PoE Shortfalls: Enough Ports Is Not Enough Power

What it looks like

A 24-port PoE switch populated with 24 APs leaves the last few rebooting or dead. Alternatively everything works by day and drops out together at peak when power draw rises.

Likely causes

Ordered from most to least common:

• Total PoE budget exceeded: enough ports, not enough watts
• Inrush current: start-up draw exceeds steady state, and simultaneous power-up trips the limiter
• Cable voltage drop: thin or long runs leave the device below its minimum operating voltage
• Mixed standards: a 24V device on a 48V port, or the reverse
• Draw above spec: power rises under full transmit load or in high ambient temperature

Diagnostic steps

Follow in order.

• Calculate demand: AP count × per-AP draw × 1.3, against the rated PoE output of the switch
• Check each device: is it standard 48V or non-standard 24V
• Measure at the AP end rather than the switch end and confirm it is within the device range
• Power up in batches: half first to check stability, then the rest — this isolates a total-budget problem from a single device
• Review per-port power readings and look for an outlier device

Fix and prevent

Fix: split the APs across more switches; power non-standard 24V devices from their own adapters or injectors, never mixed into standard PoE ports. Prevent: select on both port count and rated PoE output with a 1.3× margin, keep runs under 100m and use Cat5e or better.