Cable Sizing Practical
Undersized-by-eye estimates, forgotten derating, and mismatched termination ratings — the recurring mistakes that turn up in real installations.
Cable sizing theory is straightforward on paper. In the field, under time pressure, with a supplier who only stocks certain sizes, the same handful of mistakes keep showing up across projects and across codes.
A cable size that worked on a similar-looking job last month isn't a substitute for running the actual numbers on this job. Load current, run length, installation method, and ambient conditions all vary enough between projects that "it was fine last time" is a genuinely risky habit, not just an inefficient one.
Base ampacity tables assume a standard reference ambient temperature — commonly 30°C. A cable run through a hot ceiling space, a rooftop conduit, or a tropical outdoor installation regularly exceeds that reference significantly, and skipping the temperature correction factor means using a capacity figure the cable doesn't actually have in that real installation.
A single cable rated at a given ampacity in free air has meaningfully less capacity once it's bundled with several other current-carrying conductors in the same conduit or tray — they heat each other. Sizing as if each cable were alone in its own raceway, when it isn't, is one of the most common silent undersizing mistakes.
Covered in depth in the NEC ampacity article: using a 90°C insulation-rating column's higher ampacity figure when the actual terminations (breakers, lugs) are only rated for 60°C or 75°C conductors. The conductor's insulation rating and the termination's rating are two separate limits, and the lower of the two governs.
A cable that comfortably passes the ampacity check can still fail voltage drop on a sufficiently long run — and it's the check that's easiest to skip when time is tight, since it requires a separate calculation rather than a quick table lookup. Long runs to a detached structure, a remote pump, or a rooftop array are exactly where this omission shows up.
Run your actual load, length, and installation method through the OhmWorks Cable Sizing calculators rather than estimating.