802.3af vs 802.3at vs 802.3bt: Which PoE Class Does Your Camera Need?

PoE standards are quoted everywhere and understood rarely. The confusion is simple: the number printed on the switch is the power the switch can output, while the number the camera needs is the power it actually receives — and cable loss sits between the two. This note explains the three standards in plain terms and the checks that stop a project being either under-powered or over-priced.

1. What each standard delivers

There are three PoE standards in current use, and the difference is real power at the port, negotiated before the camera is allowed to draw:

The gap between the two columns — roughly 2.5 W to 19 W — is not marketing padding. It is the power the cable and the negotiation are expected to lose on a 100 m run. Specifying a camera at exactly 15.4 W on an 802.3af port is specifying it to fail.

2. Read the PD class, not just the wattage

Every PoE device declares a class (0–8), and the switch reserves power by class before the camera even boots. A class-4 device (up to 25.5 W) will be refused power on an 802.3af port, even if the camera would only draw 8 W in practice, because the reservation is made from the class, not from the measured draw.

• Class 0–3 map to 802.3af ceilings; class 4 requires 802.3at.
• Class 5–8 only exist under 802.3bt and need a bt-capable PSE.
• Autosensing switches fall back safely; "dumb" PoE injectors do not — they just brown out the camera.
• Check the camera datasheet for the class, not only for "max power".

3. Night draw and IR are the usual blind spot

A camera that reads 5 W on the bench can pull 9–12 W once the IR illuminator switches on. Multiply the night figure by camera count, add the switch's own consumption, and compare with the total budget with 25–30% headroom. This is the single most common cause of cameras dropping off at dusk.

4. When 802.3bt is genuinely worth it

Most surveillance sites never need bt. It earns its price in four situations: PTZ with heater and wiper, multi-sensor or multi-lens cameras, cameras that also power an LED flood or a speaker, and edge devices (LTE router, PoE-powered mini PC, access-control reader) sharing the same run.

• bt ports usually cost more per port and use heavier gauge cable — budget for both.
• bt over long runs needs Cat6 or better; thin CCA cable will fail the negotiation.
• Mixing bt and af on one switch is fine, but check the total budget under mixed load.

Our PoE switch range covers af/at as standard, and the L2/3 managed PoE switches add per-port class reporting, so the port tells you what it reserved and what it is actually delivering.

5. The four checks before you approve the BOM

• Every camera's PD class is supported by the chosen port standard.
• Total budget covers night draw plus 25–30% headroom.
• Longest run is under 100 m with the cable actually specified, not the cable hoped for.
• Spare budget exists for the two cameras the customer will add next year.

Send the camera datasheets and we will return a port-by-port class and budget table. Ask for a quote.

FAQ

Las especificaciones técnicas detalladas se presentan en inglés.