Wire size calculator

How the size, the breaker and the drop are worked

A conductor has to pass three checks, and the smallest size in Table 310.16 that passes all of them is the answer. Check one reads the table before any correction, at the noncontinuous load plus 125 percent of the continuous load. Heat and conductor-count corrections come in check two, which compares with the load itself. Check three is the breaker. Voltage drop is separate and uses the load current.

Load before corrections

I(a) = noncontinuous + 1.25 x continuousNEC 210.19(A)(1)(a) for a branch circuit and 215.2(A)(1)(a) for a feeder. Mike Holt's 2017 example is a 45 A continuous load: 56.25 A. His feeder example adds 100 A continuous and 100 A noncontinuous for 225.00 A. A continuous load runs three hours or more (Article 100).

Table value, first check

table ampacity in the lower of the insulation and terminal columns >= I(a)The correction factors are not applied here. With 75°C (167°F) terminals, 6 AWG copper is 65 A against 56.25 A, and 8 AWG is 50 A. NM-B cable stays in the 60°C (140°F) column (334.80). On 60°C (140°F) terminals the 60°C (140°F) column is used.

Corrected ampacity, second check

table ampacity in the insulation column x ambient factor x conductor-count factor >= noncontinuous + continuousTable 310.15(B)(1)(1) for the ambient and Table 310.15(C)(1) for more than three current-carrying conductors. The 90°C (194°F) value may be used here (110.14(C)(1)(a)(2)). 6 AWG THHN is 75 A x 0.80 = 60.00 A against the 45 A load. The 125 percent is not carried into this check.

Breaker, third check

breaker <= 240.4(D) limit of the size, and breaker <= next standard rating above the conductor's ampacityThe breaker is the smallest standard size at or above the first-check load (Table 240.6(A)), unless you enter one. 240.4(D) caps a breaker at 15 A on 14 AWG copper, 20 A on 12 AWG and 30 A on 10 AWG, and 15 A on 12 AWG aluminum and 25 A on 10 AWG. A 25 A breaker on a 4,500 W heater is over the 20 A limit of 12 AWG, so the answer is 10 AWG even on 75°C (167°F) terminals.

Voltage drop

VD = 2 x K x I x L / CM single phase, 1.732 x K x I x L / CM three phaseK is 12.9 for copper and 21.2 for aluminum, L is the one-way length and CM the circular mils of the size. I is the load, continuous plus noncontinuous. 10 AWG copper carrying 30 A for 200 ft at 240 V drops 14.91 V, 6.2 percent.

Continuous loads through the same formula

Each row is a continuous load with nothing else on the circuit, THHN in a dry location at 86°F with two current-carrying conductors. The breaker is the smallest standard size at or above 125 percent of the load. Copper on 60°C (140°F) terminals reads the 60°C (140°F) column, copper and aluminum on terminals listed for 75°C (167°F) the 75°C (167°F) column. At 15 A, copper is 12 AWG in both copper columns. On 60°C (140°F) terminals 14 AWG fails: 15 A in the 60°C (140°F) column, under the 18.75 A needed before corrections; and a 20 A breaker is over the 15 A limit of NEC 240.4(D) for 14 AWG copper. On 75°C (167°F) terminals it fails because a 20 A breaker is over the 15 A limit of NEC 240.4(D) for 14 AWG copper. A load the table cannot carry up to 4/0 AWG shows as over 4/0. Above a 100 A breaker the terminals count as 75°C (167°F) (110.14(C)(1)(b)), so the 60°C (140°F) column shows n/a.

Mike Holt's three published examples are checked against this code: 45 A continuous with four conductors gives 6 AWG (8 AWG fails both checks), 100 A continuous plus 100 A noncontinuous gives 4/0 AWG, and 200 A continuous needs 250 kcmil, which the page refuses. His 4,500 W appliance gives 10 AWG.

Standards and data sources

• NEC 2023, 210.19(A)(1), 215.2(A)(1), 210.20(A), 110.14(C), 240.4(B), 240.4(D), 334.80, Table 310.16, Table 310.15(B)(1)(1), Table 310.15(C)(1) and Table 240.6(A) - the branch-circuit and feeder conductor rules, the temperature columns, the breaker limits and the standard ratings. The NEC text was not read in NFPA's viewer. It was read as Mike Holt prints it in EC&M (2017 edition, with the table renumbered to 2023) and as third-party copies print it.
• Mike Holt, EC&M Code Q&A, Sizing Branch-Circuit Conductors (Feb 2, 2017) - the two conductor checks, 125 percent of the continuous load at the terminal ampacity and the maximum load after corrections, and the worked example this page opens on: 45 A continuous, four conductors, 75°C (167°F) terminals, 6 AWG.
• Mike Holt, EC&M, Conductor Sizing, Part 2 of 2 (Jun 18, 2019), and Code Q&A, Sizing Circuit Conductors for Individual Appliances (Oct 15, 2020) - the feeder rule in 215.2(A)(1)(a), the 100 A continuous plus 100 A noncontinuous example that gives 4/0 AWG, the 200 A continuous example that gives 250 kcmil, the 90°C (194°F) ampacity for corrections, and the 4,500 W, 240 V appliance at 18.75 A, 125 percent, 23.44 A and 10 AWG. The appliance article is at https://www.ecmweb.com/national-electrical-code/qa/article/21144798/code-qa-sizing-circuit-conductors-for-individual-appliances
• Ascent / IEWC, Product Specification No. 10521 rev 01 (May 16, 2025), THHN / THWN-2 building wire - where each marking may be used: THHN is a dry-location wire to 90°C (194°F), THWN-2 is wet or dry to 90°C (194°F), and THWN is wet to 75°C (167°F) or dry to 90°C (194°F). One wire is marked THHN in all sizes, also THWN-2 from 8 AWG up and THWN in 14, 12 and 10 AWG.
• HELUKABEL, Allowable Ampacity Tables, NFPA 70: NEC - 2023, and the Cerrowire ampacity chart - 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 correction and the conductor-count adjustment. Copper columns were cross-checked against Cerrowire.
• Thomas A. Domitrovich, IAEI Magazine, 100% vs 80%: Choosing the right OCPD solution (2016, 2014 NEC) - the text of 210.20(A) as printed there: the breaker is not less than the noncontinuous load plus 125 percent of the continuous load, unless the assembly is listed for operation at 100 percent of its rating. The voltage drop rows use Mike Holt's K method, as on the voltage drop calculator.

Limitations of this calculator

• This page gives preliminary numbers for estimating and for checking a quote. A licensed electrician sizes and installs the circuit under the permit and the inspection of the local authority having jurisdiction (AHJ). Nothing here is an approval of an installation.
• Sizes run from 14 to 4/0 AWG. A load that needs more is refused, with the reason. Kcmil sizes, parallel conductors and busway are not on this page.
• Not for service conductors. The 83 percent allowance for a dwelling service or feeder in 310.12 is not applied, and the load calculation of Article 220 is a separate step.
• Not for motors or air-conditioning equipment. A motor circuit is sized from the full-load current in the tables of Article 430 (430.22 and Tables 430.248 and 430.250), not from a measured load. An air conditioner, heat pump or mini split is sized from its minimum circuit ampacity under Article 440; use the AC wire size calculator.
• The conductor count is yours to enter. Which conductors count, including a neutral on a nonlinear load, is in 310.15(E) and is not worked here. The rooftop temperature adder of 310.15(B)(2) is not worked; the wire ampacity derating calculator covers the corrections in more detail.
• The insulation choice follows the location. The Ascent specification sheet rates THHN to 90°C (194°F) in dry locations only and THWN to 75°C (167°F) in wet ones, so conduit outdoors, which counts as a wet location, takes the 75°C (167°F) choice unless the wire is marked THWN-2 or XHHW-2. Check the marking printed on the wire.
• The terminal column is your choice. For circuits of 100 A or less, or equipment marked for 14 to 1 AWG, the terminals count as 60°C (140°F) unless the equipment is listed and marked for 75°C (167°F) (110.14(C)). Above that, read the marking on the breaker, the panel and the load.
• The breaker rule is the 240.4(D) limit on small conductors and the next-standard-rating rule of 240.4(B). Where 240.4(E) or 240.4(G) says otherwise, the rule differs. Where the result leans on 240.4(B), the page shows the breaker and the size without it, for a multioutlet circuit to cord-and-plug loads, which 240.4(B) does not cover. Equipment listed for continuous operation at 100 percent of its rating and fuses are not modeled; the listing and the AHJ decide.
• The NEC text was read as Mike Holt prints it in EC&M (2017 edition), with the table renumbered to Table 310.16 of the 2023 edition, and as third-party copies print it, not in NFPA's own viewer. Section numbers moved between editions, and a later edition may number them again. Check the edition and the local amendments your AHJ uses.
• The voltage drop uses the load current and one-way length times two (single phase) 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. The 3 percent figure is an Informational Note, not a requirement.

Common mistakes when sizing wire

1. Stopping after the 125 percent figure. A 45 A continuous load on 75°C (167°F) terminals needs 56.25 A before corrections, and 8 AWG gives only 50 A. It also has to carry the load after the corrections: 8 AWG THHN is 55 A x 0.80 = 44.00 A, under 45 A, with four conductors. Both checks send it to 6 AWG.
2. Running the 125 percent through the correction factors. The corrected ampacity is compared with the load, not with the 125 percent figure. A 24 A continuous load in a raceway with 7 conductors needs 30.00 A before corrections, and 10 AWG THHN is 40 A x 0.70 = 28.00 A after, which is above the 24 A load. The AC wire size calculator applies the corrections to a minimum circuit ampacity and gives 8 AWG for the same 30.00 A.
3. Calling a load continuous, or not. A continuous load runs three hours or more. A 40 A noncontinuous load on 60°C (140°F) terminals takes 8 AWG copper. The same 40 A as a continuous load is 50.00 A and takes 6 AWG. Check how long the load really runs.
4. Using the 75°C column on terminals that count as 60°C. A 50 A noncontinuous load takes 8 AWG copper in the 75°C (167°F) column and 6 AWG in the 60°C (140°F) column. The smaller size is only right where the breaker and the equipment are listed and marked for 75°C (167°F). Circuits of 100 A or less default to 60°C (140°F).
5. Sizing for ampacity and forgetting the breaker. A 4,500 W heater at 240 V is 18.75 A, 23.44 A at 125 percent, and a 25 A breaker. 12 AWG copper carries 25 A in the 75°C (167°F) column, but 240.4(D) holds a breaker on 12 AWG to 20 A, so the answer is 10 AWG. On 60°C (140°F) terminals it is 10 AWG anyway, as in Mike Holt's answer.
6. Ignoring the length of the run. Ampacity does not change with length, the voltage drop does. A 30 A load on 10 AWG copper for 200 ft at 240 V drops 14.91 V, 6.2 percent, over the 3 percent in the Informational Note under 210.19. 6 AWG brings it to 2.5 percent. Enter the length to see it.

Common questions about wire size

Related calculators

• Voltage drop calculator The drop for a chosen size, the longest run, or the size for a target drop.
• AC wire size calculator Wire for an air conditioner, heat pump or mini split from its minimum circuit ampacity.
• Conduit fill calculator The trade size of conduit for the wire this page gives.
• Equipment grounding conductor size calculator The ground wire that runs with the circuit conductors, sized from the breaker.
• Wire ampacity derating calculator The ampacity of a given wire after the heat and conductor-count corrections.

Used on the same job

• Voltage drop calculator Whether the wire is big enough for the length of the run.
• Conduit fill calculator The conduit for the wire this page gives.
• Equipment grounding conductor size calculator The ground wire for the breaker this page gives.