Running Telephone Cable in medium voltage cable tray

[avogel]

I am in the US, so hoping to get some thoughts from an NEC compliance perspective. We need to run a telephone cable and the only route is a 20kV cable tray with Type MC cable.

392.6 allows you to run Class 2 circuits in the same cable tray if the cables >600V are Type MC, however it appears article 300 is superseded by wiring methods for power-limited cables, Article 725

725.136 Separation from Electric Light, Power, Class 1,
Non–Power-Limited Fire Alarm Circuit Conductors,
and Medium-Power Network-Powered Broadband
Communications Cables.

(A) General. Cables and conductors of Class 2 and Class 3
circuits shall not be placed in any cable, cable tray, compartment,
enclosure, manhole, outlet box, device box, raceway,
or similar fitting with conductors of electric light,
power, Class 1, non–power-limited fire alarm circuits, and
medium-power network-powered broadband communications
circuits unless permitted by 725.136(B) through (I).

(B) Separated by Barriers. Class 2 and Class 3 circuits
shall be permitted to be installed together with the conductors
of electric light, power, Class 1, non–power-limited fire alarm
and medium power network-powered broadband communications
circuits where they are separated by a barrier.

So, (B) permits Class 2 to be installed with power if the circuits are separated by a barrier. But the Code has already established, MC is not defined as a "barrier."

(G) Cable Trays. Class 2 and Class 3 circuit conductors
shall be permitted to be installed in cable trays, where the
conductors of the electric light, Class 1, and non–powerlimited
fire alarm circuits are separated by a solid fixed
barrier of a material compatible with the cable tray or
where the Class 2 or Class 3 circuits are installed in Type
MC cable.

So, (G) specifically permits installing Class 2 circuits in MC cable to separate only from electric light, Class 1, and non-power-limited fire alarm circuits. Intentially leaving out "power" circuits that it listed in (B). Why is this distinction made? No where else in the NEC is "electric light" even defined or mentioned. In my mind, the rationale behind protecting the cable physically with a flimsy tin barrier would justify permitting using armored cable in the same tray with a power circuit that is also type MC.

I'm starting to wonder what use artciles 725-800 are even for, people who have no business governing electrical installations.

Any thoughts?

 

[erickench]

Just one question. Have you ever seen a telephone cable classified as Type MC? I never have. Do you know of a manufacturer that makes this type of cable? NEC 330.10 allows it's use as signal circuits but does this apply to telephone cable? There are other types of signaling equipment than telephone.

Eric Kench, P.E.

 

[erickench]

Also, article 330 of the NEC does not have any provisions for twisted pairs used inside type MC armour. Telephone cable is made up of twisted pairs. I've seen it installed inside conduit and ducts but never as an armoured cable. I think you would have to use a barrier because of the difference in voltage.

Eric Kench, P.E.

 

[7anoter4]

In my opinion even the NEC would permit I should not put the telecom cable in the same cable tray with a medium voltage cable. I think will be better to hang it in a conduit well grounded, under the cable tray.

 

[erickench]

Yes you can run it inside a conduit but I don't think grounding is necessary. A telephone circuit is very low voltage and therefore not dangerous. The NEC does'nt seen to require it.

Eric Kench, P.E.

 

[dpc]

If this is an actual telco line, the ringing voltage can exceed 50 V. Not exactly very low voltage.

If you run this in a separate conduit, I don't see an NEC issue.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[erickench]

Okay a new requirement in the 2008 NEC requires all the grounding conductors of communication circuits to be connected to an intersystem bonding terminal located at the service.

Eric Kench, P.E.

 

[7anoter4]

First of all, this is for record only, most of telephone cables may be built in twisted pair as correctly remarked erickench, but stranding elements may be pairs, triples , quins and also star-quad and DM [Dieselhorst-Martin quad], at least in Europe. See:

http://www.tkd-gruppe.de/index.php?module=Pagesetter&type=file&func=get&tid=5&fid=file01&pid=26

If the cable is built according IEEE 1613 class 2 it will be less sensible to Electro-Magnetic Interference, indeed. See:

http://www.powersystem.org/media/do...Hardened Substation LAN Equipment Growing.pdf

IEEE 487 "Recommended Practice for the Protection of Wire-Line Communication Facilities Serving Electric Power Locations": Wire-line communication facilities serving electric supply locations often require special high-voltage protection against the effects of fault-produced ground potential rise or induced voltages, or both. To achieve electromagnetic compatibility in a system the first step is to have some correction in design and second is some special methods like shielding, grounding and filtering.
That means the shield [conduit] has to be grounded as a mean against EMI.
In the past, if the cable tray was solid bottom, steel made and grounded, according to the old IEEE 422, at least 1 ft clearance had to be provided.

 

[erickench]

Some types of communication cable come with a shield built inside it that you can ground. This would certainly reduce EMI.

Eric Kench, P.E.

 

[itsmoked]

Side step the entire issue and run non-metallic fiber..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[avogel]

Actually, it appears that none of the armored telephone cables are even listed for use in Cable Tray. Looks like there is little doubt I will have to install it in conduit.

I agree with using fiber too but that's another issue I asked about, but we still are required to run twisted pairs everywhere even though we have an expanse fiber optic network. Being conerned mainly with electrical, I have little interest in usurpering the telephone guys. I will leave that up to IT.