When to use: Use this calculator when sizing a PoE switch or midspan for cameras, access control, intercoms, APs and IP speakers. It sums multiple device groups, applies the correct IEEE 802.3af / at / bt class, adds DC cable-run loss, and checks the total against the switch's PoE power budget and port count. Remember the difference between PSE power (what the switch reserves/delivers) and PD power (what the device receives after the 100 m channel). Plan for ~20% headroom.
| Device | Qty | Class / Std | PD W ea | Cable Loss | PSE W ea | Group PSE W | Ports |
|---|---|---|---|---|---|---|---|
| Fixed IP camera | 4 | 802.3af · Class 3 | 12.9 | 0.44 | 13.4 | 53.6 | 4 |
| TOTAL | 4 | 51.8 | 1.76 | 53.6 | 4 |
Within budget with 86% headroom and 4/24 ports used. Healthy margin for the PoE switch.
This calculator sizes a PoE switch or midspan injector for IP camera, access control, intercom, and wireless AP deployments by computing per-device PD power, cable run loss, and total PSE-side draw against the switch power budget. It is used when designing or auditing PoE-powered security system infrastructure.
PoE power is defined at two points: the PSE (Power Sourcing Equipment — the switch port) and the PD (Powered Device — the camera or reader). The difference is cable loss. For IEEE 802.3af: PSE = 15.4 W, PD = 12.95 W. For 802.3at: PSE = 30 W, PD = 25.5 W. For 802.3bt Type 3: PSE = 60 W, PD = 51 W.
Cable loss is calculated as I²R where I = PD_watts / 48 V and R_loop = 2 × run_length × resistance_per_foot. For Cat5e 24 AWG, R ≈ 0.0257 Ω/ft per conductor; for Cat6 23 AWG, R ≈ 0.0202 Ω/ft. The PSE-side draw per device is PD load + cable loss, and this is the value that must be summed against the switch power budget.
The switch budget is the total PoE watts the switch PSU can deliver simultaneously across all ports. Exceeding the budget causes the switch to shut down lower-priority ports. A minimum 20% headroom above the total estimated PSE draw is recommended for growth and worst-case simultaneous demand.
IEEE 802.3af (2003) defines PoE Type 1 at 15.4 W PSE. IEEE 802.3at (2009) defines PoE+ Type 2 at 30 W PSE. IEEE 802.3bt (2018) defines PoE++ Type 3 (60 W) and Type 4 (90 W PSE) using four-pair delivery. NFPA 731 (Standard for the Installation of Electronic Premises Security Systems) addresses power supply reliability requirements for security equipment including the need for battery backup on the PoE switch feeding critical security devices. NEC Article 725 classifies PoE-powered security device wiring as Class 2 or Class 3 limited energy circuits.
Always use the PSE wattage (not PD wattage) when summing against the switch budget, because the switch reserves and delivers based on class allocation. Devices can draw less than the class maximum, but the switch still budgets the full class reservation unless LLDP power negotiation reduces it.
For outdoor cameras with heaters or IR illuminators, the actual PD draw can approach or exceed the PoE+ class limit, requiring 802.3bt ports. PTZ cameras with high-speed drive motors should be classed as PoE+ minimum. Provide at least 20% headroom on the switch budget for cold-weather heater loads and simultaneous startup surge.
Enter the switch's rated PoE power budget and port count, then add device groups: select the device type, quantity, PoE class, cable run length, and cable gauge. Optionally enter the actual measured PD watts from the device datasheet to override the class default. The tool shows per-device PD power, cable loss, and PSE draw, summed to a total, and compares the total against the switch budget with a pass/fail/caution result. Use the recommendation text to specify the switch or add midspan injectors.
PSE (Power Sourcing Equipment) watts is the power the switch delivers at the port. PD (Powered Device) watts is what the device receives after resistive loss in the cable. For 802.3af, PSE = 15.4 W but PD = 12.95 W — the 2.45 W difference is dissipated as heat in the cable. Budget calculations must use PSE watts.
At 150 ft, 24 AWG Cat5e, a PoE+ device drawing 25 W: I = 25/48 = 0.52 A; R_loop = 2 × 150 × 0.0257 = 7.71 Ω; Loss = 0.52² × 7.71 = 2.08 W. The switch must source 25 + 2.08 = 27.08 W per port for this device.
Yes. A midspan injector sits between a non-PoE switch port and the device, injecting power onto the spare pairs (802.3af/at) or all four pairs (802.3bt). Each injector port has its own PSU — the total injector budget must exceed the sum of all connected device PSE draws with margin.
The switch will power-cycle or shut down lower-priority ports to stay within budget. The priority order depends on switch firmware — typically last-connected or explicitly configured lower-priority ports lose power first. Always verify the switch's port prioritization settings for security-critical devices.
Add the heater wattage to the camera's summer draw to get the worst-case PD load. A camera drawing 7 W plus a 15 W heater totals 22 W — requiring 802.3at class allocation (30 W PSE). Size the switch budget for worst-case winter conditions, not summer averages.
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