Can your PoE switch power everything?
List the devices, get the power the switch reserves and the cable loss.
From the switch's data sheet: the PoE budget, which is often less than the sum of all its ports.
Devices on the switch: power each needs at the device
Use the device's maximum power from its data sheet (a camera with heater and IR lights, an access point under load). Up to 71.3 W.
How much power does each PoE standard deliver?
IEEE 802.3af (Type 1) lets the switch (the power sourcing equipment, PSE) supply 15.4 W per port, of which the Ethernet Alliance's overview lists 13 W for the powered device (PD); other summaries give 12.95 W. 802.3at (Type 2, "PoE+") supplies 30 W and 25.5 W reaches the device. 802.3bt Type 3 supplies 60 W and 51 W reaches the device, and Type 4 supplies 90 W and 71.3 W reaches the device. One vendor article lists 73 W for Type 4; we use the Ethernet Alliance figure. The difference between the two numbers in each pair is the power lost as heat in the cable. Types 1 and 2 use two of the four cable pairs; Type 3 and 4 can use all four.
What are the PoE classes?
A class tells the switch how much to set aside for a device. In the Ethernet Alliance's tables: class 1: 4 W from the switch, 3.84 W at the device; class 2: 7 W from the switch, 6.49 W at the device; class 3: 15.4 W from the switch, 13 W at the device; class 4: 30 W from the switch, 25.5 W at the device; class 5: 45 W from the switch, 40 W at the device; class 6: 60 W from the switch, 51 W at the device; class 7: 75 W from the switch, 62 W at the device; class 8: 90 W from the switch, 71.3 W at the device. The switch normally reserves the first number per port whatever the device really draws, so a device that needs 6 W falls in class 2 and reserves 7 W, while one that needs 7 W falls in class 3, because class 2 delivers only 6.49 W at the device, and reserves 15.4 W. A class is the smallest one whose figure at the device covers what the device needs, which is why a small step in power can double the reservation.
How do I work out whether my switch can power all my devices?
Look up the total PoE budget on the switch's data sheet (it is lower than the sum of the port ratings on most switches), find the class of every device, and add up the reserved power. In the example below, four devices at 6 W, two at 12 W and one at 25 W reserve 88.8 W on a 120 W switch, leaving 31.2 W. Raise the 6 W devices from four to twelve and the reservation becomes 144.8 W, over the budget. The real draw, with the cable losses, is lower than the reservation: 74.9 W for the first case. Some switches can allocate by measured power, using LLDP or the Autoclass feature of 802.3bt; the Ethernet Alliance gives a Class 8 device that uses 65 W as an example where the reservation shrinks from 90 W to 67.8 W.
How much power is lost in the cable?
Power lost is the current squared times the loop resistance of the cable. The standard is built around a worst-case resistance of 12.5 ohms for a pair set over the 100 m channel (6.25 ohms with four pairs in use). With solid 24 AWG copper at 20 °C, 100 m measures 8.42 ohms by our calculation, so a good cable has margin; the standard's 12.5 ohms is a worst case for the whole channel, which the Ethernet Alliance calls the value that "represents the worst-case". A 12.95 W device on 100 m of 24 AWG takes 13.78 W from the switch, 0.83 W of it lost. A 25.5 W device takes 28.2 W; a 51 W device on four pairs 56.3 W and a 71.3 W device 81.7 W.
Does thinner cable or a longer run cost me power?
Yes. The loss grows with the length of the run and with the resistance of the conductors, which is higher for thin wire: 24 AWG is 84.2 ohms per kilometre (25.7 ohms per 1,000 ft) and 26 AWG, found in some thin patch cords, is higher still. Aluminum-core cables have a higher resistance than copper, so do not enter their gauge here. The calculator takes the AWG you choose; use 24 AWG for typical solid-copper cable. Cables over 100 m are beyond the Ethernet channel limit, and PoE is not guaranteed there.
Is it better to use 4-pair power?
Yes, for the cable loss. With four pairs the current splits between two pair sets, and the effective resistance is half, so the cable loss is halved for the same power. The Ethernet Alliance says the same: a Type 3 or 4 switch that powers an older device over four pairs halves the cable losses. For classes 5 to 8 four pairs are required. The calculator assumes two pairs up to 25.5 W and four above that.
How the budget is worked out
Each device you list is placed in the smallest class that covers its power at the device. The switch reserves the class figure from its budget for each port: 4 W for class 1 up to 90 W for class 8. The real draw is calculated separately for the cable you chose. The device draws power P at the device voltage V_PD, and the voltage at the switch end is V_PSE, so V_PSE − I × R = V_PD and V_PD × I = P. Solving gives I = (V_PSE − √(V_PSE² − 4 × R × P)) ÷ (2 × R), the loss is I² × R, and the switch supplies P plus that loss. This is our derivation. We use the lowest voltage a switch of the right type may supply, 44 V for devices up to 13 W, 50 V up to 51 W and 52 V above, which gives the largest loss that is still within the standard.
Check against the standard: the worst-case cable
If the equation is right, it must reproduce the standard's own numbers when the cable has the worst-case resistance. It does: a Type 1 device taking 12.95 W through 20 ohms at 44 V draws 0.35 A and loses 2.45 W, so the switch supplies 15.4 W. A Type 2 device taking 25.5 W through 12.5 ohms at 50 V draws 0.60 A and loses 4.50 W for 30 W in total. A Type 3 device taking 51 W through 6.25 ohms (four pairs) at 50 V draws 1.20 A and loses 9.00 W for 60 W. A Type 4 device taking 71.3 W at 52 V draws 1.73 A and loses 18.7 W for 90.0 W, the 90 W of the standard. So the difference between the switch figure and the device figure is the cable loss in the worst case, and a shorter cable loses less.
Limits of this method
The calculator does not know your switch: whether it allocates by class, by LLDP or by measurement, how many ports can run at full power and what its total budget is are set by the model and are in its data sheet. Switches may also limit each port to a lower type than the device asks for, in which case the device is power-demoted to a lower class, as the Ethernet Alliance describes. The cable loss uses a single run length for every device, so list devices on very different runs separately by running the calculator twice. Heat adds resistance (copper gains about 0.4% per degree Celsius), and bundled cables run warmer; the calculator uses 20 °C, so keep a margin. We did not open IEEE 802.3-2022 itself, which is a paid document, and relied on the Ethernet Alliance overview of 802.3bt (December 2019).
Sources: Ethernet Alliance, “Overview of 802.3bt Power over Ethernet” (December 2019; types, classes 1 to 8, the 12.5 ohm and 6.25 ohm loop resistances, PSE and PD voltages and Autoclass; an industry association, not the standard); IEEE 802.3-2022 Clauses 33 and 145 are the standard behind it and were not opened (paywalled). The conductor resistance is annealed copper per ASTM B258 sizes. Related: watts to amps calculator, voltage drop calculator and UPS sizing calculator for keeping the switch alive in an outage. How we check our calculators.