Cable Size and Ampacity Estimator: AWG and kcmil
Cable Size and Ampacity Estimator
A first pass at conductor sizing: enter the load current, pick the material and the insulation temperature rating, and it returns the smallest size whose table ampacity covers the load, with the next size up shown for comfort.
The derating factor multiplies the table ampacity down, so 0.8 models a bundle correction: a conductor rated 100 A carries 80 A after derating, and the tool sizes against that. Terminations are commonly rated 75 C even when the insulation is 90 C, which is why the 75 C column is the default and often the one that governs.
What this tool is, and what it is not
This is an estimation tool for a first pass at conductor sizing: it takes a load current, a material and an insulation temperature rating, and returns the smallest size whose table ampacity covers the load, with the next size up alongside. It exists so that a site conversation about whether a feeder is roughly a 3 AWG problem or a 4/0 problem can happen with real numbers in it.
It is not a design authority. Final conductor sizing belongs to the applicable electrical code, the CEC in Canada or the NEC in the US, applied with all its correction factors by a licensed electrician or electrical engineer. The tool says this on its face, and the article will keep saying it, because the gap between a table lookup and a code compliant design is exactly where electrical work goes wrong.
How to use it
Enter the load current, choose copper or aluminium and the insulation rating, and optionally apply a derating factor below 1 to model bundling or ambient corrections. The tool divides your load by the derating factor and sizes against that, so a 100 A load at a 0.8 factor is sized as a 125 A requirement, which is the direction the correction is supposed to work.
The table behind it reflects the familiar code ampacities for not more than three current carrying conductors at 30 C ambient, copper and aluminium, in the 60, 75 and 90 C columns, from 14 AWG to 500 kcmil.
Why the 75 C column is the default
Insulation ratings and termination ratings are different things. Modern 90 C insulated conductors are everywhere, but the lugs and breakers they land on are commonly rated 75 C, and the circuit is limited by its coolest rated link. That is why the 75 C column governs so often in practice, and why this tool defaults to it. The 90 C column mostly earns its keep as the starting point for derating calculations, not as a bigger number to size against directly.
Aluminium runs about one to two sizes larger than copper for the same load, which the tool shows directly: a 100 A load lands on 3 AWG in copper at 75 C and on 1 AWG in aluminium. Neither is better in the abstract; aluminium trades size for cost and weight, and its terminations demand more care.
A worked check you can repeat on paper
A 100 A load, copper, 75 C: the table reads 14 AWG at 20 A up through 3 AWG at exactly 100 A, so 3 AWG is the smallest size that covers it, with nothing to spare. Push the load to 101 A and the answer steps to 2 AWG at 115 A. That knife edge at exactly 100 A is worth noticing: a continuous load, which codes treat at 125 percent, would already have pushed past 3 AWG, which is one of several reasons the table answer is a starting point and not a design.
Mistakes that catch people out
Sizing to the 90 C column with 75 C terminations. The circuit is limited by the lugs. Fix: know the termination ratings; when unsure, 75 C.
Ignoring the continuous load factor. Loads running 3 hours or more are treated at 125 percent. Fix: this tool does not apply it; the code and the electrician do.
Forgetting voltage drop entirely. Ampacity keeps the wire from overheating and says nothing about the voltage arriving. Fix: on long runs, the voltage drop tool will frequently demand a bigger conductor than the ampacity table does.
Treating this or any online table as an installation instruction. Fix: it is an estimate for planning conversations. The person who signs the work sizes the wire.
How this was checked
The table was spot checked against the published code values, copper 75 C at 12 AWG reading 25 A and at 4/0 reading 230 A, copper 90 C at 6 AWG reading 75 A, aluminium 75 C at 1/0 reading 120 A. The selection logic was verified at the boundaries: exactly 100 A picking 3 AWG copper, 101 A stepping to 2 AWG, and the same load in aluminium picking 1 AWG. Loads beyond the 500 kcmil table entry are refused with a pointer to parallel conductor design rather than extrapolated.
Related Calculators and Reference
Ampacity is only half of sizing: the Voltage Drop Calculator covers the half that long runs care about, and on those runs it usually governs. The load current for three phase equipment comes from the Three Phase Power Calculator, and the underlying V equals I R is the Ohm Law Calculator. The full reference behind this estimator, including every AWG size, the derating factors and the termination rule that decides which ampacity column you are allowed to use, is the Wire Gauge Chart. Also see the Box Fill Calculator: whether the box at the end of the cable is big enough for the conductors, devices and clamps in it.