Fact Checking Power Over Ethernet Marketing Math

Quote of the Day

In the case of good books, the point is not how many of them you can get through, but rather how many can get through to you.

Mortimer J. Adler. He wrote a book called How to Read a Book that helped me become an effective reader.


Introduction

Figure 1: Example of Very Neat Network Cable Bundles.

Figure 1: Example of very neat network cable bundles. In PoE applications, these cable bundles can experience significant self-heating, which will reduce their load capacity. (Source)

I was reading a blog by a cable manufacturer (Belden) this morning on the advantages of using Cat 6 cable over Cat 5e for network installations going forward (Figure 1 shows a great example of network cabling). Normally, I see the cable manufacturers recommending Cat 6 to customers because it will allow them to upgrade to 10 Gbps Ethernet, at least for runs less than 55 meters long.

The blog I read this morning took a bit different approach. It was encouraging customers to switch to Cat 6 because it consumes less power in Power over Ethernet (PoE) applications. It made the following testable claims:

  • As much as 20% of the power through the cable can get “lost” in a 24-gauge Category 5e cable [relative to a Category 6 cable], leading to inefficiency.
  • As we mentioned above, losing nearly one-fifth of the total power in a 24-gauge Category 5e cable may seem like a lot of power loss – and it is. But doing the math will show you that the total dollar amount comes out to be only around $7 per year.

I will check these claims in this blog post.

Background

Table 1 summarizes some of the key characteristics of Cat5e and Cat6 cable. For this blog post, the important difference from a PoE standpoint is the wire gauge.

Table 1: Key Characteristics of Cat5e and Cat6 Cable.
Characteristic Cat5e Cat6
Max Bit Rate (bps) 1,000 10,000
Approximate Cost ($/foot) 0.3 0.5
Frequency Bandwidth (Mhz) 100 250
1000 BaseT Reach (m) 100 100
10000BaseT Reach (m)  –  55
Wire Gauge (AWG) 24 23

Analysis

Claim 1: 20% Power Loss on a PoE Line.

I was not able to confirm the 20% loss of the total power loss being attributable to the wire resistance. It is easy to confirm that as much as 15% of the total power loss is attributable to wire resistance. Figure 2 shows my calculations.  So I would say that there claim is close to true.

Figure M: I calculate 15% for the maximum loss percentage.

Figure 2: I calculate 15% for the maximum loss percentage.

Claim 2: $7 Per Year Per PoE Line Cost.

I was able to confirm their claim that each PoE line burns $7 per year in the wire resistance (Figure 3). That was a bit surprising.

Figure M: Annual Electrical Cost for a Running a PoE Line in the US.

Figure 3: Annual Electrical Cost for a Running a PoE Line in the US.

Average US Electric Power Cost Per kW-hr

Conclusion

I read marketing claims all the time. Most of the time, there is some level of reality to them. In this case, one claim is close and the other is accurate. I was surprised at the cost yearly cost incurred because of the cable resistance.

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