PoE Power Calculator: Budget, Watts, Voltage Drop and Distance

Power over Ethernet moved from powering desk phones to powering almost everything on a modern low voltage network. Access points, cameras, door controllers, speakers, displays, sensors and lighting now all pull power from the same switch that carries their data.

That shift changed how the cabling has to be designed. A switch that comfortably ran forty phones can be overloaded by twenty cameras. A cable run that passes a data certification test can still deliver too little voltage to start a heated PTZ camera at the end of a long run. Both problems are avoidable, and both come down to arithmetic done before the cable is pulled.

The calculators below are built around published IEEE figures rather than rules of thumb. Nothing is sent anywhere, everything runs in your browser, and each one shows the numbers it used so you can check the result.

1. PoE budget calculator

Use this first. It answers the question a switch datasheet does not: after you add up every device you plan to connect, plus the power lost heating the cable, is there anything left in the switch budget?

Pick your switch from the presets or enter your own budget, add the devices you plan to connect, and set the average cable run length. The calculator returns both the measured draw and the worst case class allocation, because many switches reserve power by class rather than by what a device actually uses.

2. PoE voltage drop and distance calculator

Copper has resistance, so some of the power a switch sends never reaches the device. It is lost as heat in the cable, and the voltage arriving at the far end is lower than the voltage that left the switch port. On a long run with a hungry device, the voltage can fall below what the device needs to operate.

This calculator solves for the voltage actually delivered, the power wasted in the cable, and the longest run that still works with the cable and device you selected.

3. PoE standard and class finder

Before you can budget anything, you need to know what each device requires. Pick a device type or a class and this returns the standard it needs, the power the switch must reserve, the power the device receives, and the current the cable has to carry.

PoE standards compared

Three IEEE standards define active PoE. Each one built on the last, and all of them remain in service on live networks today.

Two vendor names appear often enough to cause confusion. Cisco UPOE delivers 60 W per port over four pairs and predates 802.3bt Type 3, which it closely resembles. Cisco UPOE+ delivers 90 W per port and aligns with Type 4. Both interoperate with standards based devices, but the pre-standard implementations negotiate power differently, so confirm behaviour before mixing generations on one switch.

Full PoE class table

Classification is how a device tells the switch how much power to reserve. The switch measures the device signature during startup, assigns a class, and holds that much of its budget. These figures come directly from IEEE 802.3bt.

The gap between what the switch reserves and what the device receives is not waste in the switch. It is the allowance the standard makes for the power lost in up to 100 metres of cable. On a short run, most of that allowance is never used, but the switch still holds it against the budget unless LLDP negotiation trims the allocation down.

How the math works

Every figure in the calculators above comes from three equations. They are worth understanding, because they explain why long runs and high power devices interact badly.

Step 1: loop resistance

Current has to travel to the device and back, so the resistance that matters is the round trip, called the loop resistance. With 2-pair power, one pair carries current out and one carries it back, and each pair has two conductors in parallel:

Loop resistance (2-pair) = resistance per metre × length

With 4-pair power, two pairs carry current in each direction, so four conductors share the load in each direction and the resistance halves:

Loop resistance (4-pair) = resistance per metre × length ÷ 2

This is the single biggest reason 802.3bt uses all four pairs. Spreading the current over four pairs halves the resistance and cuts the heat produced to a quarter of what two pairs would produce at the same current.

Step 2: delivered voltage

A PoE device is a constant power load, not a constant current load. As the voltage arriving at the device falls, the device draws more current to maintain the same wattage, which increases the drop further. That feedback loop means you cannot simply multiply current by resistance. Solving it properly gives:

Device voltage = ( V_switch + √( V_switch² − 4 × P_device × R_loop ) ) ÷ 2

If the value inside the square root turns out negative, there is no stable operating point at all. The link cannot deliver that much power over that much resistance at that voltage, and the device will either fail to start or cycle repeatedly. The calculator flags this case rather than printing a meaningless number.

Step 3: power lost in the cable

Current = P_device ÷ V_device

Power lost in cable = Current² × R_loop

Power the switch must supply = P_device + Power lost in cable

Cable choice and resistance

Cable is where most PoE problems are created and where most of them can be prevented. Two properties matter: conductor gauge and conductor material.

IEEE 802.3bt sets a maximum channel loop resistance of 12.5 ohms for a 2-pair pairset, and 6.25 ohms for both pairsets in parallel when powering over four pairs. Compare those numbers against the table. A 100 m channel of solid copper Cat5e sits at 8.42 ohms with room to spare for patch cords and connectors. The same length of CCA sits at 13.05 ohms and has already failed before a single patch cord is added.

Typical device wattage

Use these as planning figures only. Actual draw varies with model, firmware, radio configuration, heater state and how many things are plugged into the device itself.

Heat, bundles and derating

Current flowing through a conductor produces heat. In a single cable that heat escapes easily. In a bundle of 96 cables tied together in a conduit, the cables in the middle have nowhere to send it, and the bundle temperature rises above ambient.

Higher temperature means higher copper resistance, which means more voltage drop and more heat. It also degrades insertion loss and shortens the supported length for data. ISO/IEC 14763-2 and EN 50174-2 give the framework for managing this, and ANSI/TIA TSB-184-A covers the same ground in North America.

The practical rules that come out of it:

• Keep bundles at 24 cables or fewer where PoE loading will be high
• Prefer larger conductors, since 22 and 23 AWG run cooler than 24 AWG at the same current
• Account for ambient temperature in ceiling voids, rooftops and unconditioned spaces, not just the occupied room below
• Avoid running fully loaded PoE bundles through thermal insulation
• Use open ladder or basket tray in place of sealed conduit where the design allows
• Reduce the maximum supported channel length when the bundle temperature rises well above 20 degrees Celsius

Common PoE design mistakes

PoE design checklist

Frequently asked questions

Technical note: These calculators use published IEEE 802.3af, 802.3at and 802.3bt figures and nominal solid copper conductor resistance at 20 degrees Celsius. Real installations vary with temperature, bundle configuration, patch cord construction, connector performance and manufacturer implementation. Treat the results as design guidance and confirm against equipment datasheets and field test results before committing to a design.