My specific use case is for an Elk M1G panel. I want to use a magnetic alarm sensor, e.g., a window open/close sensor, at a driveway gate about 1500' from the house. I can run a cat6 cable underground to the gate. Can I use one of the twisted pairs to connect the sensor to an M1G zone, or is 23AWG "not enough"? I would rather not use multiple pairs for this.
Max distance to a magnetic alarm sensor
- Thread starter Ira
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Sandpiper
Active Member
I know this sounds crazy, but I just calculated the maximum distance using one pair of a cat 6 23AWG wire is 7 miles.
Let me explain so that you-all can verify my work. Cat 6 AWG 23 has 9.38 ohms per 100m (or per 328 ft). So that is 0.0286 ohms/foot. (9.38 ohms/328 feet). Since we have a pair (supply and return leads), the equivalent ohms is double 0.0286, or 0.0572 ohms per foot of distance the contact is from the M1 input. To get maximum distance we can use the wire itself as an end-of-line-resistor. The ELK expects the EOL to be 2200 ohms. In order to get 2200 ohms from a length of CAT 6 we calculate as follows: 2200 ohms/ 0.0572 ohms per foot = 38461 feet. Divide by 5280 to get miles. 38461/5280 = 7.28 miles.
So if you have 7 miles of distance to the contact that would work perfect. However, you only have 1500 feet. So what do we do now? And what is the resistance of CAT 6 wire for 1500 feet? That would be 1500 ft x 0.0572 ohm per foot = 85.8 ohms. But we need 2200 ohms for it to work properly. So add an EOL at the contact of 2K ohms. 2200 - 85.8 = 2114.2 ohms. I think 2K will work.
Just remember to configure that input as an EOL zone i.e. zone type = type 0 (EOL Hardware). And add a 2K resistor at the contact.
Let me explain so that you-all can verify my work. Cat 6 AWG 23 has 9.38 ohms per 100m (or per 328 ft). So that is 0.0286 ohms/foot. (9.38 ohms/328 feet). Since we have a pair (supply and return leads), the equivalent ohms is double 0.0286, or 0.0572 ohms per foot of distance the contact is from the M1 input. To get maximum distance we can use the wire itself as an end-of-line-resistor. The ELK expects the EOL to be 2200 ohms. In order to get 2200 ohms from a length of CAT 6 we calculate as follows: 2200 ohms/ 0.0572 ohms per foot = 38461 feet. Divide by 5280 to get miles. 38461/5280 = 7.28 miles.
So if you have 7 miles of distance to the contact that would work perfect. However, you only have 1500 feet. So what do we do now? And what is the resistance of CAT 6 wire for 1500 feet? That would be 1500 ft x 0.0572 ohm per foot = 85.8 ohms. But we need 2200 ohms for it to work properly. So add an EOL at the contact of 2K ohms. 2200 - 85.8 = 2114.2 ohms. I think 2K will work.
Just remember to configure that input as an EOL zone i.e. zone type = type 0 (EOL Hardware). And add a 2K resistor at the contact.
Thanks for the replies. The seller said that both ends of the cable are not accessible when still on the spool (although not sure I believe him), so I would have to unspool the 1000' of cable to test it.
Their spec sheet says "Max. Conductor DC Resistance 20°C (Ω/km) 68", so it's less resistance than @Sandpiper used in his calculations, which is a good thing. I couldn't find any spec's in the Elk manual for cable length vs wire size.
I need to buy 1500' feet of the same cat6 cable to string together some network switches (different project, same conduit), so I guess I will buy the cable. Hopefully, I will have access to both cable ends while on the spool.
Their spec sheet says "Max. Conductor DC Resistance 20°C (Ω/km) 68", so it's less resistance than @Sandpiper used in his calculations, which is a good thing. I couldn't find any spec's in the Elk manual for cable length vs wire size.
I need to buy 1500' feet of the same cat6 cable to string together some network switches (different project, same conduit), so I guess I will buy the cable. Hopefully, I will have access to both cable ends while on the spool.
68 ohm sounds about right. The ELK manual doesn't give a spec for contact wire, but DOES give a spec for smoke alarms, and its 13 ohms and it uses an 820 ohm resister as opposed to 2.2K ohm.
There are two big reasons not to violate these specs. 1) The reason for an EOL is to detect wire faults, both opens and shorts. If the EOL is replaced by wire resistance, you can no longer detect shorted wires. 2) Longer wires pick up lighting and other electrical interference which can occasional cause false alarms. If you must go longer distances, use wireless and repeaters if needed.
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