A PoE budget calculator is a worksheet, not a formula to memorise: list each device, take its real draw instead of its class rating, add the cable loss factor, multiply by port count, and require that the total stays at or below 80% of the switch's nameplate PoE budget. Everything below is the fill-in version of that sentence.
Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches.
Size on measured or datasheet device draw, not on class rating. A Class 3 camera that idles at 4 W and pulls 6 W with its IR heater on should be budgeted at 6 W, not at 15.4 W and not at 4 W.
Cable loss scales with the square of current, so it is negligible at 6 W and reaches roughly 20% for a 90 W load at 100 m. Use the loss factor table rather than a flat percentage.
Require steady-state demand to be at or below 80% of the nameplate budget. That 20% gap absorbs cold-start inrush, IR heater switching and supply ageing.
If you want the theory rather than the arithmetic, read How to Calculate the Power Budget of a PoE Switch and the calculation method that avoids an underpowered design. This page assumes you already know what a class is and goes straight to the table.
The whole calculation is one row per device type and one multiplication per row.
| Column | What you write | Where the number comes from |
|---|---|---|
| Device | Camera, AP, phone, sensor | Your bill of materials |
| Quantity | Number of that device on this switch | Your floor plan |
| Class | 802.3af Class 0 to 3, 802.3at Class 4, 802.3bt Type 3 or 4 | Device datasheet; determines the per-port cap |
| PSE nominal | 15.4 W, 30 W, 60 W or 90 W | The standard, not the device |
| Actual draw | What the device really consumes in its worst mode | Device datasheet, or a clamp meter on a sample |
| Cable length | Metres from switch port to device | Your cable schedule |
| Subtotal | Quantity x actual draw x (1 + loss factor) | The arithmetic below |
The last line of the worksheet is the decision: required budget = total subtotal / 0.8. If the switch's nameplate PoE budget is at or above that number, the design passes. If not, you either split the load or buy a bigger supply.
The PSE figure is what the switch port offers. The PD figure is what the device is guaranteed to receive after 100 m of cable. Both matter, and the gap between them is the reason the loss factor exists.
| Standard and class | PSE output at the port | Guaranteed power at the PD | Typical devices |
|---|---|---|---|
| 802.3af Class 0 to 3 | 15.4 W | 12.95 W | Fixed bullet cameras, IP phones, small sensors |
| 802.3at Class 4 (Type 2) | 30.0 W | 25.5 W | PTZ cameras, 802.11ac access points, video door stations |
| 802.3bt Type 3 (Class 5 to 6) | 60.0 W | 51.0 W | Panoramic cameras, high-density APs, small displays |
| 802.3bt Type 4 (Class 7 to 8) | 90.0 W | 71.3 W | Heater-equipped enclosures, compact computing nodes |
Two practical notes. First, the class sets the cap, and a modern PSE will happily allocate less through the LLDP negotiation; that is exactly why the "actual draw" column exists. Second, on a poe switch for ip cameras the IR illuminator is the load that breaks budgets, because it switches on at dusk and adds 2 to 4 W per camera at exactly the moment every other camera does the same thing.
Loss is I squared R, so it grows with the square of current. Cat5e at 100 m has roughly 20 ohms of loop resistance per pair (24 AWG, about 8.4 ohms per conductor each way), and with all four pairs in parallel the effective loop is about 5 ohms.
The table below is computed from that resistance at a nominal 48 V and then rounded up, so it is conservative:
| Run length | 802.3af up to 15 W | 802.3at up to 30 W | 802.3bt 60 W | 802.3bt 90 W |
|---|---|---|---|---|
| 30 m | 1% | 2% | 4% | 6% |
| 60 m | 2% | 4% | 8% | 12% |
| 100 m | 3% | 7% | 13% | 20% |
The industry shorthand "allow 12 to 20% for a 100 m run" is really a statement about 60 W and 90 W loads; at 15 W the physics gives about 3%. Use the table. Where the run is genuinely 100 m and the load is high, add a further margin for connector contact resistance and for copper resistance rising with temperature, roughly 0.4% per degree Celsius above 20 °C.
Copy this blank table and fill one row per device type. The worked rows are from Example 1 below.
| Device | Qty | Class | PSE nominal | Actual draw | Cable length | Loss factor | Subtotal |
|---|---|---|---|---|---|---|---|
| IR bullet camera | 8 | Class 3 (af) | 15.4 W | 6.0 W (IR on) | 60 m | 4% | 49.9 W |
| (your device) |
Then add the switch's own consumption, which is typically 5 to 15 W for a small industrial platform depending on port count and design — take it from the datasheet rather than from this worksheet, since it varies by platform. Add it to the subtotal before applying the 80% rule.
Take a common configuration: eight gigabit PoE access ports plus four uplink ports, with all eight access ports carrying cameras. These are industry-typical planning values, not measurements of any specific product.
| Row | Value |
|---|---|
| Device | Fixed IR bullet camera, 4 W idle, 6 W with IR illuminator on |
| Class | 802.3af Class 3, PSE cap 15.4 W |
| Actual draw used in the worksheet | 6.0 W (the worst mode, because all eight switch on together) |
| Cable | 60 m average run, Cat5e |
| Loss factor | 4% |
| Per port subtotal | 6.0 x 1.04 = 6.24 W |
| Eight ports | 8 x 6.24 = 49.9 W |
| Switch own consumption | 8 W (industry-typical planning value) |
| Steady-state demand | 57.9 W |
| Required nameplate budget | 57.9 / 0.8 = 72.4 W |
A 65 W PoE budget fails this design and a 72 W budget sits exactly on the line, which is another way of saying "no margin for a hot summer". A 120 W budget passes with room to add two cameras later. Notice that the class rating would have given 8 x 15.4 = 123.2 W and pushed you to an oversized supply, while the idle figure of 8 x 4 = 32 W would have passed a 60 W supply that then collapses at dusk.
| Device | Qty | Class | PSE nominal | Actual draw | Cable | Loss factor | Subtotal |
|---|---|---|---|---|---|---|---|
| IR bullet camera | 4 | Class 3 (af) | 15.4 W | 6.0 W | 60 m | 4% | 24.96 W |
| 802.11ac access point | 2 | Class 4 (at) | 30.0 W | 13.0 W | 40 m | 4% | 27.04 W |
| Total | 6 | 52.0 W |
Add 8 W for the switch itself: demand is 60.0 W, so the required budget is 60.0 / 0.8 = 75 W. A 65 W supply is 10 W short and will show as APs that reboot under load rather than as a clean failure, because the two APs are the highest-draw devices and the first to be rationed. On an 8 port poe switch with six PoE ports in use, the two spare ports are also the easiest trap: they look free, and then someone adds two more cameras and the whole thing tips over.
Six PTZ cameras with heater and blower plus two fixed bullets, on a switch with a 120 W nameplate budget.
| Device | Qty | Class | PSE nominal | Actual draw | Cable | Loss factor | Subtotal |
|---|---|---|---|---|---|---|---|
| PTZ camera with heater | 6 | Class 4 (at) | 30.0 W | 22.0 W | 100 m | 7% | 141.2 W |
| IR bullet camera | 2 | Class 3 (af) | 15.4 W | 6.0 W | 60 m | 4% | 12.5 W |
| Total | 8 | 153.7 W |
Required budget: 153.7 / 0.8 = 192 W. The 120 W supply is 72 W short, and here is what actually happens on site rather than on paper.
IEEE 802.3at and 802.3bt define a per-port power priority, negotiated over LLDP: critical, high, medium and low. When the budget is exhausted the PSE does not brown out the whole switch. It refuses new requests and de-powers the lowest-priority ports first. Because heaters and IR illuminators switch on at dusk, the aggregate draw crosses the limit every evening: the lowest-priority port loses power, its camera reboots, the reboot draws inrush current, the spike pushes the total back over the limit, and the next port drops. The symptom is cameras cycling overnight while the daytime picture is perfect, which is why "it only fails at night" is the classic signature of an under-sized budget rather than a faulty camera.
The fixes, in order of cost: set priority correctly so the lobby camera outranks the yard camera; cap per-port draw by class so no single device can grab more than its share; move half the load to a second switch; or buy the bigger supply.
Three things eat headroom, and none of them show up in a static calculation:
Cold-start inrush. Every PD draws a surge when it powers up. If the whole switch restarts after an outage, all ports request at once and the aggregate peak exceeds the steady-state figure.
Mode changes over the day. IR heaters at dusk, PTZ motors during a patrol, and AP radios at peak client count all raise draw temporarily, and they tend to coincide.
Ageing. Electrolytic capacitors in the supply lose capacity with temperature and time, which is why a design that passed at commissioning with 5% margin can fail in year four.
Twenty percent covers all three for typical commercial and light industrial duty. For a sealed enclosure at 60 °C ambient, use 70% instead.
The worksheet predicts; the management page confirms, and on a poe switch for ip cameras the confirmation has to happen after dark. Read the per-port power draw and the total consumption from the switch UI, wait until after dusk for cameras, and compare the total with your computed subtotal. A measured total more than 10% above the worksheet means a device is drawing more than its datasheet claims, which is common with third-party cameras and with heaters that run longer than expected.
What is the quickest way to size a PoE budget?
List every device with its real worst-case draw, multiply by quantity, add 4% to 20% for cable loss depending on power and run length, add the switch's own consumption, then divide by 0.8. The result is the minimum nameplate PoE budget to specify.
Why not just use the class rating for each port?
Because the class sets a ceiling, not a consumption figure. A Class 3 port offers up to 15.4 W but a fixed camera may draw 6 W. Budgeting on class gives an oversized supply; budgeting on idle draw gives one that fails at dusk.
How much power is lost in 100 m of Cat5e cable?
It depends on current, because loss rises with the square of it. Roughly 3% at 15 W, 7% at 30 W, 13% at 60 W and 20% at 90 W over a 100 m run. The 12% to 20% rule of thumb applies to the higher power classes.
What happens if the cameras need more power than the budget?
The PSE refuses new requests and de-powers the lowest-priority ports first, using the priority negotiated over LLDP. Devices reboot, draw inrush current, and push the total back over the limit, producing cameras that cycle overnight while looking fine during the day.
Does an 8 port poe switch run out of ports before it runs out of power?
Usually power comes first. Eight 60 W class devices would need 480 W of budget, which is far beyond what a compact unit supplies. Check the port count against your device list first, then check the budget, and remember that uplink ports typically do not source PoE.
Six columns, one division, and three worked cases: that is the whole calculator. Rayin publishes port configuration and PoE budget options for its 8+4-port gigabit industrial switch line, so you can run the worksheet above against a real nameplate figure and confirm the 80% margin before the order, not after the first winter.
How to Calculate the Power Budget on a PoE Switch, Step by Step
A Power Calculation Method That Avoids an Underpowered Design
Class and the LLDP Handshake: How a Port Decides What to Give
PoE Distance and Cable: Choosing the Run That Still Delivers Power
The specifications, thresholds and standard references quoted above come from public industry sources — the IEEE 802.3af, 802.3at and 802.3bt power classes, IEEE 802.3 cable and segment limits, and published vendor datasheets — rather than from marketing copy. The method, worked examples and wording are written by Sara, who checked each figure against those sources before publication. Anything presented as an industry-typical value is labelled as such; nothing here is a measured figure from a Rayin product unless it says so.
About the author: Sara — Sara is a Customer Manager at Rayin with over 10 years of experience in the communications field. In her free time, she enjoys badminton and swimming.
About Rayin: Shenzhen Rayin Technology Co., Ltd. — Company Profile
