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Power over Ethernet (PoE) carries data and low-voltage power over the same structured cabling. A power sourcing equipment device (PSE), usually a switch or injector, negotiates with a powered device (PD) such as an access point, camera, phone, display, or controller. A valid plan needs enough aggregate watts and enough powered ports; passing only one of those checks can still leave devices offline.

PSE watts
Power allocated or delivered at the switch end of the cable.
PD watts
Power available or required at the endpoint after cable loss.
Type ceiling
The maximum PSE allocation associated with a recognized IEEE PoE type.
Power budget
The total watts the switch, stack, or injector shelf can supply across powered ports.

Endpoint datasheets, switch telemetry, and IEEE type limits describe different watt bases. Datasheet demand is often a PD figure. A switch may report measured or allocated PSE watts. Type-ceiling planning deliberately reserves more than normal draw so a device can negotiate its full class. Mixing those bases without adjustment either understates the switch load or wastes capacity.

Cable resistance turns some source power into heat, so PD demand is lower than the power required at the switch. The loss depends on conductor size, bundle temperature, connection quality, powered pairs, distance, and current. A single loss percentage is therefore a planning allowance, not a cable-engineering model.

Reserve capacity protects against uncertainty and change. Cameras may draw more when infrared illuminators turn on, access points can increase radio load, and multiple devices may renegotiate power during a restart. A fixed spare target can also protect a known future addition. Neither allowance is mandated by the IEEE type table; both are local design choices.

Priority matters when the plan does not fit. Life-safety or access-control equipment may need to remain powered while lower-priority phones or convenience devices are shed. That operating policy should be agreed before an outage, not invented while the switch is already overloaded.

How to Use This Tool:

Use the switch's published aggregate PoE budget, not its AC power-supply rating, and choose one watt basis that matches the inventory.

  1. Enter Switch PoE budget, PoE ports available, and the percentage of Planning reserve to hold back.
  2. Choose PD watts for endpoint datasheets, Switch-side for PSE measurements or allocations, or Type ceiling for conservative IEEE type reservation.
  3. Paste Device inventory rows as device, count, watts each, standard, priority, and cable length in metres. Correct any row-specific warning before trusting the total.
  4. Set Cable-loss allowance only for modes that start from endpoint watts. Add Boot-surge allowance and Minimum spare target when the site policy requires them.
  5. Check Policy spare, Boot spare, and Port spare separately. If capacity is short, use the priority order to decide what must move or shed.

Interpreting Results:

Planned draw is the aggregate switch-side demand after the selected allocation rule. Usable budget is the switch budget after the percentage reserve. Policy spare also subtracts the fixed spare target, so it is the decisive steady-state margin.

  • Healthy means the policy target fits and the remaining usable margin is at least 10%.
  • Tight means less than 10% of usable budget remains before the fixed spare target, or the modeled boot draw exceeds capacity after targets.
  • Over budget means planned draw exceeds usable capacity after both reserve policies.
  • Port shortage is independent of watts. Adding an external injector may solve a port problem without changing the switch's aggregate budget.

Technical Details:

IEEE PoE types define maximum power at the PSE and the lower power available at the PD after the channel allowance. The calculation uses those ceilings only in type-ceiling mode; entered device watts remain the demand basis in the other two modes.

Formula Core:

For each inventory row j, count is multiplied by its switch-side watts. The selected allocation mode determines that per-device watt value.

Wplanned= ∑j=1m cj×wj
Mode Switch-side watts per device Use when
PD watts Entered watts × (1 + cable-loss percentage ÷ 100) The inventory contains endpoint demand.
Switch-side Entered watts The inventory already contains PSE demand.
Type ceiling Recognized IEEE PSE ceiling Each recognized type should reserve its maximum source allocation.

Reserve and surge are applied to different quantities. Percentage reserve is held back from the switch budget, while surge is added to planned draw.

Wspare= Wbudget×(1−R100) −Wfixed−Wplanned
Type PSE ceiling PD ceiling
802.3af Type 115.4 W12.95 W
802.3at Type 230 W25.5 W
802.3bt Type 360 W51 W
802.3bt Type 490 W71.3 W

Rule Core:

Rule Exact condition or order
Over budget Policy spare < 0 W.
Tight Not over budget, and either spare before the fixed target is less than 10% of usable budget or boot spare is below 0 W.
Healthy Neither over-budget nor tight conditions apply.
Priority retention Critical, then High, Normal, and Low until capacity after reserve targets is exhausted.
Port shortage Powered-device count > available PoE ports.

Accuracy Notes:

This is a switch-budget planning model, not an electrical design, thermal study, or certification check.

  • The cable length column records context but does not calculate conductor resistance. Use a cable design supported by gauge, bundle, temperature, and installation limits.
  • Passive PoE does not use normal IEEE negotiation. Confirm voltage, polarity, pinout, and endpoint compatibility separately before connection.
  • Type ceilings do not prove that the switch supports the required type on every port or at full simultaneous load. Check the exact hardware and power-supply configuration.
  • Display precision changes visible rounding only; capacity decisions use unrounded watt values.

Worked Examples:

Watts fit but ports do not

A 740 W, 48-port switch with a 15% reserve has 629 W of usable budget. The sample inventory draws 583 W after a 10% loss allowance, leaving 46 W, but its 54 endpoints exceed the available powered ports by six. The watt status is Tight and the port check still fails.

Type 4 exact-budget boundary

One 802.3bt Type 4 device in type-ceiling mode reserves 90 W at the PSE. On a 90 W switch with no reserve, the policy spare is exactly 0 W. It is not over budget, but it is Tight because no margin remains.

References: