If you've been working with Power over Ethernet for a while, you probably know the old standards by heart — 802.3af gives you up to 12.95W at the powered device, 802.3af/at uses 2-pair delivery, and that covers most access points and IP cameras. But 802.3bt (ratified in 2018) changed the game by bringing 4-pair power delivery and pushing available power to 90W.
Here's the thing though: 802.3bt isn't just one standard. It defines two distinct types — Type 3 and Type 4 — and they behave differently in ways that matter for your design. Let's break down exactly what changed and what it means for your next PoE project.
A Quick PoE Standards Recap
Before we get into the bt specifics, let's set the baseline:
| Standard | IEEE | Type | PSE Output | PD Input | Pairs Used |
|---|---|---|---|---|---|
| PoE | 802.3af | Type 1 | 15.4W | 12.95W | 2 |
| PoE+ | 802.3at | Type 2 | 30W | 25.5W | 2 |
| PoE++ | 802.3bt | Type 3 | 60W | 51W | 4 |
| PoE++ | 802.3bt | Type 4 | 90W | 71.3W | 4 |
The jump from 30W (802.3at) to 60W or 90W (802.3bt) isn't incremental — it opens up entirely new use cases. We're talking pan-tilt-zoom cameras with heaters, small displays, 5G small cells, and even thin clients powered entirely over Ethernet.
The Big Change: 4-Pair Power Delivery
802.3af and 802.3at deliver power over just 2 of the 4 pairs in a Cat5e/Cat6 cable. Specifically, they use either the data pairs (pins 1-2, 3-6) or the spare pairs (pins 4-5, 7-8), but never both.
802.3bt uses all 4 pairs. Both the data pairs and the spare pairs carry power simultaneously, roughly splitting the load across them. This doubles the available copper for current flow, which is how you get to 60W and 90W without exceeding the per-pair current limits.
Why 4-Pair Matters
It's not just about total power. Distributing current across 4 pairs instead of 2 means:
- Lower current per pair — less heating in the cable
- Lower voltage drop — more power actually reaches the PD
- Better efficiency — less waste heat in the cable itself
The cable is no longer the bottleneck it was with 2-pair delivery at higher power levels.
Type 3 vs Type 4: The Real Differences
Both types use 4-pair delivery, but they differ in power class, current levels, and some implementation details.
Power and Current Levels
| Parameter | Type 3 | Type 4 |
|---|---|---|
| Max PSE output | 60W | 90W |
| Max PD input | 51W | 71.3W |
| Max current per pair | 360mA | 480mA |
| Voltage at PSE | 50–57V | 52–57V |
| Min voltage at PD | 42V | 42V |
Type 4 pushes per-pair current to 480mA. That's within the cable's rating for Cat5e and above, but it means every connection point — the RJ45 jack, the patch panel termination, the PD connector — needs to handle that current reliably.
Class Signaling
802.3bt introduced new power classes that the PD signals to the PSE during negotiation:
| Class | PD Power Range | Type |
|---|---|---|
| 1 | 0.44–3.84W | Type 1/2/3/4 |
| 2 | 3.84–6.49W | Type 1/2/3/4 |
| 3 | 6.49–12.95W | Type 1/2/3/4 |
| 4 | 12.95–25.5W | Type 2/3/4 |
| 5 | 25.5–40W | Type 3 |
| 6 | 40–51W | Type 3 |
| 7 | 51–62W | Type 4 |
| 8 | 62–71.3W | Type 4 |
Classes 5–6 are Type 3 territory. Classes 7–8 are Type 4 only. The PD advertises its class during the signature detection phase, and the PSE allocates power accordingly.
Dual-Signature vs Single-Signature
Here's a subtlety that trips people up. 802.3bt defines two PD architectures:
Single-Signature PD maintains one detection signature across all powered pairs. The PSE treats it as one load. This is the simpler, more common approach — and it's what most chips on the market implement.
Dual-Signature PD presents two independent signatures, one for each pairset. The PSE can power each pairset independently. This gives you some redundancy and flexibility, but it's more complex and less common in practice.
Most Type 3 and Type 4 designs use single-signature. Dual-signature shows up in niche applications where you might want to power two independent subsystems in one PD.
Cable and Connector Requirements
You can't just shove 90W through any old Cat5 cable and call it a day. 802.3bt has specific cable requirements, and for good reason.
Cable Specifications
| Parameter | Type 3 | Type 4 |
|---|---|---|
| Min cable category | Cat5e | Cat5e |
| Recommended | Cat6A | Cat6A |
| Max cable length | 100m | 100m |
| Max loop resistance | 12.5Ω (per pair) | 12.5Ω (per pair) |
The standard minimum is Cat5e, but let's be real — if you're pushing 90W through 100m of Cat5e, you're losing a lot of power to cable resistance. Cat6A has lower DC resistance and handles the heat better. Use it for new deployments.
Voltage Drop Reality Check
Let's calculate the actual voltage drop for a 90W Type 4 PD at the end of a 100m Cat5e cable.
Cable resistance per conductor: ~12.5Ω per pair (so ~6.25Ω per conductor). With 4-pair delivery, each pair carries about 480mA.
Voltage drop per pair = I × R = 0.48 × 12.5 = 6.0V
If the PSE outputs 54V, the PD sees approximately 48V. Still well above the 42V minimum, but that's 6W lost to cable heating across all pairs combined. Not terrible, but it adds up in large installations.
With Cat6A (lower resistance per pair, typically 9–10Ω), the drop improves to about 4.3V — saving you roughly 2W in cable losses.
Thermal Considerations in Cable Bundles
Here's a gotcha that's easy to miss: when you bundle multiple PoE cables together in a conduit or cable tray, the cables in the center of the bundle can't dissipate heat as effectively. At 90W per cable, that heat is real.
TIA-568.2-D (and its predecessor TSB-184) provides derating guidelines for bundled PoE cables. The key guidance:
- For Type 3 (60W): Most installations are fine without special derating in standard cable trays
- For Type 4 (90W): You need to pay attention to bundle size. Large bundles (>24 cables) may require reduced fill ratios or conduit sizing
- Ambient temperature matters: In a hot plenum space (50°C+ ambient), cable current capacity drops. Consider reducing cable lengths or upgrading to Cat6A
If you're designing a building infrastructure with dozens of 802.3bt Type 4 runs, this isn't theoretical — it can cause nuisance tripping or even cable damage over time.
PD Design Considerations
If you're designing a Powered Device (the thing receiving PoE), here's what's different with 802.3bt:
Front-End Design
Your PoE front end now needs to handle up to 57V input and potentially 71.3W. Key components:
- PoE PD controller — must support the correct type and class (e.g., TI TPS2378 for Type 3, TI TPS2379 for Type 4)
- Diode bridge — handles polarity from either pairset. At 90W, you need diodes rated for at least 1A continuous with low forward voltage to minimize losses
- Hot-swap MOSFET — limits inrush current when power is first applied
- Isolation transformer — 802.3bt PDs still need isolation per IEC 60950-1 / IEC 62368-1
Thermal Management
71.3W at the PD input means your DC-DC converter needs to handle that power efficiently. Even at 90% efficiency, you're dissipating ~7W in the PD. That needs a thermal plane, probably some thermal vias, and possibly a heatsink depending on your enclosure.
Maintain Power Signature (MPS)
The PD must periodically draw a minimum current to tell the PSE "I'm still here." If the PD goes to a very low power state, it still needs to maintain MPS or the PSE will cut power. For bt designs, this is typically 10mA minimum maintain current — check your PD controller datasheet for specifics.
Backward Compatibility
Good news: 802.3bt is backward compatible with 802.3af and 802.3at. A Type 3 or Type 4 PD will negotiate down to Type 1 or Type 2 power if connected to an older PSE. The PD controller handles this automatically — it detects what the PSE offers and adjusts.
A few things to watch:
- Class 4 PDs (25.5W max) work with all PSE types
- Class 5–6 PDs (Type 3) will fall back to Type 2 (25.5W) if connected to an at-only PSE — make sure your device can operate or gracefully degrade at that power level
- Class 7–8 PDs (Type 4) similarly fall back — you need a power management strategy for degraded mode
When to Use Type 3 vs Type 4
| Use Case | Recommended Type | Why |
|---|---|---|
| PTZ cameras with heater | Type 3 (51W) | Sufficient power, wider PSE availability |
| Small displays / kiosks | Type 3 or 4 | Depends on display size and backlight |
| 5G small cells | Type 4 (71.3W) | High continuous power demand |
| Multi-radio access points | Type 3 (51W) | Usually enough for Wi-Fi 6E/7 APs |
| Building automation controllers | Type 3 (40–51W) | Headroom for sensors and actuators |
| Thin clients / mini PCs | Type 4 (71.3W) | CPU + display output needs the power |
Type 3 is the sweet spot for most applications. You get 51W at the PD, which is plenty for the vast majority of devices that previously needed a separate power supply. Type 4 is for when you genuinely need maximum power.
Plan Your PoE Power Budget
Whether you're sizing a single run or planning a 48-port switch deployment, you need to know exactly how much power reaches your devices. The PoE Power Budget Planner at semiconductor.tools calculates voltage drop, cable losses, and available power at the PD for any combination of standard, cable type, and distance.
Stop guessing on PoE cable lengths and power budgets. Try the PoE Power Budget Planner — enter your standard (802.3af/at/bt), cable type, and run length, and get exact voltage drop and power delivery numbers. Covers all four PoE types including Type 3 and Type 4. Free, no sign-up.