How this works
Each device sits on its own switch port. The switch has to supply what the device draws, plus what the cable wastes as heat on the way. The standard sets the most a device of each class may draw, and the least the switch must supply for that class at the end of a 100 m channel.
| Class | Standard | Max at device | Min at switch port |
|---|---|---|---|
| 0 | 802.3af (Type 1) | 12.95 W | 15.4 W |
| 1 | 802.3af (Type 1) | 3.84 W | 4.0 W |
| 2 | 802.3af (Type 1) | 6.49 W | 7.0 W |
| 3 | 802.3af (Type 1) | 12.95 W | 15.4 W |
| 4 | 802.3at (Type 2, PoE+) | 25.5 W | 30.0 W |
| 5 | 802.3bt (Type 3) | 40.0 W | 45.0 W |
| 6 | 802.3bt (Type 3) | 51.0 W | 60.0 W |
| 7 | 802.3bt (Type 4) | 62.0 W | 75.0 W |
| 8 | 802.3bt (Type 4) | 71.3 W | 90.0 W |
Class 0 and 3 devices: 12.95 W in 802.3af; later editions of 802.3 round it to 13.0 W.
Cable loss
The loss is worked out from Ohm's law, with the switch at the lowest output voltage its Type allows (44 V for Type 1, 50 V for Types 2 and 3, 52 V for Type 4) and the loop resistance the standards assume for 100 m (20 Ω for Type 1, 12.5 Ω for Type 2 and up), scaled to your cable length. Four-pair classes (5 to 8) carry current on both pairsets, which halves the resistance.
P_device = (V_switch − I × R) × I Cable loss = I² × R R = loop resistance at 100 m × (length ÷ 100 m)
At 100 m this model gives back the Ethernet Alliance's published worst-case losses, and the standard's switch-port figures, for every class. Class 4, for example: 25.5 W at the device, 0.6 A over 12.5 Ω from a 50 V port, loses 4.5 W, and 25.5 + 4.5 = 30.0 W. Shorter cable loses less, which is what "by actual draw" allocation benefits from.
Multi-sensor cameras
A multi-sensor camera is one device on one port, whatever its number of sensors. It counts once against the switch's ports and once against its budget. It usually draws more than a single-sensor camera, often class 4 or above, so enter its datasheet draw. For storage and recorder channels it counts once per sensor stream; that is handled in the camera storage calculator.
What gets flagged
- A device whose datasheet draw is above its class's maximum. A switch policing by class may cut it off.
- A device whose class is above what the switch's ports can supply.
- More devices than PoE ports.
- A total above the switch's budget, less any reserve you set.
- A cable longer than the 100 m Ethernet channel.
What this does not account for
- Switch-specific behaviour: some reserve power per port in fixed steps, some let you set per-port limits, some drop ports in a priority order when over budget. The switch's manual has the final word.
- Heaters, IR illuminators and PTZ motors that raise draw in cold weather or at night. Use the datasheet's maximum, not its typical.
- Cable that is not to category spec, such as copper-clad aluminium, loses more than modelled here.
- Non-standard "passive" PoE, and vendor modes above 802.3bt. This covers IEEE 802.3 PoE only.
Common questions
- My switch says 370 W. Why can't I run twelve 30 W devices on it?
- You can, if it allocates by actual draw and the devices draw less than 30 W. If it reserves by class, twelve class 4 ports reserve 12 × 30 W = 360 W; add a device and it is out.
- What is the difference between power at the device and at the switch?
- The cable. Some power is lost as heat in the conductors, so the switch always supplies more than the device receives. At 100 m that is up to 2.45 W on class 3 and 4.5 W on class 4.
- Does a four-sensor camera need four ports?
- No. It has one network connection and one PoE interface, so it takes one port. It may need a higher class than a single-sensor camera.
- The switch runs off a battery and solar panel. How big?
- Enter the same devices in the off-grid solar calculator: it adds the loss of boosting the battery to 48 V for PoE and sizes the battery and panels.
Sources
- IEEE Std 802.3, Clause 33 (802.3af/at, Types 1 and 2) and Clause 145 (802.3bt, Types 3 and 4), as tabulated by Sifos Technologies' PoE technology overview and IEEE's 2017 PoE tutorial: class power at the switch and the device, and minimum switch voltage per Type.
- Ethernet Alliance, PoE – Cable Losses (2017), Table 2: channel resistance and worst-case cable loss per class.
- ANSI/TIA-568.0-E. The 100 m Ethernet channel that PoE is specified over.
Every source used on this site, and how each number is used.