PLTC vs ITC

[rickg121]

Hello Everyone,
There's a query that has plagued me for years. I seem to be caught between what the code states and what the world actually does. I can't seem to capture a definitive answer regard the following:

I design and build SCADA systems for various customers within the natural gas industry. These facilities mostly involve 24 VDC power supplies (15 - 20 amp typical ... 480 VA max), digital & analog signals, MODBUS, and RS-485 / RS-232. As such, I typically run digital signals on THHN/THWN, analogs on twisted shielded pairs, and the rest on Cat5, Cat6, or RS-485 cable.

My question involves the Cats and RS-485 stuff. The trick is to minimize the number of rigid steel conduits that these things are installed in. The Holy Grail is to run everything in the same raceway when possible. Obviously, pipe is money ... and takes space.

The first part involves the defination of my power supply in regards to Art. 725. Since it's not listed as a Class 2 or 3 PS, I have to assume that it's a Class 1. Art 725.49 states that conductor insulation shall be 600 volt rated on a Class 1 power supply.

Most of the RS-485 cable that I've seen is type PLTC (300 volt). Also, CAT cable is rated less than 600 volts. Most Smart transmitters use two pair ... one for RS-485 signal and one for 24VDC power. Does this mean that I have to run two separate conduits to every transmitter?

In my experience, this is never done, but I'll be darned if I can find code justification for it.

Does Art 727 (ITC cable) offer any relief here?

If it does, the second part of my question involves the cable itself. PLTC and ITC seems to be of the same construction and essentially the same stuff. Back in 1997, it was said that ITC rating could be 'grandfathered' to all existing PLTC and eventually all cable would be dual rated as PLTC/ITC. Has this ever happened?

I'm dazed & confused here guys ... I know that it can't be this difficult. What's your take on it? What am I missing?

Thanks,

Rick G.

 

[nhfire77]

Class 2 and 3 is 100VA or less on the circuit, if your 24 VDC power supply is 480VA then its over that, you can however, remove the class 2, 3 markings on all the equipment and reclassify the circuit as Class 1 as long as Class 1 wiring methods are used. Rigid is fine.

 

[rickg121]

I'm not concerned with the Class 2 or 3 rating. I need to run this 300 volt insulated stuff with the regular 24 VDC stuff in spite of 725.49(B).

Since it's so common to do so, I'm wondering how.

Thanks for the quick reply.

Rick G

 

[wireguru]

could you replace the THHNs with tray cable?

 

[rickg121]

That's a thought ... but I'd still be faced with a Class 1 power supply being used on PLTC wiring.

 

[nhfire77]

Check out this article, it addresses your situation, it may not directly answer your question, but its a start!!!!

http://ecmweb.com/mag/electric_making_sense_necs/

 

[rickg121]

Good reading ... thanks.

 

[JWCELECTRIC]

Rick,

On projects we design where there is both Power & Scada installations, we seperate the wiring into catagories: 4-20mA cables can be run in same conduit but not with discrete I/O signals. We do run power with some control cables in same conduit if running from VFD/Starter to a motor with temp switches and insulation of cables is 600V. Also we also keep the Ethernet wiring seperate form all other cables, It eliminates any interference from any one system. Most instruments that require two connections (Power & 4-20mA) unless it is loop powered we run seperate conduits.

- JWC