AC wire size calculator

How the conductor is chosen

The conductor has to carry the minimum circuit ampacity at the temperature its terminals allow, after any corrections for a hot location or a crowded raceway. The voltage drop is worked separately, because a conductor that carries the current can still be too thin for a long run.

Which column of Table 310.16

ampacity column = the lower of the terminal rating and the insulation ratingNEC 110.14(C) holds the ampacity to the lowest temperature rating among the terminals, the conductor and any device it connects to. For circuits of 100 A or less, terminals count as 60°C (140°F) unless the equipment is listed and identified for higher. A 22.5 A MCA on 60°C (140°F) terminals needs 10 AWG copper (30 A). On 75°C (167°F) terminals 12 AWG copper (25 A) carries it.

Corrections for heat and crowding

ampacity = table value at the insulation column x ambient factor x conductor-count factor, and not above the terminal columnTable 310.15(B)(1)(1) for the ambient and Table 310.15(C)(1) for four or more current-carrying conductors. In the attic example a 33 A MCA at 120°F needs 8 AWG instead of 10 AWG.

Voltage drop

VD = 2 x K x I x L / CM, K = 12.9 (copper) or 21.2 (aluminum)K is the resistance constant at 75°C (167°F), I the current, L the one-way length and CM the circular mils from Chapter 9 Table 8. With 12 AWG at 22.5 A and 80 ft the drop is 7.11 V, 2.96 percent of 240 V.

The 3 percent figure

guideline, not a requirementAn Informational Note under NEC 210.19 suggests keeping the drop near 3 percent on a branch circuit, and near 5 percent for feeder and branch circuit together, for reasonable operation. An Informational Note is guidance. It is not a requirement of the code.

The same MCA under different conditions

Eight cases run through the same function the form uses, each over an 80 ft one-way run at 240 V unless the row says otherwise. The MCA values are illustrations, not typical values for any unit.

The 75°C (167°F) row at 80 ft sits at 2.96 percent, and one more foot makes it 3.0003 percent, just over the 3 percent figure in the Informational Note under NEC 210.19. That note is a recommendation and not a requirement, and the page shows the next size that brings the drop under it. The last two rows have the same 20.8 A. The compressor unit takes 12 AWG, while the heat strip takes 10 AWG, because a heater circuit is held to the small-conductor limits of NEC 240.4(D) and its 25 A breaker is over the 20 A limit for 12 AWG copper. In the attic row, heat takes the conductor from 10 AWG to 8 AWG even though the terminals are the same.

Standards and data sources

• NEC 2023, Table 310.16 and 310.15 - the ampacities of copper and aluminum conductors in the 60°C (140°F), 75°C (167°F) and 90°C (194°F) columns, the ambient temperature correction of Table 310.15(B)(1)(1) and the adjustment for more than three current-carrying conductors in Table 310.15(C)(1).
• NEC 2023, 110.14(C) and 240.4(G) - 110.14(C) for the terminal temperature limit and 240.4(G) with Article 440 for why an air-conditioning circuit's breaker is not limited by the conductor's ampacity.
• NEC 2023, Informational Note under 210.19, and Chapter 9 Table 8 - the 3 percent and 5 percent voltage drop recommendation, and the circular mils of each conductor size used in the drop formula.

Limitations of this calculator

• The nameplate comes first. The MCA and the MOCP on the unit govern the installation, and the installation instructions may set a minimum wire size on top. NEC 110.3(B) has listed equipment installed according to its listing and labeling.
• This page gives preliminary numbers for estimating and for checking a quote. A licensed electrician sizes, pulls and terminates the conductors under the permit and the inspection of the local authority having jurisdiction (AHJ). Nothing here is an approval of an installation.
• No typical MCA is given for any tonnage or model, and none is looked up. You type in the number from the nameplate. The default example is a hand example, not a typical unit.
• The tables are the 2023 edition of the NEC. Your state or city may enforce another edition or amend it. Check the edition your AHJ uses.
• The 100 A test in 110.14(C)(1) looks at the circuit rating, which is the breaker. With a breaker above 100 A, or terminals marked for conductors larger than 1 AWG, the 75°C (167°F) rule of 110.14(C)(1)(b) applies, so check the terminal marking.
• Cable types carry their own temperature limit in their own article of the NEC. For nonmetallic-sheathed cable (NM-B) the page follows NEC 334.80: corrections start from the 90°C (194°F) value, and the result may not exceed the 60°C (140°F) value. That rule was read in the text of an earlier edition. NM cable is also not permitted in wet or damp locations (NEC 334.12, read in the 2017 text), so not outdoors and not in conduit outdoors. Other cable types, such as armored or metal-clad cable, have their own articles that this page does not apply.
• The heater circuit type applies the small-conductor breaker limits of NEC 240.4(D) and the next-higher-rating rule of 240.4(B). A heater with a single supply circuit or a kit with its own nameplate MCA and MOCP follows those numbers.
• The conductor count is yours to enter. The rules for which conductors count in a raceway, including neutrals and spares, are in 310.15(E) and (F) and are not worked here.
• Below 78°F the table allows a correction factor above 1.00 for cool air. This calculator holds the factor at 1.00 and takes no credit. Rooftop and sunlight conditions are not modeled, and an ambient above 185°F is outside the table.
• The voltage drop uses the MCA as the current, which is a little higher than the running current, and one-way length times two for the round trip. The constant K is for AC conductors up to 1/0, so at 2/0 and larger the reactance is left out and the real drop is somewhat higher.
• Single phase only. Three-phase circuits use a different drop formula, which the voltage drop calculator works. Sizes go up to 4/0, and a larger need is outside this page.
• The equipment grounding conductor, the disconnect, conduit fill and the breaker are not sized here. The breaker comes from the nameplate or the breaker calculator.

Common mistakes sizing AC wire

1. Using the 75°C (167°F) column on unmarked terminals. Where the terminals are not listed and identified for 75°C (167°F), the 60°C (140°F) column applies, even with 90°C (194°F) wire. For a 22.5 A MCA that is the difference between 10 AWG and 12 AWG copper.
2. Sizing the wire to the breaker. For an air conditioner the wire is sized from the MCA, not from the breaker. The nameplate MOCP is a ceiling for the breaker, and Article 440 lets it sit above the conductor's ampacity.
3. Ignoring a hot attic or a crowded raceway. Heat and bundling lower the ampacity. A 33 A MCA on 75°C (167°F) terminals needs 10 AWG in a cool run and 8 AWG at 120°F.
4. Checking ampacity and skipping the length. A conductor that carries the current can still drop too much voltage on a long run. At 22.5 A, 12 AWG copper drops 2.96 percent at 80 ft and 3.0003 percent at 81 ft, on either side of the 3 percent guideline.
5. Calling 3 percent a code limit. The 3 percent and 5 percent figures are in an Informational Note. They are a recommendation for reasonable operation, not a requirement, and the equipment's own voltage tolerance is what protects the unit.
6. Pulling aluminum to terminals that are not marked for it. NEC 110.14 requires terminals used to connect aluminum to be identified for it. Look for AL or CU/AL on the lugs before you land aluminum. Check first.

Common questions about AC wire sizing

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