Derating Power Based on DC-Bias Capability
In the AF standard
Using a transformer that can only tolerate this amount of DC bias reduces the maximum current the PSE can deliver without saturating the transformer to: 350 mA × (8 mA/10.5 mA) = 267 mA.
We can see that the standard is derating the maximum power so that the DC bias from PoE stays within the capability of the transformer. It is not adding BLW-based DC bias (8 mA) to the number from PoE (10.5 mA). For supporting 350 mA of PoE with 3 percent imbalance, theoretically, we would need a transformer with a DC bias capability of 10.5 mA + 8 mA = 18.5 mA. So a transformer with 8-mA bias is theoretically no good for any amount of PoE at all (zero power). The only reason we get away with ...