January 2, 2024 – If you ask an accountant what a budget is, they’ll tell you it’s a spending plan based on how much money you earn versus what you spend over a certain period.

Budgets also exist in industrial networking circles, but instead of money, the currency is power. Let me explain…

Industrial networks deploy PoE (power-over-Ethernet) switches, injectors, and media converters as power-sourcing equipment (PSE) to supply electricity to powered devices (PD) that, in turn, spend that power.

The ranks of PDs can include such devices as IP cameras, WAPs, VoIP, laptops, sensors, among others. The total wattage required by all the PDs connected to the PSE cannot exceed the maximum wattage supplied by that equipment. When power is insufficient, the devices will not boot up or may be damaged. In fact, low power is a commonly responsible for a powered device powering On and Off, or reloading intermittently midway through a run.

This leads us back to the concept of the “PoE budget”, which refers to the total amount of power that the power-sourcing equipment can supply to connected PDs. While PoE is measured by wattage, a miscalculation when developing a PoE budget will be measured in dollars—in the form of unavailable or damaged devices, production downtime, or network outages.

PoE types

Before we discuss PoE budgets, let’s review the various IEEE standards for power-over-Ethernet:

• Type 1 (PoE): 802.3af supplies up to 15.4 W per port to a PD that receives a minimum of 12.95 W* over 2 pairs.
• Type 2 (PoE+): 802.3at supplies up to 30 W per port to a PD that receives a minimum of 25 W* over 2 pairs.
• Type 3 (PoE++): 802.3bt supplies up to 60 W per port to a PD that receives a minimum of 51 W* over 2 or 4 pairs.
• Type 4 (“high power PoE”): 802.3bt supplies up to 100 W per port to a PD that receives a minimum of 71-90 W* over 4 pairs.

* Note: The loss in output from what a PoE industrial switch injects into an Ethernet cable and the actual wattage that reaches the PD is due to a percentage of the power being converted to heat.

There are industrial PoE switches in the market that do not conform to IEEE standards. These non-standard PoE switches (“passive PoE switches”) will constantly supply power over Ethernet lines at a specific voltage, regardless of whether the connected device supports PoE or not.

But when poorly matched, a passive switch will burn out a terminal device. Conversely, an IEEE-compliant PoE switch will detect and classify a connected PD to verify whether it is within capacity before it powers On the PD, thereby preventing damage. To ensure safety and interoperability, only choose products that comply with the IEEE PoE standards.

The amount of power used by a PoE device can range from just a few watts to 30 watts, depending on the device and the PoE standard used. It is important to check the powered device’s specs to determine its power requirements and ensure the network switch or midspan injector can provide enough power.

Maximum power consumption

Just because a powered device supports a PoE standard type, it doesn’t necessarily mean that PD will draw that type’s full power capacity. For instance, a wireless access point (WAP) that supports IEEE 802.3af doesn’t necessarily require the full 15.4 watts; in fact, it may only need 5 watts.

To know the WAP’s power requirements (or any powered device’s actual power requirements), you check the manufacturer’s specs for “maximum power consumption”. When the maximum power consumption for a device is far less than a port’s full power capacity, the system administrator may choose to configure the “port limit”, thereby putting a ceiling on how much power it supplies to any given PD.

Maximum power consumption is a vital parameter, as you will use it when calculating your PoE budget requirements. Thankfully, the math is simple: the maximum power consumption of all the powered devices that will populate a PoE switch is added up. (Where possible, round up the total wattage to give yourself a little buffer.)

For example, let’s say your application calls for a PoE switch to supply power to four 9-megapixel 360-deg outdoor cameras. Each camera has a maximum power consumption of 12.5 W. Therefore, the requirement side on the PoE ledger is 12.5 W x 4 devices = 50 W.

What is the total power budget?

A total power budget refers to the maximum amount of power that is available for use in a given system or application. This budget is typically defined by the power supply or energy source that is used to provide power to the system.

The total power budget is the sum of the power consumption of all the components or devices within the system, including processor, memory, storage, display, network interface, and other peripherals.

In electronic systems, the total power budget is an important consideration because exceeding the available power supply can cause the system to malfunction or fail. Designers of electronic systems must carefully balance the power requirements of each component to ensure that the total power budget is not exceeded.

Calculating the PoE budget

So, how do I know my PoE budget? How much power does PoE consume?

Let’s assume you plan on using an 802.3af Ethernet 4-port switch that supplies 15.4 watts per port. Would this switch be appropriate for the camera application?

The switch’s PoE budget is calculated by multiplying the number of ports by the wattage supplied per port. In this case, 15.4 W x 4 ports = 61.6 W. So, yes, the switch will work.

That said, it’s cutting it close in the real world of industrial networks. For one, the actual wattage delivered to the camera over copper lines will be less than 15.4 W (but not less than 12.95 W). So you’re still in the clear. However, network administrators often find it convenient to have at least one spare port for diagnostics, or several open ports to add more edge devices in the future.

Additionally, PoE’s “golden rule” is that the more power your power-sourcing equipment has, the more power you can deliver per port. Therefore, choosing a higher-power switch that can deliver total higher wattage reduces the chances of needing an upgrade later.

Another caveat: it is rare for a powered device to require its maximum power consumption, since this amount of power is only needed when the device is operated at full capacity. Yet, when a PD needs less than the maximum wattage to operate (as is often the case), the PoE switch may still reserve the maximum amount of wattage based on the PoE class, as shown in the chart. (A PD that supports PoE, but without classification, is assumed to be Class 0.)

Another concern: a powered device can accidentally be overloaded by increasing its draw after the PoE Budget has been established and an application installed. Consider what would happen were a heater to be added to the outdoor cameras mentioned above. A single camera heater can require up to 35 W to operate, bringing the camera’s power requirements to 47.5 W. This would badly deplete the 15.4 W maximum power per port of your IEEE 802.3af 4-port switch.

Finally, when planning PoE projects, remember to consider the environmental conditions to which your power-sourcing equipment and powered devices will be subjected. In an extremely hot or cold environment, power delivery can be diminished by as much as 40% of its rating. In those situations, seek out environmentally hardened PoE equipment to ensure the highest power delivery.

Henry Martel’s career began as an interior communication electrician aboard the USS Abraham Lincoln—a Nimitz-class aircraft carrier in the U.S. Navy—where he served as operator/administrator of shipboard communication system. He currently serves as field application engineer with Antaira Technologies, a developer and manufacturer of industrial networking and communication product solutions.

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