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5 Free NEC Calculators
Voltage drop, ampacity, box fill, conduit fill, and conduit bending — one app, the worked formula and code reference on every result. Free, no signup, works on your phone.
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NEC has no mandatory voltage-drop limit for most branch circuits. The 3% (branch) / 5% (total) figures are recommendations from the Informational Notes to 210.19(A) and 215.2(A). Circular mils per NEC Ch. 9 Table 8. K = 12.9 (Cu) / 21.2 (Al) ohm-cmil/ft.
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Base ampacity per NEC Table 310.16 (raceway/cable, 30°C). Ambient correction per Table 310.15(B)(1); bundling adjustment per Table 310.15(C)(1) when more than 3 current-carrying conductors. Final selection is still limited by termination rating (110.14(C)) and 240.4 — this estimates ampacity, it does not size the breaker.
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Volume allowances per NEC Table 314.16(B); box volumes per Table 314.16(A). Each conductor entering and terminating = 1 at its size; all clamps together = 1; each device yoke = 2; all equipment grounds together = 1. Non-conductor items use the largest conductor in the box. Assumes one conductor size — for mixed sizes, sum each separately.
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Fill % per NEC Ch. 9 Table 1: one conductor 53%, two 31%, three or more 40%. Conductor areas are THHN/THWN-2 from Table 5; EMT internal areas from Table 4. Assumes one conductor size and EMT only — for mixed sizes sum each from Table 5, and other raceways (RMC, PVC) use their own Table 4 areas.
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