PoE Power Budget and Cable Length: 802.3af, at, bt Power per Port, Voltage Drop on Cat5e, Cat6 and Cat6A, and Why CCA Fails on PoE
Sep 10, 2026
Summary: IEEE 802.3 PoE delivers power from the switch (PSE) to the device (PD) over the data pairs, and the standard guarantees the device its rated power only at 100 m on compliant cable. 802.3af (Type 1) supplies 15.4 W at the switch and guarantees 12.95 W at the device; 802.3at PoE+ (Type 2) supplies 30 W and guarantees 25.5 W; 802.3bt Type 3 supplies 60 W and guarantees 51 W; 802.3bt Type 4 supplies 90 W and guarantees 71.3 W. The difference is cable loss: at 90 W the current is 960 mA per pair over four pairs, and the standard's maximum loop resistance of 12.5 ohms (Cat5e, 100 m) dissipates the missing 18.7 W as heat in the cable. Cat6A has about 30 percent lower DC resistance than Cat5e and runs cooler in bundles. NEC 725.144 derates ampacity for bundles carrying more than 60 W per cable; Cat6A 23 AWG cable is exempt from the derating in bundles up to 192 cables at 0.6 A. Copper-clad aluminum cable has 40 to 55 percent higher resistance than bare copper, fails the TIA 100 m loop-resistance limit, overheats under PoE++ and is not UL-listed CM/CMR/CMP; do not use it on any PoE run.
The PoE classes
| Standard | Name | Type | Class | PSE power (switch) | PD power (device, at 100 m) | Pairs | Current per pair | Typical devices |
|---|---|---|---|---|---|---|---|---|
| 802.3af (2003) | PoE | 1 | 0 to 3 | 15.4 W | 12.95 W | 2 | 350 mA | VoIP phones, basic APs, fixed cameras |
| 802.3at (2009) | PoE+ | 2 | 4 | 30 W | 25.5 W | 2 | 600 mA | PTZ cameras, Wi-Fi 5 and 6 APs, access control |
| 802.3bt (2018) | PoE++ / 4PPoE | 3 | 5 to 6 | 60 W | 51 W | 4 | 600 mA | Wi-Fi 6E and 7 APs, video conferencing, thin clients |
| 802.3bt (2018) | PoE++ / 4PPoE | 4 | 7 to 8 | 90 W | 71.3 W | 4 | 960 mA | PTZ with heaters, LED lighting, displays, laptops |
| Cisco UPOE+ | Proprietary | 90 W | 71 W | 4 | 960 mA | Same as Type 4, pre-standard | ||
| Passive 24 V / 48 V | Non-standard | Fixed | Varies | 2 or 4 | Varies | Ubiquiti and Mikrotik radios; no negotiation, can damage non-passive devices |
PSE power is what the switch's power budget is consumed by; PD power is what the device can count on. A 48-port switch with a 370 W budget powers 24 PoE+ devices at 30 W each on paper, fewer if the ports are Type 3 or 4, and the switch's own class negotiation with each device determines the allocation. Budget by PSE watts; specify devices by PD watts.
Where the power goes
Loss in the cable is I²R. At 802.3bt Type 4, 960 mA on each of four pairs through the standard's 12.5 ohm maximum loop resistance per pair (Cat5e at 100 m, two conductors in series) dissipates 0.96² × 12.5 = 11.5 W per pair pair-set, about 18.7 W total, which is exactly the 90 − 71.3 W gap. On Cat6A (about 9 ohms loop at 100 m) the loss falls to about 13 W, so a Type 4 device at 100 m on Cat6A sees roughly 77 W rather than 71. Shorter runs lose proportionally less; a 30 m run loses under 6 W.
| Cable | Conductor | DC resistance, ohm per 100 m, one conductor | Loop resistance at 100 m | Type 4 loss at 100 m | PoE++ bundle behaviour |
|---|---|---|---|---|---|
| Cat5e bare copper | 24 AWG | 9.4 | 18.8 (limit 25 per TIA, 12.5 per IEEE pair) | about 18.7 W | Derate per 725.144 |
| Cat6 bare copper | 23 AWG | 7.5 | 15 | about 15 W | Derate per 725.144 |
| Cat6A bare copper | 23 AWG, larger OD | 7.0 to 7.5 | 14 to 15 | about 13 to 15 W | Exempt to 192 cables at 0.6 A per 725.144(A) exception; runs 4 to 5 C cooler than Cat5e |
| Cat5e CCA | 24 AWG aluminum core | 13.5 to 15 | 27 to 30 | about 27 W, and rising as it heats | Fails TIA loop resistance; not listed; overheats |
| Cat6 CCA | 23 AWG aluminum core | 11 to 12 | 22 to 24 | about 22 W | Same |
Distance
The 100 m (328 ft) channel limit, 90 m permanent link plus 10 m of patch cords, is set by the Ethernet timing and attenuation budget, not by PoE; PoE simply guarantees its PD power at that distance on compliant cable. Past 100 m, data fails before power on most switches: 1000BASE-T drops link at about 110 to 120 m, and 100BASE-T often survives to 150 m. PoE extenders (PoE-powered repeaters that regenerate data and pass through power) add 100 m per hop, typically two hops; PoE over coax converters and long-reach PoE switches at 10 Mbps push 250 to 500 m for single cameras. For a Type 4 device at distance, the honest answers are Cat6A, an extender or fiber with local power.
Bundles, heat and NEC 725.144
Since the 2017 NEC, 725.144 applies to any cable carrying more than 60 W (PoE++). Table 725.144 gives the ampacity per conductor by bundle size and conductor gauge at 30 C ambient: for 24 AWG in a bundle of 37 to 61 cables the limit is 0.45 A per conductor, below the 0.6 A that Type 3 and 4 need; for 23 AWG it is 0.6 A at the same bundle size; for 22 AWG it is 0.7 A. 725.144(A) Exception: Type 4 and Type 3 loads are permitted on 23 AWG or larger cable marked with an LP (limited power) rating, e.g., LP(0.6A), in bundles up to 192 cables without applying the table, which is why Cat6A CMP with an LP rating is the specification for Wi-Fi 6E and 7 access-point runs in ceiling bundles. Temperature rise in a bundle of 96 Cat5e cables at 0.6 A can exceed 10 C, pushing a 60 C rated cable past its insulation rating in a hot plenum.
Why CCA fails on PoE
- Resistance: aluminum has 61 percent the conductivity of copper; a copper-clad aluminum conductor has 40 to 55 percent more resistance, so loss and heat rise by the same fraction and the PD may not receive its class power.
- Loop resistance limit: TIA 568 requires 25 ohms maximum DC loop resistance per 100 m; CCA cable typically measures 27 to 30 and fails certification.
- Heat: I²R in the bundle rises with resistance; CCA in a PoE++ bundle is the documented cause of melted jackets.
- Listing: UL does not list CCA cable as CM, CMR or CMP; the markings on the jacket are not backed by a UL file. That makes it a code violation under 800.179 and an insurance problem after a fire.
- Termination: aluminum creeps under IDC contacts in jacks and plugs, so the connection loosens over time.
Design checklist
- Sum device PD watts, then add 20 percent cable loss and headroom; compare to the switch PSE budget.
- Any Type 3 or 4 device: Cat6A 23 AWG, LP-rated, bare copper. Wi-Fi 7 APs are Type 3 minimum.
- Any Type 1 or 2 device: Cat6 bare copper is fine to 100 m.
- Bundles over 60 W per cable: apply Table 725.144 or use LP-rated cable; spread bundles in cable tray rather than tight-wrapping.
- Verify with a certifier that reports DC resistance and PoE class after installation; a cheap tester that lights LEDs proves nothing about PoE.
- Never passive PoE into an 802.3 device; check the injector before plugging in a camera.
Quick answers
How many watts does PoE+ deliver? 30 W from the switch, 25.5 W guaranteed at the device at 100 m, per 802.3at.
What is the maximum PoE distance? 100 m for guaranteed power and gigabit data; an 802.3 PoE extender adds 100 m per hop.
Can I use CCA cable for PoE cameras? No. CCA fails the TIA loop-resistance limit, delivers less power, overheats in bundles and is not UL-listed for in-building use.
Does PoE++ require Cat6A? For 60 to 90 W devices in bundles, yes in practice: NEC 725.144 derates 24 AWG below the 0.6 A needed, and 23 AWG LP-rated Cat6A is exempt from the derating.
Conversions Tech stocks bare-copper Cat6 and Cat6A in CMP and CMR, Cat6 patch cables, Cat6 and Cat6A keystone jacks and patch panels, plus the 12 V to 48 V PoE injectors for vehicle and off-grid installs. See Cat6 vs Cat6A vs Cat8 and jacket ratings.
