Connecting instruments and junction box (distance 1.5m)

[coyotelive]

Hi!



I'm trying to design electrical installation according to NEC and I have some problems with it. I have to connect instruments with Junction Box. The distance beetwen JB and instruments is about 1.5m. I thought that it would be good enough to use TC-ER cables and without using conduit and cable trays. (there is too little space for cable trays) What do you think about that?

 

[mgookin]

Hi!



I'm trying to design electrical installation according to NEC and I have some problems with it. I have to connect instruments with Junction Box. The distance beetwen JB and instruments is about 1.5m. I thought that it would be good enough to use TC-ER cables and without using conduit and cable trays. (there is too little space for cable trays) What do you think about that?

What's your voltage and current from the j-box to the instrument?

 

[coyotelive]

What's your voltage and current from the j-box to the instrument?

Most of the instruments are 24V DC or 120V AC. (4-20mA)

 

[Aleman]

I am not 100% sure of the code on this subject but it is common to see instruments wired in this way. If you fuse the 24VDC at 4 amps or 100VA it will fall
under class 2. Then you can run your cable pretty much how you want. If you limit the 120V to 100VA it will also be a class 2 circuit. But what I usually will
do with something like you describe is either run conduit or pipe chases to the instrument. Or just run your cables in Karflex. Sometimes we will just put a
cord and plug to the 120V instrument and plug it into a receptacle. The 4-20mA will just be cable from device to the raceway be it tray or other raceways.

 

[don_resqcapt19]

I am not 100% sure of the code on this subject but it is common to see instruments wired in this way. If you fuse the 24VDC at 4 amps or 100VA it will fall under class 2. Then you can run your cable pretty much how you want. If you limit the 120V to 100VA it will also be a class 2 circuit. But what I usually will do with something like you describe is either run conduit or pipe chases to the instrument. Or just run your cables in Karflex. Sometimes we will just put a cord and plug to the 120V instrument and plug it into a receptacle. The 4-20mA will just be cable from device to the raceway be it tray or other raceways.

In general most AHJs want to see a listed power supply for any circuit that is used as a Class 2 circuit. There are some electronic circuit breakers where the load side is so listed (list item 3 in the code section below covers those breakers). I don't think that there are any fuses that can be used to create a listed Class 2 power supply.

725.121 Power Sources for Class 2 and Class 3 Circuits
(A) Power Source. The power source for a Class 2 or a Class 3
circuit shall be as specified in 725.121(A)(1), (A)(2), (A)(3), (A)(4), or (A)(5):
(1) A listed Class 2 or Class 3 transformer
(2) A listed Class 2 or Class 3 power supply
(3) Other listed equipment marked to identify the Class 2 or Class 3 power source
(4) Listed information technology (computer) equipment limited-power circuits.
(5) A dry cell battery shall be considered an inherently limited Class 2 power source, provided the voltage is 30 volts or less and the capacity is equal to or less than that available from series connected No. 6 carbon zinc cells.

A Class 2 power supply that is inherently limited permits you to have a maximum current of 0.005 amps at 120 volts. I have never seen a Class 2 power supply at that voltage. All of the 120 volt control circuits that I have seen were Class 1 circuits.

If the power supply is not inherently limited, you can't have voltages above 30 volts for a Class 2 circuit.

As far as the installation, around here it would be like you suggest...conduit, conduit bodies and LFMC between the junction box and the instrument.