Key takeaways
- Size on worst-case current: every fail-safe lock energized, readers at peak, plus any fail-secure door held unlocked by a schedule.
- Headroom is a planning allowance, not a surge calculation. Check simultaneous peak current and duration, temperature limits and shared battery charging current.
- Size batteries from the actual standby load and required duration using the supply worksheet and battery discharge tables. A 1.25 multiplier is only a preliminary assumption.
- For the same lock power, 24 V halves the current and cuts the percentage voltage drop to a quarter, which matters on long cable runs.
- Where alarm release is required, use the approved interface to interrupt battery-backed power-to-lock outputs as well as mains-fed power.
Power supply & battery calculator (12 V DC)
Supply target = the larger of operating load × margin and simultaneous peak load, plus shared charging current. Candidate sizes are for comparison. Confirm the supply’s continuous / surge duration limits, battery capacity range, charger and enclosure. This standby estimate does not include a separate alarm period or certify a required runtime; use the manufacturer’s worksheet and battery discharge data. Connect multiple batteries only as the supply manual permits.
Size an access control supply from operating load, simultaneous peak load and any battery charging current sharing its rating. Check the chosen voltage, temperature derating and peak duration limits. Size the battery separately from the loads that remain powered during the specified outage. The examples below are preliminary calculations, followed by the checks needed to select a compatible supply and battery.
Step 1: List every device’s current draw
Work from datasheets, not the label on the box, and record operating current, peak current and the expected standby load / duty cycle. Note when each device draws: some loads are constant, others only while a door is unlocked. The ranges below are illustrative planning values for 12 V DC hardware, not guaranteed ratings. Use the exact datasheet, including any maximum current at a different voltage.
| Device | Typical current at 12 V DC | When it draws | Notes |
|---|---|---|---|
| Maglock, single door | 250–500 mA example range | Continuously while locked | Use the actual 12 / 24 V ratings; not always an exact half |
| Double maglock | 600–1,000 mA | Continuously while locked | Two coils in one housing |
| Electric drop bolt | 200–400 mA holding | Surge each time the bolt moves, then holding current | Surge is several times the holding figure |
| Electric strike, fail-secure | 150–500 mA | Only while unlocked | Continuous if fail-safe or held open by a schedule |
| 125 kHz / 13.56 MHz reader or keypad | 50–150 mA | Continuously; peaks during reads, beeps and LED changes | Use the peak figure |
| Long-range UHF reader | Several hundred mA or more | Continuously; peaks while transmitting | Often better on its own supply |
| Door controller board | 100–300 mA | Continuously | Board only; readers powered from it add their own draw |
| Face recognition terminal | 1 A or more | Continuously; peaks with display and IR lighting | Many ship with a dedicated adapter |
| REX PIR sensor or illuminated exit button | 10–50 mA | Continuously | Small, but adds up across doors |
Three rules keep the list honest:
- Power-to-lock hardware draws while locked, so include it in standby if it remains locked during the specified outage. Magnetic locks are always in this group.
- Fail-secure locks draw only when unlocked, until a schedule holds doors unlocked through business hours. Then they are continuous loads too.
- Count readers once. If they take power from the controller’s reader terminals, include them on whichever supply feeds the controller. When choosing networked door controllers, check the maximum current each reader output can pass.
If you are still choosing the lock, our comparison of the types of electric locks sets out each type’s draw pattern.
Step 2: Check operating headroom, actual peaks and charging
A 25% operating-load allowance is a useful initial comparison; 50% may allow expansion. Neither percentage guarantees that a bolt’s start-up surge is covered. Calculate the highest credible simultaneous peak from the hardware ratings, then check its duration against the supply’s continuous and surge limits. Apply any ambient-temperature derating.
If load and charging share a rating, include the maximum charging current available under that condition. Where the manufacturer gives a separate load-only rating, do not count charging twice. For a conservative preliminary supply target, use:
Target A = max(operating A × chosen margin, simultaneous peak A) + shared charging A
This treats the entered peak as a load the candidate must support; a lower-rated unit is only an option if its documented surge performance covers that specific current and duration.
Worked example for two maglock doors:
| Item | Qty | Current each | Subtotal |
|---|---|---|---|
| Maglock, 12 V DC | 2 | 500 mA | 1,000 mA |
| Card reader | 2 | 120 mA | 240 mA |
| Controller board | 1 | 250 mA | 250 mA |
| Total load | 1,490 mA (1.49 A) | ||
| With 25% headroom | 1.86 A | ||
| With 50% headroom | 2.24 A |
The example assumes no higher simultaneous peak and no shared charging current. A 2 A load-rated supply covers 1.86 A; adding a 0.5 A shared charger raises the target to 2.36 A, suggesting a 3 A comparison candidate. Verify its actual charger, output ratings and temperature limits.
Battery backup: calculate the actual standby load
Add all loads powered by the battery: locks in their outage state, readers, controllers, sensors and the supply’s own consumption. A fail-secure door can draw during scheduled or repeated unlocking; do not assume its standby current is zero unless that operating plan is valid. Use any separate alarm period and allowance required by the selected supply’s worksheet.
For preliminary arithmetic with a constant load:
- Planning capacity (Ah) = standby load (A) × standby hours × allowance factor
- Planning runtime (h) ≈ rated Ah ÷ (standby A × allowance factor)
An assumed factor of 1.25 is equivalent to using 80% of the marked capacity, but is not a universal aging / cold-temperature rule. For example, the Altronix ACM power-supply worksheet includes internal current, standby and alarm periods, then applies a 1.8 factor. Use the worksheet for the exact model, together with the battery’s discharge data and the supply cutoff voltage.
If the complete battery load is assumed to be 1.49 A, with no separate alarm period:
- With factor 1.25, 7 Ah gives 7 ÷ (1.49 × 1.25) ≈ 3.8 hours as a planning estimate.
- A four-hour target gives 1.49 × 4 × 1.25 = 7.45 Ah. A 9 Ah unit is a comparison candidate, not a confirmed four-hour runtime.
- If the actual standby load or required factor is higher, recalculate rather than rounding this example up by habit.
Yuasa’s NP7-12 is rated 7 Ah at a 20-hour discharge and 6.4 Ah at a 10-hour discharge, with specified temperature and cutoff conditions. Faster discharge, aging, temperature and a different supply cutoff can reduce the useful capacity further.
Battery selection also requires these checks:
- Use the chemistry, capacity range, series / parallel arrangement and fuse protection permitted by the supply manual. Do not build a parallel bank simply because a single battery is too small.
- Check recharge performance, enclosure space and wire / terminal ratings for the selected capacity. A large Ah battery does not prove that the installed charger can recharge it in the required time.
- Follow the supply and battery maintenance / replacement instructions and test the installed standby system periodically.
- Use the manufacturer’s output / charging voltage. A nominal 12 V battery-backed supply may operate above 12 V; all connected devices must tolerate its full specified range. Do not raise its setting solely to compensate for cable drop.
- The accepted egress arrangement determines permitted behavior on power failure. Required releases must still interrupt lock power with the battery connected.
12 V vs 24 V and voltage drop over long cable runs
For an illustrative constant 6 W load, current is 500 mA at 12 V or 250 mA at 24 V. Real dual-voltage locks can have different power at each voltage; use their stated currents. Cable loss depends on current, so moving to 24 V halves the drop in volts and cuts it to a quarter as a percentage of supply voltage.
Voltage drop = current × loop resistance, where loop resistance = 2 × one-way cable length × conductor resistance per meter. Current flows out on the positive conductor and back on the negative, so both count.
| Conductor | Resistance per conductor (Ω/km, 20 °C) | Drop over a 30 m (98 ft) run at 0.5 A (12 V lock) | Drop over a 30 m (98 ft) run at 0.25 A (24 V lock) |
|---|---|---|---|
| 22 AWG (0.33 mm²) | 53.0 | 1.59 V (13%) | 0.79 V (3.3%) |
| 0.5 mm² | 36.0 | 1.08 V (9.0%) | 0.54 V (2.3%) |
| 0.75 mm² | 24.5 | 0.73 V (6.1%) | 0.37 V (1.5%) |
| 18 AWG (0.82 mm²) | 20.9 | 0.63 V (5.2%) | 0.31 V (1.3%) |
| 1.0 mm² | 18.1 | 0.54 V (4.5%) | 0.27 V (1.1%) |
| 16 AWG (1.31 mm²) | 13.2 | 0.40 V (3.3%) | 0.20 V (0.8%) |
| 1.5 mm² | 12.1 | 0.36 V (3.0%) | 0.18 V (0.8%) |
| 14 AWG (2.08 mm²) | 8.3 | 0.25 V (2.1%) | 0.12 V (0.5%) |
Resistances are illustrative copper cable values at 20 °C, not a specification for every cable of that size. Use the selected cable’s declared resistance and installation temperature. Percentages are of nominal 12 V or 24 V.
Keep the voltage at the lock within its rated tolerance (±10% is common) and aim for a drop under about 5% where you can. Low voltage can reduce a maglock’s holding performance, so an under-powered lock can look fine on the bench and still pull open on site. When the run is long, you have four options: switch to 24 V locks (many maglocks are dual-voltage), use a heavier cable or double up spare conductors, move a supply closer to the door, or choose an approved local / distributed supply arrangement. Do not exceed a device’s allowed voltage to recover cable losses. Readers and controllers are often 12 V only, so 24 V locks usually mean a second supply or a dual-output unit.
Drop bolt start-up surge
Electric drop bolts draw a burst of current each time the bolt moves, commonly several times the holding current for a fraction of a second. Even one bolt can exceed a small supply’s rating. Check simultaneous operation especially when mains power returns after a cut, when a fire alarm resets, or when a schedule relocks every door at the same moment.
Symptoms of an undersized supply include a controller that reboots, bolts that stall halfway or chatter, and a switch-mode supply whose overload protection trips and restarts repeatedly. The fixes:
- Size the supply for the sum of simultaneous surges, not only the holding current. A 50% allowance does not replace this calculation; check the supply’s rated peak duration and overload behavior.
- Keep lock power separate from controller and reader power, on its own supply or fused output. The reader and controller still share a common ground for data.
- Stagger relock times where the controller allows it.
- Keep bolt cable runs short and heavy enough, because voltage drop eats into the surge margin at the lock.
- Fit suppression as the lock’s datasheet specifies.
Fire-alarm input and time-delay relays
Some access control supplies / distribution modules provide an approved fire-alarm interface for selected outputs. Its trigger can be NC, NO or a specified voltage signal, and its latching / reset behavior is configurable on some models. Follow the exact supply and fire-alarm design. Where a fail-safe output must drop on alarm, the release must interrupt battery-fed power too; test this rather than assuming any FA terminal does it automatically.
Wiring points to check:
- Put locks on the FA-switched output and the controller and readers on the unswitched auxiliary output, so the system keeps logging events while doors are released.
- Confirm whether the FA input expects a normally closed contact, a normally open contact or a voltage signal, and follow the fire alarm designer’s requirements for how and when doors relock.
- Interrupting power releases power-to-lock hardware only; an alarm interface can have other approved functions, so follow the door schedule. Fail-secure strikes and bolts stay locked without power, so those doors rely on mechanical egress through the lockset. Our fail-safe vs fail-secure guide covers the door-by-door decision.
A controller unlock timer or an arbitrary time-delay relay is not automatically an independent egress release. For a sensor-release arrangement, use a device / circuit accepted for direct timed interruption; for door-hardware release, the lever or exit device directly interrupts power under that arrangement’s requirements. Our magnetic lock wiring diagrams distinguish the REX signal from the required release path.
Using the sizing calculator
The 12 V calculator separates lock operating and simultaneous peak current from battery standby load. Enter maximum reader / controller loads and any charger current sharing the rating. For standby, enter the complete per-door outage load and other loads, including supply self-consumption. Choose the duration and allowance from the manufacturer’s worksheet; the default 1.25 is only an example.
It uses max(operating load × margin, peak load) + shared charging current for a conservative supply comparison. It returns a separate standby Ah estimate and comparison battery size. Larger battery banks need manufacturer sizing rather than an automatic parallel-bank suggestion.
The calculator assumes all locks peak together. It does not model surge duration, voltage drop, temperature derating, recharge time or a separate alarm load / duration. Verify these and the supported battery capacity before ordering; a candidate result does not establish a required runtime or an accepted egress installation.
Power supply selection checklist
- Output voltage matches the locks (12 or 24 V DC), and every device accepts the full specified output range of the chosen battery-backed supply
- Continuous rating ≥ total load plus headroom; surge rating covers simultaneous bolt throws
- Charger rating, recharge performance, battery chemistry / capacity and permitted connection arrangement checked; enclosure fits the selected battery
- Fire-alarm input switches the lock output, and its input type (NC, NO or voltage) matches the fire alarm panel
- Separate, individually fused or PTC-protected outputs for locks and for controller and readers
- AC-fail and low-battery outputs wired to a controller input for supervision
- Mains input range suits the site supply
- Lockable enclosure with a tamper switch where the supply sits in an accessible space
- Voltage at the farthest lock checked against its rated tolerance
- Exact product and system listing evidence required by the adopted code / arrangement obtained (for example UL 294 or UL 1034 where applicable)
Compare output ratings, battery support and fire-alarm inputs across our access control power supplies and enclosures.
Next steps
Send us your door count, lock types and voltage, readers, controller and required standby hours. We will check the current budget, suggest a supply and battery size from our range, and ship samples for bench testing before a larger order. Request a quote and we will reply within 24 hours.
Sources
- Altronix ACM power-supply / distribution manual: ratings, charging, alarm interface, fuse limits and battery worksheet.
- Securitron BPS installation manual: output settings, emergency release and battery limits.
- Yuasa NP7-12 data sheet: discharge-rate capacity and conditions.
- ICC: electrical egress-locking arrangements; UL: locking configurations.
Frequently asked questions
How many amps does a mag lock draw?
A single-door maglock in the 280 kg (600 lb) class typically draws about 300–500 mA at 12 V DC and roughly half that at 24 V DC. It draws that current the whole time it is locked, so use the datasheet figure and double it for a double-door unit.
What size power supply do I need for two maglocks?
Two 500 mA locks, two 120 mA readers and a 250 mA controller total 1.49 A; with 25% operating headroom that is 1.86 A. A 2 A load-rated supply covers that estimate only if its peak limits also fit. Add shared charging current and check temperature derating before choosing a rating.
How long will a 7 Ah battery run an access control system?
With an assumed 80% usable capacity and a constant 1.5 A battery load, 7 Ah gives about 3.7 hours as a preliminary estimate. Actual runtime depends on the battery discharge rate, age, temperature, supply self-consumption and cutoff voltage; use the manufacturer's worksheet and discharge table.
Is 12 V or 24 V better for access control locks?
24 V draws half the current for the same lock power, which cuts voltage drop on long runs and lets one supply feed more locks. 12 V is simpler when readers and controllers are 12 V only, because everything can share one supply.
Why does my controller reset when the drop bolt locks?
Possible causes include lock inrush pulling the supply voltage down, cable voltage drop or switching interference. Measure under load, check simultaneous peaks and approved suppression, and consider separately protected lock and electronics outputs.
Should maglocks stay powered from the battery during a fire alarm?
Where the approved egress arrangement requires alarm release, that release must remove battery-backed maglock power too. Configure the fire-alarm interface from the exact manuals; its trigger may be NC, NO or a specified voltage signal.
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