PoE budget math: the number that quietly wrecks a deployment

Here's a failure we've been called in to fix more than once. A business buys a 24-port PoE switch, plugs in sixteen access points and eight cameras, and somewhere around device nineteen things start rebooting at random.

Nothing is faulty. The switch is doing exactly what it's designed to do. It ran out of power budget and started shedding load.

Port count and power budget are two different numbers, and only one of them is printed on the front of the box.

The classes

Power over Ethernet comes in tiers. Two numbers matter per tier: what the switch supplies and what the device actually receives. The difference is loss in the cable.

Budget using the switch-supplies column. That's what comes out of the power budget, and the cable loss is real whether you account for it or not.

There's also passive 24V PoE on some older gear, which isn't a standard at all and won't be powered by a standards-compliant switch. Worth checking before you assume an old access point will come up on a new one.

Working out the number

Add up what every device draws at maximum, not at idle.

A rough working set:

• Indoor access point: 15 to 25 W, higher-end models pushing PoE+ limits
• Outdoor or long-range access point: 20 to 30 W
• Fixed indoor camera: 6 to 12 W
• Camera with IR illuminators at night: considerably more, sometimes double
• PTZ camera: 25 to 60 W, sometimes needing PoE++
• Door access hub or reader: 10 to 15 W
• VoIP desk phone: 5 to 13 W

That IR line catches people. A camera measuring 7 W during a daytime walkthrough can pull 15 W the first cold night, once the illuminators come on and the heater kicks in. Size your budget during a daytime commissioning visit and you'll find this out at two in the morning.

Then add headroom

Take the total and add 25 to 30 percent. That covers:

• Devices you'll add in the next two years, and you will
• IR, heaters, and cold-weather draw
• Inrush when everything powers on at once after an outage
• The next access point model, which will draw more than the current one

A switch running at 95 percent of its budget has no room to absorb anything, and the failure mode isn't a clean error message. It's the switch dropping power to its lowest-priority ports, which is rarely the port you'd have chosen.

A worked example

A 12,000 square foot office:

  8 × access point         @ 25 W  = 200 W
 12 × indoor camera        @ 12 W  = 144 W
  4 × outdoor camera (IR)  @ 20 W  =  80 W
  2 × door access hub      @ 15 W  =  30 W
 14 × VoIP phone           @ 10 W  = 140 W
 ─────────────────────────────────────────
 Subtotal                           594 W
 +30% headroom                      178 W
 ─────────────────────────────────────────
 Required PoE budget               ~772 W

Forty ports of devices and roughly 772 W of budget. A single 24-port switch with a typical 400 W budget covers neither. You're looking at two switches, or one larger switch with a substantially higher budget, and the difference between guessing and calculating here is a four-figure line item.

Things that catch people

Budget is shared across the switch, not per port. A switch can be rated for 60 W on any individual port and still only have 400 W total. Four devices at 60 W and you've used 60 percent of the budget on four ports.

Stacked switches don't share budget. Each chassis has its own. Distribution matters, so put the high-draw devices where the power is.

Cable length costs you. The device-side numbers above assume a full 100 metre run. Short runs lose less, but never budget on the optimistic figure.

Bad cable costs you more. Undersized conductors, copper-clad aluminium, or a marginal termination all raise resistance and therefore loss. CCA is a recurring culprit. It's cheap, it passes a basic continuity test, and it performs badly under load. Use solid copper.

Injectors are a stopgap. One injector for one awkward device is fine. A closet full of them means an undersized switch and a nest nobody can trace.

Redundancy, if it matters

If the network genuinely can't go down, PoE has a specific weak point: the switch is now the power supply for every access point, camera, and door reader on it. Lose the switch and you lose physical security along with connectivity.

Options in rough order of cost: a UPS sized for the full PoE load, which is bigger than people expect since that 772 W is 772 W the UPS must also carry; a redundant power supply for switches that accept one; or splitting critical devices across two switches so no single failure takes out everything.

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None of this is difficult. It's arithmetic that has to happen before the purchase order, because the alternative is finding out during commissioning that the switch you bought can't run the devices you bought.

We do this calculation on every quote and show the working. Send us your device list and we'll size it.

• PoE
• Switching
• Design