PoE for CCTV: how to plan power budgets, switches and cable runs

Power over Ethernet makes IP camera installation far simpler. One cable carries both data and power, so you do not need a mains point at each camera. But PoE also creates a kind of fault that is frustrating to diagnose. A camera that reboots at night when its infrared turns on, or a heater that never gets enough power, can bring an installer back to a site several times.

Most of those problems can be avoided at the design stage. This guide covers what you need to plan PoE properly.

The PoE standards

These are the IEEE standards you will come across most.

The figures differ because some power is lost in the cable. The switch must supply more than the device receives so that the device still gets enough at the end of a long run.

Most fixed cameras sit comfortably in Type 1. PTZ cameras, cameras with heaters and multi-sensor models often need more. Always check the actual data sheet, including the maximum power figure with infrared and any heater on.

Passive PoE is different

Standard PoE involves a negotiation: the switch checks that the device can accept power before supplying it. "Passive" PoE simply applies voltage to the cable. Some budget or wireless equipment uses it. Connecting the wrong device could cause damage, so never mix them without checking. Some "PoE" switches sold cheaply may not fully follow the standard, so look for explicit compliance on the data sheet.

Planning a power budget

Every PoE switch has two limits: power per port and total power for the whole switch. A "16-port PoE switch" may not be able to run all 16 ports at their maximum at once.

Step 1: list each device's maximum

Use the worst-case figure rather than a typical one. Night mode with infrared on can use noticeably more than daytime operation.

Step 2: add headroom

Do not design a switch to run at the limit. Aim for headroom so that the switch is not running flat out and you have room for an extra camera later. A common rule of thumb is to stay around 70 to 80 percent of the switch's budget. On that basis, a 142 W load needs a budget of about 180 to 200 W.

Step 3: check the switch's details

• The total PoE budget, in watts
• Per-port power, and whether all ports support the higher class
• The behaviour when the budget is exceeded. Many switches shut off low-priority ports
• Port priority settings, so that important cameras stay on
• The uplink speed
• Operating temperature, especially for an enclosure in a warm area

NVRs with built-in PoE

Many NVRs have integrated PoE ports, and they work well for small sites. Check these points.

• The total budget may be lower than you assume. An 8-port NVR may provide less total power than 8 × 30 W.
• Cameras are typically placed on the NVR's own internal network. This is convenient, but remote configuration and third-party camera support can be affected.
• A failed NVR takes down power for all connected cameras. This matters on bigger jobs.
• Expansion is limited. Adding a camera later may mean adding an external switch.

For larger or more critical sites, external switches give greater flexibility.

Distance and cable

Standard Ethernet has a maximum channel length of 100 metres, including patch leads. Plan for less where you can, because long runs are where marginal problems appear.

Cable quality matters more with PoE

Copper-clad aluminium (CCA) cable is sold cheaply and looks like the real thing. Its higher resistance means a larger voltage drop and more heat, and it does not meet the usual standard for Ethernet cabling. On a long PoE run with a demanding camera, it can cause exactly the kind of intermittent fault that is difficult to find. Solid copper Cat5e or Cat6 is the safe choice. Buy from a trusted source, and be suspicious if the price looks too good.

Longer runs

If a camera is beyond 100 metres, the common options are:

• Fibre to a switch nearer the cameras, which is often the best choice for long distances or between buildings, as it also avoids surge paths through copper
• An intermediate PoE switch placed partway along the run with a power supply
• A PoE extender, which is simple but counts as another point of failure and may have power limits
• Ethernet over coax or long-reach Ethernet devices where existing cable has to be reused

Check that any "long-range PoE mode" on a switch fits your needs. These modes often reduce link speed, which could limit high-resolution cameras.

Cable bundles and heat

Higher-power PoE warms the cable, and tight bundles retain heat. For higher-power loads in large bundles or warm spaces, follow the cabling standards and manufacturer guidance. For the modest loads of most fixed cameras, this is rarely an issue, but it matters more as 802.3bt devices spread. Fire-rating rules for cable may also apply in particular building spaces, so check local requirements.

Outdoor runs and surge protection

Cable between buildings or to outdoor poles can pick up surges from nearby lightning and other events. These can damage cameras, switch ports or both.

• Consider surge protection rated for PoE at both ends of an exposed run.
• Ground it properly. A surge protector without a proper ground offers little protection.
• Use fibre between buildings where you can.
• Use outdoor-rated cable where cable runs outside. Sunlight and moisture degrade indoor cable.
• Follow your local electrical rules, and involve an electrician where required.

Keeping it running through outages

If power fails, your switch and NVR go down, and so do all your PoE cameras. Since footage may matter most during an outage, a UPS is often a sensible addition.

To size one:

1. Add up the load: the switch's own consumption, the PoE power it delivers, the NVR and any router.
2. Decide how long the system must run.
3. Use the UPS maker's runtime chart for that load. Battery figures tend to be better on paper than in old age, so leave a margin.

For a load of 200 W needing 30 minutes, for example, check the runtime chart at about 200 W rather than dividing battery capacity by hand. Replace batteries on the maker's schedule, and put a test on your service checklist.

Diagnosing PoE faults

When a camera misbehaves, these are the common causes to check.

A managed switch can help here, since you can see power use and errors port by port. It also lets you restart a camera remotely by cycling PoE on its port, which can save a site visit.

Managed or unmanaged?

Unmanaged switches are cheap and simple, and they are often fine for small, standalone systems.

Managed switches offer VLANs, port-level power information, remote PoE cycling, alarms and better security. For a business customer or anything where a site visit is expensive, the price difference usually pays off. If you also run a maintenance contract, these features can make your work noticeably more efficient.

On-site checks at commissioning

Before you leave a site, confirm the following:

• Actual PoE load is compared with the budget, with infrared on if possible
• Each link has come up at the expected speed
• Each long run has been tested and recorded
• Cameras are labelled at the switch, and the port map is documented
• Spare ports and spare budget are noted for future expansion
• Surge protection and grounding are in place where needed
• The UPS has been tested, if fitted

These records belong in your handover pack. They will help the next technician to visit the site, who may well be you, a year from now.

Putting it in your quote

Be specific about what you are supplying. Instead of "PoE switch", write the port count and the budget, such as "16-port gigabit PoE+ switch, 250 W". This allows a fair comparison with other quotes. A low-priced competitor may be offering a switch that can't really run the proposed cameras, and your clarity may help your customer see it.

Good PoE design is invisible when it works. That is the point. A few minutes with the power budget and the cable plan will spare you many return visits.