22-AP Retail PoE Budget, 105 m CCA Runs, and an Ungrounded Roof AP
A 22-AP retail refresh uses one 48-port PoE+ switch rated at a 370 W total PoE budget. The bill of materials lists 18 indoor 802.3at APs (Class 4, 30 W maximum, measured draw 12.5 W) and 4 outdoor 802.3at APs with heaters (measured 22 W in cold weather). Six cable runs to stockroom APs measure 105 m; the contractor used copper-clad aluminum (CCA) Cat6 because it was cheaper and "the link lights came on." The roof-mounted outdoor AP is fed with shielded Cat6, but the drain wire is cut at both ends, no surge suppressor is installed, and the AP chassis is bonded to a nearby HVAC conduit. The store opens in three days.
1. Does the switch power all 22 APs? Show the calculation, state the risk, and give options.
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Worst case, Class 4 reserves 30 W per port at the PSE: 22 x 30 W = 660 W, which is 290 W above the 370 W budget. Realistic steady-state draw is 18 x 12.5 = 225 W plus 4 x 22 = 88 W, or 313 W (about 85% of budget). That is under the limit but leaves only ~57 W of headroom, and PoE reservation behavior matters: many switches reserve the full class until LLDP or CDP power negotiation completes, so simultaneous boot or a cold morning with heaters can push the switch into port shedding.
Options: split the APs across two switches, use a switch with a larger PoE budget (740 W or more), enable LLDP power negotiation so actual draw replaces class reservation, and put outdoor heaters on a separate power source or a schedule. Also note the 105 m runs: horizontal cabling is limited to 90 m plus 10 m of patch cords. CCA has higher resistance than solid copper, so voltage drop over 105 m may cause PoE negotiation failures or reboot loops. Re-pull those six runs in solid copper or use fiber with local power.
2. Identify the grounding and surge-protection errors on the outdoor AP and describe correct practice.
Read the worked answer
Three errors are visible. First, the shielded cable's drain wire is cut at both ends, so the shield is floating and acts as an antenna rather than a ground path. The drain should be bonded to the building's grounding electrode system at the cable entry point, typically through a surge suppressor, and not carried through the building. Second, no Ethernet surge suppressor is installed at the building entrance, leaving the AP, switch port, and cabling exposed to induced surges. Third, the chassis is bonded to HVAC conduit, which is not a valid equipment grounding conductor; it may be isolated by fittings and can carry fault current.
Correct practice: mount an outdoor-rated surge protector at the entry point, bond it and the AP chassis to the building ground with the specified conductor, use weatherproof connectors with a drip loop, keep the AP and cabling away from AC equipment, and verify continuity to ground after installation. This is a safety and liability issue, not a performance tweak, and should be fixed before the store opens.