Running Ethernet and analogue telephone up the same cable.

[epsilonGreedy]

I have chanced upon a reel of abandoned armoured BT cable and I am thinking of using this to make a future provision for a garage based office network and phone.

 

The current situation is that BT should be connecting my static caravan soon with a normal voice and broadband connection.

4 months from now I hope to have a roofed garage and a temporary office situated about 8 meters away from the static caravan.

There is an existing mains power service trench open between the garage foundation and static caravan.

 

The current custodian of the reel of abandoned armoured BT cable thinks it is multi core so is the following plan viable?

 

Lay a 10 meter length of the BT cable in a separate duct longside the mains cable then close the trench. Then when the garage is compete use some of the cable cores for a voice telephone extension and other spare wires for a wired Ethernet link into the back of the BT hub still located in the static caravan. I would be hoping for a 30 Mb quality link between the static caravan and garage office.

[PeterW]

Lay two if you’ve got the cable as it’s cheap (free...)

[Jeremy Harris]

I suspect there may be problems because the twist and characteristic impedance of phone cable is different to Ethernet cable  Phone cable twisted pairs have a looser twist than Ethernet twisted pairs, and IIRC, are around 6 times the impedance  (Ethernet cable is around 100 ohms, phone cable is probably still around 600 ohms).

 

Ethernet cable is pretty cheap, even Cat 5e underground cable (to go in a duct) is only around £20 for a 50m reel.

[epsilonGreedy]

1 minute ago, PeterW said:

Lay two if you’ve got the cable as it’s cheap (free...)

 

 

Hmmm yes two lengths... this is an option, so it does sound that the armoured and effectively heavily shielded BT cable can function as expensive CAT 5 Ethernet cable even when laid underground.

 

The corrugated plastic ducting I have available is just 20mm and I am not sure it would accommodate two lengths of the heavy BT cable and I am worried about electrical safety if it is laid directly alongside the 40amp mains cable. 

[Jeremy Harris]

3 minutes ago, epsilonGreedy said:

 

Hmmm yes two lengths... this is an option, so it does sound that the armoured and effectively heavily shielded BT cable can function as expensive CAT 5 Ethernet cable even when laid underground.

 

The corrugated plastic ducting I have available is just 20mm and I am not sure it would accommodate two lengths of the heavy BT cable and I am worried about electrical safety if it is laid directly alongside the 40amp mains cable. 

 

 

There's a minimum separation distance required in a  trench between LV power cables and data cables; off the top of my head I think it's 300mm (need to check, I have a trench drawing somewhere).

 

I'm not sure that 600 ohm phone cable would support 100 ohm Ethernet, but it might do with a bit of a performance hit, but that may not matter.

[PeterW]

7 minutes ago, JSHarris said:

 

 

There's a minimum separation distance required in a  trench between LV power cables and data cables; off the top of my head I think it's 300mm (need to check, I have a trench drawing somewhere).

 

I'm not sure that 600 ohm phone cable would support 100 ohm Ethernet, but it might do with a bit of a performance hit, but that may not matter.

 

If it’s the BT armoured stuff it’s CCA and it’s ok to about 80-100ft for 70mbps so would be fine for broadband over 10 metres. Anything more than that and I think it would start to degrade. 

[Jeremy Harris]

Thanks @PeterW, I had a feeling it would degrade performance a bit, but wasn't sure by how much.  Looks like it's not enough degradation to worry about, if 30 Mb/s is the requirement.  However, I did a bit of digging around and found that 25m of outdoor/underground in a duct Cat 5e can be had for around £10, which makes me wonder whether it might not be better to just run Cat 5e to give a bit of future proofing.

[Ed Davies]

I'd agree with @JSHarrisand @PeterWthat the BT cable is not ideal for Ethernet but will probably work OK in practice. One thing you can do to help with this is use the cable for the whole length: don't, for example, use Cat5 from one end of the static to the other then plug the BT cable in there, take the BT cable all the way from the  broadband modem/router to whatever it's plugged into in the garage. It's the changes in impedance causing reflections which will hurt.

 

Personally, I'd just stick a few bits of Cat 5e (or Cat 6) in and be done with it.

Edited October 17, 2018 by Ed Davies

[epsilonGreedy]

47 minutes ago, JSHarris said:

(Ethernet cable is around 100 ohms, phone cable is probably still around 600 ohms).

 

 

Oh I had a feeling there would be a scientific gotcha.

 

Ok so Plan-B.

 

Run the armoured BT cable naked through the trench for the voice link.

Regular CAT-5 cable through the 20mm conduit.

 

Should I attempt to maintain a minimum distance between the mains cable in the trench and the other two cables for signal quality?

[Jeremy Harris]

Sounds good, but I'd be inclined to use outdoor Cat 5e, as it has a much tougher sheath.  I found this source of a 25m length for £10, including delivery: https://www.ebay.co.uk/itm/25m-External-Underground-Outdoor-Network-Ethernet-Cat-5-Cable-Solid-Copper/351540910858?epid=906119465&hash=item51d9789b0a:g:AVwAAOSwyQtVqWwV:rk:1:pf:0

 

The trench drawing I have shows a 300mm  minimum separation between data cables and power cables, so that seems a good guideline.

[PeterW]

I had a drum of 500m of black external full copper BT spec Cat5 and is ideal for all this sort thing - I’ve run 3 cores in 25mm rigid duct as it was cheap and was easy to install.