PoE 功率预算详解：为摄像机和 AP 供电

The most common symptom we see on CCTV service calls: everything works during the day, then around 6 pm some cameras start rebooting or the picture drops in and out. The camera is rarely faulty. The switch simply runs out of PoE budget once every camera's IR illuminators switch on at the same time.

This guide explains how a switch actually counts power, and how much headroom you need.

Two terms first

Power over Ethernet (PoE) carries data and power over a single network cable. IP cameras, access points, IP phones and face-recognition terminals don't need their own outlet, and everything can ride on one UPS in the rack.

• PSE (power sourcing equipment): the switch or injector supplying power
• PD (powered device): the device receiving it

The standards

The gap between PSE and PD is the worst-case loss over 100 m of cable. Maximum PoE distance is the same 100 m as ordinary Ethernet.

Watch out for "passive PoE" (24 V or 48 V applied without any negotiation). It is not an IEEE standard. Some outdoor wireless gear still uses it, and plugging the wrong device into it can kill that device instantly.

A switch has two limits, not one

The first is maximum power per port, for example 30 W for PoE+. The second is the total budget the whole switch can deliver at once, for example 370 W on a typical 24-port model.

A 24-port PoE+ switch cannot deliver 30 W on every port simultaneously (24 × 30 = 720 W). The box says "24-Port PoE+" in large print. The total budget is on the last page of the datasheet. Read that page before you order.

The part most people miss: how the switch allocates power

Many switches don't deduct from the budget based on what a device actually draws. They reserve power by class, as declared by the device. A camera that announces itself as Class 4 gets 30 W reserved, even if it really draws 9 W. The switch then reports the budget as exhausted and refuses power to the next port, while real consumption is far below the limit.

Most managed switches let you choose the mode (class-based, actual/consumption-based, or LLDP negotiation). Know which mode your switch uses before you calculate. Unmanaged PoE switches usually can't be configured and tend to allocate by class.

How we calculate on site

1. List every device with its maximum draw from the device datasheet (for cameras, the figure with IR on; for outdoor cameras with heaters, the figure with the heater running)
2. Note each device's class, in case the switch allocates by class
3. Add it all up, then add 20–30% headroom
4. Compare with the switch budget, and confirm each port supports the standard that device needs

Example: a mid-sized warehouse

A 370 W switch is not enough. Choose a model with roughly 500 W or more, or split the load across two switches (cameras on one, APs and phones on the other). The 40 W PTZ cameras exceed PoE+, so they need 802.3bt ports.

The same warehouse, as a class-based switch sees it

Each device declares a class when it connects. In class mode, the switch reserves the full class amount at the PSE port:

Now redo the warehouse. The 12 bullet cameras declare Class 3: 12 × 15.4 = 184.8 W. The two PTZs declare Class 6: 2 × 60 = 120 W. The six APs declare Class 4: 6 × 30 = 180 W. The four phones declare Class 2: 4 × 7 = 28 W. Reserved total: 512.8 W, against a real draw of 316 W.

So on a 500 W switch running in class mode, the last device to boot gets no power, even though the switch is only delivering about 60% of its budget. Two fixes: switch the allocation to actual consumption or LLDP negotiation where the switch supports it, or size the budget for the reserved figure. On unmanaged switches only the second fix is available.

Checking a switch that is already running

Every managed PoE switch shows, per port, the class detected, the power allocated and the power actually consumed, plus the total remaining budget. Look at that page twice: once at midday and once after dark with IR on. If the remaining budget at night is under 15% of the total, you have no room for the next camera. Write both readings into the site documentation so the next engineer has a baseline.

If you'd rather not do it by hand, try our PoE budget calculator.

What causes the "random" outages

• Reading "PoE+" on the box and never checking the total budget
• Not knowing the switch allocates by class, calculating from real draw, and then the last few ports get no power
• IR cameras draw more at night, so the fault only appears after sunset, and a technician who visits during the day finds nothing
• Cheap CCA (copper-clad aluminium) cable has higher resistance, heats up and drops voltage under high-power PoE. Use solid copper only
• A switch in an unventilated cabinet overheats, and some models cut their PoE budget to protect themselves

Don't forget the UPS

All PoE power comes from the switch. When mains fails, cameras and Wi-Fi go dark with it. In Vientiane and many provinces we regularly see brownouts and flicker between 17:00 and 19:00 when load peaks, which is exactly when IR switches on.

Size the UPS for the switch's own consumption + the actual PoE load (not the maximum budget) + everything else in the rack, such as the NVR and firewall. For switches that power security cameras we recommend an online UPS, or at minimum a line-interactive unit with AVR. You can check runtime with our UPS runtime calculator.

Cameras or APs more than 100 m away

Fence lines, mine gatehouses, car parks: plenty of points sit more than 100 m from the nearest switch. There are three proper options:

Don't stretch copper past the standard just because the camera came up during testing. Testing happens in daylight. At night, with IR on, or on a hot April afternoon, the voltage at the far end won't be enough.

Our view for Lao sites

Keep at least 25–30% headroom. A rack cabinet without air conditioning easily reaches 40°C inside, and most sites end up adding cameras later.

For outdoor cabinets at mines or car parks, use industrial PoE switches rated for wide temperature ranges, surge protection on both the AC side and the LAN side, and a dust-tight enclosure (IP55 or better). In the rainy season, lightning destroys more PoE ports than everything else combined.

One more thing: when switches come in from Thailand, check the plug and input voltage, and keep the datasheet for the exact model you bought. Sub-models in the same series often have different budgets.

Send us your device list

Take a photo of your camera and AP list (brand, model, quantity) and send it on WhatsApp. Our engineers will check each device's class, work out the budget and UPS, and tell you whether the switches you already own can still do the job.