A network port fails on power long before it fails on data. As Power over Ethernet climbs from 15 W to 90 W, the humble RJ45 jack — the 8P8C female socket soldered to your board — becomes a current-carrying part, not just a data doorway. The question every hardware engineer eventually asks is simple: how much current can an RJ45 jack really carry, and when does it get too hot?
Short answer: A standard RJ45 (8P8C) jack built to IEC 60603-7 is rated at roughly 1.5 A per contact with all eight contacts energized (measured near 70 °C ambient). Even the most demanding PoE tier, IEEE 802.3bt Type 4 (PoE++), pushes only up to about 960 mA per pair — so the amp rating is rarely the wall. The real limit is temperature rise: I²R heating in the contacts, plus winding DC resistance in an integrated-magnetics jack, adds to the enclosure's ambient. Pick a jack for its thermal headroom and contact resistance, not just its nameplate current.
Three linked properties decide how much current a female RJ45 socket can carry safely:
1. Contact current rating. IEC 60603-7 — the detail specification for 8-way modular connectors — defines the current a single contact may carry continuously with every contact loaded. For unshielded and shielded RJ45 jacks this is commonly 1.5 A per contact, verified by a temperature-life test at elevated ambient. That figure assumes the worst case where all eight contacts carry current simultaneously.
2. Contact resistance. Every mated contact has resistance — typically specified at ≤ 20 mΩ on a new IEC 60603-7 jack. Current through that resistance dissipates power as heat (P = I²R). Low, stable contact resistance is what keeps a high-current jack cool, and it depends directly on contact material and gold plating thickness.
3. Temperature rise. The current rating is really a thermal statement in disguise. The self-heating from I²R losses raises the contact temperature above the surrounding air; the connector's total temperature (ambient + rise) must stay within its rated operating range. This is why the same jack carries more current in a cool, open chassis than in a hot, sealed, high-port-density switch.
These are representative catalogue values for a quality 8P8C jack. Always confirm against the specific part datasheet.
| Parameter | Typical value | Reference / basis |
|---|---|---|
| Current rating (per contact) | ≈ 1.5 A | IEC 60603-7, all 8 contacts energized |
| Contact resistance | ≤ 20 mΩ | IEC 60603-7 low-level test |
| Insulation resistance | ≥ 500 MΩ (at 500 VDC) | IEC 60603-7 |
| Dielectric withstanding voltage | 1000 V AC rms, 1 min | IEC 60603-7 (contact-to-contact) |
| Durability (mating cycles) | ≥ 750 cycles | IEC 60603-7 minimum |
| Operating temperature | −40 °C to +85 °C (industrial) | VOOHU SYT-series integrated RJ45 |
To size a jack you compare its per-contact rating against the current each PoE tier drives through the pairs. The values below are the nominal per-pair currents defined by the IEEE 802.3 PoE amendments.
| PoE tier | Standard | PSE power | Pairs used | Max current / pair |
|---|---|---|---|---|
| Type 1 (PoE) | 802.3af | 15.4 W | 2 | ~350 mA |
| Type 2 (PoE+) | 802.3at | 30 W | 2 | ~600 mA |
| Type 3 (PoE++) | 802.3bt | 60 W | 4 | ~600 mA |
| Type 4 (PoE++) | 802.3bt | 90 W | 4 | ~960 mA |
The takeaway: a jack rated ≈ 1.5 A per contact clears even Type 4's ~960 mA per pair on paper. But note how the demand jumped almost 3× from Type 1 to Type 4. A socket that was comfortable at 350 mA in a 15 W design now dissipates roughly seven times more heat per contact (heat scales with I²), which is exactly where under-specified jacks fail. For the full pin map of which contacts carry that current, see the RJ45 jack pinout for PoE.
Because heating scales with the square of current, the move to 4-pair PoE++ is a thermal problem first. Three heat sources stack inside a modern RJ45 jack:
Contact I²R. At 960 mA through a 20 mΩ contact, each contact dissipates about 18 mW; across eight loaded contacts that is on the order of 0.15 W of heat generated right at the connector interface — modest alone, but it never sits alone.
Magjack winding DCR. An integrated-magnetics jack (magjack) routes PoE current through the center taps of its transformers. The winding DC resistance adds its own I²R loss inside the connector body, where there is little airflow. This is why PoE++ magjacks use heavier center-tap wire than data-only parts.
Ambient stacking. The rated 1.5 A / 70 °C figure already assumes a warm environment. Inside a fan-limited PoE switch with dozens of powered ports side by side, the local ambient can be well above room temperature before the jack adds its own rise — so the usable current must be derated as ambient climbs toward the connector's maximum rated temperature.
Practically, this means two jacks with the same "1.5 A" nameplate can behave very differently: the one with lower contact resistance, lower winding DCR, an industrial −40 to +85 °C grade and better internal heat-spreading will hold full PoE++ current where a commercial 0–70 °C part throttles or degrades. Durability matters too — as plating wears over the rated mating cycles, contact resistance rises and so does the heat.
VOOHU manufactures RJ45 jacks / female modular sockets (母座) — standard 8P8C sockets and integrated-magnetics (magjack) types with built-in transformers and common-mode chokes — in 1×1, 1×N and 2×N sink-type modules, with 90° and 180° mounting and speed support from 100M through 10G and HDBaseT. For PoE and PoE++ designs, VOOHU's SYT-series integrated RJ45 sockets (for example SYT111B002CA2A1D and SYT111B002FA2A1D) provide center-tapped magnetics for phantom-power injection and are graded to an industrial −40 °C to +85 °C operating range, giving thermal margin exactly where high-power ports run hottest. Because VOOHU controls both the connector and the magnetics, the winding DCR and contact plating are matched to the target PoE class rather than left to chance. If you are choosing between a magjack and a discrete jack plus external transformer, the integrated-magnetics guide walks through the trade-offs, and multi-gig links add their own constraints in the 2.5G/5G RJ45 jack guide.
A standard RJ45 (8P8C) jack built to IEC 60603-7 is typically rated at about 1.5 A per contact with all eight contacts energized, at an ambient near 70 °C. That gives comfortable headroom over Power over Ethernet: even IEEE 802.3bt Type 4 (PoE++) draws up to only about 960 mA per pair. The practical limit is not the amp number itself but the temperature rise the jack sees at that current inside a hot, densely packed enclosure.
Heat comes from I²R losses. Each contact has a small resistance (typically 20 mΩ or less when new), and in an integrated-magnetics jack the transformer and common-mode choke windings add DC resistance. When PoE current flows, that resistance dissipates power as heat, raising the jack temperature above ambient. The rise stacks on top of the enclosure ambient and worsens with high port density, so a jack rated for 350 mA can overheat when asked to carry the 960 mA of a Type 4 design.
Yes, in practice. A PoE++ socket must carry up to about 960 mA per pair across all four pairs without excessive temperature rise, provide center-tapped magnetics for phantom power, and hold adequate isolation for the DC bus. An integrated-magnetics RJ45 jack (magjack) with an industrial temperature grade and low winding DCR is the safe choice. VOOHU SYT-series integrated RJ45 sockets provide center-tapped magnetics and are graded to a −40 to +85 °C operating range for exactly this duty.
Contact resistance is the electrical resistance across a mated contact pair, typically specified at 20 mΩ or less on IEC 60603-7 RJ45 jacks. It matters because it directly sets the I²R heat and voltage drop in a PoE path. As plating wears or corrodes over the connector's mating life the resistance climbs, so contact resistance, gold plating thickness and durability rating together decide how much current a jack can carry reliably over time.