[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"article-ieee-802-3bt-explained-type-3-type-4-poe":3},{"id":4,"slug":5,"title":6,"category":7,"summary":8,"tags":9,"publishTime":15,"views":16,"seoTitle":17,"seoDescription":18,"seoKeywords":19,"content":20},13,"ieee-802-3bt-explained-type-3-type-4-poe","IEEE 802.3bt Explained: Type 3, Type 4 and 90W PoE","standards","A practical guide to IEEE 802.3bt: what Type 3 and Type 4 deliver, how 4-pair power works, single- and dual-signature PDs, Autoclass, and backward compatibility.",[10,11,12,13,14],"IEEE 802.3bt","Type 3","Type 4","PoE++","4-pair PoE","2026-07-02",7,"IEEE 802.3bt Explained: Type 3 vs Type 4 and 90W PoE","How IEEE 802.3bt delivers up to 90W over four pairs: Type 3 and Type 4 power levels, single vs dual-signature PDs, Autoclass, and compatibility with 802.3af\u002Fat.","IEEE 802.3bt, Type 3 PoE, Type 4 PoE, 90W PoE, PoE++, 4-pair PoE, single signature PD, dual signature PD, Autoclass","\u003Cp>IEEE 802.3bt, ratified in 2018, is the third generation of standardized Power over Ethernet. It roughly triples the power available to a device compared with 802.3at and, for the first time, makes powering all four pairs of a twisted-pair cable a standardized behaviour rather than a vendor extension. If you are specifying equipment for PTZ cameras, Wi-Fi 7 access points, PoE lighting or thin clients, 802.3bt is the specification that matters.\u003C\u002Fp>\u003Cimg src=\"\u002Fbrand\u002Fnet\u002Fpatch-panel.jpg\" alt=\"structured cabling patch panel\" loading=\"lazy\" \u002F>\n\u003Ch2>What 802.3bt Actually Changed\u003C\u002Fh2>\n\u003Cp>Earlier PoE generations pushed power down two of the four pairs in the cable. 802.3bt energises all four, which halves the effective loop resistance and cuts conduction losses for a given delivered wattage. On top of that it adds two new power Types, extends the classification ladder from five classes to nine, and introduces mechanisms that let a PSE reclaim power a device is not actually using. The standard also formalises operation alongside 2.5G, 5G and 10GBASE-T, which the older documents never addressed.\u003C\u002Fp>\n\u003Ch2>Type 3: 60 W at the Source\u003C\u002Fh2>\n\u003Cp>Type 3 covers classes 5 and 6 and allows a PSE to source up to 60 W per port, guaranteeing 51 W at the powered device after worst-case channel losses. Classes 5 and 6 require 4-pair operation, while a Type 3 PSE must still support the lower classes for backward compatibility. Most enterprise switches marketed as \"PoE++\" or \"60W PoE\" are Type 3 devices.\u003C\u002Fp>\n\u003Ch2>Type 4: 90 W and the 100 W Question\u003C\u002Fh2>\n\u003Cp>Type 4 adds classes 7 and 8, taking the PSE up to 90 W per port with 71.3 W guaranteed at the PD. Type 4 is always 4-pair. You will sometimes see \"100 W PoE\" in marketing copy; that figure refers to the theoretical maximum a Type 4 port can source into the cable under ideal conditions, not to a standardized guarantee. When you compare datasheets, compare the PD-side number, because that is the power your device can actually consume.\u003C\u002Fp>\n\u003Ctable>\u003Cthead>\u003Ctr>\u003Cth>Parameter\u003C\u002Fth>\u003Cth>Type 1 (802.3af)\u003C\u002Fth>\u003Cth>Type 2 (802.3at)\u003C\u002Fth>\u003Cth>Type 3 (802.3bt)\u003C\u002Fth>\u003Cth>Type 4 (802.3bt)\u003C\u002Fth>\u003C\u002Ftr>\u003C\u002Fthead>\u003Ctbody>\n\u003Ctr>\u003Ctd>Classes\u003C\u002Ftd>\u003Ctd>0-3\u003C\u002Ftd>\u003Ctd>4\u003C\u002Ftd>\u003Ctd>5-6\u003C\u002Ftd>\u003Ctd>7-8\u003C\u002Ftd>\u003C\u002Ftr>\n\u003Ctr>\u003Ctd>Max PSE power per port\u003C\u002Ftd>\u003Ctd>15.4 W\u003C\u002Ftd>\u003Ctd>30 W\u003C\u002Ftd>\u003Ctd>60 W\u003C\u002Ftd>\u003Ctd>90 W\u003C\u002Ftd>\u003C\u002Ftr>\n\u003Ctr>\u003Ctd>Guaranteed PD power\u003C\u002Ftd>\u003Ctd>12.95 W\u003C\u002Ftd>\u003Ctd>25.5 W\u003C\u002Ftd>\u003Ctd>51 W\u003C\u002Ftd>\u003Ctd>71.3 W\u003C\u002Ftd>\u003C\u002Ftr>\n\u003Ctr>\u003Ctd>Pairs used\u003C\u002Ftd>\u003Ctd>2\u003C\u002Ftd>\u003Ctd>2\u003C\u002Ftd>\u003Ctd>2 or 4\u003C\u002Ftd>\u003Ctd>4\u003C\u002Ftd>\u003C\u002Ftr>\n\u003Ctr>\u003Ctd>PSE output voltage\u003C\u002Ftd>\u003Ctd>44-57 V\u003C\u002Ftd>\u003Ctd>50-57 V\u003C\u002Ftd>\u003Ctd>50-57 V\u003C\u002Ftd>\u003Ctd>52-57 V\u003C\u002Ftd>\u003C\u002Ftr>\n\u003Ctr>\u003Ctd>Minimum cabling\u003C\u002Ftd>\u003Ctd>Cat3 \u002F Cat5\u003C\u002Ftd>\u003Ctd>Cat5e\u003C\u002Ftd>\u003Ctd>Cat5e\u003C\u002Ftd>\u003Ctd>Cat5e\u003C\u002Ftd>\u003C\u002Ftr>\n\u003C\u002Ftbody>\u003C\u002Ftable>\n\u003Ch2>Single-Signature and Dual-Signature PDs\u003C\u002Fh2>\n\u003Cp>802.3bt defines two ways a powered device can present itself on four pairs. A single-signature PD shows one detection signature and one classification signature across both pairsets, and the PSE treats it as a single load. A dual-signature PD presents an independent signature on each pairset, so the two halves can be detected, classified and powered separately. Dual-signature designs exist mainly for devices that internally split power between two subsystems, and for certain PoE lighting fixtures. The distinction matters when you buy a midspan or a splitter: a PSE that only supports single-signature PDs will not correctly power a dual-signature load.\u003C\u002Fp>\n\u003Ch2>Autoclass and Power Reclamation\u003C\u002Fh2>\n\u003Cp>Classification tells the PSE which class a device belongs to, but a class is a band, not a measurement. A class 6 device is allocated 60 W even if it never draws more than 38 W, and in a fully loaded switch that wasted allocation is expensive. 802.3bt adds Autoclass, in which the PSE measures the PD's actual peak draw during a short window after power-up and then reserves only that amount plus a margin. On a 24-port switch feeding mixed loads, Autoclass can free enough headroom to avoid a second power supply.\u003C\u002Fp>\n\u003Ch2>Backward Compatibility\u003C\u002Fh2>\n\u003Cp>An 802.3bt PSE will correctly detect, classify and power 802.3af and 802.3at devices; the detection and classification sequences were extended, not replaced. The reverse is not true. Plugging a class 8 device into an 802.3at switch produces either no power at all or a device that boots into a reduced-function mode, depending on how the PD firmware handles an under-powered supply. Many PTZ cameras and multi-radio access points behave exactly this way, which is a frequent source of \"the camera works but the heater and the zoom motor do not\" support tickets.\u003C\u002Fp>\n\u003Ch2>Specifying Type 3 and Type 4 Equipment\u003C\u002Fh2>\n\u003Cp>When you write a specification, pin down five things: the Type and class per port, whether the per-port maximum can be sustained on every port simultaneously or only on a subset, the total system power budget, whether the PSE supports single- and dual-signature PDs, and whether Autoclass and LLDP negotiation are implemented. A switch advertising \"90 W PoE\" that can only deliver it on two of twenty-four ports is a legitimate product, but it is not the product most buyers think they are getting.\u003C\u002Fp>\n\u003Cp>Cabling deserves its own line in the specification. Four-pair power at Type 3 and Type 4 levels pushes considerably more total current through a bundle, and the resulting temperature rise limits large installations far more often than any single port's rating does. Cat6 or Cat6a with a documented maximum bundle size is the safe default.\u003C\u002Fp>"]