PoE switch has the function of PoE power supply and the ordinary switch’s communication and data exchange function. It can provide DC power to network devices such as network cameras (IPC), wireless APs, VoIP phones, etc., thus saving them from complicated power wiring. Therefore, the power supply issue of the PoE switch is one of the core functions of the PoE switch. After PoE switches are widely used in today’s weak power projects and even in smart homes (home applications), we need to understand more about the technology and knowledge of PoE switches regarding power supply. So we can better choose the right PoE switch products for our scenario to avoid unnecessary losses. So today, Mr. Hua talked to you about how to select the power of the PoE switch on communication issues.
PoE power consumption and PoE standards IEEE802.3af/at/bt technical analysis
IEEE802.3af (PoE), IEEE802.3at (PoE +), and IEEE802.3bt (PoE ++) were introduced in 2003, 2009, and 2018 by the IEEE (International Institute of Electronics Industry) PoE of the three international standards. Scientific regulations and requirements are made regarding PoE power supply power. Let us use the PoE function to enjoy the core value of energy-saving and security brought by PoE products. The following is an analysis of the power supply issues in the three international standards introduced as follows.
IEEE802.3af main power supply parameters.
DC voltage is between 44 and 57V, with a typical 48V； Typical operating current of 10 to 350mA； and Typical output power: of 15.4W. The overload detection current is 350 to 500mA. Under no-load conditions, the maximum required current is 5mA. Four Class levels of electrical power requests from 3.84 to 12.95W are provided for PD devices.
IEEE802.3af classification parameters.
- Class 0 devices require a maximum operating power of 0 to 0.44W.
- Class 1 devices require a maximum operating power of 0 to 3.84W.
- Class 2 devices require operating power between 3.85W and 6.49W.
- Class 3 devices have a power range of 6.5 to 12.95W.
IEEE802.3at standard value and power supply parameters.
Since the IEEE802.3af standard limits PoE power consumption on powered devices (PDs) to 12.95W, this limits the scope of Power over Ethernet cable applications. To solve the PoE power budget limitation,
a new standard is introduced: IEEE802.3at (also known as PoE+), which defines devices with power requirements higher than 12.95W as Class 4, extending the power level 25W or higher. IEEE802.3at can output more than two times the power compared to IEEE802.3af of power, and the output power of each port can be 30W or more. The powered device PD can go up to 29.95W, and PSE will provide it with 30W or more DC power. The PD responds with Class4 graded current to tell PSE if it can give it the higher power specified by IEEE802.3at.
- IEEE802.3at (PoE+) main power supply parameters:
- DC voltage between 50 to 57V, the typical value is 50V.
- The typical operating current is 10 to 600mA,
- standard output power: 30W.
- Powered device PD supports Class 4 grading.
IEEE802.3bt main power supply parameters.
| Class | PD Available Watts |
| 0 | 0.44W |
| 1 | 3.84W |
| 2 | 6.49W |
| 3 | 13W |
| 4 | 25.5W |
| 5 | 40W |
| 6 | 51W |
| 7 | 62W |
| 8 | 71W |
IEEE802.3bt specification introduces four new high-power PD classifications (Class), bringing the total number of single feature classes to nine. Class 5~8 are new to the PoE standard and translate into PD power levels from 40.0W to 71W. IEEE802.3bt is backward compatible with IEEE802.3at and IEEE802.3af. A lower power IEEE802.3at or IEEE802.3af PD can connect to a higher power IEEE802.3bt PSE without any problem. When a higher power IEEE802.3bt PD connects to a lower power IEEE802.3at or IEEE802.3af PSE, the PDs only need to operate at their respective lower power states is called ” Downgrade.”
Select cables based on PoE switch power
Based on HoweVision‘s many years of field experience, the power supply and communication effect of the PoE switch is very much related to the correct communication cable application. PoE application is a system application, not only with PSE and PD but also with the communication channel. Based on HoweVision’s experience with PoE field applications, what power conditions, and specifications of communication cable should be used is summarized as follows.
| Cable type\ power consumption by port | 15.5W | 15.4W~30W | 30W~90W |
| CAT 5E | √ | √ | × |
| CAT 6 | √ | √ | × |
| CAT 6A UTP | √ | √ | × |
| CAT 6A FTP | √ | √ | × |
| CAT 7 S/FTP | √ | √ | √ |
| CAT 7A S/FTP | √ | √ | √ |
| CAT 8.2 S/FTP | √ | √ | √ |
Port output power is not equal to the power consumption required by the PD
HoweVision PoE switch often encounters many problems of insufficient power in the practical application of PoE by customers. After Mr. Hua’s careful communication, he found that many users did not clearly distinguish the relationship between the power supply power of PoE switch ports and the power consumption required by the equipment when choosing PoE switches. Ignore the PoE power supply loss in the cable, for example, when the power needed by the terminal equipment is 30W. User friends only choose the IEEE802.3at standard PoE switch with port power supply power of 30W, plus the line loss will not meet the required power consumption of the end device. To avoid this technical problem, Mr. Hua summarizes the power supply and line loss law as follows.
| IEEE802.3af | IEEE802.3at | IEEE802.3bt(60W) | IEEE802.3bt(90W) |
| Distance=100meter | Distance=100meter | Distance=100meter | Distance=100meter |
| R=20ohm | R=20ohm | R=10ohm | R=10ohm |
| I=350mA | I=570mA | I=1140mA | I=1667mA |
| P2=I2R | P2=I2R | P2=I2R | P2=I2R |
| P2=2.45W | P2=6.5W | P2=13W | P2=27.8W |
| P1=15.4W | P1=32W | P1=64W | P1=95W |
| P=P1=P2 | P=P1=P2 | P=P1=P2 | P=P1=P2 |
| P=12.95W | P=25.5W | P=51W | P=67.2W |
PoE switch power selection summary
PoE switches total power selection problem.
Once the port power is selected, the total power consumption of the PoE switch needs to be calculated.
For example, the power required for the port is 30W, 24-port PoE switch all ports need to support a single port 30W, PoE switches to achieve full PoE function, the total power can not be less than 24 × 30 = 720W. Plus the power consumption of the PoE switch itself, the actual power consumption of the PoE switch chooses 800W power. Currently, many PoE switches do not support the entire load with their total power configuration on the market. Users need to select the real power of the PoE switch according to their field application requirements.
The problems of MCU PoE switch.
Many non-standard PoE switches and MCU PoE switches on the market lack a systematic power management mechanism and distribution mechanism for PoE switch power supply. Therefore the cottage version of the PoE switch power application does not comply with the international standard IEEE802.3af/at/bt power characteristics, with very large randomness. Therefore, there are often various kinds of problems such as insufficient power or large heat generation (power supply overload phenomenon). Therefore, Mr. Hua strongly recommends that users choose international standard PoE products, the usual common, power safety, energy-saving, and environmental protection.
PoE switch power deficiency problem.
According to Mr. Hua’s experience in the PoE field for more than ten years and his understanding of the current PoE business, there are more than 80% of manufacturers PoE switch power false label problem, 65W total power false label for 90W, 90W false label for 120W and other problems. It is very troublesome for PoE switch users to choose the right PoE switch. Therefore Mr. Hua strongly recommends that users and friends select PoE switch; please choose HoweVision class professional PoE brand, as the power is guaranteed.
HoweVision says communication, I am HoweVision, 100% technology, 100% originality, talking about PoE technology and stories, and jointly promoting communication technology to a broader future! Today’s “PoE switch power how to choose” share here. What kind of views do you have on PoE switch power selection? You are welcome to leave a message for Haoyuan. Thank you for checking, for more PoE stories, and so on. See you again next time!
