PoE af vs at vs bt: Power at Both Ends
Quick answer / TL;DR: Short answer: 802.3af delivers up to 15.4 W at the source and 12.95 W at the device; 802.3at delivers 30 W and 25.5 W; 802.3bt delivers 60 W and 51 W for Type 3, and 90 W and 71 W for Type 4. The difference between source and device power is cable loss. bt uses all four pairs, which halves the current per conductor for a given power and is why it is required above 30 W.
Three PoE adapters: injector adds power, splitter removes it, midspan sits in the run.Power over Ethernet is specified in three tiers, and the numbers quoted in marketing refer to the source while the device receives less. That gap is the first thing to understand, because a device rated at 25.5 W needs the higher tier even though the source figure is 30 W.
The second thing is that higher tiers change the cable requirement, because the current flows through the same small conductors that carry data.
The Three Types
The source power is what the switch or injector must provide. The device power is what arrives after cable loss, and that is the figure the powered device's specification refers to.
| Standard | Type | Source power | Device power | Pairs used |
|---|
| 802.3af | Type 1 | 15.4 W | 12.95 W | 2 |
| 802.3at | Type 2 | 30 W | 25.5 W | 2 |
| 802.3bt | Type 3 | 60 W | 51 W | 4 |
| 802.3bt | Type 4 | 90 W | 71 W | 4 |
The pair count is the mechanism, not a detail. Sending 60 W over two pairs means roughly twice the current per conductor compared with four pairs, which doubles the resistive loss and the heating in the bundle.
What It Means for Cable
Higher power means higher current, and current through a small conductor produces heat. That heat accumulates in a bundle, which raises the conductor temperature and therefore the resistance.
- Use at least Cat5e for af and at; Cat6 or Cat6A for bt is common practice
- Prefer 23 AWG over 24 AWG for bt, because resistance per metre is lower
- Avoid large bundles of high-power cables without considering temperature rise
- Keep the channel within the 100 metre limit, including patch cords
- Check that the switch supports the class the device requests, not just the total budget
The bundle temperature point is the one that gets missed. A single cable carrying 90 W is fine; forty of them in a closed tray is a thermal design problem that the cable datasheet alone does not answer.
Negotiation and Compatibility
PoE negotiates. The source detects a valid device, classifies it, and only then applies power. Higher types are backward compatible in the sense that a Type 3 source can power a Type 1 device, but not the reverse.
| Source | Type 1 device | Type 2 device | Type 4 device |
|---|
| 802.3af switch | works | insufficient power | insufficient power |
| 802.3at switch | works | works | insufficient power |
| 802.3bt switch | works | works | works |
Real-World Applications
Ceiling access points and cameras
Most access points sit within Type 2 at 25.5 W, which is why 802.3at switches are the common baseline. Cameras with heaters and pan-tilt mechanisms can exceed that, and those need Type 3 or a local supply.
Digital signage and displays
Large displays exceed any PoE tier and need mains power. Small information panels can run on Type 3, which simplifies installation where an outlet is not available.
Industrial sensors and controllers
Type 4 with four-pair delivery suits devices that need both power and a reliable data link in one cable. Verify the device's class and the switch's per-port budget, and remember that total switch budget is often lower than the sum of port maximums.
Standards and Verification
The tiers, the detection method and the cable requirements are all specified in the Ethernet amendment.
- IEEE 802.3af: Power over Ethernet, Type 1, 15.4 W at the source
- IEEE 802.3at: Type 2, 30 W at the source, with classification enhancements
- IEEE 802.3bt: Types 3 and 4, four-pair delivery, up to 90 W at the source
- IEC 61156 series: cable test methods including the temperature and resistance behaviour relevant to heating
- TIA-568.2-D and ISO/IEC 11801-1: channel requirements that PoE does not change but does stress
Verification is a measurement at the device end. A PoE tester or an inline meter shows the negotiated class and the delivered power. If the device reboots under load, the delivered power is below its requirement, and the cause is usually cable loss, a long run, or a switch port budget that is shared with other devices.
Related reading: PoE cable: power over Ethernet requirements — blogs/news/poe-cable
Related reading: Ethernet cable guide: categories, construction and runs — blogs/news/ethernet-cable
Related reading: Cat6 cable: what the category actually promises — blogs/news/cat6-cable
Frequently Asked Questions
What is the difference between PoE af, at and bt?
They are the three power tiers: af provides 15.4 W at the source and 12.95 W at the device, at provides 30 W and 25.5 W, and bt provides 60 W or 90 W at the source using all four pairs.
Why is device power lower than source power?
The difference is cable loss. Power is lost in the resistance of the cable, so the device receives less than the source supplies, and the gap grows with length and current.
Do I need Cat6 for PoE bt?
Cat6 or Cat6A is common practice because lower resistance reduces loss and heating. Cat5e can carry bt over short runs but has less margin.
Can I power a 30 W device from an af switch?
No. The port cannot supply enough power, and the device will either not start or will reboot under load. Use an at or bt source.
Does PoE affect network speed?
No, the data pairs are unchanged. Higher power does raise cable temperature, which affects loss and therefore the power budget, not the data rate.
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