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The detailed guide below is currently available in English.
Where the formula comes from
Every conductor resists current, and that resistance eats voltage along the run. The circular-mil method starts from the wire's cross-sectional area in circular mils and a material constant K — about 12.9 for copper and 21.2 for aluminum in ohms·cmil per foot. Because a circuit has two conductors out and back, DC and single-phase drops double the one-way loss: VD = 2 × K × I × L ÷ CM. Three-phase systems share the return path across phases, so the factor becomes roughly √3/2 of that.
The 3% and 5% guidelines
The NEC does not mandate a drop limit for most circuits, but its informational notes — and a century of field practice — aim for no more than 3% on a branch circuit and 5% from service to load. Sensitive electronics, motors under start-up strain, and low-voltage lighting feel small drops the most, since on a 12 V system 5% is barely half a volt of headroom.
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Why does length matter so much at low voltage?
Drop is a fixed number of volts per foot of wire, but its impact is relative. A 1.3 V drop is 10% of a 12 V supply yet under 1% at 208 V, which is why 12 V landscape and automotive runs need much heavier wire than mains circuits of the same length.
Should I enter the one-way or round-trip length?
One-way. The formula already counts the return conductor by doubling the factor for DC and single-phase, so enter just the distance from panel to load.
Does aluminum really drop more voltage than copper?
Yes, about 64% more per circular mil, which is why aluminum conductors are usually installed one or two sizes larger than the copper equivalent for the same circuit and length.