Applies to: Altronix

Summary

Calculating voltage drop in a paired wire run (two-conductor cable for power and return) is critical in access control systems. These systems often use low-voltage DC (typically 12V or 24V) to power devices like electric strikes, maglocks, card readers, and exit devices. Excessive voltage drop can cause unreliable operation, such as locks failing to release, readers malfunctioning, or devices overheating. This article describes how to calculate voltage drop in paired wire runs.

Calculating Voltage Drops

Use this process for every powered device or group of devices on a single wire run. To calculate voltage drops:

1.  Determine the maximum allowable voltage drop by starting with the supply voltage, then working out how much drop the run can tolerate. The industry guideline is 5% or less.

   Supply voltage	Maximum drop (5%)	Minimum voltage at device
   12V	0.6V	11.4V
   24V	1.2V	22.8V

2. Check the device's datasheet for its minimum operating voltage. If that value is stricter than the 5% guideline, use the datasheet tolerance value.
    
3. List every device powered by this wire run, such as the lock(s), reader(s), and other peripheral devices.
    
4. Verify each device’s current draw from the device’s datasheets.
    
5. Add all devices Amperage current draw together.
    
6. Multiply the sum of the devices by 1.2 to 1.3, which adds 20 to 30% margin for in-rush current and any future expansion.
   
   Example: One 12V Peripheral Device rated at 0.4A × 1.25 = 0.5A design load.
    
7. Measure the cable path from the power supply to the farthest device, in feet. Use the actual cable route, including vertical rises, drops, and service loops, rather than the straight-line distance.
    

8. Find the resistance of the wire gauge you plan to use.
    

   Wire gauge	Resistance (Ω per 1,000 ft)	Drop per amp per 100 ft of run*
   22 AWG	16.14	3.23V
   20 AWG	10.15	2.03V
   18 AWG	6.39	1.28V
   16 AWG	4.02	0.80V
   14 AWG	2.53	0.51V
   12 AWG	1.59	0.32V

   *Already includes both conductors (the round trip). Values are for solid copper at about 68°F (20°C).
    

9. Calculate the voltage drop.

   V_drop = (2 × I × L × R) ÷ 1,000

   I = load current in amps

   L = one-way run length in feet

   R = wire resistance in ohms per 1,000 ft 

   2 accounts for the round trip through the power and return conductors

   ÷ 1,000 converts R from "per 1,000 ft" to "per foot"

   NOTE: V_drop = I × (L ÷ 100) × the "drop per amp per 100 ft" value from the 
   table.

   IMPORTANT: If your calculated drop is at or below the maximum from Step 1, 
   the design works. If it is too high, try one or more of these fixes, then 
   recalculate: Use a heavier wire gauge (a lower AWG number).
   Switch to 24V devices. Drawing half the current for the same power cuts the 
   drop by half, and the allowable drop doubles.
   Move the power supply closer to the load.
   Split the load across separate wire runs.

10. After installation, measure the voltage at the device terminals while the device is operating (lock energized, reader active). The reading should be at or above the minimum voltage from Step 1.

Worked example

A 12V maglock rated at 0.4A, located 250 ft from the power supply.

Maximum drop: 12V × 5% = 0.6V

Load: 0.4A × 1.25 = 0.5A

Run length: 250 ft
Calculate by gauge:
 

Result: Use 12 AWG. Alternatively, a 24V version of the lock would allow 14 AWG or lighter.

Alternative View: Maximum One-Way Distance for Common Gauges (5% Drop)