Voltage drop
Most calculators compute I×R and stop. On AC that understates the drop, so this one uses the full impedance and shows you both.
I × R is the DC answer
On an AC circuit the conductor is not just a resistor. It has inductive reactance, and leaving it out always understates the drop — which is the direction that gets you a phone call.
The drop per amp per unit length is the conductor impedance projected onto the current phasor: R·cos φ + X·sin φ. Two terms, and which one dominates depends entirely on the load.
At a power factor of 0.95 the resistance term carries almost all of it and the simple calculation is close enough. At 0.7 — a feeder to motor load, say — sin φ is over 0.7 and the reactance term becomes a large share of the total. The same is true as conductors get bigger: resistance falls with cross-section while reactance barely moves, so on large feeders reactance takes over.
The result is a failure mode that only shows up on exactly the runs where it matters most: long, large, low power factor. A calculator that ignores reactance is at its least accurate precisely where you needed it to be right.
So this page shows both
The resistance-only figure and the full-impedance figure, side by side, with the reactance share stated as a percentage. Seeing the gap is the point. If it is negligible on your run, you will see that too, and you can stop worrying about it.
The 3% figure is a recommendation, not a rule
It appears in an informational note, and the code says informational notes are explanatory only and are not enforceable as requirements. No run is safe purely by virtue of coming in under it either — motor starting current and voltage-sensitive equipment have their own requirements.
One thing to check before you lean on that: a jurisdiction can adopt an informational note as a mandatory local amendment, and some have. "Not enforceable" describes the national code, not necessarily the rules your inspector is working to.
Plenty of calculators print a red FAIL at 3.01%. That is not what the code says, so this one states in words where the run sits against the 3% branch figure and the 5% combined figure, and leaves the judgement where it belongs.
Where the numbers come from
The AWG areas are a formula, not a table. The gauge system is defined by diameter = 0.005 × 92^((36−n)/39) inches, so every area is arithmetic — 4/0 comes out at exactly 211,600 circular mils. Resistivity is a published physical constant.
Reactance is asked for, not shipped. It depends on conductor spacing and whether the raceway is steel, so there is no single right value, and the published impedance table is copyrighted. It is on your cable data sheet. Leave it blank and you get the resistance-only answer with a warning attached, rather than a silent zero.
Questions electricians actually ask
Why is your answer higher than every other voltage drop calculator?
Because most of them compute I × R and stop, which is the DC answer. On an AC circuit the conductor also has inductive reactance, and the drop is the impedance projected onto the current phasor: R·cos φ + X·sin φ per amp per unit length.
Leaving the reactance out always understates the drop, which is the unsafe direction — the calculator says the run is fine and the equipment at the far end browns out. This page shows both figures side by side so you can see exactly what the omission costs.
When does reactance actually matter?
Two things drive it: conductor size and power factor.
Resistance falls as the conductor gets bigger, but reactance barely changes — so on large conductors reactance becomes a larger share of the total impedance. And the sin φ multiplier grows as power factor falls: at 0.95 the resistance term carries almost everything, while at 0.7 feeding motor load the reactance term can be a large fraction of the drop.
Small conductors at a high power factor: ignore it safely. Large feeders to motor loads in steel conduit: ignoring it is how you get a nuisance call.
Why do I have to supply the reactance myself?
Because there is no single correct value to compute. Reactance depends on the spacing between conductors and on whether the raceway is magnetic — the same cable in steel conduit and in PVC has different reactance. The published impedance table is also a copyrighted compilation.
The figure is on your cable data sheet, and manufacturers publish it freely. Leave the field blank and the tool gives the resistance-only answer and says plainly that it is optimistic, rather than quietly pretending reactance is zero.
Where does the resistance come from, then?
Computed from the conductor cross-section and the metal’s resistivity, corrected to the operating temperature you set. The AWG areas are not looked up in a table either — the gauge system is defined by a formula, so every area follows from arithmetic. 4/0 comes out at exactly 211,600 circular mils.
The figure is for a solid conductor, so a real stranded cable runs a few percent higher: the strands are longer than the cable, and AC current crowds towards the surface. If you have the data sheet resistance, type it in and it will be used instead.
Is 3% a legal limit?
Not in the code itself. The 3% branch circuit and 5% total figures appear in informational notes, and the code states plainly that informational notes are explanatory only and are not enforceable as requirements.
The caveat: a jurisdiction can adopt an informational note as a mandatory local amendment, and some do. So check what your AHJ has actually adopted before you argue the point with an inspector — "it is only a note" is a statement about the national code, not about your county.
That cuts both ways. You do not have to redesign a run sitting at 3.2% because a website printed FAIL in red. And staying under 3% is not a guarantee either — motor starting current and voltage-sensitive electronics have requirements that no single percentage covers.
This tool prints the percentage, states in words where it sits against both the 3% and the 5% recommendation, and stops there. It does not pretend to be an inspector.
Why does three phase drop less than single phase at the same current?
The multiplier on the run length differs. Single phase and DC count the conductor twice, because the current goes out along one and back along the other. Three phase uses √3, because the return path is shared among the other two phases.
So at identical current, length and conductor, the three-phase drop is √3 ÷ 2, about 87%, of the single-phase figure. That is a real effect of the geometry, not a fudge factor.
Does DC use the reactance?
No. Reactance only exists because the current is changing — there is no phase angle on DC, so sin φ is zero and the reactance term vanishes. The tool ignores both the reactance and the power factor when DC is selected, even if you have typed values into them.
For 12 V, 24 V and 48 V battery and off-grid work the resistance-only answer is the correct one, which is also why voltage drop matters so much more there: 0.5 V on a 12 V system is over 4%.
Notes
No table from NFPA 70 is reproduced here. The code itself is free to read from the NFPA. Conductor areas are computed from the AWG definition, resistance from published resistivity and the temperature coefficient, and the drop from the standard impedance relation. Reactance is entered by you.
The computed resistance is for a solid conductor. Stranding adds length and AC current crowds towards the surface, so a real cable measures a few percent higher; the field is overwritable with a data sheet figure for that reason.
The 3% and 5% figures are the recommendations stated in informational notes, which the code itself declares to be explanatory only and not enforceable as requirements — subject to whatever a local jurisdiction has amended. This tool states where the run sits against both figures in words and does not present either as a pass or a fail. Tell me if a figure here disagrees with your own impedance data.