Cables, Grounding, and Insulation Systems
Ampacity and derating
Conductor ampacity starts with the Table 310.16 value at the reference ambient and is then corrected for ambient temperature and adjusted for the number of current-carrying conductors in a raceway. Continuous loads are sized at 125% of load. A common trap is applying the 125% continuous-load factor and then also forgetting the derating multipliers, or counting a neutral as a current-carrying conductor when it is not (it is, on a wye circuit with nonlinear loads).
Voltage drop
Three-phase: VD = √3 × I × R × L (with R in ohms per unit length). Single-phase: VD = 2 × I × R × L. Feeder plus branch totals are usually held to about 3% and 5%.
Worked example
A 200 A, 480 V three-phase feeder runs 250 ft with 0.052 Ω per 1000 ft. VD = 1.732 × 200 × 0.052 × 0.25 = 4.5 V, or 0.94% - well within limits. Note the √3 for three-phase and the factor of 2 for single-phase; swapping them is the standard distractor.
Grounding and bonding
Separate the grounding electrode system (earth connection, NEC 250.50–250.66) from equipment bonding (250.122). The neutral is a current-carrying grounded conductor; the equipment grounding conductor is not. Ground-fault return path impedance - not earth resistance - determines touch voltage during a fault. For substation grids, IEEE 80 governs step and touch potential, and a measured grid resistance that looks acceptable can still leave unsafe surface gradients.
Insulation
Know insulation resistance testing, the polarization index (R at 10 min / R at 1 min, typically >2.0), and that readings must be temperature-corrected and trended rather than judged against a single absolute value.