Wire Size Calculator

Find the smallest copper or aluminum wire that meets both NEC Table 310.16 ampacity at 75°C and your voltage-drop target, with an option for the 125% continuous-load rule.

Shows whether ampacity or voltage drop sets the size.

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Results will appear here once you enter your values.

Two tests, one answer

A conductor has to pass two independent checks, and the calculator reports the larger of the two sizes:

  • Ampacity: the smallest size whose NEC Table 310.16 rating in the 75°C column is at least the load current. For continuous loads (running 3 hours or more), 210.19(A)(1) and 215.2(A)(1) require sizing for 125% of the current. Small conductors are also capped by 240.4(D): 14, 12 and 10 AWG copper are limited to 15, 20 and 30 A, and 12 and 10 AWG aluminum to 15 and 25 A, even though the table lists higher values.
  • Voltage drop: the smallest size whose drop, VD = 2 × I × R × L ÷ 1,000 (1.732 instead of 2 for three-phase), is at or below your target. R is the Chapter 9 Table 8 resistance in ohms per 1,000 ft.

On short runs ampacity usually governs. On long runs voltage drop takes over.

Worked example: a 24 A, 240 V load

A 24 A noncontinuous 240 V single-phase load on copper:

  • Ampacity: 12 AWG is listed at 25 A but limited to 20 A by 240.4(D), so it fails. 10 AWG (30 A) passes.
  • At 50 ft: 10 AWG drops 2 × 24 × 1.21 × 50 ÷ 1,000 = 2.90 V, or 1.21%. Ampacity governs and 10 AWG is the answer.
  • At 150 ft: 10 AWG drops 8.71 V (3.63%) and fails 3%. 8 AWG (0.764 Ω/kft) drops 5.50 V (2.29%). Voltage drop governs and the answer becomes 8 AWG.
  • At 300 ft: 8 AWG reaches 4.58%, so voltage drop pushes the size to 6 AWG (2.95%).

If the same load were continuous, ampacity would need 24 × 1.25 = 30 A, which 10 AWG copper still meets.

Where the 75°C column does not apply

The calculator uses the 75°C column because most modern breakers, lugs and THHN/THWN-2 or XHHW-2 wire are rated for it. Check before relying on it:

  • Terminations: under 110.14(C)(1)(a), circuits of 100 A or less and 14 through 1 AWG conductors use the 60°C column unless the equipment is listed for 75°C. NM-B (Romex) cable is always limited to 60°C ampacity by 334.80.
  • Hot or crowded conditions: ambients above 30°C (310.15(B)) and more than three current-carrying conductors in a raceway (310.15(C)(1)) reduce ampacity. Rooftop conduit and attic runs are common cases.
  • Motors, HVAC and EV chargers have their own sizing rules (Article 430, 440 and 625) based on nameplate data rather than a plain amp figure.

Common sizing mistakes

  • Forgetting the 125% factor on continuous loads such as EV charging, water heaters and lighting that stays on all evening. A 40 A EV charger needs a 50 A circuit and conductors rated for 50 A.
  • Using the table rating for 12 AWG as 25 A. 240.4(D) caps the overcurrent device at 20 A, so treat it as a 20 A conductor for everyday sizing.
  • Swapping copper for aluminum at the same gauge. Aluminum carries less current and has about 65% more resistance, so it typically needs to be one or two sizes larger. Use terminals rated AL/CU.
  • Checking drop on the branch only. The 5% recommendation covers feeder plus branch. A subpanel fed with 2% drop leaves only 3% for the circuits after it.

Frequently Asked Questions

Common questions about the Wire Size Calculator

For a 30 A circuit on copper, 10 AWG is the minimum under 240.4(D) and Table 310.16. Long runs may need 8 AWG or larger to keep voltage drop under 3%. Aluminum needs at least 8 AWG.

Sources & References

NFPA 70: National Electrical Code (Chapter 9 Table 8, Table 310.16, 210.19, 215.2)

The NEC publishes conductor areas and DC resistance (Chapter 9 Table 8), allowable ampacities (Table 310.16) and the informational notes recommending 3% branch and 5% total voltage drop.
→Source: NFPA

Voltage Drop Calculations

Worked examples of VD = 2 x K x I x D / CM for single-phase and 1.732 x K x I x D / CM for three-phase circuits, including the 6 AWG copper single-phase and 1 AWG aluminum three-phase examples used to check this calculator.
→Source: Mike Holt Enterprises

Southwire Voltage Drop Calculator

Manufacturer voltage-drop and conductor-sizing calculator based on NEC resistance and reactance data, useful for cross-checking large AC feeders.
→Source: Southwire Company

Wire and Cable Ampacity and Sizing Resources

Manufacturer ampacity charts and voltage-drop tables for copper and aluminum building wire based on the NEC.
→Source: Cerro Wire

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