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6 mistakes to avoid when designing a PoE cabling system

poe patch cord cables plugged into a device

Power over Ethernet (PoE) utilizes balanced twisted-pair Ethernet cabling infrastructure to deliver DC power to a range of devices. PoE has revolutionized the way modern networking infrastructures operate, enabling a range of devices to be connected and powered conveniently and cost-effectively. PoE is particularly well suited to low-power devices like IoT sensors and wifi access points, but the latest Standards allow for up to 90W (71.3W delivered) to be carried. This opens up the scope of PoE to new types of devices and functionality.

While PoE uses Ethernet cabling and components familiar to many IT professionals, there are additional factors that must be taken into account when designing a network that will be expected to carry power.

1. Thinking only in the short term – planning for “current requirements”

Cabling infrastructure is unlikely to be future ready if you only consider the applications you want to support today. The same is true of any network or technology installation.

When thinking about PoE, customers may be considering their immediate requirement for devices like phones, sensors and lights, which have relatively low power and data requirements. However devices are becoming increasingly sophisticated and powerful, and IoT-based smart building systems are becoming common not just in offices but in schools, hospitals and industry 4.0. PoE Types 3 and 4 can supply more than 60W of power, and as well as powering a much wider range of building and IoT devices, this enables equipment such as audiovisual equipment, workstations and point-of-sales to be powered by the same network– if the right infrastructure is in place.

Installing a higher category or bigger gauge of cable can provide the headroom for more sophisticated uses of PoE to be incorporated with relative ease, whenever there is the requirement.

2. Failing to adequately plan for heat buildup

Heat will always be generated within PoE cables. The more power applied, the greater the temperature rise; and the more cables in a bundle, the less that heat is able to dissipate, especially at the center of the bundle. Some amount of temperature increase is inevitable, but it can and must be managed.

The impact of heat rise on cables can be significant. Heat increases the Insertion Loss (IL) and therefore will reduce the overall performance of the cabling system, lowering the signal strength (dB) which may contribute to higher bit error rates and lower networking performance. An increase of just 2dB of IL results in half the available signal strength. Insertion Loss increases with temperature, typically 0.4% per 1°C for communication cables. Finally, heating effect is proportional to the square of the current flowing in the conductors. Therefore, a 50% increase in the source current would result in a 125% increase in the power that is dissipated as heat within the cable.

Excessive heat also accelerates the physical wear of the cable, along with its integrity.

As per International Standards recommendations, the maximum cable bundle size is 24, and the maximum temperature rise at the center of the cable bundle must be no more than 10°C / 50°F. Note that these are the maximum values permitted by these Standards, and depending on the type of application, cable and installation, the maximum cable bundle size may be less than this. Cables with metallic or foil shielding also dissipate more heat than unshielded UTP cables. Adequate spacing between cable bundles must also be provided, preferably maintained by physical barriers to prevent unintended movement.

3. Ignoring design rules when using 28 AWG cords

28 AWG patch cords are narrower than 24 or 26 AWG, making them ideal for installations where space is minimal. Narrower gauge cables can be used for PoE but there are important additional design considerations.

Typically, cables with a smaller diameter (cables with a higher AWG) have higher insertion losses than thicker cables, so channels featuring these cable types must be additionally derated to ensure sufficient signal-to-noise ratio. For 28 AWG stranded patch cord, the derating factor is 1.95.

Thinner gauge cables also have higher DC resistance and as a result generate more heat per unit of current. In some environments the thinner cable size can aid airflow, but as soon as cables are bundled this benefit is lost. The maximum bundle size must be reduced to ensure the cables do not exceed their temperature rating.

International Standards specify the temperature rise for various cable bundle sizes and different current levels, they provide recommendations about cable bundle sizes and spacing.

Explore the Molex range of copper cables here.

4. Using Copper Clad Aluminum cables

Copper clad aluminum cables have been present in the marketplace for some time as manufacturers try to develop cheaper alternatives to copper. However in Ethernet installations these cables are not only non-standards compliant, they can present severe problems and safety risks.

The resistance of a solid aluminum cable is about 55% greater than for a copper cable of the same diameter. The greater resistance will result in increased heat within the cable and lower voltage availability at the powered device.

Both ANSI/TIA and ISO/IEC standards require twisted pair data cable to be 100% copper.

5. Expecting a cabling system to support the relevant PoE applications because the cable is UL LP rated

UL LP (Limited Power) testing determines how many amps a conductor can carry. For 4-pair Category cables, typical results range from 0.5 amps to 0.9 amps.

However, an LP rating is intended to assess cabling from a safety standpoint only. It does not guarantee the Ethernet or remote power performance of the product.

An LP rated cable is certified by UL as one that doesn’t exceed its temperature rating under certain conditions, but regardless of the cabling (LP certified or not), if higher temperatures are introduced then the cable’s reach and performance will be negatively impacted due to insertion loss – not to mention the accelerated physical wear of the cable.

6. Ignoring damage to contacts caused by connections and disconnections when cabling is under load

As per International Standards, the design and operation of a channel must take into account the impact of mating and demating under load.

The greater the contact resistance, the higher the losses and the hotter the contacts. Therefore, the connector/cable contacts are of increased significance for 4 Pair PoE. An insulation displacement contact (IDC) is far superior to an insulation piercing connector (IPC) in terms of contact reliability, with IDC technology offering higher long-term stability. Insulation displacement technology creates a connection comparable to a solder joint. In contrast, IPCs merely piece the insulation and produce a loose contact. Over time, the stability of an IPC contact will reduce. This can become a serious problem if contacts are impaired by fine arcs when being disconnected under load. The contact design is also a concern in this context.

During disconnection, the current eventually flows over a small remaining contact area. When the contact is pulled out, sparks are produced. This creates plasma with extremely high temperatures that can cause local damage to the contact. High-quality plug connections like the Molex Connected Enterprise Solutions DataGate jacks are therefore made in such a way as to create sufficient distance between the pull-out point and the nominal contact area, minimizing this risk.

PoE Implementation Guide  

There are a number of resources available to Molex CSP users to help when working with PoE, including the PoE Implementation Guide and the PoE Calculator. The PoE Calculator is a free excel-based tool which streamlines many of the necessary calculations for your PoE network, alerting you when you may be risking non-compliance or unsuitable design choices. If you have not already downloaded a copy, log in to CSP now to access. The PoE calculator can be downloaded from “Electronic Tools” and the PoE Implementation Guide can be found under “Technical Bulletins”.

If you are not already a CSP user, visit https://csp.molex.com/ to get started.

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