SizingKitConductor sizing
Wire size calculator
This returns the smallest conductor that satisfies every limit at once — the NEC Table 310.16 ampacity after the ambient and bundling corrections, the 110.14(C) termination ceiling, the 240.4(D) small-conductor cap, and the voltage drop you are willing to spend on the run. Then it says which of them decided, and prints the arithmetic that got there. It is free, takes no account, and works in a basement with no signal.
- 100% free
- No signup
- 21 sizes, 14 AWG to 1000 kcmil
- 60/75/90 °C columns
- Names the binding limit
The circuit
Load, run and metal decide the answer; the conditions below decide how much of the table you are allowed to keep.
Amperes. A motor circuit takes its figure from NEC Table 430.248 or 430.250, not from the nameplate.
The nominal voltage of the circuit, not the open-circuit reading at the panel.
Panel to load, measured one way. The return conductor is already in the arithmetic.
Aluminum needs about 1.6 times the area of copper for the same resistance, which is usually two gauge sizes.
Drop is multiplied by 2 on a single-phase circuit and by √3 on a balanced three-phase one.
THHN, THWN-2 and XHHW-2. Dry-location 90 °C is a starting point for correction, not a load limit — 110.14(C) still holds the final figure to the termination column.
Conditions and assumptions75 °C terminations · 30 °C ambient · 3 current-carrying · 3% drop budget · K 12.9
Default 75 °C. NEC 110.14(C): the temperature rating used for a conductor's ampacity may not exceed the lowest temperature rating of any connected termination, conductor or device. 110.14(C)(1)(a) puts circuits of 100 A or less, and equipment marked for 14 AWG–1 AWG, on the 60 °C column unless the equipment is listed for higher; 110.14(C)(1)(b) puts larger circuits on 75 °C. Equipment marked 90 °C at its terminals is rare outside of specific listings — do not assume it.
Default 30 °C (86 °F) — NEC Table 310.16 heading: 'Based on Ambient Temperature of 30°C (86°F)'. An attic in July and a boiler room are not that.
Default 3, which is the basis of the table and needs no adjustment. Grounding conductors never count; a neutral counts only when it carries more than the unbalance.
Percent. Default 3 — A recommendation, not a rule. Project specifications and some local amendments tighten it to 2%; motor and lighting design guidance sometimes differs from both.
Default 12.9 for copper. Rises with conductor temperature: about 11.1 at 20 °C, 12.9 at 75 °C, 13.7 at 90 °C. A lightly loaded conductor runs cooler than 75 °C and drops less than K predicts.
Smallest conductor that clears every limit
6 AWG
Cross-sectional area
13.3 mm²
26,251 circular mils · 0.162 in (4.12 mm) of metal
Ampacity alone allows 8 AWG. The 3% drop budget over 140 ft is what forced 6 AWG.
How the ampacity was arrived at
| Table 310.16, 90 °C copper | 75 A |
|---|---|
| × ambient factor at 30 °C, Table 310.15(B)(1) | 1 |
| × 3 current-carrying, Table 310.15(C)(1) | 1 |
| = corrected ampacity | 75 A |
| 110.14(C) ceiling, 75 °C column | 65 A |
| 240.4(D) small-conductor cap | none at this size |
| Usable — the smallest of the three | 65 A |
Against a load of 40 A.
And the drop it actually has
| Budget, 3% of 240 V | 7.2 V |
|---|---|
| Area the budget needs at 40 A | 20,067 cmil |
| Drop on 6 AWG over 140 ft (42.7 m) | 5.5 V · 2.29% |
Drop is computed at 40 A, the current the load actually pulls — not at 125% of it. The 125% multiplier is an overcurrent-device rule from 210.19(A)(1); a conductor only drops the voltage that flows through it.
Ampacity is read from NEC Table 310.16 as printed, corrected by the band of Table 310.15(B)(1) — generated here from the 310.15(B) equation, which reproduces all 48 printed factors — and adjusted by Table 310.15(C)(1). The cap is the 7 rows of 240.4(D), and it caps the protective device rather than the wire: a motor branch circuit under 240.4(G) is one of the places it does not apply at all.
The K method models the conductor as pure resistance. It ignores inductive reactance, which on large conductors is a large share of the impedance, and it ignores power factor, conductor spacing, and whether the raceway is steel or PVC. It is close enough on small branch circuits — 14 AWG through about 1/0 at unity power factor — and progressively wrong above that, understating drop on long feeders of 250 kcmil and larger and on any load below about 0.9 power factor. Where the answer matters, use the AC resistance and reactance of NEC Chapter 9, Table 9, which tabulates both for steel and PVC conduit, or the impedance figures from the cable manufacturer.
A size returned here is what the tables permit for the conditions you typed. Whether those conditions are the ones the installation will see, and whether the design needs a stamp, is a judgment an engineer and the authority having jurisdiction make, not a calculator.
How to size a conductor to a load and a distance
The example loaded is a 40 A load 140 ft from the panel on 240 V single phase — the case where drop and ampacity give different answers.
Enter the load and the run
Type the current the load draws, in amperes, and the one-way distance from the panel to it. Tick the continuous box if the load runs at its maximum for three hours or more, which multiplies the figure by 1.25 before any table is read. A motor takes its current from NEC Table 430.248 or 430.250 rather than from the nameplate.
Set the metal and the insulation column
Copper or aluminum, then the temperature rating your conductor's insulation actually carries. THHN and XHHW-2 are 90 °C, and the 90 °C column is a legitimate starting point for the corrections — but the terminations field decides the ceiling, because 110.14(C) holds the final figure to the lowest-rated thing the conductor lands on.
Read which limit bound the answer
The result names the one constraint that decided: corrected ampacity, the termination ceiling, the 240.4(D) cap, or the drop budget. Relaxing any of the others changes nothing at all until that one moves, which is the difference between guessing at a size and knowing what to argue about.
Technical specifications
| Sizes searched | 21, from 14 AWG to 1000 kcmil — the rows NEC Table 310.16 is carried for here. 700, 800 and 900 kcmil and everything above 1000 are deliberately absent, so a search that runs past 600 kcmil lands on 750 and over-sizes rather than guessing |
|---|---|
| Limits applied to every candidate | Four: the Table 310.16 value at the insulation rating, the Table 310.15(B)(1) ambient correction, the Table 310.15(C)(1) conductor-count adjustment, and the lower of the 110.14(C) termination ceiling and the 240.4(D) cap |
| Ambient range | Table 310.15(B)(1) runs from 10 °C or less through 85 °C in five-degree bands. Where the ambient reaches the conductor's own rating there is no allowable current, and the page returns nothing rather than a small number |
| Bundling adjustment | 80% at 4–6 current-carrying conductors, 70% at 7–9, 50% at 10–20, 45% at 21–30, 40% at 31–40, 35% at 41 and above. Three or fewer is the basis of the table and takes no adjustment |
| 240.4(D) caps enforced | 15 A on 14 AWG copper, 20 A on 12 AWG copper, 30 A on 10 AWG copper, and 15 A and 25 A on 12 and 10 AWG aluminum. This is why the 90 °C column never puts 14 AWG on a 20 A circuit |
| Voltage drop method | K method, defaulting to 12.9 Ω·cmil/ft for copper and 21.2 for aluminum at 75 °C, both editable. Resistance only — no reactance, no power factor, no raceway material |
| Drop budget default | 3%, the branch-circuit figure from the informational note at NEC 210.19(A). Editable, because it is a recommendation rather than a requirement and project specifications often tighten it to 2% |
| Where the arithmetic happens | In this browser tab. Loads, lengths and nameplate figures are never uploaded or stored anywhere |
Frequently asked questions
Why will it not put 14 AWG on a 20 A circuit when the 90 °C column says 25 A?
Because NEC 240.4(D)(3) caps the overcurrent device on 14 AWG copper at 15 A regardless of what any ampacity column says. The cap is applied after the ambient and conductor-count corrections, and it caps the protective device rather than the conductor — but for a calculator sizing to a load the two come to the same limit. This is the single most common error in wire size calculators on the web: they read 25 A off the 90 °C column and hand back 14 AWG for a 20 A circuit, which is a code violation. The exits are real but narrow: 240.4(E) through (G) send tap conductors, transformer secondaries and motor, air-conditioning, welder and fire-alarm circuits to their own articles, which is why a motor branch circuit on 14 AWG protected well above 15 A is normal and legal.
Should I read the 60, 75 or 90 °C column?
Both, which is why this page asks twice. Your conductor's insulation rating sets the column the correction factors start from, and NEC 110.14(C) sets a separate ceiling at the lowest temperature rating of any termination, device or conductor in the circuit. Most modern breakers and lugs are marked 75 °C, so 90 °C insulation buys you headroom against a hot attic or a full raceway without ever letting the final figure exceed the 75 °C value. Equipment marked 90 °C at its terminals is rare outside specific listings, and assuming it is how an installation ends up over-loaded on paper that looks correct.
Do I apply the 125% continuous multiplier before or after the derating factors?
Before, and to a different number. The 125% belongs to the load: 210.19(A)(1) and 215.2(A)(1) require the conductor to be sized for the noncontinuous load plus 125% of the continuous load, so it scales the amps you are trying to carry. The ambient and bundling factors belong to the conductor and scale the Table 310.16 value in the other direction. They meet in the comparison, and confusing the two is how a 40 A continuous load in a 6-conductor raceway gets sized as though it were 40 A in free air.
Which current does the voltage drop use — the load, or 125% of it?
The load, and this page is explicit about it. Voltage drop is a physical consequence of current flowing through resistance, so it happens at the current that actually flows. The 125% multiplier is a rule about overcurrent device ratings and conductor ampacity, not a prediction that more current will flow. Calculators that apply it to the drop calculation report roughly a quarter more drop than the run really has, which quietly buys a conductor size nobody needed.
My load is in watts, kilowatts or kVA. Where do I enter it?
Convert it first, on one of the pages built for that. This calculator takes amperes because the ampacity tables are written in amperes and because the conversion needs a voltage and a power factor that would otherwise be buried inside a sizing tool. The kW to amps calculator, the kVA to amps calculator and the amperage calculator each handle one of those conversions, and the three phase power calculator handles the balanced three-phase case where the √3 belongs in the power identity rather than in the drop formula.
Does this size the equipment grounding conductor as well?
No — the grounding conductor is sized from the overcurrent device, not from the load. NEC Table 250.122 takes the rating of the breaker or fuse ahead of the circuit and gives a minimum grounding conductor for it, and the upsizing rule applies proportionally when the ungrounded conductors are increased in size for voltage drop. Size the protective device first, then read 250.122 against that rating.
Why does aluminum come back two sizes larger than copper?
Because 1350-H19 aluminum wire is specified at 61.0% IACS conductivity against copper's 100%, so it needs about 1.64 times the cross-sectional area for the same resistance. In gauge terms 1.64 times the area is a little under two steps, and since ampacity and voltage drop both scale with area, both limits move together. The trade-off is real rather than a defect: aluminum feeders are lighter and cheaper per ampere, and the extra size mostly costs raceway.
About sizing a conductor to a load
Sizing a conductor looks like a lookup and is actually an argument between four rules that do not know about each other. NEC Table 310.16 gives an ampacity for three current-carrying conductors at 30 °C, which almost nothing in a real building is: the ambient correction of Table 310.15(B)(1) and the bundling adjustment of Table 310.15(C)(1) both cut it, and they multiply, so six conductors in a 40 °C attic keep 73% of the printed figure. Then 110.14(C) caps whatever survives at the temperature rating of the terminations, which on ordinary breakers and lugs is 75 °C no matter what the wire is rated for. Then 240.4(D) overrides all of it on the four smallest sizes. And none of those four rules has heard of the run length, which is where the fifth constraint comes from.
The fifth is voltage drop, and it is the one that most often decides. Ampacity does not care how far the load is; drop is directly proportional to distance, so a circuit that is comfortable at 40 ft can need two sizes more at 200 ft while the ampacity answer never moves. The 3% figure this page defaults to is a recommendation in an informational note rather than an enforceable rule — NEC 90.5(C) says so explicitly — which is exactly why it is an editable field here. If you want to see the trade on its own, without an ampacity table getting in the way, the voltage drop calculator prints how far each size reaches at a given load, and the wire gauge chart carries the geometry and the raw 310.16 columns behind both pages.
What sizing does not settle is everything at the two ends of the conductor and everything around it. The device that protects it is a separate calculation with its own rounding rules, on the breaker size calculator; whether the conductors physically fit the raceway is Chapter 9 arithmetic on the conduit fill calculator; and whether the service upstream can carry the new load at all is an Article 220 calculation on the electrical load calculator. The case that exercises all of them at once is an EV charger, which is a continuous load at 125%, usually at the far end of a long garage run, and increasingly the reason somebody looks a conductor up at all — what it then costs to run is on the EV charging cost calculator. None of these is a stamped design. They are the tables and the arithmetic, printed so you can check them; the local code amendments, the field conditions and the signature belong to a licensed electrician and the authority having jurisdiction.
Where these load figures go
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
That includes the parts of a job that identify it. A run length, a panel voltage and a load schedule together describe a specific building, and none of the three leaves this tab — there is no server to send them to, because the whole site is static files and the arithmetic is JavaScript.