What is the difference between PoE, PoE+ and PoE++?

They are 802.3af, 802.3at and 802.3bt. PoE reserves up to 15.4 W at the switch and delivers 12.95 W to the device; PoE+ reserves 30 W and delivers 25.5 W; PoE++ reaches 60 W or 90 W reserved as Type 3 and Type 4, delivering 51 W and 71.3 W. The gap in each case is loss allowed for in 100 metres of cable.

Why does my switch run out of PoE budget so quickly?

Because it reserves by class, not by measurement. A camera drawing 4 W that classifies as Class 3 takes 15.4 W out of the budget and holds it. Forcing the class, or using LLDP-based allocation where the hardware supports it, reclaims the difference.

Why does a PoE device work on a short cable but not the installed run?

Resistive loss. Long runs, undersized conductors, or copper-clad aluminium all drop enough voltage that the device sees too little at the far end. CCA cable is the most common culprit and the hardest to spot, since it tests fine on the bench.

Which pairs does PoE use?

Mode A powers the data pairs on pins 1/2 and 3/6, Mode B powers pins 4/5 and 7/8, and 802.3bt uses all four pairs at once. Four-pair delivery is what makes 60 W and 90 W thermally possible.

Field guide · Cabling & low-voltage

PoE standards

802.3af, at and bt in the terms that matter on site: what each class reserves from the switch against what actually reaches the device, which pairs carry the power, and the faults behind a device that will not power up — in the order worth checking them.

Plate 01Diagram

The switch reserves the class, not the draw.

Look at

What each class reserves, and what reaches the device.

PoE classes, reserved and delivered power
Class	Standard	Switch reserves	Device receives	Lost to the cable
Class 0	802.3af	15.4 W	12.95 W	2.45 W
Class 1	802.3af	4.0 W	3.84 W	0.16 W
Class 2	802.3af	7.0 W	6.49 W	0.51 W
Class 3	802.3af	15.4 W	12.95 W	2.45 W
Class 4 · PoE+	802.3at	30.0 W	25.50 W	4.50 W
Class 5 · PoE++	802.3bt Type 3	45.0 W	40.00 W	5.00 W
Class 6 · PoE++	802.3bt Type 3	60.0 W	51.00 W	9.00 W
Class 7 · PoE++	802.3bt Type 4	75.0 W	62.00 W	13.00 W
Class 8 · PoE++	802.3bt Type 4	90.0 W	71.30 W	18.70 W

The budget trapA switch reserves the class maximum, not the measured draw. A camera pulling 4 W that classifies as Class 3 costs the switch 15.4 W of budget and keeps it — which is why a 48-port switch with a 370 W supply runs out long before anybody expected.

The lost wattsThe gap between the two columns is resistive loss in 100 m of cable. It is not waste you can design away — it is the standard’s allowance for a worst-case channel, and it is why cable category and bundle temperature matter at the top classes.

Fig. 01What each class takes out of the switch against what arrives at the device, which conductors carry it, and the faults behind a device that will not power — ordered by how often they turn out to be the answer.

Plate 02Questions

Questions people actually ask

What is the difference between PoE, PoE+ and PoE++?: They are 802.3af, 802.3at and 802.3bt. PoE reserves up to 15.4 W at the switch and delivers 12.95 W to the device; PoE+ reserves 30 W and delivers 25.5 W; PoE++ reaches 60 W or 90 W reserved as Type 3 and Type 4, delivering 51 W and 71.3 W. The gap in each case is loss allowed for in 100 metres of cable.
Why does my switch run out of PoE budget so quickly?: Because it reserves by class, not by measurement. A camera drawing 4 W that classifies as Class 3 takes 15.4 W out of the budget and holds it. Forcing the class, or using LLDP-based allocation where the hardware supports it, reclaims the difference.
Why does a PoE device work on a short cable but not the installed run?: Resistive loss. Long runs, undersized conductors, or copper-clad aluminium all drop enough voltage that the device sees too little at the far end. CCA cable is the most common culprit and the hardest to spot, since it tests fine on the bench.
Which pairs does PoE use?: Mode A powers the data pairs on pins 1/2 and 3/6, Mode B powers pins 4/5 and 7/8, and 802.3bt uses all four pairs at once. Four-pair delivery is what makes 60 W and 90 W thermally possible.

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