PoE budget: can your switch power all your devices?
A PoE switch has two limits: per-port power, and the GLOBAL budget — the one everyone forgets. Exceed it and ports drop without warning, usually at night, usually the camera that mattered. This tool adds up your devices and tells you where you stand.
Required PoE power
54W
- Remaining budget margin
- 66W
- Budget utilisation
- 45%
Comfortable margin, room to grow.
The method
A switch’s PoE budget is the total power it can deliver across all ports — almost always less than “port count × per-port power”. An 8-port “30 W per port” switch with a 65 W budget canNOT feed eight 15 W devices.
The sum uses devices’ REAL draw (their spec sheet), not the negotiated PoE class: a class-3 camera (15.4 W reserved) often consumes just 6.
On long runs, copper resistance dissipates part of the power: up to 10% over 80–100 m. The allowance covers that case.
In the formula: B the switch budget (W), nᵢ and pᵢ each device type’s count and draw, c = 1.1 for long cables.
total = (n₁ × p₁ + n₂ × p₂) × c · margin = B − total
Good to know
- Beware of non-standard “passive PoE” injectors and switches: they push power without negotiating and can FRY a device built for standards-based PoE (802.3af/at/bt). Always check for the standard’s mention.
- The standards cap per-port power: 802.3af ≈ 13 W usable, 802.3at (PoE+) ≈ 25 W, 802.3bt (PoE++) 51–71 W. An “at” device on an “af” port will not boot, budget or not.
- Account for start-up surges: PTZ cameras and access points briefly pull 1.5× their nominal draw at power-on — hence the value of a real margin.
- Heated cameras (outdoor, frost) double or triple their draw in deep cold: size on the “heater active” spec line.
- This switch may be powering your surveillance cameras: the matching storage calculation lives in this same category.
FAQ
- What actually happens when the budget is exceeded?
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The switch protects its power supply by shedding ports, per its policy: often by port-number priority (the last ones drop), sometimes the most recently connected device. Typical result: a camera boot-looping or an access point “randomly” dying — symptoms blamed on everything except the PoE budget.
- Real draw or PoE class: which should I add up?
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Both exist: some switches reserve power by negotiated CLASS (static allocation), others meter actual draw (dynamic). This calculation uses actual draw — the modern-switch case. If yours allocates by class, the sum of classes (15.4 W for class 3, 30 W for class 4) fills the budget much faster: check its datasheet.
- Is PoE dangerous for non-PoE devices?
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Standards-based PoE, no: the switch tests for an electrical signature before applying power, and a PC on a PoE port only gets data. The danger comes from PASSIVE PoE (cheap gear, wireless bridges) that pushes 24 or 48 V permanently without asking — that is what fries equipment.
- Do I need special cable for PoE?
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Solid-copper Cat5e or better handles af and at up to 100 m. Two real traps: CCA cable (copper-clad aluminium sold as cheap “Cat6”) which heats up and loses far more, and tight bundles of PoE++ cables in conduit, where heat accumulates. For 802.3bt, use pure-copper Cat6.
- My switch claims 30 W per port AND a 65 W budget: which is true?
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Both: each port CAN deliver 30 W, just not all at once. It is a shared pool: two 30 W devices plus one at 5 W saturate the 65 W. That asymmetry is normal (the internal power supply is the expensive part) and it is exactly why this calculator exists.