Wire Size Calculator

Enter the current, voltage and one-way distance. The calculator checks NEC ampacity, the small-conductor breaker limits and voltage drop, then tells you which one set the size.

Quick answer: 14 AWG copper for 15 A, 12 AWG for 20 A, 10 AWG for 30 A. Past about 46 ft on a 20 A, 120 V circuit, voltage drop rather than the breaker decides the size, and you move up one gauge.

Wire size by breaker: the short table

This is the minimum copper or aluminum size for each common breaker before you account for distance. NM-B cable is limited to the 60 °C column by NEC 334.80, while individual THHN conductors in conduit can use 75 °C when every terminal in the circuit is rated 75 °C.

How the calculator chooses a size

Every result passes three checks, in this order, and the calculator reports which one set the size.

1. Ampacity. The conductor's allowable current from Table 310.16 in the temperature column that matches your cable. For a continuous load (one that runs three hours or more) the wire must carry 125% of that load, per 210.19(A)(1) for branch circuits and 215.2(A)(1) for feeders.
2. Small-conductor limits. 14, 12 and 10 AWG copper may not be protected above 15, 20 and 30 A, even though the table lists higher numbers. Aluminum 12 and 10 AWG stop at 15 and 25 A.
3. Voltage drop. Using the DC resistance from Chapter 9 Table 8, the calculator works out the volts lost in the run and steps up until the drop is at or below your target: 3% for a branch circuit is the usual design figure, 5% for feeder plus branch combined.

The answer is the smallest size that clears all three. If the drop check forced it up, you see a note saying so, which is the part most quick charts leave out.

Which temperature column applies to you

Table 310.16 lists three ampacities for each size, and choosing the wrong one is the most common sizing mistake.

• 60 °C: NM-B (Romex) and UF-B cable in a house, and any circuit whose breaker or device terminals are rated 60 °C. Under 110.14(C), equipment rated 100 A or less, or wired with 14 through 1 AWG, defaults to 60 °C unless it is marked 75 °C.
• 75 °C: THHN/THWN-2 conductors in conduit, SER and other feeder cables, when the breaker and lugs carry a 60/75 °C or 75 °C marking. Most modern residential breakers are marked 60/75 °C.
• 90 °C: only for derating. If a conduit holds more than three current-carrying conductors or passes through a hot rooftop, you start from the 90 °C number, apply the correction factors, and the result may not exceed the 75 °C or 60 °C value of the terminals.

Worked example: a 40 A EV charger in the garage

A Level 2 charger drawing 40 A is a continuous load, so the wire is sized for 40 × 1.25 = 50 A on a 50 A breaker. With an 80 ft run at 240 V:

• THHN copper in conduit at 75 °C: 8 AWG, voltage drop 4.98 V (2.07%).
• NM-B copper at 60 °C: 6 AWG, voltage drop 3.14 V (1.31%).

The distance is short enough that ampacity decides. Compare a 20 A, 120 V outlet circuit run 120 ft to a detached shed: the breaker alone says 12 AWG, but the drop check pushes it to 6 AWG so the far outlet still sees 117.6 V under full load.

Copper or aluminum

Copper is the default for 15–60 A branch circuits: it is smaller for the same current, easier to terminate on receptacles and switches, and less sensitive to loose connections. Aluminum (AA-8000 series alloy) is the normal choice for service entrance cable, feeders to subpanels and long runs where copper would cost two to three times more. Use lugs and breakers marked AL/CU, apply anti-oxidant compound where the manufacturer calls for it, and torque connections to the listed value.

What this calculator does not cover

• Ambient temperature correction (Table 310.15(B)(1)) and adjustment for more than three current-carrying conductors (Table 310.15(C)(1)).
• The 310.12 dwelling service and feeder table that allows smaller conductors for 120/240 V services.
• Motor circuits, which are sized from Article 430 full-load current tables, and HVAC equipment, which uses the nameplate MCA and MOCP.

Codes are adopted locally and many jurisdictions amend them. Treat these numbers as a design check and confirm with your inspector or a licensed electrician before you pull wire.

Related calculators and charts

• Voltage drop calculatorVolts lost over any run, copper or aluminum, single- or three-phase.
• AWG wire gauge chartDiameter, mm², resistance and ampacity from 4/0 to 40 AWG.
• What size wire for 50 ampRanges, EV chargers, subpanels and RV pedestals, by distance.

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