NEC FAQ

Common NEC Cable Sizing Questions From First-Time Users

The recurring questions that come up the first time someone opens the NEC calculator — answered directly.

The same handful of questions come up repeatedly from first-time NEC calculator users. Here are direct answers.

"Which temperature column should I use — 60, 75, or 90°C?"

Whichever matches the lowest-rated termination in your circuit, per NEC 110.14(C) — not automatically the highest column just because your conductor's insulation supports it. See the dedicated Table 310.15(B)(16) article for the full explanation.

"Is the NEC's voltage drop limit mandatory?"

Technically an Informational Note, not a numbered rule — but treat it as mandatory unless you have specific confirmation otherwise. See the dedicated article on this distinction.

"AWG or kcmil — which do I use for my size?"

AWG for smaller conductors, transitioning to kcmil (thousand circular mils) for larger ones — the calculator handles this transition automatically based on your required size.

"Why does the tool ask about my termination rating separately from insulation rating?"

Because they're genuinely two different limits, and the lower of the two governs — a distinction covered in depth elsewhere in this series.

Try the NEC Cable Sizing calculator with your own project's numbers.

Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks

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