When to use: Size the access-control power supply by summing the DC current of every connected device — readers (OSDP/Wiegand), request-to-exit (REX) sensors, door contacts, electric strikes or mag-locks, and the controller board. Add a margin for inrush and simultaneous unlock, then pick the next standard PSU amperage above the total.
This tool calculates the total DC current draw of an access control system by summing the load of all connected devices — readers, door contacts, REX sensors, electric locks, and controller boards — so engineers can correctly size the power supply unit and battery backup.
Each device on a 12 VDC or 24 VDC access control bus draws a specific continuous current. OSDP readers typically draw 80–150 mA; Wiegand readers 60–100 mA; REX / PIR sensors 20–50 mA; door contacts 5–10 mA; electric strikes 150–350 mA continuous (higher inrush); maglocks 400–600 mA; and the controller board itself 200–450 mA.
The total current is summed, then an inrush margin (typically 20–30%) is added to account for simultaneous lock energization and startup current spikes. The result is compared against standard PSU amperage ratings (2.5, 4, 6, 8, 10, 12.5 A) to select the appropriate supply.
For systems with battery backup, the same current draw at load determines the battery Ah requirement: Ah = amps × hours of standby. UL 294 Level II requires 4 hours standby minimum.
UL 294 governs access control system power supply requirements including standby power levels. NFPA 101 requires that maglocks release on loss of power (fail-safe), which means the locking current must be maintained from the supply continuously. SIA OSDP / IEC 60839-11-5 specifies the electrical interface for OSDP reader connections. Local electrical codes (NFPA 70 / NEC Article 725) classify access control wiring as Class 2 limited energy, imposing current and voltage limits on field wiring.
Always verify individual device current draws from manufacturer datasheets — the values in this tool are typical industry estimates and real products vary. Apply a 25% inrush margin as a minimum; use 30–35% for systems where multiple doors may unlock simultaneously (e.g. fire alarm mass-release of all maglocks).
For 24 VDC systems, the current draw of each device is roughly half that of an equivalent 12 VDC device, reducing wire voltage drop on long cable runs. Long runs to mag-locks in particular benefit from 24 VDC to minimize resistive loss. Account for voltage drop on runs longer than 100 feet using the correct AWG for the current.
Select the system voltage (12 VDC or 24 VDC) and enter the desired inrush margin percentage. Then add each device type and quantity for the system — readers, REX sensors, door contacts, strikes, maglocks, and the controller board. The tool sums the raw current, applies the margin, calculates total power, and recommends the next standard PSU amperage. Use the result to specify the power supply and size the battery backup using the UPS Battery Backup Sizing tool.
A standard single-door maglock draws 400–600 mA at 12 VDC or 200–300 mA at 24 VDC continuous holding current. Double-door maglocks draw approximately double. Always verify against the specific manufacturer datasheet, as ratings vary from 250 mA to 750 mA depending on the holding force rating.
A minimum 25% margin is standard practice. For systems where fire alarm releases all maglocks simultaneously, use 35–40% to account for the simultaneous inrush of all lock coils. Inrush current on a maglock can be 2–3× the steady-state holding current for the first few milliseconds.
Yes, as long as the total current draw with margin stays within the PSU rating. Distribute the load so that a single PSU failure does not leave a large portion of the facility uncontrolled. Critical doors (server rooms, cash offices) should be on dedicated or redundant supplies.
24 VDC systems draw half the current for the same power, reducing resistive losses on long cable runs and allowing use of smaller gauge wire. However, some legacy readers and locks are 12 VDC only. Many modern systems support both; confirm device compatibility before selecting the system voltage.
The battery backup must supply all device loads during a power outage. Size the battery using: Ah = total amps × standby hours / usable depth of discharge (80% for SLA batteries). UL 294 Level II requires 4-hour standby, so a 2 A system load needs at least a 10 Ah battery (2 × 4 / 0.8).
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