PEC Cable Sizing
A concrete walkthrough of sizing a branch circuit cable under the Philippine Electrical Code — from design current to the final, derated recommendation.
Formulas are easier to trust once you've walked through a real example. Here's how a branch circuit cable actually gets sized under the Philippine Electrical Code, using the same logic as the OhmWorks PEC Cable Sizing calculator.
| Parameter | Value |
|---|---|
| Load current | 25 A, continuous |
| Installation method | Conduit, 3 current-carrying conductors |
| Ambient temperature | 35°C |
| Run length | 22 meters, one-way |
| System voltage | 230V, single-phase |
For a continuous load, a safety margin is applied before anything else. This protects the conductor from a load that runs at or near full current for hours at a time, not just an instantaneous peak.
The applicable PEC ampacity table gives a base current rating for each conductor size, keyed to insulation type and installation method — this is the starting point before any derating is applied.
Two corrections get multiplied against that base rating: an ambient temperature factor (since 35°C is above the table's standard reference), and a grouping factor (since three current-carrying conductors share the conduit). The result is the conductor's real, derated ampacity in this specific installation — usually meaningfully lower than the table's headline number.
Separately, the 22-meter run length gets checked against the code's voltage drop limit at the design current. On a longer run like this, voltage drop can easily overtake ampacity as the limiting factor — which is exactly why both checks matter, not just the one that's easier to eyeball.
Whichever check — derated ampacity or voltage drop — demands the bigger conductor determines the final size. This is the same four-step logic running behind every result in the calculator.