Wire Size Calculator
Minimum conductor size by ampacity and voltage drop, with the NEC table.
About
Wire Size Calculator
Two separate limits decide a conductor size, and the correct answer is whichever demands the larger wire. Checking only one is how a circuit ends up code-compliant and useless, or adequate on paper and overheating in the wall.
Limit one: ampacity
The wire has to carry the current without its insulation exceeding its rated temperature. Those values are published in NEC Table 310.16, and they depend on the conductor material and the insulation rating. Copper THHN at the 75°C column carries 30 amps on 10 AWG; the same gauge in aluminium carries 25.
A detail that trips up people reading the table for the first time: you often cannot use the 90°C column even with 90°C wire. NEC 110.14(C) limits the rating to the lowest-rated component in the circuit, which is usually the terminal on the breaker or device. For most equipment rated 100 amps or less, that means the 60°C column, and for larger equipment the 75°C column. The 90°C rating is used for derating calculations rather than for picking the final ampacity.
Ampacity also gets reduced by conditions. More than three current-carrying conductors in one raceway, high ambient temperature, or a run through insulation all require adjustment factors that lower the usable figure.
Limit two: voltage drop
The wire also has to deliver usable voltage at the far end. Resistance turns some of it into heat along the way, and the loss grows with distance and current.
For a single-phase run:
Vd = 2 × K × I × L ÷ CM
where K is resistivity in ohm-circular-mils per foot (12.9 for copper, 21.2 for aluminium at 75°C), I is current in amps, L is the one-way length in feet, and CM is the conductor area in circular mils. The 2 accounts for the current travelling out and returning. For three-phase, the 2 becomes the square root of 3.
The NEC's guidance sits in an informational note to 210.19(A), recommending 3 percent on a branch circuit and 5 percent for the whole path from service to outlet. Informational notes are not enforceable requirements, which surprises people, but the recommendation exists because the consequences are real: motors run hot and lose torque, LED drivers behave oddly, and heating elements deliver less than their rating.
Which limit wins
Short runs are almost always decided by ampacity. Long runs are almost always decided by voltage drop, and the crossover comes sooner than people expect.
A 20 amp 120 volt circuit run 40 feet in copper is fine on 12 AWG on both counts. The same circuit at 150 feet drops over 6 volts on 12 AWG, more than 5 percent, and needs 10 AWG or larger for the drop alone even though 12 AWG carries the current comfortably. That is the situation the table at the bottom of this page is designed to make obvious.
Low voltage makes it worse, because the same absolute drop is a larger percentage. A 12 volt run losing 1 volt has lost more than 8 percent; a 240 volt run losing 1 volt has lost 0.4 percent. This is why solar, RV and landscape lighting wiring uses conductors that look absurdly large for the current.
Copper or aluminium
Aluminium carries roughly 61 percent of the current of copper for the same area and has 1.64 times the resistivity, so an aluminium conductor is typically two AWG sizes larger for the same job. It is also markedly cheaper by weight, which is why service entrance and feeder cables are commonly aluminium while branch circuits are copper.
Aluminium needs different practices: terminals listed for aluminium (marked AL or CU-AL), antioxidant compound on the connection, and correct torque. The problems aluminium had in mid-century residential branch wiring came largely from connections rather than the metal.
Continuous loads
A load running three hours or more continuously must be sized at 125 percent of its rating, per NEC 210.19(A)(1). A 16 amp continuous load is therefore a 20 amp circuit, not a 16 amp one. Lighting, EV charging and heating are the everyday cases.
What this calculator is for, and what it is not
It is a planning tool, useful for checking whether a run is likely to need a larger conductor before you buy the cable or dig the trench.
It is not a design. Conduit fill, ambient temperature correction, bundling adjustments, breaker coordination, grounding and bonding, and the specific requirements of your local amendments all matter, and none of them are here. Electrical work is inspected because getting it wrong causes fires. Have the design checked and the work signed off by a licensed electrician.
The ampacity figures used here are the widely published NEC Table 310.16 values. Where your local code differs, or a newer edition applies, the local code governs.
Common questions
Frequently asked questions
12 AWG copper for a typical short run, per NEC Table 310.16. On a long run the voltage drop rather than the current decides it: at 150 feet on 120 volts, a 20 amp circuit needs 10 AWG to stay inside the recommended 3 percent drop.
6 AWG copper at the 75°C column carries 65 amps, so it covers a 50 amp circuit for ampacity. Check the voltage drop as well: a long run to a workshop subpanel often pushes it to 4 AWG.
Vd = 2 × K × I × L ÷ CM for single phase, where K is 12.9 for copper or 21.2 for aluminium, I is amps, L is the one-way distance in feet and CM is circular mils. The 2 accounts for the return leg. Three phase uses the square root of 3 instead of 2.
The NEC recommends 3 percent on a branch circuit and 5 percent overall, in an informational note to 210.19(A). It is guidance rather than an enforceable requirement, but exceeding it means dim lighting, motors running hot and heaters underperforming.
NEC 110.14(C) limits the circuit to the lowest-rated component, which is usually the terminal on the breaker or device. For most equipment at 100 amps or less that is the 60°C column. The 90°C rating is used for derating calculations, not for the final ampacity.
Yes, when used with terminals listed for it, antioxidant compound and correct torque. Aluminium is standard for service entrances and feeders. Its poor reputation comes from mid-century branch circuit installations where the connections, not the metal, were the problem.
Typically two AWG sizes for the same job. Aluminium carries about 61 percent of the current of copper for the same area and has 1.64 times the resistivity.
Very often, yes. Short runs are decided by ampacity, long runs by voltage drop, and the crossover arrives sooner than expected. Low voltage systems such as 12 volt landscape lighting are almost always decided by drop.
A load expected to run for three hours or more without interruption. NEC 210.19(A)(1) requires sizing at 125 percent of its rating, so a 16 amp continuous load needs a 20 amp circuit. Lighting, EV charging and electric heat are the usual examples.
No. It is a planning tool for checking whether a run will need a larger conductor. Conduit fill, ambient temperature, bundling, breaker coordination, grounding and local amendments all affect the answer and are not included. Have the work designed and inspected by a licensed electrician.
Method
How this is worked out
Before you act on this: This is a planning figure, not a circuit design. Conduit fill, ambient temperature, the number of conductors sharing a raceway and continuous-load derating all change the answer, and local amendments override the national tables. Have the design checked and the work inspected by a licensed electrician: undersized conductors start fires.
The formula on this page follows:
- NFPA 70 National Electrical Code, Table 310.16 — Conductor ampacity by insulation rating
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