Wiring to an MCC with Type IIB or IIC control wiring

[nogienugz]

Hi,

Got a question on bringing a multi-core control wiring from the PLC cabinet to MCC with type 2B or 2C control wiring. It says in the definition of NEMA that there is interconnection between control terminal blocks among different buckets in the MCC. Does that mean that neutral/common terminals of the buckets are interconnected (daisy chained) to the neutral/common of the master control terminal block in the horizontal/vertical wireway?

If yes, then, will it be OK to just bring 1 common/neutral (just used 1 core from the multi-core cable ) from my PLC panel (say 1 common from 120 VAC signals) and terminate it to the common of the master TB instead of bringing in 1 common to each of the buckets?

Thanks,

 

[controlsdude]

I have had this discussion so many times with field equipment wiring. I had one horror story where electricians had tied one field area to one neutral. Caused a lot of problems with too many devices on one neutral and all the splices were loose which caused more problems. I had to go back and run a few more neutral wires for different areas.

You can daisy chain a neutral up to a point of it carrying up to its wire size amperage.
Example: If the neutral is shown on a plc card and lets say 16 devices. I would wire these 16 devices to one neutral if the amperage is not exceeded on the neutral.

Another thing to consider is if some areas of the prints have different breakers for certain areas of prints. Its better practice to run a neutral with this circuit breaker devices. If some engineer has shown it that way on the prints they probably went thru how many devices can fit on that breaker, hence the neutral would be pulled for these devices and daisy chained the neutral for these devices.

Does that make sense?

 

[nogienugz]

Hi Controlsdude,

I get your point about overloading the capacity of the neutral and who easy it is to mess things up there is one neutral for all those PLC signals.

I haven't wired it myself, I usually deal with type 1B MCC's where control TB in the buckets are not interwired. But going back to my question, for type 2C and 2B, is the common/neutral interwired among buckets?

When you say prints, do you mean PLC wiring diagrams?

Let me give a simple example:

Say for 120 VAC inputs, they "usually" allot 1 breaker/fuse per 1 PLC input card (16 addresses). But I can fit 6~8 buckets in (1) 120VAC input card. E.g. assuming everything is hardwired to the PLC, [1 FWD, 1 REV, 1 FAULT, and other statuses, ects,]. So can I run 16c-#14 awg multi conductor cable and just use 1 core for the neutral?


Cheers,

 

[jraef]

I think that you are conflating the NEMA wiring designations with the concept of “separate control”, they are different issues. Class II just means that the factory provides any inter-wiring between units and drawings show those interconnections as well as field connection points. With Class II wiring, the only time you need a neutral terminal wired out to field devices is if those devices need a neutral, for example external pilot lights or solenoids.

Type B means that this is done just to the terminal blocks in the buckets, but it is only with Type C that you get a master terminal block in each section (or sometimes to a “marshaling cabinet” at the end of the lineup). The factory per-wiring to those master terminals would only be for any required field connections to the buckets.

Aside from that is the issue of where the control power comes from for each bucket. It must be one of the following: “common control” in which the control source is derived from the line power in each bucket, ie 480V coils and control circuits (which is virtually unheard of now); “unit CPTs” (control power transformers) in which each bucket has its own CPT attached to its line source; or “separate control” in which buckets get their control power from an external source, such as a panel board or large transformer inside or outside of the MCC, then brought into each bucket through a door interlocked aux contact of the breaker or disconnect switch.

So the only time you would have a common neutral for all buckets is if you are using separate control. If you have Type C wiring and you have field devices that require neutrals, you would have a row of common neutral terminal blocks so that you have enough places to land individual wires. But if you have unit CPTs, each control circuit is like a “separately derived source” in that the neutrals are all current carrying conductors of separate transformers, so each set of master terminals from each bucket must have its own separate set of neutrals for control devices in the field that need it.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[nogienugz]

Hi Jraef,

I'm aware that with type IIB and IIC, there is a pre-wired interconnection between control TB among the MCC buckets. I also know that it's only with Type 2C that you get a "marshalling" section at the end of the line up. I also know that CPT for each bucket has its own neutral to power the pilot lights, coils, etc in a specific bucket.

I think I'm not being specific about what I'm trying to ask here so I uploaded on of the schematic diagram drawings just to make sure that I'm getting my ideas across. Let's just focus on the 120VAC MCC outputs to the PLC (so PLC input in the drawing) and say that my MCC control wiring is TYPE 2C. Let us just focus on the control TB. See those TB that are jumpered? (4, 5, 6). They are the common TB for the FWD, STP, REV statuses of the actuator. That's one bucket. Let's say you have 10 of these loads, now in a type 1B, I would have needed 1 conductor for each MCC bucket as a common, so 10 conductors in all.

But for Type 2C wiring, can I just use 1 conductor (common) for all the 10 buckets, that is, 1 conductor for all those status signals? Does the master control terminal block in the marshalling section have an internal wiring (pre-wired by vendor) that connects a common terminal to all those MCC buckets? BTW, these internal control TB wiring is not shown in the Vendor drawings, and I'm looking at Eaton's drawings.

Cheers,

 

[jraef]

Whatever you tried to attach didn't work.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[nogienugz]

Hi,

I took a picture and tried attaching the schematic and wiring again. Let me know if it worked.

Cheers,

 

[nogienugz]

Hi, ideas anyone?


Regards

 

[jraef]

Still too hard to see clearly. But certainly if the "common" you are referring to is the 120V common of a group of inputs on a PLC card, you can run it just once. But if the commons on the PLC cards have separate fuses, you can't do just one common for everything.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[nogienugz]

Thanks for the input Jraef. Will go to site and talk to the electricians tomorrow, we'll see what's actually there and will also check how they wired their PLC inputs.

Cheers,