Automatic Door Voltage Drop
Will this automatic-door or access-control device receive enough voltage at the end of this wire run?
Field voltage at the device
Enter the physical one-way cable distance, wire size, load current, and the minimum voltage the device still needs. The calculator uses outgoing and return conductors and compares estimated device voltage to the minimum you entered — not a generic percent-drop rule.
Field result
PASS
At 125 ft one-way on 18 AWG with a 0.42 A total load, the estimated device voltage is 23.3 VDC from a 24.0 VDC source.
Estimated device voltage23.3 VDC
Voltage drop0.7 V (2.8%)
Entered minimum20.0 VDC
Margin+3.3 V
Loop length used250 ft (2 conductors)
Loop resistance1.60 Ω
How the loop length is handled
One-way distance is the run from the power source to the device. Loop resistance uses that distance times two conductors. Do not enter round-trip footage unless you are deliberately using a different conductor count in a later advanced tool.
Modes
- Check an existing run against the device minimum voltage.
- Find the maximum one-way distance on a chosen wire size.
- Find the smallest supported AWG that still meets that minimum.
FAQ
- Does PASS mean the installation meets code or OEM spec?
- No. PASS only means the estimated voltage is at or above the minimum you typed. Use the installed device’s documented requirement.
- Why doesn’t load type fill in the amps?
- Activation sensors, maglocks, and strikes draw very different current. The category is a label for your notes. You still enter the actual load.