PoE Power Supply Budget Calculation

08 Apr 2021 | Filed under: Industry Knowledge

The use of Power over Ethernet (PoE) is expanding due to its proven reliability when powering devices over low-voltage ethernet network cables. For the end-user, PoE is more or less hassle-free. That is until they connect a new device, and it won’t power on.

Power budget depletion is a real problem, and it’s frustrating for clients. Which is why getting the PoE power budget calculation correct is so important.

In this guide, we’ll cover the steps to follow to calculate your PoE power budget properly.

Step 1: Add Up The Demand For PoE In Watts

The first thing to consider is the devices connected to the PoE switch and how much power they’ll require. The draw varies depending on the device. For example, an IP camera with PTZ movement and a built-in heater consumes significantly more energy than a wireless router.

Add up the expected demand for power for each Powered Device (PD) in watts. Allow for the maximum power and upper end of PD classification. Any unspecified devices should be assumed Class 0.

For example, an IEEE802.3af, Class 0 device may consume 9 watts; however, as it’s Class 0, assume 15.4 watts.

Round the numbers up, occasionally, to account for the additional consumption of the UTP (unshielded twisted-pair) ethernet cable that runs between the PD and PoE switch.

As another example, if an IEEE802.3at Class 4 IP camera consumes 25.5 watts, round to 30 watts, which adds a buffer to balance out the loss between the PoE switch and the device.

Include room for future capacity. It’s convenient to have at least one spare port for diagnostics, troubleshooting, or monitoring. And many clients want extra ports for the option to add more PD devices in the future. However, as long as devices are appropriately selected and integrated, accounting for spare ports isn’t required for a PoE power budget calculation.

Step 2: Scale For The Operating Environment

It’s important to account for environmental conditions when performing a PoE power budget calculation.

Accommodate for the conditions. Expect the long-term performance of a power supply to be 70% of its rating in a benign/conditioned environment (somewhere with steady temperatures between 32°F/0°C and 120°F/50°C). In a benign environment, divide the total wattage from step one by 0.7.

If the power supply is subject to a harsh environment (cold temperatures less than 32°F/0°C or heat above 120°F/50°C), plan for diminished performance. Divide the total wattage from step one by 0.6 for this type of setting.

When in extreme conditions, choose a rugged, “hardened” device, such as ComNet’s PS-DRA Series of environmentally hardened, DIN rail mountable, DC 48V power supplies.

Take this harsh scenario, for instance:

A switch and its power supply will be stored in a metal enclosure, exposed to direct sun, at a site in the northeastern United States. In winter, the temperature inside the enclosure could be as low as –10°F/–24°C. And in summer, it could be as high as 140°F/60°C. Accounting for the temperature inflections, expect the power supply to operate at 60% of its power rating.

It’s always safe to assume a conservative long-term performance drop of 50%, no matter the conditions. That means totaling the anticipated power demand (step 1) and dividing by 0.5 (step 2) to get a power budget in watts.

Step 3: Select The Power Source

After determining the demand for PoE power and accounting for the surroundings, it’s time to select an appropriate supply. ComNet has DC 48- to 56-volt supplies with ratings from 30 watts to 480 watts.

Select a supply that meets your PoE power budget calculation from the lists below:

For conditioned or benign environments:

• CONSIDER
• PS-DRA30-48A
• PS-DRA60-48A
• PS-DRA120-48A
• PS-DRA240-48A
• PS-DRA480-48A
• WHEN STEP 2 IS BETWEEN
• 0 W and 20 W
• 20 W and 40 W
• 40 W and 80 W
• 80 W and 160 W
• 160 W and 320 W

For unconditioned or harsh environments:

• CONSIDER
• PS-DRA30-48A
• PS-DRA60-48A
• PS-DRA120-48A
• PS-DRA240-48A
• PS-DRA480-48A
• WHEN STEP 2 IS BETWEEN
• 0 W and 20 W
• 15 W and 30 W
• 30 W and 60 W
• 60 W and 120 W
• 120 W and 240 W

Alternatively, contact the ComNet Design Center for further assistance, as needed, at your convenience.

The Final Step: Get Ready For Installation

After determining the PoE power budget, you’re soon set for installation. Remember to plan for purchases such as the power supplies themselves, the labor to install cabling for the supplies, and the cost for integration (ex. sourcing material, voltage drop calculations, and device compatibility checks).

ComNet will gladly help you get the best results

Our technical support and sales team has extensive experience working with network components. We’ll help ensure your PoE power supply can deliver the right amount of power for your needs.