725.45 OCPD of motor control conductors.

[Dsg319]

I have 1 single phase 120volt (20amp OCPD) motor starter where the control for it is tapped from the line side of the contactor.

I have to run this class 1 control circuit back to the PLC to go through a relay to be controlled remotely from a PLC output.

Can I use 16AWG as long as the load does not exceed it’s listed ampacity. Or am I going to have to use 14AWG.

I read 725.45 (C) and I see that with 14AWG and larger I’m allowed to have a tapped conductor from the load side of the branch circuit as long as the rating of the OCPD does not exceed 300% of my conductors ampacity.

 

[Dsg319]

Bump

 

[augie47]

Based on the wording in 40.72(A),, It appears to me that your circuit would fall under 430.72 not 725.

 

[Dsg319]

Thank you I will read it.

 

[tom baker]

Some motor starters have a fuse kit for the control wiring , this takes you from the rules in 430 to 725 , then you can use 16 awg with a 10 amp fuse

 

[paulengr]

Need to follow chapters 1-6 as this is not a power limited circuit. I have no idea why PLC people seem so enamored with chapter 7. It’s easier to use it only if you can’t meet chapters 1-6 not the other way around. Please reread NEC introduction.

Chapter 7 ONLY applies to power limited circuits. If there is no current limiting power supply or you can’t find one, it’s general purpose wiring, period. If there is even a question, it’s not chapter 7.

Normally you would see this wire (or both hot and neutral) go to a fuse block then to the control wiring as the power source. The short jumper falls under NEC tap rules, 240.21(B). Generally this would be a tap less than 10 feet long that does not extend beyond the equipment.

You are extending beyond the equipment so you have three choices. You can locate an isolation relay inside the starter enclosure and use this to control the control power locally. If you are worried arc shock or arc flash just use a 24 VDC coil operated from the PLC. Or the second choice is add your own control power fuses and do the dry contact control on the PLC side.

If there is no room to add either one you have only the third choice. Disconnect the controls from local power and refeed it from your control system. Be careful of the required control power current (VA rating). These days larger contactors are largely electronic but older ones can take 10 A or more to close so you may need a substantial isolation relay or even a smaller contactor to operate it from a PLC. As a rule I try to just use 10 A outputs and check ratings only if the contactor is size 5 or larger.

What you cannot do is use the system as is unless another tap rule allows it.