Access Control Power Supply: Sizing by Door Count

Four doors on one job, and the client wants battery backup on every reader in case the power drops mid-shift. That's the moment a PSU stops being a line item and becomes a decision: how many channels, how many amps, and whether one box covers the lot or you're running two. Access control draws current differently to CCTV -- strikes and maglocks pull hard for short bursts, readers sit on a steady low draw, and battery backup adds its own charging load on top.

Calculating the Real Current Draw for Your Door Count

Nameplate max ratings on a strike or maglock tell you the worst case, not the steady state. In practice, a mag-lock typically draws in the 300-500mA range continuously once energised, while a fail-secure strike draws more but only for the brief moment it's released. Readers sit low, usually under 200mA each, but they're always on. The logic is the same amp-budgeting approach we use for how many amps you need, just with different load types on each channel.

Where access control gets trickier than CCTV is the inrush. If a fire panel triggers a mass door release, every strike on that PSU energises at once -- and that peak, not the steady-state total, is usually what trips an undersized supply. Battery backup adds a third load on top: the charging current the PSU pulls to keep the SLA battery topped up, which as a general rule runs a few hundred mA extra depending on battery size.

Single PSU or Dual-Channel: Sizing by Door Count

For one or two doors on the same board, a single-channel PSU with a dedicated battery charger circuit is usually enough, provided the readers and lock aren't sharing the channel with anything else. Once you're past two doors, or the doors need to fail independently (one fail-safe for fire egress, one fail-secure for a plant room), a dual-channel unit earns its keep -- it isolates the loads so a fault or a fire-trigger on one channel doesn't drag the other down.

Treat this table as a starting point, not a substitute for adding up the actual loads on the job -- a site with three maglocks and heavy readers can outgrow a 3A channel faster than the door count alone suggests.

Battery Backup and Fire Egress: What You're Actually Providing For

Battery backup on an access control PSU isn't just about keeping the doors working through a blackout -- on fail-safe egress doors, it's usually what keeps the door unlocked long enough for occupants to exit safely before backup runs out. As a general rule, sizing the battery for runtime (not just charge current) is the part that gets skipped when a PSU is chosen on amps alone.

Common Mistakes

• Sizing off nameplate max instead of steady-state draw. A strike's peak rating looks alarming on paper, but it's the continuous draw across all doors plus charging current that actually determines the PSU size.
• Forgetting the mass-release inrush. A fire panel triggering every door at once is the load that actually trips an undersized PSU, not day-to-day access swipes.
• Skipping battery runtime sizing. Plugging in whatever SLA battery fits the enclosure, without checking it covers the required egress runtime, defeats the point of having backup at all.
• Running fail-safe and fail-secure doors off the same channel. A fault on one door shouldn't be able to drag down a door with a different fail mode -- that's what dual-channel isolation is for.
• Treating the fire egress requirement as a PSU spec question. Runtime and fail mode are set by the fire engineer, not read off a datasheet after the fact.

Frequently Asked Questions

Sizing & Selection

How much current does a mag-lock actually draw compared to a strike?

A mag-lock typically draws a steady 300-500mA while energised, while a fail-secure strike draws more but only briefly on release -- so a mag-lock is usually the bigger continuous load, while a strike is the bigger inrush spike.

Can one PSU run four door strikes and readers?

It depends on the total continuous draw and whether the doors need independent fail modes. A higher-current dual-channel unit like the Ness POW246 24V 6A dual-channel PSU can cover four doors on many sites, but confirm the actual load first.

What's the difference between a dedicated access control PSU and a generic 24V supply?

A dedicated unit like the Hikvision DS-K7P02 access control power supply integrates the battery charger, AC-fail monitoring, and isolated outputs designed for lock loads -- a generic 24V supply usually leaves you sourcing those functions separately.

Installation & Compliance

Do I need a licence to wire battery-backed access control into a fire egress door release circuit in NSW?

Yes -- wiring into a fire egress or door release circuit is electrical work and needs to be carried out by a licensed electrician, and any interface with the fire system typically also needs sign-off from the fire contractor or certifier.

How long should battery backup run an access control PSU during an outage?

Required runtime is set by the building's fire egress requirements, not a fixed industry number -- confirm the figure with the fire engineer or certifier before sizing the battery.

What happens to fail-safe locks if the PSU battery backup fails silently?

On a fail-safe door, a dead battery combined with a mains outage means the lock releases -- which is the intended safety behaviour, but it also means a failing battery you didn't know about leaves the door unsecured well before an actual outage. A PSU with an AC-fail and low-battery output, like the Ness POW240, lets you monitor that condition instead of finding out the hard way.

Shop Access Control Power Supply at Schnap

Trade pricing across dedicated and generic PSU options, dispatched same day from Kingsgrove NSW.

• Dedicated Access Control PSU
• Hikvision DS-K7P02 Access Control Power Supply -- integrated battery charger and AC-fail monitoring

• Single-Channel PSUs
• Ness POW240 13.5V DC 2.5A PSU -- AC-fail and low-battery monitoring outputs
• Ness 106-029 3A DC PSU PCB Adaptor -- 13.8V output with LED status indication

• Dual-Channel PSUs
• Ness POW244 24V DC 3A Dual-Channel PSU -- isolated outputs for independent fail modes
• Ness POW246 24V DC 6A Dual-Channel PSU -- higher-current option for larger door counts

See the full access control power supply range at Schnap and get same-day dispatch from Kingsgrove NSW.