Wire Size Calculator (NEC Ampacity and Voltage Drop)

Background.

There are two completely different reasons a wire can be too small, and almost every argument about wire sizing comes from someone thinking about one of them while someone else thinks about the other. The first reason is heat. Current warms a conductor, the conductor warms its insulation, and if the insulation runs above its rating for long enough it hardens, cracks and eventually stops being insulation. That is what the National Electrical Code's ampacity tables exist to prevent, and it is the half that an inspector enforces. The second reason is voltage. Every foot of conductor has resistance, so some of the supply voltage is spent getting to the load rather than delivered to it. Nothing catches fire when this happens. The lights just dim, the motor runs hot and slow, the well pump takes longer to start, and the run is legal the whole time.

This calculator answers both, separately, and then gives you the larger of the two. On a short run inside a house, heat almost always wins and the code table is the whole answer. On a long run — a shop at the back of a property, a well, a barn, a gate operator, a parking-lot light — voltage drop wins, often by two or three trade sizes, and a conductor picked from an ampacity chart alone will be perfectly compliant and perfectly useless.

The ampacity half is more involved than a single lookup, which is why so many charts on the internet are wrong in the same way. Four separate rules stack on top of Table 310.16. Ambient temperature corrects the table figure, because the table assumes 30 °C air and a summer attic or a rooftop raceway is nothing like 30 °C. The number of current-carrying conductors bundled together adjusts it again, because conductors in a bundle heat each other. NEC 110.14(C) then caps whatever survives at the temperature rating of the equipment terminals — this is the rule that catches people, because it means a 90 °C conductor connected to a 60 °C-rated breaker is treated as a 60 °C conductor for the final selection, no matter what the jacket says. And 240.4(D) puts a hard ceiling on the overcurrent device protecting 14, 12 and 10 AWG regardless of what the table says those conductors could carry. Miss any one of these and you get an answer that is one size small in exactly the conditions where being one size small matters.

The page has a second mode for one-family dwelling services and feeders, because NEC 310.12 gives those a genuinely different rule: the conductors may be sized at 83% of the service rating rather than at 100% of it, on the reasoning that a house never actually draws its full service rating continuously. That single sentence is why a 200 A house service is wired in 2/0 copper or 4/0 aluminium rather than the 3/0 copper an ordinary 200 A feeder would need. Applying the 83% and reading the 75 °C column of Table 310.16 reproduces every published row of Table 310.12 exactly, for both materials, from 100 A to 400 A — a check this calculator's test suite runs on every build.

Two scope limits, stated here rather than buried in an FAQ. This is AC premises wiring. Low-voltage DC — 12 V, 24 V and 48 V battery, solar, RV and automotive systems — behaves differently enough that it has its own page: at 12 V a 3% budget is 0.36 V, so voltage drop governs essentially every run and the ampacity table barely participates. And boats are not NEC territory at all; they are governed by ABYC E-11, which uses its own ampacity table, its own resistance constant, and a 3%/10% split between critical and non-critical circuits.

The last limit is the one that matters most. This is a design aid. The edition of the NEC your jurisdiction has adopted may not be the 2023 edition this page implements, local amendments routinely override the tables, and no calculator has seen your panel, your terminals, your derating conditions or your inspector. A licensed electrician has to sign off before any of this gets installed.

What is wire size calculator?

Wire size, in North American practice, is a conductor's cross-sectional area expressed as an American Wire Gauge number or, above 4/0, in thousands of circular mils. The gauge scale runs backwards — 14 AWG is small, 4/0 AWG is large — and each three steps down the gauge roughly doubles the area. A circular mil is the area of a circle one thousandth of an inch across, which makes it an awkward unit until you meet the voltage-drop equation, where it turns out to be exactly the unit that makes the arithmetic simple.

Ampacity is the current a conductor can carry continuously without exceeding its temperature rating. It is a property of the conductor in its installed conditions, not of the conductor alone: the same 8 AWG THHN is a 55 A conductor in the 90 °C column of Table 310.16, a 50 A conductor once its 75 °C terminations are taken into account, and a 38.5 A conductor if it shares a raceway with eight others. Ampacity is what the code enforces and what the overcurrent device protects.

Voltage drop is the loss along the run, and it is governed by Ohm's law rather than by the code. The trade equation is VD = 2·K·I·L/CM, where I is the current, L is the one-way length in feet, CM is the circular-mil area, and K is a resistance constant — 12.9 for copper and 21.2 for aluminium, both at 75 °C, both derived from NEC Chapter 9 Table 8. The 2 accounts for the fact that current has to come back; on a three-phase circuit it becomes √3.

The two constraints are independent and they do not scale together. Doubling the length doubles the drop and does nothing at all to the ampacity requirement. Bundling ten conductors halves the usable ampacity and does nothing to the drop. That is why a conductor has to be checked against both, and why the answer is whichever of them demands more.

How to use this calculator.

1. 01Choose the sizing basis. Use the dwelling-service mode only for a service or feeder carrying the entire load of one dwelling unit on a single-phase 120/240 V system at 100–400 A. Everything else — every branch circuit, every subpanel feeder in a commercial building, every three-phase feeder — is the branch-circuit mode.
2. 02Enter the real load current, not the breaker size. Sizing to the breaker is how you end up with an expensively oversized run, and it is not what NEC 210.19(A)(1) asks for. If you only know watts, divide by the voltage first.
3. 03Set the system voltage. This is what the drop percentage is measured against, so getting it wrong quietly scales the whole voltage-drop answer.
4. 04Pick the conductor material and its insulation rating. Modern building wire is nearly always 90 °C dual-rated THHN/THWN-2 — check the printing on the jacket rather than assuming.
5. 05Set the termination rating. If you can read the breaker and panel labels and they say 75 °C, choose 75 °C and you will often save a trade size. If you cannot, leave it on 'auto', which takes the conservative 110.14(C) default.
6. 06Say whether the load is continuous. Anything running three hours or more — EV charging, commercial lighting, electric heat — is continuous, and the conductor has to be sized for 125% of it.
7. 07Enter the real ambient temperature and the real conductor count. An attic in July is not 30 °C, and a raceway with three circuits in it is not three conductors.
8. 08Measure the run along the route the cable actually takes, not straight-line, and be generous — bends, drops and the loop inside the panel all count.
9. 09Read both minimum sizes, not just the recommendation. Which one governs tells you what to change: if ampacity governs, better terminations or cooler routing help; if voltage drop governs, only more copper does.
10. 10Take the result to a licensed electrician, and confirm which NEC edition and which local amendments your jurisdiction has actually adopted before anything is bought or pulled.

The formula.

The ampacity side starts with the load and ends with a table row, and there are four steps in between that all have to happen in the right order.

Step one is the code minimum. NEC 210.19(A)(1) requires a branch-circuit conductor to have an ampacity of at least the non-continuous load plus 125% of the continuous load. A 40 A load that runs for three hours or more therefore needs 50 A of conductor ampacity, not 40 A. On a one-family dwelling service the rule is the opposite direction: NEC 310.12 permits 83% of the rating, so a 100 A service needs 83 A of ampacity rather than 100 A.

Step two is the two derating factors, and they multiply. Table 310.15(B)(1) corrects for ambient temperature relative to the table's 30 °C basis — a 90 °C conductor keeps 96% of its rating at 31–35 °C, 82% at 46–50 °C and 29% at 81–85 °C. Table 310.15(C)(1) adjusts for bundling: 80% for four to six current-carrying conductors, 70% for seven to nine, 50% for ten to twenty. Both are applied to the figure in the conductor's own insulation column, which is why a 90 °C conductor is worth buying even when it is going to be connected to 75 °C terminals — you derate from the bigger starting number.

Step three is the termination limit, and it is where the answer usually gets decided. NEC 110.14(C) says the conductor's ampacity for selection purposes cannot exceed the figure in the column matching the equipment terminals: 60 °C for equipment rated 100 A or less unless it is listed and identified for 75 °C, and 75 °C above 100 A. So the usable ampacity is the smaller of the derated figure and the termination-column figure. In the worked example, 8 AWG copper is 55 A in the 90 °C column with no derating at all, but its 75 °C figure is 50 A, and 50 A is what it is worth.

Step four is NEC 240.4(D), which caps the overcurrent device on 14, 12 and 10 AWG at 15, 20 and 30 A for copper and at 15 and 25 A for 12 and 10 AWG aluminium, whatever the table says those conductors could carry. This is why a 25 A required ampacity cannot be run on 12 AWG copper even though the 75 °C column says 25 A: the breaker protecting it may not exceed 20 A.

The voltage-drop side is a single equation with one subtlety. VD = 2·K·I·L/CM, where K is 12.9 for copper and 21.2 for aluminium — those are the DC resistance of a 1000-circular-mil conductor 1000 ft long at 75 °C, taken from NEC Chapter 9 Table 8. The 2 is the out-and-back path; on three phase it becomes √3. Rearranged to solve for the conductor, CM = 2·K·I·L / VD_allowed, and the calculator takes the smallest standard size whose circular-mil area reaches that number. The subtlety is which current to use: the 125% continuous adjustment is a code minimum for heating, not a prediction of current, so voltage drop is always calculated on the real running current. Using the inflated figure would push the conductor up for no physical reason.

The two answers are then compared and the larger one wins, because a conductor has to satisfy both simultaneously. Every comparison — which correction band, which conductor size, which standard device rating — is made on the unrounded value, and rounding to ten decimal places happens once, at the return boundary. The ambient correction band is a step function on whole degrees, so 30.0 °C is the 26–30 band and 30.1 °C has already stepped into 31–35; that boundary is tested explicitly.

One approximation is worth naming. The DC-resistance method ignores reactance and assumes unity power factor. It is the method the trade uses and it is accurate for the runs most people are sizing, but from about 2/0 upward self-inductance starts to matter, and for a long large three-phase run at a poor power factor the more exact route is NEC Chapter 9 Table 9, which tabulates AC resistance and reactance for conductors in PVC, aluminium and steel conduit separately.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Fire Protection Association, NFPA 70 — National Electrical Code, 2023 edition. Table 310.16, "Ampacities of Insulated Conductors with Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30 °C (86 °F)". Source of every ampacity in this calculator. The code text itself is gated behind NFPA's free-access registration; the values used here were read from two independent reproductions and cross-checked against each other (see the dossier). Retrieved 2026-07-29.
2. [2]NECA/IBEW Electricians, reproduction of NEC Table 310.15(B)(16) (formerly Table 310.16), "Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60 °C Through 90 °C". Read directly from the PDF on 2026-07-29. Used as the primary transcription source for the ampacity table. This is the 2011-edition numbering; every value is identical to the 2023-edition Table 310.16, which is the cross-edition check recorded in the dossier.
3. [3]Conduit.site, reproduction of NEC Table 310.15(B)(1), "Ambient Temperature Correction Factors Based on 30 °C (86 °F)", 2020 NEC. Retrieved 2026-07-29. Source of all thirty-nine correction factors implemented here, across the 60 °C, 75 °C and 90 °C columns and the sixteen ambient bands from '10 °C or less' to '81–85 °C', including the points at which the 60 °C and 75 °C columns terminate and the conductor may no longer be used.
4. [4]Mike Holt Enterprises, "Voltage Drop Calculations". Retrieved 2026-07-29. Source of the K constants used here — 12.9 ohm-circular-mils per foot for copper and 21.2 for aluminium, both defined as the DC resistance of a 1000-circular-mil conductor 1000 ft long at 75 °C, derived from NEC Chapter 9 Table 8 — of the VD = 2·K·I·L/CM and √3·K·I·L/CM forms, and of the stated limit that the method is suitable up to 1/0 with self-inductance needing a Q adjustment at 2/0 and above.
5. [5]Zing², reproduction of NEC Table 310.15(C)(1), "Adjustment Factors for More Than Three Current-Carrying Conductors", noted as unchanged across the 2020, 2023 and 2026 editions. Retrieved 2026-07-29. Source of the 80 / 70 / 50 / 45 / 40 / 35 percent bands and of the rule that grounding conductors and the neutral of a balanced three-phase system are excluded from the count.
6. [6]WireRef, "NEC Article 240 — Overcurrent Protection", 2023 edition with 2026 updates noted. Retrieved 2026-07-29. Source of the 240.6(A) standard ampere ratings list (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600 A and above), of the 240.4(D) small-conductor limits for copper (14, 12 and 10 AWG at 15, 20 and 30 A — the page does not reproduce the aluminium limits of 15 A on 12 AWG and 25 A on 10 AWG, which are stated in 240.4(D) itself), and of the 240.4(B) next-higher-standard-rating allowance.
7. [7]American Boat and Yacht Council, Standard E-11, "AC and DC Electrical Systems on Boats", excerpts published by Paneltronics. Read directly from the PDF on 2026-07-29. Cited as the scope boundary rather than as an input: E-11.14.2.6 requires 3 percent maximum drop on main feeders, bilge blowers, electronic equipment and navigation lights and 10 percent on non-critical circuits, and E-11.14.2.7 sizes conductors from ABYC's own Tables VI, IX and X. Marine wiring must be sized from those tables, not from the NEC tables this page implements.