Why Is My Electromagnetic Door Holder Weak?
A magnetic door holder that "slips" or fails to hold a door open isn't just a convenience issue—it's a maintenance red flag for your fire safety system. While these devices have no moving parts, their performance depends entirely on electrical integrity and physical alignment.
The Direct Answer: Top 3 Causes for Weak Hold
If your magnet feels weak or the door is pulling away too easily, the problem is almost always one of these three:
- Voltage Drop: The device isn't receiving its full rated power (often due to thin wire gauge).
- Armature Misalignment: The catch disk isn't making 100% flat contact with the magnet face.
- Blown Relay or Fuse: A partial failure in the fire alarm interface is limiting the current.
1. The Voltage Guide: Are You Under-Powered?
Magnetic holders are fail-safe, meaning they require constant power to stay "on." If the voltage drops even slightly, the holding force plummets.
- 24VDC (Standard): Most common in modern systems. If you measure less than 22V at the magnet, your wire run may be too long or the wire gauge too thin.
- 24VAC: Often found in older mechanical systems. Ensure your multimeter is set to the correct mode when testing.
- 120VAC: High-voltage units used in retrofits. Warning: Never attempt to wire a 24V magnet to a 120V line; this will instantly burn out the coil.
Pro Tip: Always measure voltage under load (while the magnet is energized). A "ghost" reading on an idle wire can be misleading.
2. The "Thru-Bolt" Tip: Stop the Wobble
Standard wood or sheet metal screws can loosen over time as the door is pulled and released hundreds of times. A "wobbly" armature base is the leading cause of poor alignment.
Always use thru-bolts (also called sex bolts) to secure the armature base to the door. Thru-bolts clamp through the entire thickness of the door, providing a rock-solid foundation that won't strip out. This ensures the catch disk stays perfectly parallel to the magnet for maximum suction.
3. Maintenance Checklist: The 5-Minute Fix
Before replacing an expensive magnet, perform these three maintenance steps:
- Clean the Faces: Dust, oil, and metallic shavings can create a microscopic gap between the magnet and the catch disk. Wipe both surfaces with a soft cloth and a mild, non-abrasive cleaner.
- Check the Swivel: Most armatures have a ball-joint or swivel. It should move freely so it can "self-align" when the door hits the magnet. If it’s stuck or painted over, the door won't stick.
- Sand Away Burrs: If the magnet face has a "burr" or high spot from a hard impact, a light pass with fine-grit sandpaper can restore the flat contact surface.
Recommended Replacement Hardware
If your testing confirms a dead coil, these are the industry-leading replacements:
- Rixson 996M Series: The workhorse of commercial corridors. Known for high holding force and durable tri-voltage coils.
- ABH 20020-628 Armature Assembly: If your magnet is fine but your hardware is failing, this heavy-duty armature kit includes the swivel catch disk needed for a perfect hold.
Preventing Voltage Drop: Selecting the Correct Wire Gauge
To prevent voltage drop in electromagnetic door releases, selecting the correct wire gauge is critical. If the wire is too thin for the distance of the run, the magnet will not receive enough current to maintain its rated holding force, leading to doors that "drift" or fail to stay open.
Below is the Wire Gauge Chart for standard 24VDC magnetic door holders.
24VDC Wire Gauge Selection Chart
This table calculates the maximum one-way distance (in feet) between the power supply and the door holder to maintain a 5% maximum voltage drop.
| Wire Gauge (AWG) | 100mA Load (Single Door) | 500mA Load (Typical Bank of 5 Doors) | 1000mA (1A) Load (Large Corridor) |
| 22 AWG | 150 ft | 30 ft | 15 ft |
| 20 AWG | 240 ft | 48 ft | 24 ft |
| 18 AWG | 385 ft | 77 ft | 38 ft |
| 16 AWG | 610 ft | 122 ft | 61 ft |
| 14 AWG | 970 ft | 194 ft | 97 ft |
| 12 AWG | 1,500 ft | 300 ft | 155 ft |
Key Takeaways for Installation
- Standardize on 18 AWG: Most low-voltage contractors use 18/2 (18 gauge, 2 conductor) wire as a standard. It provides a great balance of flexibility and current-carrying capacity for most commercial corridors.
- Account for Total Load: If you are "daisy-chaining" five doors together on one circuit, you must add their current draws together (e.g., 100mA x 5 = 500mA) to determine the correct gauge for that run.
- The 5% Rule: For a 24VDC system, you should never allow the voltage at the magnet to drop below 22.8V. Anything lower can cause the magnetic coil to hum, overheat, or lose its "stick."
