PoE Budget and Camera Expansion on a 48-Port Switch
A distribution warehouse is adding 14 fixed IP cameras and 3 PTZ cameras to an existing 48-port managed PoE switch. Current load: 18 fixed cameras at 12.9 W each, 4 access-control panels at 15.4 W each, and 2 PTZ cameras at 25.5 W each. The switch datasheet lists a 370 W total PoE budget, 802.3af/at per port, and no PoE++ support. New fixed cameras draw 12.9 W; new PTZs draw 25.5 W and require 802.3at. The integrator proposes replacing 8 existing fixed cameras with new models to 'free up ports' and using passive PoE injectors for two PTZs because 'they only need 24 V.' The IDF has no spare electrical circuits. The security manager asks whether the proposal is technically sound and what must be verified before ordering.
1. Calculate current and proposed PoE load, assess the proposal, and state whether port count or power is the binding constraint.
Read the worked answer
Current: 18 x 12.9 = 232.2 W; 4 x 15.4 = 61.6 W; 2 x 25.5 = 51.0 W; total 344.8 W, leaving 25.2 W. Proposed: add 14 x 12.9 = 180.6 W and 3 x 25.5 = 76.5 W; total 601.9 W. That is 231.9 W over the 370 W budget. Ports used: 18 + 4 + 2 = 24; adding 17 gives 41, still under 48, so power is binding. Replacing 8 fixed cameras with same-draw models frees no PoE watts and is irrelevant to the budget; it only changes port count, which is not the constraint. Passive PoE injectors are not a compliant 802.3at solution for the PTZs and still require AC power at the IDF; with no spare circuits this is not viable. The proposal is technically unsound. Verify per-port class, cable, switch power supply, UPS, and thermal load before ordering.
2. Recommend two viable remediation paths and the decision criteria, given no new circuits and a limited budget.
Read the worked answer
Path A: replace the switch with a higher-budget PoE switch or add a PoE midspan, but both add AC load. Calculate AC input: 601.9 W PoE plus switch overhead of roughly 10-15% is about 680-700 W; confirm the existing circuit, breaker, and UPS can carry it. If not, the 'no new circuits' constraint blocks this path. Path B: reduce PoE demand: use lower-power fixed cameras (for example 7 W models) and limit PTZ count or use local power where a circuit exists; recalculate. For example, 32 fixed at 7 W = 224 W plus 5 PTZs at 25.5 W = 127.5 W plus panels 61.6 W = 413.1 W, still over budget; further reduction or a higher budget is needed. Decision criteria: life-safety/UPS priority, per-port 802.3at for PTZ, cable category and length, physical space, heat, and total cost. Do not use passive injectors. Confirm with a load calculation and vendor power tables.
3. List acceptance tests and records for the final design.
Read the worked answer
Before energizing, record the as-built port map, PoE class, cable test results, and switch/UPS capacity. After energizing, use LLDP or the switch PoE table to confirm negotiated class and actual draw per port; verify PTZs receive 802.3at and boot under load; measure total draw against budget with margin; test UPS runtime with the full load; check IDF temperature and airflow; test camera streams and recording; simulate a switch reboot and a single power-supply failure; confirm alerts for PoE overload or port denial. Document firmware, configuration backup, test dates, and any derating. Acceptance should require at least 10-15% headroom and no port over its class limit.