Analysis Of Cabling Requirements For Ieee 802.3Bt Type 4 Devices - Cisco Catalyst 9400 Series Hardware Installation Manual
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Analysis of Cabling Requirements for IEEE 802.3bt
Type 4 Devices
The National Electrical Code (NEC) in its 2017 edition describes the restrictions on Power over Ethernet
(PoE) systems. The restrictions are based on the per-conductor current and are targeted to minimize the thermal
rise that may occur in large bundles of cable delivering Power over Ethernet.
Note
The NEC defines a cable bundle as a group of cables that are tied together or in contact with one another in
a closely packed configuration for at least 1.0m (40 in).
A standard Category cable used in Ethernet systems consists of eight conductors. IEEE 802.3af and 802.3at
systems use only four of these conductors to carry current. Cisco UPOE and IEEE 802.3bt systems use all
eight conductors to carry current. The following table lists the maximum current allowed under each standard
and the associated per-cable and per-conductor current.
Table 27: Maximum Current prescribed for various Standards
Standard
802.3af
802.3at
Cisco UPOE
802.3bt
Analysis of a cabling system for compliance to the NEC starts with section 840.160:
Installation of the listed 4-pair communication cables for a communication circuit or installation where
4-pair communication cables are substituted for Class 2 and Class 3 cables in accordance with 725.154(A)
shall comply with 725.144.
Exception: Installing communications cables in compliance with 725.144 shall not be required for listed
4-pair communications cables where the rated current of the power source does not exceed 0.3 amperes
in any conductor 24 AWG or larger.
This, in effect states that 802.3af, 802.3at, and Cisco UPOE systems do not require any further consideration
when the conductors are 24AWG or larger. Also, TIA-568 compliant horizontal cables require a minimum
A P P E N D I X
Cable Current
350mA
600mA
1200mA
1732mA
Cisco Catalyst 9400 Series Switches Hardware Installation Guide
A
Conductor Current
175mA
300mA
300mA
433mA
179
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