A practical rule of thumb can help estimate the size of a neutral conductor based on the overcurrent protection device and phase conductor size. 15 (E), harmonic-load checks, and worked residential plus commercial examples. Neutral conductor sizing looks simple until a project mixes 120V branch circuits, 120/240V split-phase feeders, or 208Y/120V. However, in systems with non-linear loads, the neutral conductor size should be equal to or larger than the phase conductor, depending on the level of harmonic distortion. Let's consider a three-phase 4-wire.
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