Cable Sizing All Codes
A cable has to pass both an ampacity check and a voltage drop check — whichever demands the bigger conductor wins. Here is why skipping either one is a real risk.
Ask a new apprentice how to size a cable and you'll usually hear one answer: "match the wire to the current." That's half right. Every cable sizing method in the Philippine Electrical Code, AS/NZS 3008.1.1, and the NEC actually runs two independent checks, and the final recommended size is whichever check demands the larger conductor.
The cable's derated current-carrying capacity has to meet or exceed the design current. Base ampacity comes straight from the code's rating table for that conductor's material and insulation, then gets reduced by two correction factors:
Derated ampacity = Base ampacity × Temperature factor × Grouping factor Pass condition: Derated ampacity ≥ Design current
Even a cable that easily passes the ampacity check can still fail on voltage drop over a long run. Every extra volt lost in the conductor is a volt — and real energy — the load never receives.
Voltage drop (V) = Resistance factor × Design current × Length / 1000 Percentage = Voltage drop / System voltage × 100
Each code sets its own acceptable limit, typically in the 3–5% range depending on circuit type. A cable that's oversized for ampacity but run 60 meters from source to load can still fail this check outright.
If ampacity says 4mm² is enough but voltage drop demands 6mm² over that length, the cable is 6mm² — full stop. This is exactly why OhmWorks' cable sizing calculators run both checks automatically and tell you which one actually governed the result, instead of just handing you a number with no explanation.
Try it yourself with your own load, length, and installation method in the OhmWorks Cable Sizing calculator.