Access Control Power Supply Sizing: Lock Loads, Readers and Battery Backup

Access control power supply sizing starts with a load schedule, not the number of doors on the controller. Locks, readers and control equipment can draw current at different times, and backup performance depends on which loads remain energised during an outage. This guide explains a practical DC sizing method for NZ access control projects.

For New Zealand projects, confirm the final power design against the approved project documentation, manufacturer instructions, and the requirements of the contractors responsible for electrical, fire and egress interfaces. Site conditions and consent requirements vary, so use this guide as a sizing method rather than a substitute for the project's approved design and commissioning requirements.

Separate standing load from operating load

A continuously energised maglock contributes to the standing load, while an electric strike or motorised lock may draw its highest current only when operating. Readers and controller electronics usually remain active. Use the selected product's documented figures and duty cycle. Do not substitute a generic current for all locks or assume that a fail-secure lock always draws no current when the door is secured.

Work through a stated example

Suppose four locks each draw 0.45 A continuously, four readers each draw 0.12 A and a controller draws 0.35 A. The illustrative standing load is 2.63 A. If another accessory draws 0.20 A, it becomes 2.83 A. These figures are invented design inputs for the calculation, not specifications of the products linked below. Replace them with verified values before selecting equipment.

Allow for concurrent operation and charging

If a motorised device needs an additional 1.2 A during operation, check whether it can start while the other loads remain active. The example would reach 4.03 A before any further charging allowance. Apply the power supply manufacturer's rules on output capacity and battery charging, because some ratings combine these differently. Spare capacity must cover an identified design requirement; a fixed percentage alone cannot prove suitability.

Check voltage drop at the lock

For a two-conductor DC circuit, the outgoing and return paths both contribute resistance. As an illustrative case, a total loop resistance of 0.8 ohm carrying 0.5 A drops 0.4 V. Measure the loaded voltage at the device and compare it with the allowed operating range. A shared return or poorly made joint can change the outcome. Do not raise the cabinet voltage without checking all connected devices.

Make battery assumptions visible

A 2.83 A continuous load for four hours requires 11.32 Ah before derating and reserve. If the documented usable fraction for the design were 0.70, the mathematical minimum would be about 16.2 Ah. This is an illustration, not a battery recommendation. Temperature, ageing, discharge rate, low-voltage cutoff and charger capacity affect the selected battery. Verify the actual supply's supported battery range and recharge performance.

Choose monitored distribution

Where supported and required, separate protected outputs help a fault on one circuit avoid taking down unrelated doors. Specify monitoring for mains and battery faults and show where those events are received. The PSU-135-6 and PSU-135-10 listings offer cabinet-mounted supply options to investigate, while a switchmode supply is not automatically a complete battery-backed access supply. Confirm voltage compatibility, charging and fault outputs before ordering.

Apply the design to the actual project

Create separate columns for normal operation, maximum simultaneous operation and battery-only operation. A device's highest current may occur in only one of those states, but the supply still needs to support the state the design permits. Put the calculation beside the door identifiers and preserve the source of each load figure. If the lock changes during procurement, recalculate before accepting the substitution, even when its nominal voltage is unchanged.

Review the disconnect and protection arrangement with the installer responsible for the supply. A branch fault should produce the documented effect, and the maintainer should be able to identify the affected circuit without guessing. Check the relationship between lock release, fire interface and battery power in the approved design. Do not assume that removing mains drops a battery-backed lock output. Demonstrate the actual release condition safely and record which element removes or changes power, with the relevant building professionals confirming the intended result.

Document the complete load and cable schedule

List each connected device with its supply voltage, continuous current, maximum current, operating temperature and required protection. Put device identifiers beside the figures so that the schedule can be checked against the installation. Keep mains equipment, low-voltage supplies and communication equipment distinct. Count accessory loads such as heaters, fans and communications routers that are easy to overlook. Confirm which loads remain powered in the intended outage condition, rather than assuming the whole cabinet behaves as one device.

Check the complete current path. Cable length, conductor size, connections and protective devices influence voltage at the equipment. Measure at the furthest connected device with the representative worst-case load running. An unloaded voltage reading is insufficient. If the design changes during installation, update the calculation and equipment schedule before handover. Do not compensate for cable loss by increasing the source voltage beyond another connected device's allowed operating range.

Commissioning acceptance checklist

Agree the expected result before testing. Use the installed configuration and retain evidence of each outcome, including a failure that must be corrected. The following checks supplement the exact manufacturer procedures and the approved project design.

Repeat affected checks after a material equipment, software or site change. Keep the test record with the as-built schedule so the next maintainer can understand which configuration passed and which assumptions still need review.

Equipment candidates and specification checks

These are relevant active catalogue candidates for a project review, rather than a universal recommendation or a promise of immediate stock. Use the full product page and current manufacturer information to confirm the supplied variant and its role in your design.

• PSU-135-6 - PSU 13.5VDC 6A rated with fused outputs, with charger, in a cabinet. Confirm the exact supplied configuration, relevant interfaces and installation requirements before including it in the equipment schedule.
• PSU-135-10 - PSU 13.5VDC 10A rated with fused outputs, with charger, in a cabinet. Confirm the exact supplied configuration, relevant interfaces and installation requirements before including it in the equipment schedule.
• ITPSA12-5AMP - Regulated - Switchmode Power Supply. Confirm the exact supplied configuration, relevant interfaces and installation requirements before including it in the equipment schedule.

Review the relevant Essential Security Supplies equipment range alongside the intended workflow. For further context, read our related installer guide.

Frequently asked questions

Can I size a supply by door count?

Only after calculating the actual loads. Different locks and reader arrangements can give very different current demands.

Does a larger battery always increase useful backup?

It may increase capacity, but the charger, enclosure, discharge characteristics and supported battery range must also suit the design.

Is a nominal 12 V supply suitable for every access device?

No. Check the operating voltage range of each connected device, including its limits while the battery is charging.

Primary references and technical scope

Use the primary manufacturer or technical reference for the claims it covers. General design guidance does not replace the selected product's installation manual. Confirm current firmware, model variants and the approved project requirements before ordering or commissioning.

Get specification help from Essential Security Supplies

Send your site layout, equipment schedule, intended workflow and any existing model details to Essential Security Supplies for specification help and a project quotation. Identify the functions that must be demonstrated and the interfaces owned by other contractors. Our enquiry route supports a documented equipment review without publishing prices or assuming unverified compatibility.