Control* CALCULATING POE POWER LOSS DUE TO CABLE RESISTANCE In PoE applications, there are power losses due to cable resistance. That is the reason why the PoE standard defines a higher PoE output voltage for PSE (Power Sourcing Equipment) than the PoE output voltage at the destination, which is a PoE Powered Device (PD). The voltage dictates how much power is available at a PoE powered device. To meet PoE standard, PoE injector ports and PoE powered devices have to meet the following voltage, wattage, and current requirements: Standard PoE parameters and comparison In any PoE applications, it is critically important that a cable power loss calculation be performed to ensure the PoE PD devices will have required power available to them. The following method can be used to calculate the power loss due to cable resistance and the available power at the PoE powered device: • Step 1 - Calculate Output Current o Icurrent = Pout/ Vout Where: Icurrent = Output current from PoE Switch Pout = Output power from PoE Switch out = Output voltage from PoE switch • Step 2 - Calculate voltage drop across the cable due to cable resistance o Vdrop = Icurrent * Rcable Where: o Vdrop = Voltage drop over the cable length due to cable resistance o Rcable = Cable resistance • Step 3 - Calculate the actual voltage (Vpd) at the destination (PoE PD device) o Vpd = Vout-Vdrop • Step 4 - Calculate the actual wattage (Pfinal) available for the PoE PD device o Pfinal = Vpd * Icurrent

• Example 1 - PoE Application (15.4-watts) using RocketLinx ES7510 PoE port Given the above information, the power loss and the actual wattage at the PoE PD (Pfinal) can be calculated as follows: o Step 1: Icurrent = Pout/ Vout = 30/ 53= 0.56A = 560mA o Step 2: Vdrop = Icurrent * Rcable = 0.56 * 12.5 = 7VDC o Step 3: Vpd = Vout - Vdrop = 53 - 7 = 46VDC o Step 4: Pfinal = Vpd * Icurrent = 46 * 0.56 = 25.7-watts As illustrated above, the available power at the destination (PoE PD) will be 25.7-watts with the installation shown in the above table. Warranty Information Sales Support Technical...