The same method the Texas Journeyman exam tests: VD = K × I × L ÷ circular mils. Free, no signup.
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NEC has no mandatory voltage-drop limit for most branch circuits. The 3% (branch) and 5% (total) figures are recommendations from the Informational Notes to 210.19(A) and 215.2(A). Conductor circular mils per NEC Chapter 9, Table 8. K = 12.9 (copper) / 21.2 (aluminum) ohm-cmil/ft, the standard exam constant.
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