Power and cabling
The PoE power budget, and why it bites when you add cameras
Most commercial cameras take power and data down one cable. That is Power over Ethernet, and it is the reason a straightforward four-camera addition to a system that works perfectly well sometimes turns into a new switch. The constraint is not the ports. It is the budget shared across them.
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.
Common questions
Power and cabling questions
Why would adding four cameras mean replacing a switch that is working fine?
Because the switch has two separate capacities and only one of them is about ports. It has a number of PoE ports, and it has a total power budget shared across all of them. A switch can have free ports and no free power. When that happens the symptom is not a clean refusal: the ports are physically there, the cameras plug in, and the system misbehaves in ways that look like anything except a power problem. Cameras restart under load, or the ones drawing most power drop first, or everything is stable until a cold night puts the heaters on and several devices ask for more at once. That is why the budget is checked before anything is ordered, not diagnosed afterwards.
Related Structured cabling
Can we run cameras further than the standard cable distance if we use better cable?
No, and better cable is the wrong lever. TIA/EIA permits a 100 metre channel for copper, and the figure includes the patch cords at both ends, not only the cable in the wall, which is the part most people miss. That leaves roughly 90 metres for the permanent link, and less again once you count pathway routing, vertical drops and slack. A run near the limit is the commonest reason a link fails certification later. Where a position sits beyond it the answer is a change of design rather than a change of cable: an intermediate location, or fibre where the path crosses separately grounded structures. We settle that at survey against the actual route, because the route is what decides it, not the straight-line distance on a floor plan.
Related Structured cabling
Does higher-power equipment change what cable we should be pulling?
It changes the argument for Cat6a, and the reason is heat, not speed. Cat6a supports 10 Gigabit to the full 100 metre channel, which is the headline, but its construction also handles the heat generated by higher-power loads in a bundle. Cable does not sit alone in a commercial building; it sits in a bundle in a tray with a lot of other cable, and a bundle carrying power warms up. That is a genuine input into the choice, alongside what the run has to carry. Cat6 remains appropriate for general workstation and voice. The decision is made per run against what that run has to do, not once for the whole building.
Related Structured cabling
Why will you not tell us the wattage figures for the PoE standards?
Because we could not source them to IEEE. The standards themselves are paywalled and the free abstract pages do not carry the per-class power numbers, which means every figure in circulation traces back 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 PoE wattages are not an exception just because they are widely repeated. In practice it costs you nothing: the number that decides your project is not the theoretical maximum of a class, it is what your actual switch can deliver across all its ports at once with your actual devices connected. That is a site figure, and it is established at survey.
Related Structured cabling
When it bites
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