Power over Ethernet (PoE) Installation Best Practices

Written by Don Schultz, Senior Technical Marketing Specialist, Fluke Networks Certified Copper/Fiber CCTT, BICSI TECH, INSTC, INSTF Certified

You just bought a nice PoE (Power over Ethernet) switch with cameras and access points. You realize you need to buy Ethernet cable to handle this, but you are a bit worried about which cable is the right kind and if there are any special techniques you should use. Not to mention, how about accessories? You start looking through products and documentation out there and get a lot of conflicting information along with answers to questions you don’t fully understand. This can end in a great deal of frustration.

What is PoE?

First, let’s talk a bit about what PoE is. One of the biggest advantages of copper twisted pair Ethernet cable (also called Category cable) is its ability to perform two critical functions at the same time:

• Data transmission
• Supply power to a remote device on your network for said data transmission

When these functions are simultaneously performed, it is known as PoE or Power over Ethernet. A single cable is used to do it, which means you don’t need to find an AC power outlet near your end point device. Metallic Ethernet cable is the only choice for this as fiber optic cable, also capable of carrying Ethernet packets, is unable to transmit electricity since it makes use of light pulses over glass. For those not in the know, glass is quite horrible at transmitting electricity! Power over Ethernet, therefore, will be with us for some time to come and it will actually get even more pervasive as we automate our lives.

PoE is not new, but it certainly is being used in new ways and is being called upon to power up devices you may not be aware of. Common uses for PoE include:

• Stand alone WiFi Access Points
• IP surveillance cameras
• Smart lighting
• Remote Ethernet switches (more powerful PoE Ethernet switch powering up one or more smaller Ethernet switches that are not located near any electrical outlets)
• IP phones
• Signage
• HVAC controls (smart environmental control)
• Yes, even computers (laptops, usually)

Take all of those devices and hook them into the same Ethernet network and now you have something you can centrally manage with nothing more than an app on your smartphone. Pretty cool, huh? For a more detailed look at the technology behind PoE see The Power of PoE.

Although a great deal has been written about the types of PoE out there, very little is devoted to the best way to install the physical cable so you don’t run into any issues with performance. Remember, although PoE is considered low voltage it is still voltage with wattage and those requirements are increasing, not decreasing. With that being said, heat may become an issue! We are going to cover:

• PoE Types and Classes — What 802.3af, 802.3at, and 802.3bt Actually Mean
• Cable Selection — Choosing the Right Cable for PoE
• Termination Accessories and Spark Arc Damage
• Bundling PoE Cable Safely
• Do’s and Don'ts for PoE
• PoE Quick Rule Thumb

PoE Types and Classes — What 802.3af, 802.3at, and 802.3bt Actually Mean

There are a number of variations of PoE. PoE has evolved over the years to work with faster connections and devices that require more power. All PoE uses DC wattage.

• 802.3af (PoE, Type 1, Class 0 - 3) is the original IEEE PoE standard with up to 15.4W injected from the power source, resulting in ~12.95W usable at the device after cable loss at full length.* This type of PoE covers basic devices like IP phones, static surveillance cameras, and simple access points. Connection speeds are up to 100Mbps, and two out of the four pairs are used for power.
• 802.3at (PoE+, Type 2, Class 4) doubles the power injected and increases speeds with up to 30W injected from the power source, resulting in ~25.5W usable at the device after cable loss at full length.* This type of PoE covers higher-draw devices like PTZ moving cameras and more powerful “business grade” access points. Connection speeds are up to 1Gbps, and two out of the four pairs are used for power.
• 802.3bt (4PPoE/PoE++, Type 3 and Type 4)
  • Type 3 (Class 5 and 6) : Up to 60W injected, ~51W usable due to DC wattage cable loss at full length.* This type of PoE covers more demanding devices such as smaller PoE powered downstream switches, high-performance “enterprise grade” access points, LED lighting, building controls, etc. Connection speeds are up to 10Gbps and all pairs are used for power and data simultaneously.
  • Type 4 (Class 7 and 8): Up to 100W injected, ~71W usable due to DC wattage cable loss at full length.* This type of PoE covers the most demanding devices such as laptops, terminals, larger PoE powered downstream switches, LED lighting, outdoor signage, televisions, etc. Connection speeds are up to 10Gbps and all pairs are used for power and data simultaneously.

Full length means 328 feet channel length and assumes solid copper conductor cable Cat5e or higher (22 to 24AWG) is used for the permanent link and then two patch cords using 24AWG stranded copper conductors at each end. Be aware, however, that other factors may influence your maximum length of 328 feet such as ambient temperature.

• Each PoE Type (except Type 2) can supply different DC wattage levels (called Class) up to the default maximum. What Class is used is dependent upon the device that needs power. For example, a surveillance camera and small WiFi AP both may require PoE Type 1 (802.3af in other words) but the surveillance camera may require the Class 0 (full default power) where the small WiFi AP may require less power with Class 2. When the two devices negotiate via the 802.3af protocol, they agree upon the Class as well. Not all devices require the full default power, which results in power savings.

Power Over Ethernet Summary Table

Category cable from Cat5e upward, in pure copper construction, can technically carry Type 4 PoE++ up to 100W — the type ladder determines power delivered, not which cable category is required outright, but be aware Category and conductor gauge will affect bundle size, which is where limits come in.

Cable Selection — Choosing the Right Cable for PoE

On the outside most Ethernet cable looks the same. Cut it open and you start to find the differences. Some differences matter more than others. Here are some generalizations:

• Thinner copper AWG is not as capable at handling heat as thicker copper conductors
• Solid copper conductors are always superior to stranded copper conductors due to less resistance
• Higher Category rated Ethernet cable is “better” for PoE, often due to thicker copper conductors found in the higher ratings although overall cable construction plays a role too
• Shielded Ethernet cable allows for better heat dissipation
• Select LP rated Ethernet cable as it will greatly simply installation

The above generalizations need to be taken in context, however. That context is the specific application and installation, and usually no two are precisely alike. This is one of the reasons why there is a great deal of conflicting and misinformation out there about PoE. Combine this with “closely held beliefs” and other anecdotal advice and you can start to see the problem.

One of my favorite sayings regarding the technicalities of Ethernet cable installations is “usually, but not always”. What I mean is that the answers are often not concrete and vary by your installation and your goals. There can be more than one correct answer. Some answers are more correct than others.

Here are some concrete cable selection guidelines:

• Any Category of Ethernet cable from Cat5e upwards is capable of carrying IEEE Type 4 Class 8 PoE (802.3bt, and referred to as PoE++) up to 100W as long as the construction is of copper (stranded or solid).
• As you increase the number of PoE cables in a bundle the number of cables may need to be reduced and is relative to copper AWG size and Category, especially in conduit or other spaces that will trap heat.
• As you increase the current (PoE Class/Type) the number of cables in a bundle may also need to decrease relative to copper AWG size and Category.
• It is better to not construct larger bundles of Ethernet cables carrying power, as air circulation around the cable for heat dissipation is your friend.
• Ethernet cable bundles carrying PoE that are put inside conduit are even more limited as to cable count, so use conduit with caution unless your cable happens to be “LP” rated--the LP rating addresses 60W or higher PoE in cable bundles. For example, a LP 0.5A rating means you can safely install bundled PoE cable of 60W or higher in conduit and up to 192 count without performing complex calculations found in the NEC/NFPA 70 assuming these conditions are met. Professional installers covet LP rated cable as a result!

One item of great importance concerns CCA (Copper Clad Aluminum) cable. Don’t use it! You risk serious equipment damage due to underpowered devices. CCA has very high DC resistance.

For more on why you should never (ever) use CCA please see Copper Clad Aluminum vs Copper: The Ultimate Test on a Fluke Versiv DSX CableAnalyzer.

Termination Accessories and Spark Arc Damage

As for termination accessories, all of trueCABLE’s Ethernet keystone jacks, field termination plugs, Ethernet couplers, and RJ45 8P8C connectors handle Type 4 Class 8 IEEE 802.3bt, PoE++ up to 100W with no issue. As it turns out, the limiting factor on PoE ratings for accessories has to do with a phenomenon known as “spark arc damage”. This happens by plugging and unplugging live PoE circuits.

Each time you plug and unplug a live PoE connection, a tiny spark is created. Over time it will accumulate in the form of minute pitting and burns on the surfaces of RJ45 8P8C contact wires and the associated contact springs inside keystone jacks or couplers. Accumulation of this (not always visible) damage will contribute to performance degradation and possible termination hardware damage.

Note, we are not talking about a few plugs and unplugs. We are talking about hundreds. In other words, this is not a huge worry but we will address that later too as the problem can be avoided.

Bundling PoE Cable Safely

PoE Cable Bundle Guidelines for Non LP-Rated Ethernet

* Type 3 and Type 4 are also known as 4PPoE or 4 Pair PoE. Type 2 PoE is 2 Pair PoE.

Notes for using this table:

• This table does not apply to LP rated Ethernet, and calculations are not required to determine bundle size based on PoE type/wattage within the LP rated cable amperage capacity
• Bundles sizes, even for LP rated Ethernet may need to be reduced due to conduit size limits for obvious reasons
• Bundles of mixed Category cables carrying PoE should be rated with the worst case scenario in mind (for example, if you have Cat5e and Cat6 in the same cable bundle then use the figures for Cat5e)
• Cat5e conductor gauge is assumed to be 24 AWG
• Cat6 and Cat6A conductor gauge is assumed to be 23 AWG
• All figures are for solid copper conductors
• Ambient temperature is assumed to not exceed 113℉ or 45℃
• This table is constructed to avoid a temperature rise exceeding 27℉ or 15℃ above ambient so as to not exceed the cable operating temperature

Here’s a pro tip (and shortcut!). If you base all of your maximum run lengths off of the guidelines found in Temperature's Effect on Ethernet Cable Length and then secondarily use the table above for bundle sizes you are good to go for the worst case scenario with non LP rated Ethernet!

If you base your bundle size and run lengths using the above tip and then combine that with Conduit Fill Chart for Ethernet and Coaxial Cable then you have all you need. Simply cross reference the two charts and go with the LOWER maximum bundle size.

Do’s and Don'ts for PoE

Do:

• Select LP rated Ethernet, it makes life a lot easier and you don’t have to lug around a copy of the NEC/NFPA 70 for doing calculations on bundle size
• Opt for higher Category cable (which will typically have thicker copper conductors) when feasible for PoE applications.
• Opt for F/UTP (shielded) Ethernet cable when working with large numbers or very high wattage PoE runs (or both) especially if bundled.
• Avoid bundling when possible, but follow the table guidelines above if you cannot.
• Use velcro cable straps to bundle cables if bundles are necessary
• Avoid conduit and similar pathways if possible, otherwise defer to ENT or similar metallic conduit for better heat dissipation and follow the table guidelines above if you cannot. Metallic conduit should be properly bonded and grounded to AC ground for safety.
• If bundling cannot be avoided, follow the table guidelines above and also cross reference with Conduit Fill Chart for Ethernet and Coaxial Cable
• Keep bundles separate by at least 1.5” in cable tray to allow for air movement

• Power down PoE ports prior to plugging or unplugging end equipment or working on the cable

  Do NOT:

• Don’t use CCA or copper clad aluminum conductor Ethernet cable (ever).

• Don’t use zip ties to secure Ethernet cable. Cable damage over time will occur which will degrade performance, and cables cannot dissipate heat nearly as well.
• Never put 26 AWG or 28 AWG stranded copper Ethernet cable into conduit when it is running PoE. Open air bundles are OK.
• Do not use cable combs. Avoid the urge to achieve perfect alignment and visual appeal. Although this is a trend as of late, it turns out that cable combing for visual appearance will not only cause potential immediate alien crosstalk (ANEXT) between cables but also long term cable damage due to bundle sag and lay. Lastly, perfectly combed out cable bundles also do not dissipate heat generated by PoE as well as randomized bundles.
• Avoid using any of the advice in this blog if your PoE application type is manufacturer specific (proprietary). Defer to that manufacturer’s advice for that cable. installation considerations. This blog assumes IEEE 802.3 style PoE is in use, which is industry standard.
• Avoid repeatedly plugging or unplugging RJ45 8P8C connectors or field termination plugs from female mating interfaces when the PoE circuits are energized as accumulated damage will result and may degrade performance over time or even damage end point devices.

PoE Quick Rule of Thumb

As luck would have it, the vast majority of Ethernet switches and patch panels come in rows of 24. Although you may be dealing with a 48 port PoE switch or 48 port patch panel, it will be two rows of 24. Well, it turns out that 24 is below the absolute worst case scenario of 31 Cat5e cables bundled into a conduit when running at 100W on 802.3bt PoE++. Simply keep any bundle to 24 or less and then keep those bundles separated by 1.5” and at no point will you ever run into a problem. For large commercial installations, the side benefit is any code inspector coming out to inspect your work will have a much easier time figuring out what you have done and you will be able to pass code inspection easily--and that will save you time and hassle. Yes, the NEC does indeed address PoE now, therefore your inspector can hold you to it.

Alternatively, just use LP rated Ethernet and skip the worries! Code inspectors are now actively looking for LP ratings to make their job easier. They have to do the same calculations you did to confirm your installation is up to Code. Your inspector will be in a much better mood if you don’t make him/her go through hundreds of calculations and look-ups.

Conclusion

And there you have it. The average DIY installer and/or homeowner is not likely to bump into the maximum worst case scenarios outlined in this blog, but PoE is becoming more and more important to IoT and smart devices designed to save energy and provide convenience. It is best practice to follow these guidelines for any installation as you never know what the future will bring. With that I will say…

HAPPY NETWORKING!

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