TL;DR: PoE and PoE+ are two IEEE standards for sending power down an ethernet cable. PoE (802.3af) delivers 15.4 W at the switch and PoE+ (802.3at) delivers 30 W. The number that matters is what reaches the device after cable losses, which is lower in both cases. Pick the standard from the device’s draw, not the switch’s label. Our network infrastructure team sizes the power budget before the switch is ordered.
A common PoE mistake in commercial buildings is not choosing the wrong standard. It is reading the wattage on the switch datasheet, assuming the device at the far end receives that figure, and finding out at commissioning that it does not. This post covers what each standard actually delivers, where the difference bites, and how to size a deployment properly.
What is the difference between PoE and PoE+?
PoE and PoE+ differ in how much power the switch port can supply. PoE, defined in IEEE 802.3af, supplies up to 15.4 W per port. PoE+, defined in IEEE 802.3at, roughly doubles that to 30 W per port. Both run over the same ethernet cabling and use the same RJ45 connector, so the cable does not change.
What changes is the device you can hang off it. A basic wireless access point, a desk phone or a fixed indoor camera usually sits inside the PoE budget. A pan tilt zoom camera, a camera with a heater for outdoor use, a tri band access point or a small display is normally PoE+ territory.
The standards are backward compatible in the useful direction. A PoE+ switch negotiates with the device and supplies only what that device requests, so plugging an older PoE device into a PoE+ port is safe and normal.
PoE wattage: what the device actually receives
Every standard quotes two numbers, and the smaller one is the one that matters. Power is lost as heat in the copper between the switch and the device, so the figure available at the far end is always below the figure leaving the port.
The Fiber Optic Association’s overview of Power over Ethernet sets them out side by side.
| Standard | Common name | At the source | At the device |
|---|---|---|---|
| 802.3af | PoE | 15.4 W | 13 W |
| 802.3at Type 2 | PoE+ | 30 W | 25.5 W |
| 802.3bt Type 3 | PoE++ | 60 W | 51 W |
| 802.3bt Type 4 | PoE++ | 99 W | 71.3 W |
This is where datasheets mislead. A camera rated at 25 W will not run reliably on PoE, because PoE puts 13 W at the device, not 15.4 W. It needs PoE+. Sizing from the source column is how installations end up with equipment that boots, runs for a while, then drops under load.
The same FOA reference notes that “PoE uses a 48 volt power supply and requires cable of Cat 5 rating or higher”, which rules out running standards based PoE over older or damaged cable and expecting it to behave.
Is PoE++ worth specifying?
PoE++ is the common name for IEEE 802.3bt, which comes in two types delivering 51 W and 71.3 W at the device. It exists because a growing number of commercial devices exceed what PoE+ can supply, and it uses all four pairs in the cable rather than two.
In commercial buildings we specify it for outdoor cameras with heaters and wipers, high density tri band access points, powered displays and signage, some access control door controllers, and LED lighting where a building is running power over the data cabling.
The cost question is usually about the switch rather than the cable. Cat6 and above already carry PoE++ in most installations, so the decision is whether to buy switch capacity now or re-do the electronics later. On a building that will outlive several generations of switches, that is the same argument we make about Cat5e versus Cat6.
The part that catches people out: heat in the bundle
Power in a copper cable produces heat, and cable in a commercial building does not sit in open air. It sits bundled, often tightly, in trays and conduit where heat has nowhere to go.
The FOA reference is blunt about it, warning that “UTP cables are often bundled by the hundreds in cable trays where heat is hard to dissipate” and that bundling hundreds of powered cables together is “likely to cause serious heating”. Heat raises conductor resistance, which increases loss, which produces more heat.
The practical consequences are bundle sizes, pathway design and cable choice, not the switch. Larger conductors dissipate better, smaller bundles run cooler, and a design that was fine at PoE can misbehave once every drop is carrying PoE+. This is why we treat a PoE rollout as a cabling question first, which is the same discipline behind our approach to the network closet and the network rack.
How to size a PoE deployment properly
Start from the devices, not the switch. List every powered device, take the manufacturer’s draw figure, and add headroom for the ones that draw more at startup or in cold conditions than they do idling.
Then check the switch’s total power budget, not just its per port rating. A 24 port PoE+ switch almost never supplies 30 W on all 24 ports at once. The budget is a shared pool, and a switch advertised as PoE+ can run out long before every port is loaded. It is one of the most useful numbers on the datasheet and one of the most often skipped.
Finally, look at run lengths and bundles. Long runs lose more, and bundled powered runs get hot. Where a device sits beyond comfortable reach, a PoE injector or a local switch is usually a better answer than pushing the budget.
For background on how PoE works before you get to the numbers, our guide to Power over Ethernet covers the fundamentals, and managed versus unmanaged switches covers whether you will be able to see per port power draw at all.
Getting the power budget right in Southwest Florida
For commercial clients across Fort Myers, Cape Coral, Naples, Bonita Springs and Estero, we size PoE against the actual device list and the actual cable routes, then specify the switch. Warehouses get extra attention, because run lengths are longer and pathways are shared with equipment that generates its own heat.
CommX Networks has spent nearly two decades designing and installing structured cabling, fiber, enterprise wireless and security systems for commercial clients in the region. If you are planning a camera rollout, an access point refresh or a building full of powered devices, talk to our team, or read more about our network cabling work and the areas we serve.
Frequently asked questions
Do I need PoE or PoE+?
Take the device manufacturer’s power draw and compare it to what reaches the device, which is 13 W for PoE and 25.5 W for PoE+. Anything above 13 W needs PoE+. Fixed indoor cameras and desk phones usually fit PoE, while pan tilt zoom cameras, outdoor cameras with heaters and tri band access points usually need PoE+.
Is PoE++ backwards compatible with PoE+?
Yes. A PoE++ switch port negotiates with whatever is plugged into it and supplies only the power that device requests, so PoE and PoE+ equipment works normally on a PoE++ port. The compatibility does not run the other way: a PoE+ port cannot supply a device that requires PoE++.
Is PoE+ 24V or 48V?
Standards based PoE and PoE+ use a 48 volt supply. The 24 volt PoE found on some wireless equipment is passive PoE, which is not an IEEE standard, does not negotiate, and can damage a device that was not built for it. Never assume a passive injector and a standards based device will work together.
Does PoE work over Cat5e?
Yes, within limits. The standards require Cat5 rated cable or better, so Cat5e carries PoE and PoE+ in most installations. The cautions are run length, bundle size and cable quality, which matter more as power levels rise. Cat6 and above handle the heat of higher PoE classes better.