Wire Size Calculator

Find the wire gauge for a load from NEC Table 310.16 ampacity with 125% continuous load, ambient and bundling derating, then check voltage drop. AWG and mm².

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About this tool

What is the Wire Size Calculator?

Enter the load current, say whether it runs for three hours or more, and this wire size calculator picks the smallest copper or aluminium conductor that the NEC ampacity table allows. It then runs a voltage drop check over your cable length and moves up a size if the drop would be too large, so the answer covers both questions an electrician asks: will the wire stay cool, and will the load get enough voltage?

Behind the single answer you can see each step. The design current includes the 125% factor for continuous loads, the ampacity is read from the column your terminals allow, the 90 °C column is corrected for hot rooms and for more than three conductors in a raceway, and a short table compares the sizes on either side so you can see which test each one fails.

Switch to IEC mode for metric cable. It uses typical current-carrying capacities from IEC 60364-5-52 for PVC or XLPE copper cable clipped direct or in conduit, with the standard ambient and grouping factors, and the same voltage drop check with IEC 60228 resistances.

  • NEC 2023 Table 310.16 copper and aluminium, 60/75/90 °C columns
  • 125% continuous-load rule, 110.14(C) terminal limit and 240.4(D) breaker caps
  • Ambient (°C or °F) and bundling correction factors
  • Voltage drop check that upsizes the conductor automatically
  • IEC mode with typical mm² capacities for PVC and XLPE cable
  • Step-by-step working, copy and share links
Read the guide: How to Size Electrical Wire: Ampacity, Derating and Voltage Drop

How to use the Wire Size Calculator

  1. Choose NEC for AWG and kcmil sizes or IEC for mm² cable.
  2. Enter the load current in amps and say whether it is continuous (on for three hours or more, such as an EV charger, water heater or office lighting).
  3. Pick copper or aluminium, the insulation rating (THHN is 90 °C, NM cable is held to 60 °C) and the terminal rating printed on the breaker or equipment.
  4. Set the ambient temperature and the number of current-carrying conductors sharing the raceway or cable. Neutrals that carry only unbalanced current and grounding conductors do not count.
  5. Fill in the voltage drop check: circuit type, voltage, one-way length and the maximum drop you accept. Read the recommended size and which rule governs it.

Worked examples

40 A continuous load, 100 ft at 240 V

Input

Copper, THHN (90 °C), 75 °C terminals, 30 °C, 3 conductors, single-phase 240 V, 100 ft, 3% limit

Output

8 AWG, 50 A usable, 2.59% drop (6.22 V)

40 A × 1.25 = 50 A, and 8 AWG copper is exactly 50 A in the 75 °C column. The drop over 100 ft is under 3%, so ampacity governs.

48 A EV charger 150 ft from the panel

Input

48 A continuous, copper, 75 °C terminals, 240 V, 150 ft, 3% limit

Output

6 AWG copper (65 A at 75 °C), 2.95% drop

The charger needs 60 A of ampacity, which 6 AWG covers. At 150 ft it lands just under the 3% target; a longer run would push it to 4 AWG.

Nine conductors through a hot attic

Input

16 A non-continuous, 12 AWG THHN, 45 °C ambient, 9 current-carrying conductors

Output

12 AWG: 30 A × 0.87 × 0.70 = 18.27 A, still above 16 A

Derating from the 90 °C column leaves 18.27 A. 14 AWG would only manage 15.23 A, so it fails even though 15 A breakers are common on it.

32 A metric circuit, 25 m

Input

IEC, PVC, method C, 2 loaded conductors, 30 °C, 32 A, 230 V, 25 m, 3% limit

Output

4 mm² carries 36 A, but 6 mm² is needed for drop (2.56%)

Here voltage drop governs: 4 mm² passes on current but loses about 3.8% over 25 m.

Background and insight

In the United States conductor ampacity comes from NFPA 70, the National Electrical Code. Table 310.16 lists allowable ampacities for insulated conductors with not more than three current-carrying conductors in a raceway, cable or earth at 30 °C (86 °F) ambient. This calculator uses the 2023 edition values: 12 AWG copper is 20, 25 and 30 A in the 60, 75 and 90 °C columns, 8 AWG is 40, 50 and 55 A, and 4/0 aluminium is 150, 180 and 205 A. Those columns are the most widely adopted and match the long-standing table; check the edition your jurisdiction enforces.

Three rules shape the answer. Section 110.14(C) limits you to the 60 °C column for circuits of 100 A or less (or 14 to 1 AWG) unless the equipment is marked for 75 °C, and to 75 °C above that, because the breaker and lug are rated lower than modern 90 °C insulation. Sections 210.19(A) and 215.2(A) require branch circuits and feeders to carry 125% of a continuous load before any adjustment. And section 310.15 applies ambient correction factors (Table 310.15(B)(1)(1), for example 0.91 at 36 to 40 °C for 90 °C wire) and adjustment for bundling (Table 310.15(C)(1): 80% for 4 to 6 conductors, 70% for 7 to 9, 50% for 10 to 20). THHN may be derated from its 90 °C value, but the result can never exceed the termination column.

Finally, section 240.4(D) caps the breaker on small conductors regardless of the table: 15 A on 14 AWG copper, 20 A on 12 AWG and 30 A on 10 AWG (15 A and 25 A for 12 and 10 AWG aluminium). Voltage drop is not a hard NEC rule; informational notes to 210.19 and 215.2 suggest about 3% on a branch circuit and 5% overall, and this tool lets you set the limit. Resistances come from NEC Chapter 9 Table 8 at 75 °C, reused from our voltage drop calculator.

Common use cases

  • Sizing a new branch circuit

    Check the gauge for an EV charger, heat pump, hot tub or shop subpanel before buying cable.

  • Reviewing a derated installation

    See how many amps a conductor can really carry in a hot attic or a crowded conduit.

  • Long runs to outbuildings

    Find the size where voltage drop, not ampacity, decides the cable for a barn, well or detached garage.

  • Exam and apprentice practice

    Work through 110.14(C), 125% continuous load and correction factors with every step shown.

Tips and limitations

  • Use the terminal rating, not the insulation rating, for the final ampacity. A 90 °C wire on a 60 °C breaker is a 60 °C conductor.
  • NM-B (Romex) is 90 °C insulation but section 334.80 limits its ampacity to the 60 °C column; pick 60 °C insulation for it.
  • Count only current-carrying conductors. A neutral of a balanced three-phase wye circuit feeding non-linear loads does count, because harmonics flow in it.
  • Motors, air conditioners and welders have their own sizing rules (Articles 430, 440 and 630) based on nameplate or table currents. Use those for the ampacity input.
  • Aluminium is fine for feeders when terminated with listed lugs and the correct torque; it needs about two sizes larger than copper for the same current.
  • Ampacity and voltage drop are separate checks. A short run can use the minimum size; a long one often needs one or two sizes more.

Frequently asked questions

What size wire do I need for a 40 amp continuous load?

Size for 125%, so 50 A. With 75 °C terminals that is 8 AWG copper (50 A) or 6 AWG aluminium (50 A). With 60 °C terminals you need 6 AWG copper (55 A).

Can I use the 90 °C column for THHN wire?

Only as the starting point for derating. You may apply ambient and bundling factors to the 90 °C ampacity, but the final ampacity cannot be more than the 60 or 75 °C value of the terminals it connects to.

What does the 125 percent rule for continuous loads mean?

A load expected to run for three hours or more must be supplied by conductors and a breaker rated for at least 125% of it, so a 40 A continuous load needs 50 A of capacity. It keeps the breaker and wire from running at full rating for long periods.

How many current-carrying conductors before I must derate?

More than three. Four to six conductors in a raceway or cable take 80% of the table ampacity, seven to nine take 70%, and 10 to 20 take 50%, per NEC Table 310.15(C)(1).

Why is 14 AWG wire limited to a 15 amp breaker when the table says 20 A?

Section 240.4(D) sets the maximum overcurrent device for small conductors: 15 A for 14 AWG copper, 20 A for 12 AWG and 30 A for 10 AWG. The table value is still used for derating calculations.

Does wire size depend on the length of the run?

Ampacity does not, but voltage drop does. Past roughly 50 to 100 feet on a branch circuit, the drop often decides the size. The calculator checks both and reports which one governs.

Is the IEC mode valid for BS 7671 or AS/NZS 3008?

It uses typical IEC 60364-5-52 values, which are close to but not identical with BS 7671 Appendix 4 or AS/NZS 3008 tables. Use it for estimates and confirm against the national standard you design to.

Is there an in-depth guide to the Wire Size Calculator?

Yes. “How to Size Electrical Wire: Ampacity, Derating and Voltage Drop” covers the background, how it works, common pitfalls and worked examples. Read the full guide.

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