How to use this calculator
- Enter the system voltage. Choose 12 VDC or 24 VDC to set the voltage-drop and watt-hour basis.
- Enter opening loads. Add powered openings, simultaneous locks, lock current, reader current, accessory current and controller current from datasheets.
- Check supply size. Review total active load, design current with margin and the next listed power-supply size.
- Check the farthest run. Choose the farthest-door or trunk load basis, wire size and one-way length, then compare voltage drop with the target.
- Size standby capacity. Enter desired runtime and usable battery capacity to estimate required amp-hours and watt-hours.
How it works
Access-control power sizing starts with the load schedule. The calculator sums the lock current for the locks energized at the same time, reader and REX/accessory current for every powered opening, and the controller or panel current. The supply recommendation is the next listed DC supply size above that load after the entered margin.
The wire check uses the same voltage-drop method as the voltage drop calculator, but narrowed to copper two-wire DC access-control runs and smaller AWG sizes. Use the farthest-opening basis for separate home runs, or total panel/trunk load if one cable segment carries the whole panel load. For raceway work, pair this with the conduit fill calculator, electrical box fill calculator and conduit bend offset calculator. For longer bundled pulls, use the conduit pull tension calculator. For remote gates, sheds or standalone cabinets, use the off-grid solar calculator before finalizing the 12/24 VDC load budget.
Worked example
Verified against the live calculator
Four 24 VDC openings with four continuous locks at 300 mA each,
four readers at 120 mA each, four REX/accessory loads at
40 mA each, and a 250 mA controller draw
2.09 A. With a 25% margin, the design current is
2.61 A, so a listed 3 A supply is the first fit.
On a #18 AWG copper farthest-door run carrying one active opening
for 150 ft one-way, the voltage drop is about 1.10 V,
or 4.58% at 24 V, so the selected wire passes a 5%
target in this screen.
Reference data
Low-voltage copper wire sizes used by the access-control voltage-drop screen. The area is circular-mil area converted to kcmil for consistency with the electrical calculators.
| Copper wire size | Area (kcmil) |
|---|---|
| #22 AWG | 0.642 |
| #20 AWG | 1.02 |
| #18 AWG | 1.62 |
| #16 AWG | 2.58 |
| #14 AWG | 4.11 |
| #12 AWG | 6.53 |
| #10 AWG | 10.38 |
| #8 AWG | 16.51 |
Source: Common AWG circular-mil areas; verify cable construction, listing, jacket, fire rating and manufacturer limits for final design.
Frequently asked questions
How do you size an access-control power supply?
Add the current draw for locks that can be energized at the same time, all powered readers and accessories, and the controller or panel load. Then apply a design margin before selecting the next available listed DC supply size.
Should simultaneous locks equal the number of doors?
For fail-safe maglocks or other continuously powered locks, use all powered openings. For electric strikes or intermittent locks, use the worst-case number expected to energize together, then verify inrush and duty cycle from the hardware datasheet.
How is access-control wire voltage drop calculated?
This page uses the copper two-wire DC circular-mil shortcut: VD = 2*K*I*L/CM. K is 12.9 for copper, I is the farthest-leg current, L is one-way cable length in feet and CM is conductor circular-mil area.
Is 24 V better than 12 V for long access-control runs?
Often, yes. For the same wattage, 24 V carries about half the current of 12 V, so voltage-drop percentage is lower. The final choice still depends on the listed devices, locks, power supplies and controller outputs.
Does this satisfy code or fire/life-safety requirements?
No. It is a planning screen only. Final access-control, egress, fire alarm interface, standby power and listed-equipment requirements must come from the project documents, adopted codes, manufacturer instructions and the local AHJ.
Method & assumptions
- Uses 12 VDC or 24 VDC low-voltage access-control power only.
- Uses copper conductor K = 12.9 and the two-wire DC circular-mil voltage-drop shortcut.
- Uses one-way wire length from supply to the farthest checked load or trunk end.
- Does not model inrush, charger recharge time, battery aging curves, temperature, fire alarm interface, door egress, listed power-limited circuits or AHJ requirements.
- Use manufacturer datasheets for every lock, reader, REX, PIR, controller and accessory current draw before ordering hardware.