One cable, two jobs

Power over Ethernet works by injecting power into the Ethernet cable, alongside the data, so the cable carries both, and the powered device draws its power from the cable, while receiving and sending data over the same cable. The switch is the power source, the cable is the delivery, and the device is the load, and all three must agree on the power standard, the power budget, and the cable type, because a mismatch on any of those will leave the device unpowered, underpowered, or unreliable, however well the data flows.

PoE suits devices mounted where power outlets are scarce: cameras on ceilings, access points on walls, phones on desks, and sensors on poles, where running a power cable to each device would cost more than the device itself, and where a single Ethernet cable, already run for data, can also carry power. A buyer who needs PoE should start from the devices, their power draws, and their locations, because the devices decide the switch, not the other way round.

The power budget: the total matters

A PoE switch's power budget is the total power it can deliver across all its ports, and a buyer must ensure the budget covers all the devices at once, not just one at a time, because a budget that covers each device singly but not all together will fail when all devices draw power at once, with devices dropping offline, rebooting, or failing intermittently, in failures that appear as network faults but are really power faults. Add the power draw of every device, add a margin for growth, and choose a switch whose budget covers the total, because a switch cannot deliver more power than its budget allows.

A buyer who counts only the largest device, or only the average draw, will underestimate the budget, because devices draw their rated power when they need it, not their average, and because a network whose devices all draw power at once — at boot, at peak use, or in an emergency — needs a budget that covers the peak, not the average. Count the peak, add a margin, and choose a switch that covers the peak, because a switch that covers the peak will serve, while a switch that covers only the average will fail at the peak.

Ports: count devices, uplinks, and spares

Count the devices, add the uplinks, add a spare port for growth, and choose a switch that covers the total, because a switch cannot gain ports after installation, and because a network that outgrows its switch must replace it. A PoE switch's ports are its capacity, and a switch with too few ports forces a second switch, with its own power, its own management, and its own cost, while a switch with too many wastes money, but wastes less than a second switch, because a spare port costs little, while a second switch costs much.

A buyer who counts ports should also count PoE ports separately from non-PoE ports, because a switch may have ports that carry data but not power, and a device that needs power must connect to a PoE port, not just any port. Check which ports deliver power, and how much each delivers, because a device that needs more power than its port can deliver will not run, however many ports the switch has, and because a port's power limit is a fact of the switch, not a guess from its port count.

Cable and distance: the same rules

The Ethernet cable must suit the power and the data, because a cable that carries data well may not carry power well, and a cable that is too long will lose power to resistance, leaving the far-end device underpowered, however well the switch's budget covers it. Use cable rated for PoE, keep runs within the rated distance, and check the cable's type, because a cable that is not rated for PoE may overheat, lose power, or fail, and because a run that exceeds the rated distance will deliver less power than the device needs, however good the switch and the cable appear.

A buyer who needs PoE over a long run should confirm the cable type and the distance for the exact installation, in the inquiry, naming the devices and their power draws, so the supplier can confirm whether the run will deliver enough power, rather than assuming it from a general claim. A long run that delivers too little power is a device that fails intermittently, and an intermittent failure is the hardest to diagnose, because it appears and disappears with the load.

What PoE does not do

PoE does not power every device, because some devices draw more power than PoE can deliver, and a device that needs more power than its port can supply must be powered separately, however convenient PoE appears. PoE does not replace a power supply for high-power devices, for devices with motors, heaters, or high-power radios, and a buyer who tries to power such a device over PoE will find it unpowered, underpowered, or unreliable, because PoE's power limit is a fact of the standard, not a limitation to be worked around.

PoE also does not simplify a network whose devices are already powered, because a device with its own power supply gains nothing from PoE, and a switch that delivers PoE to already-powered devices wastes its budget, while adding cost and complexity. Use PoE where it saves a power cable, where it enables a location without an outlet, and where the device's power draw fits the port's limit, and use separate power where it does not, because PoE is a tool, not a default, and a tool used where it does not fit is a tool that fails.

Choosing a PoE switch: the checklist

Choose by the power budget first: add every device's peak draw, add a margin, and choose a switch whose budget covers the total. Choose by the port count second: count the devices, add the uplinks, add a spare, and choose a switch that covers the total, with PoE ports for every device that needs power. Choose by the rate third: match the switch's rate to the devices' rates, because a switch cannot make a device faster than it is, and a faster switch for slower devices wastes money, while a slower switch for faster devices limits the network.

Choose by the management form fourth: unmanaged for a simple network that must simply work, managed for a larger network that needs configuration, monitoring, or traffic control, because an unmanaged switch cannot gain management after installation, and a managed switch costs more than an unmanaged one, for features a simple network will not use. Confirm all four — budget, ports, rate, management — for the exact devices, in the inquiry, before ordering, because a switch chosen on fewer than four will mismatch on the one not checked.

Installing PoE: the practical points

Install PoE switches where their power can be delivered, their heat can be dissipated, and their cables can be run, because a PoE switch draws power for itself and for its devices, and it dissipates that power as heat, so a switch in a hot, unventilated cabinet will overheat, throttle, or fail, however good its budget appears. Ventilate the cabinet, keep the cable runs within their rated distance, label the PoE ports, and test every device at peak draw, before commissioning, because a device that works at idle but fails at peak will fail in service, and a test at peak is the test that matters.

Test the installation as a whole, not device by device, because a switch whose budget covers each device singly may fail when all devices draw power at once, and because a network whose devices are tested singly but not together will fail together, at the worst time. Power all devices at once, check that all stay online, and check that the switch's temperature stays within its rating, because a test that passes singly but fails together is a test that has not tested the installation.

What to confirm, before ordering

Before payment, confirm the specification for the exact model, including its power budget and its PoE ports, the minimum order quantity for that model, the lead time with its meaning, the warranty terms, the payment and shipping terms, and the certificates that apply to your market, by name, with documents attached. Name the switch type, the devices it must serve, their power draws, the quantity, and the market, so the supplier can confirm for your installation, rather than for a general case, and so the confirmation you receive is the confirmation you can rely on.

A buyer who confirms the budget, the ports, the rate, and the management, in writing, before payment, has done the work that prevents the common failures: the under-budgeted switch, the port-short switch, the rate-mismatched switch, and the unmanaged switch that later needs management. A buyer who skips that work has saved an hour and risked a shipment, and the hour saved is rarely worth the shipment risked.