker protects the circuit from excessive current while the conductor's ampacity describes what it can safely carry. Small conductor rules like the one above exist specifically to keep these two figures properly matched.

How to Use This Chart Correctly?

Reading the right row is only half the job, running through these checks in order keeps the chart from giving a false sense of certainty.

• Confirm the design current — use the equipment's rated draw rather than a guess & add 25% if the load runs continuously for three hours or more.
• Match the temperature column to your conductor's actual insulation rating then re check it against the termination rating per NEC 110.14(C).
• Apply bundling or ambient corrections if more than three conductors share a raceway or the run passes through unusually hot spaces.
• Check voltage drop separately for any run long enough that resistance becomes meaningful this chart doesn't account for distance at all.

Conclusion

Choosing the correct wire size by amperage starts with matching the circuit's required current to an appropriate conductor ampacity.

For common US applications an AWG ampacity chart provides a useful starting point with examples such as 12 AWG for many 20 A circuits, 10 AWG for many 30 A circuits and 8 AWG as a common starting point for 40 A copper circuits.

However the chart is not the entire sizing process. Continuous loads, termination temperature, ambient temperature, conductor grouping, conductor material, equipment requirements and voltage drop can all affect the final wire size.

Use the amperage chart for a quick reference then verify the complete installation before selecting the conductor and overcurrent protection.

FAQs

How do I calculate wire size for amps?

Match your circuit's current draw to the smallest wire size in the ampacity table that meets or exceeds it then confirm the insulation temperature rating you're using matches the correct column.

Does the wire size chart change for 240V circuits?

No. Ampacity depends on current not voltage a 30A circuit needs the same wire size whether it's wired at 120V or 240V. Voltage does affect how much power that current delivers and how significant voltage drop becomes over distance.

How do I convert kW to amps for wire sizing?

Divide the load in watts by the circuit voltage to get amps (for example 3,600W ÷ 240V = 15A) then use that amp figure to find the wire size in the chart above.

Is the highest ampacity column always the right one to use?

No. The 90°C column often looks best but the final ampacity has to be coordinated with your terminals and breaker which are commonly rated for only 60°C or 75°C so the 90°C figure is mainly a starting point for correction calculations not the answer itself.

Does a larger wire reduce voltage drop?

Yes. A larger conductor has lower resistance so it produces less voltage drop for the same current and cable length which is why a long run can require a bigger wire than the ampacity chart alone suggests.