wire size for plc control circuits

[pwhite]

this is a 3 part question, each related to this topic.

i have heard about a job where the contractor used #18 tffn wire from his control panel to the field devices in rigid conduit. the distance was anywhere from 25 ft to 200 ft away. his control voltage was 24 volts dc and 110 volts ac.

these devices are photo cells, encoders, and solenoid valves.

these devices are controlled by a programmable controller. the maximum output current from the plc is 4 amps (4 outputs per common fuse).

article 725 allows #18 for power limited circuits of 30 volts or less and 1000 va max.

Question #1.
isn't a 110 volt ac circuit on a # 18 awg stranded wire in violation of article 725 or have i missed something.

Question 2.
what should the wire size be for these devices?
my answer is #14 based upon Nec 2002 article 310.5.

Question 3.
what is the wiring classification of the plc inputs/outputs? class 1,2,3, power limited?

thank you in advance.

 

[tom baker]

Re: wire size for plc control circuits

1. The 18 TFFN has to be protected at it ampacity of 7 amps see table 430.72(B), its the same if its a Art 725 control circuit.
2. 310-16 does not apply if its a Art 725 type circuit see section 90.3
3. You'll have to determine the classification of the cirucits based on the design. However a 120 V circuit would be a class 1. A class 2 or 3 circuit is determined by the power supply.

I wire a lot of PLC's and use 14 or 16 AWG, its a bit stronger than 18 AWG

Here is a link to a good article by Noel Williams
http://ceenews.com/ar/electric_classification_circuits_article/index.htm

 

[don_resqcapt19]

Re: wire size for plc control circuits

725.27(A) permits the use of #16 and #18 for Class 1 remote control and signaling circuits. 725.21(B) permits voltages up to 600 volts for these circuits.
Don Ganiere

 

[jtb]

Re: wire size for plc control circuits

I always used #14AWG, but we were in a hot environment.

 

[pwhite]

Re: wire size for plc control circuits

tom,

thank you for the link.