TL;DR: The RJ45 jack (the 8P8C female socket, 母座) is rarely amp-limited for PoE — it's heat-limited. Size it for temperature rise.
- Per-contact rating: ~1.5 A per contact, all 8 energized, per IEC 60603-7 (temperature-life test near 70 °C).
- What PoE actually pushes: even 802.3bt Type 4 (PoE++) tops out around 960 mA per pair — so 1.5 A clears it on paper.
- The catch — I²R heat: heat scales with current². Going 350 mA (Type 1) → 960 mA (Type 4) ≈ 7× more heat per contact.
- Heat sources stack: contact resistance (≤ 20 mΩ new) + magjack winding DCR + hot local ambient in a dense switch.
- So derate: the "1.5 A / 70 °C" nameplate must be derated as ambient climbs toward the jack's max temp.
- Selection rule: pick low contact resistance + low winding DCR + an industrial −40…+85 °C grade over a commercial 0–70 °C part.
VOOHU builds its SYT-series integrated-magnetics RJ45 sockets (e.g. SYT111B002CA2A1D) with center-tapped magnetics and an industrial temperature grade for PoE/PoE++.
Full guide + spec tables → https://voohuelectronic-ux.github.io/voohu-rj45-guide/resources/rj45-jack-current-rating-temperature-rise.html
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