Most alarm wire on a prewire is plain unshielded cable, and most of the time that is the right call. The question of shielded vs unshielded alarm wire comes up on the runs that carry data rather than a simple contact loop, or that share space with electrical noise. This guide covers when the shield earns its place, how to handle the drain wire, and why the panel maker’s manual has the last word.
When do you need shielded vs unshielded alarm wire?
Use unshielded alarm wire for contacts, motion detectors and most short runs. Choose shielded cable for keypad and data bus runs that are long or pass near AC wiring, motors or fluorescent ballasts, unless the panel manual says otherwise. Ground the drain wire at the panel end only, keep the shield continuous, and insulate it at the far end.
What shielded alarm wire is
Shielded alarm cable has the same conductors as standard alarm wire, with a foil wrap around them under the jacket. A bare drain wire runs alongside the foil and touching it, so the shield can be terminated with an ordinary screw terminal. Retail listings often note that this drain is not counted in the conductor total, so a shielded 22/4 still has four insulated conductors.
The foil intercepts electrical noise coupled onto the cable from nearby wiring and drains it to ground instead of letting it reach the signal conductors. That only works when the drain is terminated correctly, which is where most shielded runs go wrong.
Shielded vs unshielded alarm wire compared
| Factor | Unshielded | Shielded |
|---|---|---|
| Construction | Insulated conductors under a jacket | Same conductors plus foil and a bare drain wire |
| Typical use | Contacts, motion detectors, sirens, short runs | Keypad and data buses, long or noisy runs |
| Noise rejection | Relies on routing and distance | Foil intercepts electric field noise when grounded |
| Termination | Conductors only | Conductors plus the drain, at one end |
| Handling | Thinner and easier to strip | Stiffer; foil must be trimmed cleanly |
Conductor count and gauge are a separate choice. Our guide to 22/2 vs 22/4 alarm wire covers which devices need four conductors and how gauge affects voltage drop.
When to use shielded cable
A zone loop is a slow, simple circuit: the panel looks for a change in resistance. A keypad or expander bus carries data between the panel and the device, and noise on those conductors can corrupt it. That is why shielding questions nearly always come back to bus wiring.
Shielded alarm cable for keypads and data buses
Some keypad installation instructions recommend two-pair twisted, shielded data cable for the bus, with the shield grounded at one end. Consider it where the bus run is:
- Long: a longer run has more length exposed to noise.
- Near line voltage: running beside AC circuits, in shared ceilings or along panelboards.
- Near noisy loads: motors, compressors, overhead door operators and fluorescent lighting ballasts.
- In commercial space: where you cannot control what the electricians run next to it later.
Intermittent keypad troubles, lost communication or garbled displays that come and go with a nearby load switching on are the classic symptoms of a noisy bus.
When unshielded alarm wire is fine
Door and window contacts, glass break detectors and motion detectors on short residential runs rarely need a shield. Their loops are low speed, and the panel’s resistance supervision tolerates noise far better than a data bus. Unshielded cable is also thinner, cheaper to pull and quicker to terminate.
There is a stronger reason to stop and check: not every panel maker wants a shield. At least one manufacturer’s bus wiring guide states plainly that twisted pair or shielded wire must not be used for its keypad and expansion bus circuits. Pulling shielded cable “to be safe” can therefore be the wrong cable for that system.
Drain wire: ground the shield at the panel end only
A shield grounded at both ends gives stray current a path between two ground points that are rarely at the same voltage, the same ground loop problem described in our guide on how to fix ground loop hum. For low-frequency circuits the usual practice is a single ground point. A NASA electrical grounding practice sums up the principle: single-point grounding works best at low frequencies and small dimensions, and multiple-point grounding at high frequencies and large dimensions.
- At the panel: land the drain on the terminal the manual names, usually an earth or common terminal.
- At the device: cut the drain and foil back and insulate them with heat shrink or tape so they cannot touch anything.
- In between: do not bond the drain to boxes, conduit or device terminals along the way.
- At every splice or daisy-chain point: join the drain wires so the shield stays continuous to the last device.
Splice methods themselves are covered in how to splice alarm wire. Before power-up, check with a meter that the drain reads open to every conductor and, with the panel end lifted, open to ground.
A shield does not replace good routing
Foil is good at stopping electric field noise. It does far less against low-frequency magnetic fields from heavy AC currents, which are reduced by twisting the conductors and by distance. A shielded bus cable strapped to a power feeder can still have problems.
- Keep alarm cable away from line voltage, and cross it at right angles where it must cross.
- Do not share a raceway or box with power wiring unless the code and a listed barrier allow it.
- Route around motor controllers, lighting ballasts and transformers rather than past them.
- Use twisted pairs for the bus where the manual calls for them.
Separation distances come from the electrical code your authority having jurisdiction enforces and from the equipment manual, so check both rather than relying on a rule of thumb.
Follow the panel maker’s install manual first
The control panel installation manual is the document that decides the cable for each circuit. It can specify conductor size, maximum bus length, twisted or untwisted pairs, shielded or unshielded cable, and the exact terminal for a drain wire. If a manual and this guide disagree, the manual wins.
Fire alarm circuits are a separate case. Their cable type, shielding and termination follow the fire alarm system documents and the authority having jurisdiction; the listings themselves are explained in what is FPLR cable. A shielded burglar alarm cable is not a fire alarm cable unless its jacket carries that listing.
Where to find shielded and unshielded alarm cable
The security and alarm wire category lists alarm and control cable, and keypads and sensors sit in intrusion alarms, both in the Security, Surveillance and Control division. For a shielded construction to match a panel manual, and for pricing, ask your sales representative through the Bursma contact page or the Wyoming, MI branch near Grand Rapids, or open a Bursma dealer account.
Frequently asked questions
Do alarm keypads need shielded wire?
Not always. Some keypad instructions recommend twisted, shielded data cable for the bus, while other panel makers say not to use shielded or twisted cable. Follow the installation manual for the panel and keypad you are installing.
Should the drain wire be grounded at both ends?
For alarm bus and sensor wiring, no. Ground it at the panel end only and insulate it at the device, so the shield cannot form a ground loop between two ground points.
Can I use shielded cable for door contacts?
Yes, it works electrically, but contact loops rarely need it. Terminate or insulate the drain properly, because a loose drain touching a terminal can cause faults of its own.
Does shielded alarm wire stop interference from fluorescent lights?
It helps with electric field noise from ballasts and nearby wiring, but distance and twisted pairs matter as much. Route the cable away from the fixtures where you can.