Wire Size Calculator

How to choose the right wire size

Wire is sized so that it never gets hotter than its insulation allows. The current a conductor can carry without overheating is called its ampacity, and it depends on the metal, the size, the insulation temperature rating, and the conditions it runs in. This calculator walks through that in the same order an installer would on paper: find the load, add the continuous-load margin, apply derating, pick the size, then check voltage drop.

Step 1: Find the load current

If you know the current, enter it directly. If you only have power in watts, switch the input to watts and enter the supply voltage; the calculator divides power by voltage. For motors, heaters and cameras with IR lights, use the maximum running current from the nameplate or datasheet, not an idle figure.

Step 2: Continuous loads need 125%

A load that runs for three hours or more counts as continuous. The conductor and its overcurrent device must be sized for 125% of that load. A 16 A continuous load therefore needs a conductor with at least 20 A of usable ampacity. Tick the box and the calculator does the multiplication for you.

Step 3: Pick the temperature column

Table 310.16 lists three ratings: 60°C, 75°C and 90°C. The conductor can only be used at the lowest rating of anything it connects to. Most breakers and lugs are rated 60°C or 75°C, so 75°C is the safe default for most circuits. The 90°C column is mainly useful as a starting value when you have heavy derating, because the derating is applied to the higher number.

Step 4: Apply derating

Table 310.16 assumes a 30°C ambient and no more than three current-carrying conductors sharing a raceway. Hotter surroundings and crowded conduit both make it harder for a wire to shed heat, so its allowable current goes down. The calculator multiplies the table value by an ambient factor and a bundling factor.

Step 5: Small-conductor limit

Even when the table ampacity looks higher, the NEC caps overcurrent protection for the smallest sizes. Copper 14 AWG is limited to 15 A, 12 AWG to 20 A and 10 AWG to 30 A (aluminum 12 AWG to 15 A, 10 AWG to 25 A). The calculator will not choose a size that breaks this rule, which is why a 20 A circuit lands on 12 AWG and not on 14 AWG even though 14 AWG has a 75°C table value of 20 A.

Step 6: Check voltage drop on long runs

A wire that is safe for heat can still be too thin for a long run, because resistance grows with length and the load ends up with less voltage. Many designers aim for about 3% on a branch circuit and 5% overall, but your equipment datasheet or local rules come first. Enter the one-way length and the calculator steps up to a thicker size until the drop is within your limit. For AC power factor, conduit effects, or three-phase work, use the full Voltage Drop Calculator.

Worked examples

Example 1: 20 A circuit, copper, 75°C

Load 20 A, not continuous, 30°C, three conductors. Required ampacity is 20 A. 14 AWG has a 75°C value of 20 A but is limited to 15 A of protection, so the result is 12 AWG (25 A table value).

Example 2: 20 A continuous, hot space, long run

Load 20 A continuous, 40°C ambient, six conductors, 150 ft at 120 V, 3% allowed drop. Required is 25 A. The derating factor is 0.88 × 0.80 = 0.70, so 8 AWG (50 A at 75°C) gives about 35 A and passes on heat. At 150 ft it drops more than 3%, so the calculator moves up to 6 AWG, which lands at about 2% drop.

Common mistakes

• Forgetting the 125% rule on loads that run for hours, such as lighting, heaters and EV chargers.
• Using the 90°C column to size the circuit when the breaker or terminal is only rated 60°C or 75°C.
• Counting only the circuit you are adding and ignoring other current-carrying conductors in the same conduit.
• Using the round-trip length instead of the one-way length for voltage drop.
• Ignoring a hot attic, roof run or sun-exposed conduit where the ambient is well above 30°C.

Frequently asked questions

How do I choose the right wire size?

Work out the load current (multiply by 1.25 for continuous loads), then pick the smallest conductor whose derated ampacity is at least that value. Derating covers high ambient temperature and more than three current-carrying conductors in one raceway. Finally, check voltage drop on long runs and move up a size if it is too high.

What is the difference between the 60°C, 75°C and 90°C columns?

They are the insulation temperature ratings in NEC Table 310.16. Most breakers and terminals are rated for 60°C or 75°C, so the 90°C column is normally used only as a starting value for derating, not for sizing the circuit on its own. If you are unsure, choose 75°C.

Why does the calculator use 125% for continuous loads?

A continuous load runs for three hours or more. The NEC requires the conductor and overcurrent device to be sized for 125% of that load, which is why the checkbox multiplies your current by 1.25.

Does this include voltage drop?

Yes, optionally. Enter the supply voltage and one-way length and the calculator checks the drop for the ampacity-based size, then steps up to a thicker size until the drop is within your limit. It uses DC resistance at 20°C for a two-wire run.

Can I use this for aluminum wire?

Yes. Aluminum has a lower ampacity than copper for the same size and no 14 AWG row, so the calculator starts at 12 AWG for aluminum.

Is this a substitute for the electrical code?

No. It is a planning aid based on NEC Table 310.16 for conductors in raceway or cable at 30°C ambient. Always confirm sizing against the code edition and local rules that apply to your job.

Related tools

After sizing the wire, run the full Voltage Drop Calculator, check enclosure space with the Electrical Box Fill Calculator, or plan low-voltage camera runs with the PoE Budget Calculator. For a plain-language walkthrough, read How to Choose the Right Wire Size.

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