Ground and Common sizes

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Thread Starter

ChuckyIII

I will be wiring 9 outputs in a J-box. These outputs are 115 vac at .2 amps each. It's possible that up to four of these will be energized simultaneously. Each should be grounded. The question is how many and what size grounds and commons should be run between the J-box and the control panel? (a distance of about 15'). I also have 8 inputs (24vdc) that i'd like to wire in the same J-Box and would like to know how many commons i'll need for that. Any help/education would be greatly appreciated.
 
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Bob Peterson

There is no conceivable way anyone can give you an absolute answer to this question. There are all kinds of things that might affect the answer.

However, based on NEC, and UL508a type standards as a bare minimum you would need:

A single ground wire sized according to NEC table 250.122.

If the solenoids are all on the same branch circuit a single common wire. The size of the common wire is based on the ampacity required. This can be calculated from NEC table 310.15(B)(16). pay special attention to the note under the table with double asterisk.

if the 24VDC inputs are all on the same power supply, only a single common wire needs to be run to the junction box.

--
Bob
http://ilbob.blogspot.com/
 
With little detail as to your wiring diagram, here goes....

If your outputs have a common power source - say a PLC output module, I would fuse the outputs individually to protect the module, if nothing else. Given the load power, #18 wire would suffice for each load wire. If the common/white serves all loads, it has to be sized to carry the unbalanced load. If your outputs are fused at 0.5A, a #18 white would be sufficient. I would pull in a #16 to allow for future modifications.
 
FWIW, only one grounding conductor is required and it is never required to be larger than the largest circuit conductor. If using a metallic raceway wiring method, in most cases the raceway can serve as the grounding conductor provided sections are solidly connected (most notable restrictions are flexible raceway types over 6')... making a wire grounding conductor redundant, but it must still comply with NEC requirements for minimum size.
 
Thank you for the advice. I'm no electrician, but was confident that they could share the commons and ground. That said, this PLC has on board I/O rather than an output module. Further, it uses internal relays and there is one common per (4) relays. I'm sure it doesn't matter, but wanted to throw that out there just in case. The reference to the code above doesn't help me much as i don't have access to it unless it's publicly posted? I will definitely go with the #16 common as i have plenty of wire and conduit room.Is it commonplace to fuse each individual output from a PLC? Seems strange that PLC mfgs wouldn't have protection built in.

Thanks again guys.
 
It's normally a good idea to fuse outputs from the PLC because most newer, modular units offer no protection for individual loads. If your loads are solenoids for example, problems can occur if the solenoid gets stuck. It can burn itself up trying to activate. This current outrage can burn out the output point on your processor. An interposing relay is cheaper than swapping processors. Have the processor output turn on the relay. Run the fused power to your load thru a set of normally open contacts. A little more work and expense perhaps, but this way, worst case is change a fuse.
 
Thanks again guys. All very good advice. All of my outputs are solenoids. I've also read that transient voltage spikes could result from de-energizing the coils and that components should be used to mitigate the possibility. The suppressor works out to be only about $4/output. I guess better safe than sorry. Right?

Anyhow, thanks again for all the information guys. I wouldn't have been able to do this efficiently without it. Off to the shop to implement this today!
 
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