LowVoltKit

PoE Budget and Distance

Whether a switch carries the devices hung off it, how much budget is left, and whether each run is inside the Ethernet channel.

Each class lists two wattages. The PSE figure is what the switch allocates at the port. The PD figure is what the standard guarantees at the device. The difference is the standard's worst-case allowance for cable loss over a full-length channel. The switch budget is spent at the PSE figure, not the sum of the device datasheets. Four-pair delivery (802.3bt) puts two conductors on each polarity, which halves loop resistance against the same length of two-pair PoE.

PoE budget and distance calculator

Whether a switch carries the devices hung off it, how much budget is left, and whether a run is inside the Ethernet channel. Inputs stay in the URL.

Starting points

Inputs

Devices

Typical starting classes. Confirm each against the datasheet.

802.3at Type 2, 2-pair. Switch spends 30 W PSE; 25.5 W is guaranteed at the device.

W

Datasheet watts, when the camera publishes a figure below the class ceiling.

Reserves 240.0 W at the switch

Switch

W

From the switch datasheet. The budget is spent at the class PSE figure.

W

The highest class the port is rated to grant.

%

Share of budget to stay within. Default 80.

Cable and distance

m

One representative run. The 100 m channel is a data limit, not a power limit.

24 AWG is typical of Cat5e and many Cat6. 23 AWG is common on Cat6 and Cat6A.

Results

240.0 W at the switch. 240.0 W is inside the 80% headroom limit of 296.0 W on a 370 W budget.

PoE load at the switch

240.0W

Pass

240.0 W is inside the 80% headroom limit of 296.0 W on a 370 W budget.

Budget

Class reservation

240.0W

PD total

204.0W

Budget remaining

130.0W

Percent of budget

64.9%

Percent of headroom

81.1%

PoE ports

8 / 24

This run

Length

90.0 m of 100 m

Channel

Inside the 100 m data limit

Estimated cable loss

2.73W

Estimated power at the device

27.3W

Class 4 PD

25.50W

Against the PD figure

At or above the class PD figure

100 m is a data limit, not a power limit. Power runs further than Ethernet. A long run uses a PoE extender, a media converter with local power, or fiber with an injector at the far end.

Cable loss is why the standard reserves the difference between PSE and PD power. It is not subtracted from the switch budget a second time.

PoE dissipates heat in the switch as well as in the cable. A fully loaded PoE switch in a closed enclosure runs hot. Enclosure ventilation is part of the design.

Related

Video bandwidth for bitstream and uplink after the ports are powered.

Worked example

Eight Class 4 cameras reserve 240 W at the switch. On a 370 W budget that leaves 130 W and sits at 65% of budget, inside an 80% headroom limit.

How to use

  1. List each device, its PoE class, and the quantity. The optional actual-draw field is the datasheet watts when that figure sits below the class ceiling.
  2. Enter the switch PoE budget, port count, PoE ports available, and per-port maximum from the datasheet. Headroom is the share of budget to stay within. Default 80.
  3. The switch spends the class PSE figure. The PD figure is what the standard guarantees at the device after worst-case cable loss. Some switches allocate by class. Others allocate by measured draw.
  4. Enter one representative run: length, category, and gauge. The 100 m channel is a data limit. Power continues past that length. Options on a long run are a PoE extender, a media converter with local power, or fiber with an injector at the far end.

Assumptions

  • Class allocations are IEEE 802.3af, 802.3at, and 802.3bt figures. Type 1 uses 44 V. Type 2 and above use 50 V.
  • The switch budget is spent at the PSE figure. An entered datasheet draw is reported beside that reservation and does not change the class total.
  • Switches vary in whether they allocate by class or by measured draw. The switch datasheet governs.
  • Cable loss uses the solid-copper table at 20 °C. Actual cable, pair count, and temperature change the figure. The estimate is not subtracted from the switch budget a second time.
  • The 100 m channel is commonly 90 m of solid horizontal cable plus 10 m of patch cords. That is a data limit, not a power limit.
  • The switch and camera datasheets govern. The result is an estimate to verify against manufacturer data, the AHJ, and current applicable code.

FAQ

Why can a switch with enough ports still run out of power?
Port count and watt budget are separate limits. Twenty-four Class 4 ports reserve 720 W. A 370 W budget is the watt limit, not the port count.
What happens when the budget is exceeded?
The switch stops granting power to additional ports, or it drops ports already granted, depending on how that model allocates. The switch datasheet is the source for that behaviour.
Does Cat5e carry 802.3bt?
802.3bt Type 3 and Type 4 use four pairs. Cat5e is an IEEE medium for 1000BASE-T over a 100 m channel. Current and heat in a bundle are a separate check against the cable datasheet.
How to get power past 100 m?
Power continues past the Ethernet channel. Options are a PoE extender, a media converter with local power, or fiber with an injector at the far end.

Disclaimer

Field estimate only. Manufacturer data, the authority having jurisdiction (AHJ), and current applicable code govern material and whether a circuit is energized. See the disclaimer.