For access control on egress doors, fire alarm systems, and most security-monitored installations — yes, code requires it. Battery backup runtime is typically 4 hours minimum for access control (24 hours for fire alarm). For surveillance cameras alone, battery backup is optional but recommended; a 30-minute UPS bridge handles most utility-power glitches and prevents reboot cycles that affect recorded video continuity.

If using a PoE switch, sum the per-camera PoE class budget: 8 cameras at 802.3at (PoE+, 30W max) need a switch with at least 240W total PoE budget. Add 20-30% headroom. Many 8-port PoE+ switches advertise 130W total budget — undersized for full-load 802.3at use. For 802.3bt (PoE++, 60-90W) devices like multi-imagers or large PTZs, plan even more aggressively.

Multi-output supplies (Altronix AL600ULPD8, LifeSafety FPO75) provide both 12VDC and 24VAC outputs from a single chassis, each individually fused. Mixing the two on a single output is not safe — voltage and waveform are incompatible. For larger sites, separate dedicated supplies per voltage simplify troubleshooting and isolate failures.

Voltage drop depends on conductor gauge, current draw, and cable length. For 12VDC at 2A over 100 feet, 18 AWG drops roughly 1V (5%) — borderline acceptable. 16 AWG drops half that. For runs over 100 feet at sustained current, use a 24V supply at the source and step down at the device, or move to PoE which tolerates longer runs at typical commercial distances.

A traditional power supply delivers DC (or AC) to a device through a dedicated power cable. A PoE injector adds DC power to the unused pairs of an Ethernet cable, so the device receives both power and data on the same Cat5e/Cat6. PoE simplifies installation and is the dominant choice for IP cameras and modern access readers. Traditional supplies are still preferred for high-current devices like maglocks and analog camera installations.