Low Voltage Load
Device-list demand for a 12 or 24 VDC supply, listed sizes, and remaining capacity.
Low voltage load calculator
Device list to demand, listed supply sizes, and remaining capacity. Inputs stay in the URL.
Starting points
Inputs
Supply
A mixed-voltage job is sized per supply, one supply at a time.
Covers battery charging and growth.
Devices
Typical draws. Confirm each against the datasheet.
Unit
Duty
Subtotal 0.50 A
Unit
Duty
Subtotal 0.15 A
Unit
Duty
Subtotal 0.03 A
Unit
Duty
Subtotal 0.45 A
Results
1.47 A demand at 12 VDC. 7 listed sizes cover 1.47 A.
Demand at 12 VDC
1.47A
7 listed sizes cover 1.47 A.
Currents
Continuous
0.68A
Momentary
0.45A
Peak
1.13A
Headroom applied
1.47A
Total watts
17.63W
Related
Voltage drop for the highest per-device current (0.50 A) at 12 VDC. Length stays on that page.
UPS and backup battery for a panel at 12 VDC with standby 0.68 A.
Worked example
Four maglocks at 500 mA is 2.0 A. At a headroom factor of 1.30 the demand is 2.6 A. A 3 A supply carries that at 87% loading. A 2.5 A supply does not cover it.
How to use
- List every device from its datasheet: name, quantity, current or watts, and whether the current is continuous or momentary.
- Add the peak currents. Continuous is the sum of the continuous rows. Momentary is the sum of the momentary rows. Peak is those two totals added together.
- Apply the headroom factor. Demand is peak times that factor. Listed supply sizes are compared to that demand.
Assumptions
- Currents are the values entered on each row. Milliamps divide by 1,000. Watts divide by the selected system voltage.
- Momentary devices are not assumed energized continuously. Peak is continuous plus momentary.
- The headroom factor is an allowance, not a code requirement. The default is 1.30.
- Charging current depends on the charger. The optional line is (amp-hours × 1.2) ÷ recharge hours. Demand for the size table stays peak times headroom.
- Standby duration comes from the listing, the specification, and the AHJ. This page does not size battery runtime.
- The result is an estimate. Manufacturer data, the AHJ, and current applicable code govern material and whether a circuit is energized.
FAQ
- Why are fail-secure strikes not summed the same way as maglocks?
- A fail-secure strike is energized only while the lock is released, so that current is momentary. A maglock that stays energized while the door is secure is continuous. Peak is the two totals added together; the continuous total does not include the strike.
- Where does the 30 percent go?
- The default headroom factor is 1.30, which is 30 percent above peak. The factor is an allowance for battery charging and growth, not a code requirement. The optional charging line shows the current from the entered amp-hours and recharge hours so that figure can be compared to the allowance.
- Why is the supply rating not the whole story once batteries are charging?
- A listed amp rating is the supply output. While batteries recharge, the charger draws additional current on the same supply. That charging current is a separate line. Demand for the size table is peak times the headroom factor; charging is not added a second time into demand.
- What to check when a supply runs hot or drops out under load?
- Loading percent and amps left are on the size table for each listed rating. Voltage at the load is a separate calculation from this list. Charging current is on its own line when amp-hours and recharge hours are entered.
Disclaimer
Field estimate only. Manufacturer data, the authority having jurisdiction (AHJ), and current applicable code govern material and whether a circuit is energized. See the disclaimer.