Calculating your PoE power budget

Power over Ethernet is constrained by two separate ceilings: the per-port maximum and the switch-wide power budget. A switch can have plenty of per-port capability and still refuse to power a device because the total budget is exhausted. Sizing this correctly before install is the difference between a stable deployment and intermittent, hard-to-diagnose device dropouts.

The two ceilings

• Per-port maximum — the most power a single port can deliver, set by the PoE standard the port supports.
• Total PoE budget — the sum of power the switch’s PoE power supply can deliver across all ports at once. This is the number that bites you. It is almost always less than (per-port maximum × number of PoE ports), because the power supply is sized for realistic loads, not every port at maximum simultaneously.

Check both on your unit:

show poe

This prints the total budget, current draw, and per-port consumption and class.

Know the standards and their limits

PoE power is delivered at the port (PSE) but consumed at the device (PD), and the two numbers differ because of cable loss. The relevant standards and their per-port figures:

Standard	Common name	Power at port (PSE)	Power at device (PD)
802.3af	PoE	15.4 W	12.95 W
802.3at	PoE+	30 W	25.5 W
802.3bt Type 3	PoE++	60 W	51 W
802.3bt Type 4	PoE++	90 W	71 W

Budget against the power-at-port figure, not the device figure. The switch must source the higher number; the difference is dissipated in the cable.

Power classes

Powered devices negotiate a class that tells the switch how much to reserve. A class-3 device reserves up to 15.4 W at the port even if it usually draws less. The switch reserves against the class, so your budget is consumed by what devices request, not only what they currently draw. Plan against class.

Worked example

A switch lists a 370 W total PoE budget. You want to power:

• 8 × Wi-Fi access points, class 4 (802.3at, 30 W at port) → 8 × 30 = 240 W
• 6 × IP cameras, class 3 (802.3af, 15.4 W at port) → 6 × 15.4 = 92.4 W
• 4 × VoIP phones, class 2 (7 W at port) → 4 × 7 = 28 W

Total reserved: 360.4 W against a 370 W budget.

That fits — but only just, at roughly 97 percent utilization. That is too tight. A single additional device, or a firmware change that bumps an AP to a higher class, pushes you over budget and the switch will deny power to the lowest-priority device. Size to leave headroom — aim to keep planned reservation at or below 80 percent of the budget. Here you are over that line and should either move some devices to a second switch or choose a higher-budget model.

What happens at overload

When demand exceeds the budget, the switch does not split the difference — it sheds whole devices by priority. Set priorities so the right devices win:

configure
set poe interface ethernet 1 priority high
set poe interface ethernet 12 priority low
commit
save

High-priority ports keep power; low-priority ports are denied first when the budget is exceeded. Assign high to anything that must never drop — security cameras, door controllers — and low to the convenient-but-not-critical.

Cap a port if you need to

You can cap a port’s allocation to protect the budget from a misbehaving or mis-classed device:

configure
set poe interface ethernet 1 max-power 25
commit
save

Checklist before you deploy

1. List every powered device and its class and port-side power.
2. Sum the port-side figures and compare to the switch’s total budget.
3. Confirm the planned total is at or below 80 percent of the budget.
4. Set priorities so critical devices survive an overload.
5. After install, run show poe under real load and confirm actual draw matches your plan.