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Overview: This document discusses the calculation of power loss in Power over Ethernet (PoE) applications due to cable resistance. It explains why PoE standards specify higher output voltages for Power Sourcing Equipment (PSE) compared to the voltage at the PoE Powered Device (PD).

PoE Standards and Parameters:

802.3af (Type 1): Power available at PD is 12.95 W, with a maximum power delivered by PSE of 15.40 W. Voltage range at PSE is 44.0–57.0 V, and at PD is 37.0–57.0 V. Maximum current is 350 mA, with a maximum cable resistance of 20 Ω for Category 3 or lower cables.
802.3at (Type 2, PoE+): Power available at PD is 25.50 W, with a maximum power delivered by PSE of 34.20 W. Voltage range at PSE is 50.0–57.0 V, and at PD is 42.5–57.0 V. Maximum current is 600 mA, with a maximum cable resistance of 12.5 Ω for Category 5 or higher cables.

Calculation Method:

Calculate Output Current: I_current = P_out / V_out
Calculate Voltage Drop: V_drop = I_current * R_cable
Calculate Actual Voltage at PD: V_pd = V_out - V_drop
Calculate Actual Wattage at PD: P_final = V_pd * I_current

Examples:

Example 1 - PoE Application: Using a 15.4-watt PoE port with CAT5 cable (12.5Ω resistance) over 100 meters, the available power at the PD is calculated to be 13.98 watts.
Example 2 - PoE Plus Application: Using a 30-watt PoE port with CAT5 cable (12.5Ω resistance) over 100 meters, the available power at the PD is calculated to be 25.7 watts.

Contact Information: For support and sales, contact Comtrol Corporation via email at [email protected] or call +1.763.957.6000. Warranty includes a 30-day satisfaction guarantee and a 5-year limited warranty.

Catalog excerpts

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

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• 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...

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