How to size an APFC panel
An Automatic Power Factor Correction panel adds capacitor steps as load changes so the plant runs near unity power factor. Sizing it right avoids both penalties and over-correction.
Step 1 — find the kVAr needed
Required kVAr = kW × (tanφ1 − tanφ2), where φ1 is the present power factor angle and φ2 the target.
Example: 500 kW load at 0.80 PF, target 0.99. tanφ1 = 0.750, tanφ2 = 0.142. Required = 500 × 0.608 ≈ 304 kVAr, rounded up to the next standard size: a 325 kVAr panel.
Step 2 — choose the steps
Split the total into steps so the relay can follow the load closely, for example 325 kVAr as 1×25 + 2×50 + 2×100. The smallest step should be smaller than the plant's lightest running reactive load.
Step 3 — check harmonics
VFDs, UPS, LED lighting and furnaces create harmonics that can overload plain capacitors. Where non-linear load is significant, use detuned reactors (typically 7% or 14%) with capacitors rated for the higher voltage, usually 480 V or 525 V on a 415 V system.
Step 4 — choose the switching
- Contactor switching — for steady loads such as pumps, compressors and lighting.
- Thyristor (real-time) switching — for fast-changing loads such as welding, cranes and press lines.
Where the DISCOM bills on kVAh, every point of power factor lost shows up directly in the bill, so a correctly sized APFC panel usually pays back quickly.
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