Category primers covering the underlying engineering — core concepts, the standards landscape, and how each category's calculators fit together — 10 so far, growing toward one per category. For step-by-step numeric problems, see Worked Examples instead.
How overcurrent, differential and arc-flash protection actually fit together — the core theory, the standards behind it, and a sensible order to work through this category's 9 calculators.
Read the guide →Why a cable has to pass two independent checks to be correctly sized, how MV cables are sized completely differently from LV ones, and why fill and pulling-tension limits matter just as much as the electrical sizing itself.
Read the guide →Why a low earthing resistance doesn't automatically mean a safe touch voltage, how the rolling sphere method actually works, and why bonding resistance and spark energy are two different hazards that both need checking.
Read the guide →Why CTs and VTs behave like opposite kinds of electrical sources, how a transformer's rating changes with ambient and altitude, and what separates sizing new equipment from assessing the condition of equipment already in service.
Read the guide →Why fixing power factor can introduce a new harmonic problem, how demand-factor sizing accounts for diversity, and why utility tariffs are the one part of this category with no universal standard behind them.
Read the guide →Why a motor circuit needs two genuinely different protective devices doing two different jobs, how overload relay trip class is chosen, and why VFD energy-savings estimates are deliberately conservative.
Read the guide →Why backup power is really two different technologies covering two different timescales, how NFPA 110's Type and Class ratings describe two independent things, and why nameplate battery capacity always overstates what's actually usable.
Read the guide →Why 4-20mA loops deliberately never use zero for zero, how the intrinsic safety entity concept lets you combine certified components without a full system drawing, and why thermocouples and RTDs fail in completely different ways.
Read the guide →Why a PV string has to clear three genuinely independent inverter constraints at once, and why an EV charge point's earthing system can outright disqualify a design rather than just derate it.
Read the guide →Why this category's calculators often need a non-electrical answer — heat transfer, photometry, mechanics, or economics — before the electrical sizing question can even be asked.
Read the guide →