The one idea
Ports are not the constraint. The shared budget is
A PoE switch has two capacities that people routinely treat as one. It has a number of ports that can deliver power, and it has a total amount of power it can deliver across all of them at the same time. Those are different numbers, and the second one is the one that stops projects.
A switch with free ports can have no free power. Nothing about it looks wrong: the ports are there, the cameras plug in, and the failure that follows does not announce itself as a power failure. It shows up in three ways:
- Devices restarting under load.
- The highest-drawing camera dropping first.
- A system that is completely stable until the first cold night, when several devices ask for more at once and the budget runs out.
If you have the figures to hand, the PoE power budget calculator adds them up and shows the headroom, and the cable run length checker tests a proposed run against the channel limit. Both take every number from your own documentation instead of assuming any.
That is the whole reason a survey looks at the comms room before it looks at the wall you want a camera on. What is already drawing on the switch is the number that decides whether your addition is four cameras or four cameras plus infrastructure, and it is not a number anyone can give you over the phone.
The standards
What IEEE actually publishes, and what we will not repeat
Power over Ethernet is standardised by IEEE as a series of amendments to IEEE 802.3. The named ones are 802.3af, 802.3at and 802.3bt.
The most recent of the three, IEEE 802.3bt-2018, is titled "IEEE Standard for Ethernet Amendment 2: Physical Layer and Management Parameters for Power over Ethernet over 4 pairs." Its scope states that "The maximum Powered Device (PD) power available is increased by this amendment to IEEE Std 802.3-2018 by utilizing all four pairs in the specified structured wiring plant."
Two things follow from that sentence and we will state only those two. 802.3bt delivers power over all four pairs instead of two, and it raises the maximum power available to the powered device relative to the base standard. That is what the publisher says, and it is enough to explain why newer equipment can ask for more than an older switch was built to give.
This page publishes no wattage figures, deliberately. The numbers everyone quotes for these standards are not on IEEE's free abstract pages, and the standards themselves are paywalled. Every figure in circulation traces to a vendor or reseller page, not to the body that publishes the standard. This site does not print a number it cannot attribute, and widely-repeated numbers are not an exception to that. It costs you nothing in practice: the figure that decides your project is what your switch delivers across all its ports with your devices connected, which is a site measurement, not a specification.
Source: IEEE Standards, IEEE 802.3bt-2018: standards.ieee.org/ieee/802.3bt/6749. Retrieved: 4 September 2026. Title and scope quoted as published. Per-class power levels are not stated on that page and are therefore not stated here.
The other half
The cable is a constraint too, and it is shorter than you think
Power and data share the cable, so the cable's limits are the system's limits. TIA/EIA permits a 100 metre channel for copper, roughly 328 feet. The figure that matters is what that 100 metres includes: the patch cords at both ends, not only the cable in the wall. That is the detail buyers most often miss, and it is the one that turns a run that measured fine into a run that fails.
The practical permanent-link limit is therefore about 90 metres, and shorter again once pathway routing, vertical drops and slack are counted. A camera position that is 70 metres away across a floor plan is frequently a good deal further than that once the cable has gone up, along a tray, around an obstruction and back down. Runs near the limit are the commonest cause of a link failing certification later.
Cable choice follows from what the run has to carry, not from a building-wide policy. Cat6a supports 10 Gigabit to the full 100 metre channel, and its construction handles the heat generated by higher-power loads in a bundle, which is a real consideration when cable sits packed in a tray with a lot of other cable instead of on its own. Cat6 remains appropriate for general workstation and voice work. Fibre is for backbone, inter-building runs, and anywhere the path crosses separately grounded structures.
All of that is settled per run at survey, against the actual route. The detail on how a cabling job is scoped, labelled and certified is on structured cabling.
When it bites
Six moments the power budget becomes your problem
Almost never at installation. Almost always later, when somebody assumes the system has room in it.
The building takes an adjacent suite
New floor area means new positions, and the new positions get plugged into the switch that is already there because it has free ports. The budget is not part of the conversation until something restarts.
A yard or car park gets added
Exterior positions frequently draw more than interior ones, because of what they need to do at night and in weather. Adding two outside is not the same load as adding two inside.
Cameras get replaced with newer ones
A like-for-like count is not a like-for-like load. Newer equipment can ask for more than the equipment it replaced, which is exactly what 802.3bt exists to allow.
The first genuinely cold night
Devices that draw more in cold conditions all draw more at the same time, which is the worst case the budget has to survive. A system sized with no headroom passes every mild day and fails once.
Something else is sharing the switch
Access control readers, intercom stations, wireless access points and phones can all sit on the same PoE infrastructure. The camera project is not the only claim on the budget.
Nobody wrote down what was installed
The underlying condition for all five above. Where the estate is not documented, the budget is unknown, and the first measurement anybody takes is the one taken after a fault.
How we handle it
Checked before it is ordered, not diagnosed afterwards
On an addition to an existing system, the comms room is part of the survey and not an afterthought. What is on the switch, what it can deliver, what else shares it, and what headroom is left with everything running at once. If the answer is that your addition needs infrastructure as well as cameras, you get told that before anything is ordered, not after it is installed.
We will not tell you how many cameras a given switch supports without seeing it, and we will not name a device as adequate on the strength of a datasheet. Those are the same discipline: a specification describes a product under conditions that are not your building.
Where the work is a new system rather than an addition, the budget is a design input from the start and there is nothing to discover, which is set out under commercial camera installation. Where a system is already in place and nobody has documented it, that documentation is part of what a CCTV service and repair visit produces. Where readers and entry stations share the same infrastructure, the same budget question covers them, which is why access control and camera work are usually surveyed together.
Two boundaries, as on every page here. We work on commercial premises only and not on private residences. And we do not hold a C-10 or a C-16, so we do not work on fire alarm systems; that is coordinated with your own licensed fire alarm contractor.