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How to Calculate PoE Budgets

PoE is one of the greatest things in networking. It allows you to power a device with an Ethernet cable. This cuts on infrastructure costs and gives you a ton more flexibility in how you design a network.

But, PoE does have limitations. You can only draw so much power from a single switch, and figuring that out is essential for network design. To help you with that, you can calculate your PoE budget, and that’s what this tutorial is all about.

What Is a PoE Budget?

Let’s start with the obvious question. What is a PoE budget? How does it work?

For starters, we’re not talking about the financial budget you set before shopping for devices. Instead, this is a power budget that helps you figure out exactly which devices and in what configurations you can connect all of your stuff to a PoE switch.

The budget is an idea to help you look at all of the power available from a PoE switch and how you can best utilize it. To do that, you have to consider the device as a whole, the ports available, and the devices that you want powered when they connect to your power sourcing equipment. Not to worry, all of that is going to be covered in detail.

Types of PoE

Before we can really get into PoE budgets, we first have to talk about the different types of PoE available on the market right now. For the most part, there are four types. While they technically do have numbers (Type 1, 2, etc.), they are most commonly denoted with acronyms and plus signs.

The important thing to remember is that each type of PoE is designed around different power constraints, so you need to know what type of PoE you’re working with before you can calculate your power budget.

PoE

The original power over Ethernet configuration is usually denoted by the simple “PoE.” There are no plus signs or extra notation. This form supports up to 15.4W per port. That is the lowest theoretical limit on port power for any type of PoE. Everything on the list below can handle more power.

This is also a good moment to pause and talk about how power draw works. The amount of power that flows from the PoE switch to the connected device is determined by the endpoint device. If you connect a security camera that needs 10W to operate, then that’s how much power is going to flow out of the port.

This is the concept of power draw, in that a device draws power from a power source. The source doesn’t push the power through. The important takeaway here is that a 10W device will not use up all 15.4W available through a PoE port. It will only use 10W. That’s important when calculating your budget.

PoE+

Next up is PoE+. These days, it’s more common on devices than PoE, and it supports 30W per port. You’ll notice when you browse devices that increasing the power capacity per port also increases the price of the switch. PoE+ switches, on average, cost more than PoE switches, and the trend continues from there.

PoE++

PoE++ is going to be common on medium and high-end networking switches. While this does push the price tag up a bit, you get a lot more freedom with your design. That’s because PoE++ supports 60W per port. That enables the vast majority of PoE-capable devices to function just fine. You won’t find nearly as many PoE devices that need more than this amount of power — though some do exist.

High Power PoE

High Power PoE is actually less common than PoE++ right now. This is simply because there are fewer devices that actually need this much power. That’s a trend that could change in the future, but in many cases, the 100W per port that you get with High Power PoE just isn’t necessary.

It’s also a cost-prohibitive technology at the moment. Outside of niche applications, you probably don’t need it, so why spend the money? Still, if you are utilizing this technology, then you’ll need to know the power limitations to calculate your budget.

How to Calculate the Budget

Now that we have the necessary information, we can get into calculating PoE budgets. To do this, you have to consider four things: the limit of each port, the theoretical limit of the switch, the actual power capacity of the switch, and the power draw from your powered devices.

The limits of the ports are the easiest part. You can figure that out with the sections above. These limits range from 15.4W to 100W, depending on your PoE type. When you’re calculating your budget, you know that you can’t connect any devices that need more power than is available from a single port. You can’t splice two ports together to double your power. So, you need to ensure that you’re getting switches where each individual port has enough power capacity for the devices you want to connect.

Next up, we can calculate the theoretical limit of the switch. This is a pretty easy math problem. You take the total number of PoE ports and multiply them by the maximum power per port. When you’re doing this, remember that the number of ports on a switch is not always equal to the number of PoE ports on the switch. Plenty of switches have a handful of PoE-enabled ports while the rest are non-PoE. Take that into account for the calculation.

The purpose of calculating this theoretical limit is to rule out hardware that obviously can’t work. If you have 1000W of equipment that you need to connect, and the theoretical limit of your switch is 600W, it’s an obvious no-go, and you can figure that out without digging into specification manuals.

The next aspect of your budget is the physical limit of the switch. You could have a 24-port PoE+ switch. The theoretical maximum budget for that switch is 720W. But, that switch might have a power supply that only provides 500W. This is often the most important part of figuring out your PoE budget. If the power supply only provides 500W, then you can’t use every port at the maximum draw. In fact, you would only be able to use 16 of the ports at the maximum draw at one time. We’ll get into this more with a real-world example.

The last consideration is your connected devices. If none of them are drawing at the maximum capacity, then it gives you more flexibility with your network. Likewise, if a device needs more power than a port can provide, you need to upgrade your PoE type.

A Real World Example

So, you can look at the specs of your switch to see how much power is available in order to figure out how you can connect your endpoint devices.

Let’s use a real example with the WS-C3750X-48PF-S. This is a 48-port PoE+ switch. So, the maximum power for each port is 30W. The maximum theoretical capacity is 1,440W, but the switch actually only has 800W to work with.

So, you can only connect 26 maxed-out ports at one time.

Or, using a more realistic example, you might wonder if you can run 7 devices at 30W and 12 devices at 15.4 W. If you did, that would draw a total of a little less than 400W, which means you’re fine. You still have half of your power budget available for expansion later.

This example shows how you can compare the different specifications to figure out how to plan your PoE networks. With that information, you can find economical solutions that power everything adequately and create a reliable, powerful network.

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