{"id":13117,"date":"2026-09-21T07:35:00","date_gmt":"2026-09-21T05:35:00","guid":{"rendered":"https:\/\/www.lukaswojcik.com\/blog\/?p=13117"},"modified":"2026-08-31T15:16:40","modified_gmt":"2026-08-31T13:16:40","slug":"tutorial-sizing-a-poe-budget-before-the-switch-is-ordered","status":"publish","type":"post","link":"https:\/\/www.lukaswojcik.com\/blog\/en\/it-networks\/tutorials-en-it-networks\/tutorial-sizing-a-poe-budget-before-the-switch-is-ordered\/","title":{"rendered":"Tutorial: Sizing a PoE Budget Before the Switch Is Ordered"},"content":{"rendered":"

A PoE switch states one number on its box: the total power it can deliver. Adding up the devices to be connected and comparing the sum against that number looks like the whole job, and it produces the wrong answer often enough to be worth doing differently.<\/p>\n

The reason is that a switch does not allocate power by what a device draws. It allocates by what the device says it might draw, and the gap between those two figures is regularly a factor of two.<\/p>\n

\n\"Two
The same fifteen devices, two numbers. Only the upper bar decides whether a sixteenth one can be connected.<\/figcaption><\/figure>\n

Two Numbers per Device, and Why They Differ<\/h2>\n

Every PoE specification carries two power figures, and confusing them is the most common source of a plan that does not fit.<\/p>\n\n\n\n\n\n\n\n\n
Standard<\/th>\nName<\/th>\nAt the switch<\/th>\nAt the device<\/th>\n<\/tr>\n<\/thead>\n
802.3af<\/td>\nPoE, Type 1<\/td>\n15.4 W<\/td>\n12.95 W<\/td>\n<\/tr>\n
802.3at<\/td>\nPoE+, Type 2<\/td>\n30 W<\/td>\n25.5 W<\/td>\n<\/tr>\n
802.3bt<\/td>\nPoE++, Type 3<\/td>\n60 W<\/td>\n51 W<\/td>\n<\/tr>\n
802.3bt<\/td>\nPoE++, Type 4<\/td>\n90 W<\/td>\n71.3 W<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The difference between the two columns is the cable. The standard assumes up to a hundred metres of twisted pair, and the loss over that distance is what the switch has to supply on top. A budget is always counted in the left column, because that is what leaves the switch.<\/p>\n

One warning that belongs here rather than anywhere else. Passive 24-volt PoE, as used by some older access points and adapters, is not one of these standards. It puts voltage on the line without any negotiation, and a standards-compliant device connected to such a port can be damaged. On a switch that offers both, the mode is set per port, and it is worth checking the setting before plugging anything in rather than after.<\/p>\n

Classes, and the Reservation That Follows<\/h2>\n

When a device is connected, the switch measures a resistance in it and reads a class from that. The class is a promise about the upper limit, not a measurement of consumption.<\/p>\n

Class 0   15.4 W    the default when no class is signalled\nClass 1    4.0 W\nClass 2    7.0 W\nClass 3   15.4 W\nClass 4   30.0 W    Type 2 and above\nClass 5   45.0 W\nClass 6   60.0 W\nClass 7   75.0 W\nClass 8   90.0 W<\/code><\/pre>\n
A camera that draws nine watts and reports class 4 occupies thirty watts of the budget. It does that whether or not it ever uses them, because the switch has to be able to keep the promise at the moment the device asks.<\/p>\n
Class 0 is the one to watch. A device that signals no class at all is treated as if it might want 15.4 watts, and a cheap unmanaged device that draws three watts therefore blocks five times its own consumption.<\/p>\n
There is a second, more precise negotiation on top of the class: over LLDP, a device can ask for an exact figure after the link is up, and the switch can then lower the reservation to that. Whether this happens depends on both sides and on a switch setting, and it is the single lever that turns the upper bar in the diagram into something close to the lower one.<\/p>\n
Adding It Up for a Real Switch<\/h2>\n
The calculation is a table with two columns, and the second one is the one that matters.<\/p>\n\n\n\n\n\n\n\n\n\n
Device<\/th>\nCount<\/th>\nClass<\/th>\nReserved<\/th>\nMeasured, at night<\/th>\n<\/tr>\n<\/thead>\n
Outdoor camera with IR<\/td>\n8<\/td>\n4<\/td>\n240.0 W<\/td>\n100.0 W<\/td>\n<\/tr>\n
Access point, Wi-Fi 6<\/td>\n4<\/td>\n4<\/td>\n120.0 W<\/td>\n52.0 W<\/td>\n<\/tr>\n
IP telephone<\/td>\n2<\/td>\n2<\/td>\n14.0 W<\/td>\n8.0 W<\/td>\n<\/tr>\n
Door station<\/td>\n1<\/td>\n3<\/td>\n15.4 W<\/td>\n8.0 W<\/td>\n<\/tr>\n
Total<\/strong><\/td>\n15<\/strong><\/td>\n<\/td>\n389.4 W<\/strong><\/td>\n168.0 W<\/strong><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Against a switch with a 400-watt budget, this installation is at 97 per cent before a single frame has been transmitted – and at 42 per cent by the meter. Both figures are correct, and which one governs is decided by whether LLDP negotiation is active on that switch.<\/p>\n
The practical planning rule that follows: size against the reserved column, and treat anything above eighty per cent of the budget as full. The remaining twenty per cent is not a safety margin against the devices, but room for the next one – and there is always a next one.<\/p>\n
The Loads That Are Not in the Data Sheet<\/h2>\n
Three consumers regularly turn a calculation that worked into one that does not, and none of them appears in the figure printed on the box.<\/p>\n
The first is infrared. A camera specified at nine watts draws that figure by day; at dusk the illuminator switches on and the draw rises by three to five watts per camera. The peak load of a camera installation is therefore at night, which is precisely when nobody is looking at the switch. Eight cameras make a difference of thirty watts between the two measurements.<\/p>\n
The second is heating. An outdoor dome with a heater draws twenty-five to thirty watts extra below freezing, and it does so on all cameras at once, because they all sit in the same weather. A budget that fits in October can fail on the first cold night of the year.<\/p>\n
The third is the moment of switching on. A switch that comes back after a power cut brings up all ports at once, and the inrush of fifteen devices is higher than their steady-state total. Some switches stagger the ports for exactly this reason; on those that do not, the first minute after a restart is the tightest of the whole year.<\/p>\n
What Happens When the Budget Runs Out<\/h2>\n
A switch that cannot supply another port does not fail. It refuses power to that port, and the device stays dark – which looks like a defective camera, a defective cable, or a defective port, and is investigated in that order for about an hour.<\/p>\n
Which port loses is not random. Ports have a priority, and where it is set to critical, high or low, the lowest priority is dropped first. Where nothing was set, the tie-break is usually the port number, so it is the physically last device that stops – regardless of whether it is the least important one.<\/p>\n
The setting is worth making deliberately and in a specific order. The door station and the network’s own uplink devices are critical; cameras at entrances are high; a camera on the garden shed and the access point in the guest room are low. That way, an exhausted budget produces a documented gap rather than a search.<\/p>\n
Checking It After Installation<\/h2>\n
The controller shows both numbers, and comparing them once is worth the minute it takes.<\/p>\n
Per port, the display shows the current draw; for the switch as a whole, the total consumption and the remaining budget. If the remaining budget is far smaller than the difference between the total budget and the sum of the measured values, then the switch is reserving by class and LLDP is not lowering it.<\/p>\n
Two more checks belong to the same session. Once at night, because of the infrared. And once with a deliberately disconnected and reconnected switch, because that is the case in which the reservation is rebuilt from scratch and a plan that only just fitted turns out not to.<\/p>\n
What all of this leaves out is the switch itself. Its own consumption is not part of the PoE budget and is not covered by it, but it does count towards the socket, the fuse and the UPS behind it. A 400-watt PoE budget plus the electronics is a device that draws over 450 watts at full load – which is a fact for a different plan, and a fact worth having before the rack is wired.<\/p>\n
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Sources<\/h2>\n