PoE++ Type 4 (IEEE 802.3bt) 90W — Installation Guide for UK Electricians

Power over Ethernet (PoE) is the family of IEEE standards that allows electrical power to be carried on the same twisted-pair structured cabling that already carries Ethernet data. The standards are ratified by the IEEE 802.3 working group and have evolved through four distinct generations, each one increasing the deliverable power, and each one widening the scope of what an electrician will be asked to install.

1. PoE Type 1 — IEEE 802.3af (2003). 15.4W at the power sourcing equipment (PSE), 12.95W at the powered device (PD). Uses two pairs only. Designed for VoIP phones and early wireless access points.
2. PoE Type 2 (PoE+) — IEEE 802.3at (2009). 30W at the PSE, 25.5W at the PD. Still two-pair, but higher current. Designed for pan-tilt-zoom IP cameras and dual-band Wi-Fi access points.
3. PoE Type 3 (PoE++ / 4PPoE) — IEEE 802.3bt (2018). 60W at the PSE, 51W at the PD. Uses all four pairs (4PPoE) for the first time. Designed for video conferencing, high-power APs, and modest PoE lighting.
4. PoE Type 4 (Higher-Power PoE / Hi-PoE) — IEEE 802.3bt (2018). 90W at the PSE, 71.3W at the PD. Four-pair, full-current operation. Designed for laptops, LED luminaires, high-power digital signage and motorised access control.

The leap from Type 2 to Type 4 is large. It triples the deliverable power, doubles the number of conductors carrying current, and roughly doubles the heating effect inside the cable bundle. From an electrician's perspective, PoE Type 4 is the point at which Power over Ethernet stops being "just data" and becomes a small DC distribution system that needs proper design.

Every PoE link has two roles. The power sourcing equipment (PSE) provides the DC supply and the data — typically a PoE-enabled network switch, a PoE midspan injector, or in larger systems a dedicated PoE power shelf. The powered device (PD) consumes the supply — a luminaire, camera, access point, intercom, or display. The IEEE 802.3bt standard defines a handshake (the LLDP-based classification protocol) that allows the PSE and PD to negotiate the correct power class before any current is delivered.

A standard four-pair UTP cable (Cat5e, Cat6, Cat6a, Cat7) contains eight conductors arranged as four twisted pairs. The original 10/100BASE-T Ethernet standard used only two of those pairs for data (pins 1/2 and 3/6). The original PoE Type 1 standard could therefore put power on the unused pairs (Mode B) or share power with the data pairs (Mode A) — but in both cases, only two pairs carried current.

With Gigabit Ethernet (1000BASE-T) and faster, all four pairs are used for data simultaneously. PoE Type 3 and Type 4 therefore had to define a way to share DC power on all four pairs while the same pairs still carry differential Ethernet signalling. The result is 4-pair power, often called 4PPoE: the DC supply is split across all four pairs, each carrying roughly a quarter of the total current.

IEEE 802.3bt is written conservatively: it states that Type 3 and Type 4 will work over a Class D channel (Cat5e) up to 100m. In practice, UK installers should treat Cat6a as the realistic minimum for Type 4 90W operation, and reserve Cat5e only for the lowest power classes. The reason is heat.

Cat5e has a conductor cross-section of typically 24 AWG (0.205 mm²) with a higher DC loop resistance than Cat6a. At 960 mA per pair, the I²R heating per metre is higher, and in a bundle of 24 or 48 cables each carrying near-maximum current, the bundle core temperature rises. A higher cable temperature increases insulation resistance creep, raises conductor resistance further, and reduces signal-to-noise ratio. BS EN 50174-2 includes installation rules to manage this.

PoE voltage drop is calculated differently to BS 7671 mains volt drop because it is a low-voltage DC system with a defined input tolerance at the load. The PSE puts out 52–57V DC; the PD must work down to 41.1V DC. The available voltage drop budget is therefore around 10–16V DC across the cable, which sounds generous until you do the arithmetic on 100m of 23 AWG conductors carrying 960 mA per pair.

1. Calculate the DC loop resistance — for Cat6a at 23 AWG, approximately 0.083 Ω/m per pair (round-trip). For Cat5e at 24 AWG, approximately 0.094 Ω/m per pair. Manufacturer datasheets are authoritative.
2. For 4PPoE, four pairs in parallel quarter the effective loop resistance — so on Cat6a, roughly 0.021 Ω/m total for the load current return path.
3. At 90W output with 53V at the PSE, total current is approximately 1.7A (split across four pairs as 425 mA per pair). Over 100m of Cat6a the voltage drop is approximately 1.7A × (0.021 Ω/m × 100m) = 3.6V DC — well within budget.
4. On a Cat5e installation at the same length, total voltage drop rises to approximately 4.0V DC — still within budget but with less margin and more thermal stress.
5. For long runs above 90m, always model the actual cable resistance from the datasheet, and verify the PD continues to operate at minimum input voltage with a worst-case bundle de-rated temperature.

The 100m channel length limit in BS EN 50173 is an Ethernet signalling limit, not a PoE limit. PoE itself could go further on heavy cable, but the data side will fail first. Never exceed 100m end-to-end (patch + permanent link + patch) on a PoE channel. Where greater distances are required, a fibre uplink to an intermediate PSE switch is the correct approach, not a longer copper run.

When modelling worst-case voltage drop, use the cable manufacturer's maximum DC loop resistance figure (typically quoted at 20°C) and apply a temperature correction factor for the expected bundle core temperature. Copper conductor resistance rises by approximately 0.4% per degree Celsius above 20°C, so a 50°C bundle core temperature adds roughly 12% to the loop resistance — a meaningful effect on a long run near the limit.

The single biggest installation risk with PoE Type 4 is heat. Every conductor carrying current dissipates I²R heat. A single cable in free air can dissipate this easily. A bundle of 24 cables in a closed plastic trunking, each running near full Type 4 current, behaves more like a small heating element — the inner cables of the bundle cannot lose heat efficiently to the environment, and the bundle core temperature rises.

BS EN 50174-2 addresses this directly with bundle size guidance for installations carrying remote powering. The relevant guidance is that bundles of cables carrying high PoE currents should be limited in size, kept loose rather than tightly tied, and routed in containment that allows airflow.

BS 7671:2018+A4:2026 (published 15 April 2026) applies to electrical installations including those operating at extra-low voltage (ELV). Type 4 PoE at 52–57V DC sits firmly in the ELV band (BS 7671 defines ELV as not exceeding 50V AC or 120V ripple-free DC, and PoE is well below the DC ceiling). The presence of mains-powered PSE upstream brings PoE within the broader scope of BS 7671 wherever it shares containment with low-voltage circuits or supplies fixed building services.

For PoE-powered lighting in particular, see our dedicated PoE lighting guide and the Section 715 ELV lighting A4:2026 update for the latest amendment-specific guidance.

Type 4 PoE is not for everything — for a single 9W ceiling pendant it is overkill. It earns its place in installations where centralised low-voltage DC distribution, single-cable convenience, network integration and remote management together justify the slightly higher infrastructure cost.

PoE circuits are exposed to two surge sources: induced surges on the structured cabling itself (from nearby lightning strikes, switching transients on adjacent LV circuits, or static discharge at outdoor terminations) and surges propagating from the PSE's mains supply. Both must be considered, particularly where cabling leaves the building envelope to feed external cameras, gate intercoms, or car-park lighting.

Testing a PoE installation requires both BS 7671 verification of the LV supply to the PSE and structured-cabling certification of the PoE-carrying channels. These are two distinct exercises with two distinct sets of test equipment.

1. BS 7671:2018+A4:2026 inspection and testing of the supply circuit feeding the PSE — continuity, insulation resistance, polarity, earth fault loop impedance, RCD testing (if used), and prospective fault current. Recorded on an EIC or Minor Works Certificate.
2. Structured-cabling permanent-link or channel certification to BS EN 50173 / BS EN 50346 — wire map, insertion loss, return loss, near-end and far-end crosstalk, propagation delay, delay skew. Recorded on a Fluke / Viavi / TREND test report.
3. PoE power test — many modern cable certifiers (Fluke DSX, Fluke DSX-8000, Versiv) include a PoE option that simulates a Class 8 Type 4 load and confirms the channel can sustain it without excessive voltage drop or overheating.
4. Continuity of earthing on shielded cable (Cat6a F/UTP, S/FTP) — verify the screen is continuous and bonded at one end only (typically the rack end) to avoid earth loops.
5. PD operation verification — power on the PSE, confirm the PD enumerates, confirm it operates at full load over a sustained period (15 minutes minimum), and measure cable temperature at the bundle core if possible.

Many UK contractors split the work: an electrician handles the PSE supply and the BS 7671 certification, and a structured-cabling subcontractor handles the data cable certification. Where one contractor takes both, the documentation pack should include both certificates plus the PoE power test result.

PoE Type 4 puts more current through each conductor than any previous PoE generation. That current passes through every termination: the IDC contacts at the patch panel, the RJ45 plug at each end, and the modular jack inside the PD. A poor termination at any of these points is a hot spot — and under Type 4 currents, hot spots can damage the contact, cause intermittent disconnections, and in worst cases arc on disconnect.

A compliant UK PoE Type 4 installation will reference several overlapping standards. The structured cabling standards govern the physical infrastructure and its certification; the IEEE standard governs the electrical behaviour of PSE and PD; BS 7671 governs the supply and any shared containment; and product standards govern the safety of the PSE and PD as items of equipment.

For broader structured cabling context, see our structured cabling and BS EN 50173 guide for electricians and the dedicated BS EN 50174 data cable installation guide.

PoE Type 4 sits at the intersection of two trades — structured cabling and electrical — and the design effort is usually under-quoted. A 200-luminaire PoE lighting scheme, a 64-camera CCTV install, or a 40-AP enterprise Wi-Fi deployment each involves PSE sizing, bundle planning, voltage-drop modelling, BS 7671 supply design, and a documentation pack that satisfies both BS EN 50346 cable certification and BS 7671 inspection and testing.