Read the chart as a starting point

The table below compares base ampacity values from NFPA 70 Table 310.16. It assumes no more than three current-carrying conductors in a raceway or cable, a 30°C (86°F) ambient temperature, and terminations rated for the temperature column shown. It also reflects NEC 240.4(D) small-conductor overcurrent rules for 14, 12, and 10 AWG copper.

Never size a conductor from breaker amperage alone. Verify the actual load, continuous-load 125% multiplier, terminal temperature rating, ambient heat correction, conduit fill adjustment, and voltage drop before pulling wire.

How do you calculate wire size?

The size of an electrical conductor refers to its cross-sectional area (measured in circular mils (CM) or AWG in the US). The larger the cross-sectional area, the more current the wire can transmit with lower resistance—very similar to water flowing through a wider pipe with less pressure loss. We derive the required conductor area from Ohm's law and conductor resistivity:

V=I · R
(Ohm's law)
CM=2 · K · I · LVdrop
(Single-phase / DC)

where:

  • V_drop – Voltage drop calculated between the supply source and the farthest end of the wire, measured in volts (V_drop = V_source × (% drop ÷ 100));
  • I – Maximum current running through the conductor, measured in amps (convert power using the watts to amps calculator);
  • K – Specific resistance constant of the conductor material at 75°C (12.9 Ω·CM/ft for copper and 21.2 Ω·CM/ft for aluminum);
  • L – One-way length of the wire run from panel to load, measured in feet; and
  • CM – Cross-sectional area of the conductor, measured in circular mils (where 1,000 CM = 1 kcmil).

Calculating wire sizes for three-phase AC

For a balanced three-phase AC circuit, three current-carrying phase conductors are used instead of a two-wire loop. The calculator accepts line-to-line voltage and line current, modifying the required area of a single conductor to:

CM=√3 · K · I · LVdrop
(Three-phase AC)

The factor of √3 ≈ 1.732 converts between phase and line quantities in a balanced three-phase system, replacing the factor of 2. The calculator automatically selects the next AWG size up from its listed 14 AWG – 4/0 table so the voltage-drop limit is never exceeded. To check apparent power feeding that circuit, use the kVA to amps calculator.

Wire size by amps: copper and aluminum

Smaller AWG numbers indicate thicker wire. You cannot simply pick the 75°C column to buy thinner wire; both the conductor insulation and the equipment terminals at each end must carry a 75°C listing.

This chart stops at 100 A because larger service entrances and subpanel feeders involve additional NEC rules. Always run a full feeder calculation for circuits above 100 A.

Planning reference for 15 to 100 amperes. A dash means this guide intentionally does not offer a small aluminum-conductor shortcut for that row.
CurrentCopper 60°CCopper 75°CAluminum 60°CAluminum 75°C
15 A14 AWG14 AWG——
20 A12 AWG12 AWG——
30 A10 AWG10 AWG8 AWG8 AWG
40 A8 AWG8 AWG6 AWG8 AWG
50 A6 AWG8 AWG4 AWG6 AWG
60 A4 AWG6 AWG3 AWG4 AWG
70 A4 AWG4 AWG2 AWG3 AWG
80 A3 AWG4 AWG1 AWG2 AWG
90 A2 AWG3 AWG1/0 AWG2 AWG
100 A1 AWG3 AWG1/0 AWG1 AWG

Why 60°C versus 75°C changes the size

  1. 01

    Identify the conductor and insulation

    Read the cable jacket or conductor print (such as THHN/THWN-2 or NM-B) to find its insulation temperature rating.

  2. 02

    Check both terminations

    Breaker lugs and equipment terminals often limit circuits 100 A and under to the 60°C column unless marked for 75°C.

  3. 03

    Apply ambient correction

    Base table values assume 30°C (86°F). Attics, rooftops, and hot mechanical rooms reduce allowable ampacity.

  4. 04

    Adjust for conductor count

    Bundling more than three current-carrying conductors in one conduit requires an ampacity adjustment factor.

  5. 05

    Verify overcurrent protection

    Match the breaker or fuse rating to the governing NEC protection rule for that specific load.

Worked example: ampacity passes, voltage drop changes the candidate

Take a 240 V single-phase feeder carrying 60 A over a 150 ft one-way copper run with a 3% drop target. Under 75°C base table conditions, 6 AWG copper handles 60 A of current.

Now check voltage drop. At 150 ft and 60 A, 6 AWG copper loses about 8.85 V (3.69%). Stepping up to 4 AWG cuts the loss to 5.56 V (2.32%), keeping the run under the 3% target.

Checklist before treating any AWG as final

  • Calculate the actual design load instead of copying only the breaker label.
  • Identify whether continuous-load, motor, HVAC, service or feeder rules apply.
  • Confirm copper, aluminum or copper-clad aluminum is permitted by the equipment listing.
  • Verify insulation type, terminal ratings and conductor temperature limitations.
  • Apply ambient-temperature correction and current-carrying-conductor adjustment where required.
  • Check one-way distance and voltage drop with the expected operating current.
  • Confirm raceway, cable, grounding, protection, local amendments, permits and inspection requirements.
  • Have a licensed electrician approve the complete conductor selection before installation.