Circuits
- Voltage Drop CalculatorPick a wire size and the calculator looks up its resistance for you, then says whether the drop …
- Wire Size CalculatorEnter the load and the distance. This checks ampacity and voltage drop separately and tells you …
- Watts, Amps and Volts CalculatorConvert watts to amps or back, single or three phase, and get the breaker size including the con…
- Ohm's Law CalculatorEnter the two values you know, choose what to solve for, and the formula used is shown with the …
Raceway
- Conduit Fill CalculatorPick the conduit and the conductors. This looks up both areas and applies the right fill limit f…
- Conduit Size CalculatorTell it what you are pulling and it finds the smallest conduit of that type which passes the fil…
- Box Fill CalculatorCounts conductors, devices, grounds and clamps the way NEC 314.16(B) requires, then checks the t…
Reference
- Wire Ampacity ChartNEC Table 310.16 for copper and aluminium, all three temperature columns, with the small-conduct…
- Wire Resistance ChartNEC Chapter 9 Table 8 resistance for copper and aluminium, which is the number every voltage-dro…
- Conduit Size ChartInternal area and usable fill for every common conduit type and size, from NEC Chapter 9 Table 4…
- Voltage Drop ChartHow far you can run each wire size at a given current before the 3% recommendation is exceeded.…
The tables are built in
Most electrical calculators online ask you for the conductor’s resistance per thousand feet, or the cross-sectional area of the wire, or the internal area of the conduit. Those numbers live in NEC Chapter 9 Tables 4, 5 and 8 — and if you already have the code book open at the right page, you can do the arithmetic yourself. The lookup is the work.
So these calculators hold the tables. Pick “12 AWG copper” and it knows 1.93 ohms per 1,000 ft. Pick “1/2 inch EMT” and it knows 0.304 square inches. Pick “THHN” and it knows 0.0133 square inches at 12 AWG.
And every limit is checked
A fill percentage with no limit beside it is not an answer. Neither is a voltage drop. Each calculator states the code limit that applies to your specific case and says plainly whether you pass:
- Conduit fill against NEC Chapter 9 Table 1 — 53% for one conductor, 31% for two, 40% for three or more. The two-conductor case is the one people get wrong, and it is the tightest of the three.
- Voltage drop against the 3% branch-circuit recommendation, with the correct √3 multiplier for three phase rather than the 3 that inflates the answer by 73%.
- Wire size against both ampacity and voltage drop, with NEC 240.4(D) applied — so it will never tell you 12 AWG copper is fine for 25 amps.
- Box fill against NEC 314.16, counting device yokes at two volumes and grounds and clamps once each, at the largest conductor present.
What this site is not
It is reference data and planning arithmetic. It is not a design, not an inspection, and not a licensed electrician. It does not apply ambient temperature correction, conductor bundling adjustment, motor sizing rules, or your jurisdiction’s local amendments — any of which can change the answer, usually upward.
Electrical work is dangerous and regulated. Where a result here matters, check it against the NEC edition your jurisdiction has actually adopted, and where the work is beyond your competence or needs a permit, have a licensed electrician do it.