Practical Cable Sizing
A calculated cable size doesn’t always match what’s actually available — here is the right way to handle the gap.
A calculated minimum cable size doesn't always match what your supplier has in stock. Here's how to handle the gap correctly.
If the exact calculated size isn't available, move to the next larger standard size — never the next smaller one, even if the calculated result was only marginally above the smaller size's threshold. A larger conductor is always electrically safe where a smaller one was calculated to be the exact minimum; the reverse is never true.
A larger cable automatically satisfies the ampacity check (it was already the limiting factor being solved for), but it's worth explicitly re-confirming voltage drop still passes too — it will, since a larger conductor has lower resistance and therefore less voltage drop, but confirming this explicitly avoids any doubt on the documentation.
Never substitute a smaller size "because it's close" or because the calculated result was barely above the smaller size's cutoff — safety margins exist precisely to cover real-world variation, and eroding them through a stock-driven compromise defeats their purpose.
If the final installed size differs from the calculator's exact minimum recommendation, note the actual installed size on your project documentation — a small habit that prevents confusion during a future inspection or modification.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks