The methodology behind cable sizing, voltage drop, PV/solar DC sizing, and earthing — the same four calculations, applied under three different electrical codes.
Every calculator in OhmWorks follows the same underlying engineering logic, whether you're working under the Philippine Electrical Code, AS/NZS Standards, or the NEC. What changes between codes is the reference data — ampacity tables, derating factors, safety margins, and grounding conductor sizing tables — not the physics. This page walks through how each calculation actually works.
A cable has to satisfy two independent requirements simultaneously, and the larger of the two governs the final size:
The cable's derated ampacity must meet or exceed the design current. Base ampacity comes from the applicable code's current-rating table for the conductor's material (copper or aluminium), insulation type, and installation method. That base figure then gets reduced by two correction factors:
Derated ampacity = Base ampacity × Temperature factor × Grouping factor Pass condition: Derated ampacity ≥ Design current
Independently of whether a cable can safely carry the current, it also has to deliver that current without an excessive voltage loss along its length. Every extra volt lost in the cable is a volt (and, cumulatively, real energy) the load never receives.
Voltage drop (V) = (mV/A/m or resistance figure) × Design current × Length / 1000 Percentage = Voltage drop / System voltage × 100
Each code sets its own acceptable voltage drop limit (commonly around 3–5% depending on circuit type and code). Whichever of the two constraints — ampacity or voltage drop — requires the larger cable size determines the final recommendation. OhmWorks shows you which one governed, so you understand why a particular size was recommended, not just what the size is.
Separately from cable sizing, OhmWorks includes a standalone voltage drop calculator for when you already know the cable size and just need to verify it against length, current, and load — useful for checking an existing installation, a supplier-recommended size, or a design constraint that isn't purely "pick the minimum size."
DC string and array cabling has its own sizing logic, built around the module's short-circuit current (Isc) rather than a simple load current:
Minimum cable ampacity ≥ 1.25 × Iₛₛ (a common safety-margin multiplier used across PEC, AS/NZS 5033, and the NEC's Article 690, accounting for the fact that a PV module can briefly produce more than its rated Iₛₛ under high-irradiance conditions)
That ampacity requirement then gets checked against temperature-derated cable ratings, exactly like AC cable sizing above — DC cable on a hot rooftop needs the same kind of temperature correction as AC cable in a hot ceiling space. Voltage drop is checked too, typically against a tighter limit than AC circuits (commonly 1–3%), since DC string voltage drop directly reduces the energy actually reaching the inverter.
Two related but distinct checks live under this calculator:
Zs = Ze + R1 + R2 Ze = external earth loop impedance (supply transformer to the installation) R1 = resistance of the phase/active conductor R2 = resistance of the circuit protective (earth) conductor
The resulting Zs is checked against the maximum permitted value for the installed protective device (breaker or fuse) and its required disconnection time — the lower the Zs, the faster and more reliably the device clears a fault before it becomes a shock or fire hazard.
Rather than a fixed fraction of the active conductor, codes specify minimum earth/ground conductor sizes via a lookup table keyed to the active conductor's size (or, for the NEC, the upstream overcurrent device rating). OhmWorks looks up the correct minimum size from the applicable code's own table rather than approximating it with a ratio.
This one sits a layer above pure cable/electrical calculations — it's a system-level sizing and financial estimator, not a code-compliance check:
Daily Energy (kWh) = System kWp × Peak Sun Hours × System Efficiency Monthly Savings = Daily kWh × 30 × Electricity Rate Payback Period (years) = Total System Cost ÷ Annual Savings
Panel and inverter counts are derived using a standard 1:1.3 DC-to-AC oversizing ratio, a common industry practice for improving production during non-peak sun hours.
Several jurisdictions (the NEC most explicitly, via Article 210/215) require a 1.25× safety margin be applied to continuous loads before sizing conductors and overcurrent protection — the same principle behind the PV 1.25×Isc rule above, applied more broadly to any load expected to run at or near full current for three hours or more. Where a code requires this, OhmWorks applies it before the ampacity check, not after.