# PLC/control cabinet wiring methods



## MotoGP1199 (Aug 11, 2014)

I use 18 for a lot of panels and never had an issue. A lot of newer PLC' I/O cards, meters, drives control terminals, etc don't fit 16ga too well in my experience.


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## mburtis (Sep 1, 2018)

That's a good point. Back in the plc 5 days everything was big and there was plenty of room. Landing 16 gauge on a 16 channel controllogix card is messy at best.


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## SWDweller (Dec 9, 2020)

I worked in a place where the internal wiring was cabling with some of the cable deliberately left for spares. It did make a better looking job inside the cable channels. 

Thoughtful layout and separation of power and control is a good thing. I tend to over size as my boxes are all gasketed and power supplies and control transformer do not like it when they get hot. Environment is such venting is not a good idea.


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## splatz (May 23, 2015)

Good question. 

As for wire gauge - I don't think I'd go below 18 gauge, the wires get too fragile and harder to work with. 

I think cabling from the PLC cards to terminal blocks is a good idea, ideally you'd only terminate on the card once when initially installed and do all future maintenance on the terminal block. 

I have been considering just always automatically using two-level feed-through terminal blocks like these. I'd only use the bottom level on the initial install / design and leave the top level for testing and modifications in the future. Make sure the slotted duct is bigger than it needs to be and make sure there's open space on the DIN rails. I don't think you could ever come up with a perfect solution for spare terminals or expansion in general. But, this is simple, straightforward, and doesn't use any space. 

Screw Double-level Feedthrough Terminal Block: 20/pk, gray, 26-12 AWG (PN# KN-D12X) | AutomationDirect


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## mburtis (Sep 1, 2018)

Using double level terminals and leaving the top level empty seems like a pretty good idea. I've been looking at going to doubles or even triples in a few cabinets just to gain some room back.


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## mburtis (Sep 1, 2018)

Does anyone get real wound up about wire colors per nfpa 79 or other standard or just run whatever makes sense because well we can? I try to save blue for dc, white and green obviously for ground and neutral, ac power typically black or red. I'm not opposed to pulling colored control wiring out to equipment though.


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## gpop (May 14, 2018)

Depending on the plc. If its AB then i like to use there pre-wired cards. (weird wiring colors)

As most of my wiring is plc i will use double layered blocks with a joining strip in the bottom. That way i have a row of power terminals and a place for inputs


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## glen1971 (Oct 10, 2012)

mburtis said:


> That's a good point. Back in the plc 5 days everything was big and there was plenty of room. Landing 16 gauge on a 16 channel controllogix card is messy at best.


They make a premade cable for the controllogix, which is about all I ever used after I first found them! Before that was #16 and was a PITA!


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## mburtis (Sep 1, 2018)

The first plc project I was ever involved with used the zip link cables for an automation direct plc. I thought they were great. I'm afraid to ask how much the crooks at AB want for theirs.


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## Jlarson (Jun 28, 2009)

We typically use 18 MTW if we aren't using something like Ziplinks, that's the preferred way, so much quicker.

When we do individually one we take all the inputs to terminals or input SSR's and all the outputs to either thin relays or interface SSR's. AI's and AO's go to a fuse and suppressor at a minimum but more and more all the analog stuff has been going to sig conditioners. People really like the conditioners with the screens where you can see what's up with the signal.


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## paulengr (Oct 8, 2017)

Prewired terminal blocks are the bomb! Even with AB if you consider the labor involved especially if you added all the bells and whistles (fused terminal blocks, adding relays for form C outputs) the cost vastly exceeds prewired terminal blocks. The only downside I can see is that the big fat home run wiring to the PLC IO cards is hard to route and make it look halfway pretty.

The only problem I've had with prewired terminals is this. If you plan your panel in the traditional way you would rarely use them. In other words first make a list of your IO, then select a PLC and IO cards to match it, then look up which ones have prewired terminals in the catalog. Done this way most of the time I might use maybe one or two prewired terminals because for instance it doesn't support CPU's with anything other than basic 24 VDC inputs and outputs, AC I/O cards, or high powered 24 VDC cards. BUT if you work the system backwards and select the prewired terminal blocks that match your IO, and then select the CPU and IO cards to match THAT selection, you can probably use prewired terminals 90% of the time or better.

As to single level vs. 2 or 3 level IO's I've always done single level in the past but after seeing another scheme I'm switching over to 2 or 3 level. Say for instance I have a bunch of 24 VDC inputs. Typically I'd have a row of 24 VDC outputs, a row of commons (for 3 wire devices), and the inputs themselves. If I use traditional approaches I'd use a bus bar strip to connect all those 24 VDC and common terminals together. Incoming wiring has to be split and routed as required. But with a 3 level terminal block I have a row of 24 VDC, a row of commons, and a row of inputs all together. The cabling does not get spread out. AC outputs work the same way.

I don't really like the NFPA 79 color scheme. It's a mess and field wiring is NEVER going to be color coded the way they want it. But I do quite a bit of color coding. I try to make all my 24 V terminal blocks blue and use grey for everything else which mostly means AC is grey. I suppose I could take it a step further and make the analogs yet another color. With the terminal blocks I use grounds are green. I do use the European color coding for 24 VDC where +24 VDC supplies are brown and basically everything else is blue. It's kind of opposite of the 120 VAC wiring where the neutrals are all white and grounds are green (with either voltage) but again it helps easily identify things.

The other big thing is that I'd say most of the control panels I've worked with or seen or built over the years fall into one of 3 design categories. First there is the "remote IO" panel, whether it has only remote IO or even a PLC is immaterial. There is little to no power distribution in the panel. All it contains is blocks of IO protected from the elements and say a control breaker or two and some fuses and terminal blocks. That's it. Everything else is external. Stepping up from there are the traditional "PLC panels". These typically don't have 480 V in them but they do have massive amounts of IO. The panels themselves are free standing 6 to 8 feet tall single or double door, between 36 and sometimes I've run into some that are over 12 feet wide, often 12-24 inches deep with trays underneath or all the field wiring on one side with a ton of conduits out either the top or sides. Third is the pump panel. This has 480 V running directly to it. The PLC, IO, power, everything is all in one. Everything is very compact. In the case of the traditional PLC and the "remote panel" style ones, you will usually find either an MCC or a panelboard or both for 480 V power, plus a 480:240/120 transformer, and a 240/120 distribution panel.

I've taken to building a lot of machine control projects lately using more of the pump control panel layout. Contactors have gotten so small that you can fit a massive amount of starters in a control panel instead of an MCC. The only thing I try to do is push the transformer into a sealed weatherproof design outside the PLC panel and push all VFD's into an external panel if they get to be too many. The goal is that everything in the PLC panel does not generate very much heat. The VFD panel gets its own cooling system. This layout does all the power distribution and MCC functions in the same panel making it a much more compact design, without compromising wiring space. If I reach a point where I need to go to very large panels, I'll just separate some things out and go to a remote IO panel design.


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## gpop (May 14, 2018)

On analog inputs i now make the extra effort to add everything i need for loop power even if that wastes a little space. A lot of instruments that required power are now being supplied as loop powered devices which can be a pain when there is no space available on the din rail.


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## splatz (May 23, 2015)

The multi-levels I posted above the top is connected to the bottom, it's just more terminals in the same DIN rail space. I have also been using these, top, middle, and ground: 

Screw Triple-level Feedthrough Terminal Block: 20/pk, gray, 24-12 AWG (PN# KN-TG12) | AutomationDirect










I like these for the power supplies - it's a compact install, easy to wire, hard to cross up one with another. 

As for color coding, I am pretty close to giving up on color coding terminal blocks, just use all gray and color code the wire and the labels. Color coding the terminal blocks makes inventory too hard. 

I could possibly see using the two-level that are connected top to bottom in gray and the ones that aren't in black or something like that to keep it easy to tell which is which. 



paulengr said:


> As to single level vs. 2 or 3 level IO's I've always done single level in the past but after seeing another scheme I'm switching over to 2 or 3 level. Say for instance I have a bunch of 24 VDC inputs. Typically I'd have a row of 24 VDC outputs, a row of commons (for 3 wire devices), and the inputs themselves. If I use traditional approaches I'd use a bus bar strip to connect all those 24 VDC and common terminals together. Incoming wiring has to be split and routed as required. But with a 3 level terminal block I have a row of 24 VDC, a row of commons, and a row of inputs all together. The cabling does not get spread out. AC outputs work the same way.


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## Jlarson (Jun 28, 2009)

We just use all grey, I think all the different blocks looks a little odd. Only exception is blue for IS.


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## mburtis (Sep 1, 2018)

I really like the idea of using 2 or 3 layer blocks and essentially setting them up as busses. Seems like a great way to keep the incoming wiring to the panel condensed and clean.


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## CoreyOnTheBrink (Apr 9, 2021)

Size the box slightly larger than you think. Unless wall space is slim there’s no reason to try to cram everything. Cut and mount din rail to span complete from duct to duct, even if you’re not going to fill it. Follow manufacturer directions on spacing components. Leave extra room on your terminal din rail for future installation of more. Install extra positive and negative out terminals for future field devices. Only use the thin-fingered wire duct. Like Splatz linked, the double-stack terminals are handy space savers. Either mount your terminal din rail on the angled stand-offs (check AutomationDirect) or leave plenty of space below them for the field wiring install. Use cable ties around duct if needed to keep wiring in place. Obviously my pic was a work in progress but even at this point I was proud of the cabinet.


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## LARMGUY (Aug 22, 2010)

From back in the day...I tried to keep some semblance of uniformity but what really saved the day with 5 different techs playing inside was the assistance upon labeling both ends of the wires and cables and using an alpha numeric code I cannot remember now, LOL. Give me five minutes with that panel and I could figure it out by refreshing my memory of the code.










As I remember the PLCs were for man trap doors and special timed bank vault features such as hydraulic floors swinging out of the way and hydraulic locks for vehicle gates to open then the gate can open. Then a vehicle drives in and trips a loop causing the gate to shut. Then and only then will the next gate start it's opening procedure.

Some really grainy pics of add on panels


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## SteveBayshore (Apr 7, 2013)

CoreyOnTheBrink said:


> Size the box slightly larger than you think. Unless wall space is slim there’s no reason to try to cram everything. Cut and mount din rail to span complete from duct to duct, even if you’re not going to fill it. Follow manufacturer directions on spacing components. Leave extra room on your terminal din rail for future installation of more. Install extra positive and negative out terminals for future field devices. Only use the thin-fingered wire duct. Like Splatz linked, the double-stack terminals are handy space savers. Either mount your terminal din rail on the angled stand-offs (check AutomationDirect) or leave plenty of space below them for the field wiring install. Use cable ties around duct if needed to keep wiring in place. Obviously my pic was a work in progress but even at this point I was proud of the cabinet.
> View attachment 157512


Nice and neat but how come I can't see any numbers on the terminated wires? Is it built for job security?


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## just the cowboy (Sep 4, 2013)

mburtis said:


> Using double level terminals and leaving the top level empty seems like a pretty good idea. I've been looking at going to doubles or even triples in a few cabinets just to gain some room back.


Avoid double or triple if it is not the same wire. getting to the bottom is a PITA if it has a wire on top.


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## CoreyOnTheBrink (Apr 9, 2021)

SteveBayshore said:


> Nice and neat but how come I can't see any numbers on the terminated wires? Is it built for job security?


Hmmmm… someone must have taken them off.


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## just the cowboy (Sep 4, 2013)

CoreyOnTheBrink said:


> Hmmmm… someone must have taken them off.


No they were wrap arounds and they fell off.


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## wiz1997 (Mar 30, 2021)

Here's an example of the color code the manufacturer, that built a majority of the equipment where I currently work, used in the control panels.
These machines were built in Holland.
The PLC's are wired with grey wire on both inputs and outputs.
There is also NO wire markers on any of the wires, other than those that leave the enclosure. Which is why the pandit covers are always off or missing and the panel looks like a rat's nest.


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## SteveBayshore (Apr 7, 2013)

Boy, they spell funny in Holland.


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## paulengr (Oct 8, 2017)

Sounds like FAA rules. All control wiring must be white and hand stamped with a wire number every 3 feet. Good luck finding anything,


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## mburtis (Sep 1, 2018)

Some of these color codes make me laugh just a little. The majority of our stuff from the early 90s is just pink for 90 percent of stuff, yellow is foreign voltage, blue is dc. Apparently pink was a cheap color in the late 80s early 90s, the big hydraulic press I helped install built in 89 was all pink too.


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## GladMech (Sep 18, 2020)

I couldn't find any specific amperages in the NEC, just permission to use smaller gauge in industrial control panels. However, UL508A has a chart for small gauge wire.


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## gpop (May 14, 2018)

paulengr said:


> Sounds like FAA rules. All control wiring must be white and hand stamped with a wire number every 3 feet. Good luck finding anything,


I work building custom spot welders years ago and the company had a wire printer that also did a single color strip. It was something they built in house. 

Load a roll of white wire then select a program and hit run. Come back and you have a pile of wire numbered and cut to length. 
One of my jobs was to build the first unit then after quality control i would strip it back to bits and measure every single wire for the printer/cutter.


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## mburtis (Sep 1, 2018)

I've gone back and forth on wire colors. Highly detailed color codes may make sense for oems. From an inventory and simplicity point it seems a more basic approach with good labeling would be just as good, ie black for ungrounded conductors, white for grounded conductors, green for ground, maybe blue for ungrounded 24 v. 

Safety wise it's an interesting topic. If it was truly universal than complex coloring may improve safety, such as if you see yellow you know it's foreign power, blue is low voltage,etc. However nothing really mandates any of this so there's nothing keeping a guy from running 120v in blue. Which could cause an issue if somebody just started messing in panel without looking at it close enough and just assumed blue was low voltage. If you open a panel and it's just black, white, and green you would have to take the extra 5 min to study the wiring. Good labeling would be essential which rarely happens.


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## paulengr (Oct 8, 2017)

Blue or blue/brown is what I see most for 24 V.

Analogs tend to be black/white or black/red and/ir with white.

Brown/orange/yellow is almost universal for three phase in the US even in plants with 240 V systems. But since control will be #14 or smaller and not paired with orange and yellow it’s easy to figure out.

Black is pretty universal outside control panels but inside I see more red for AC than any other color.

In the van I keep: some larger gauge wire mostly for jumpers or repair, all blackjack; black/white/green #14, red #14, blue and brown #16, some 4 conductor shielded signal conductor for analogs, RTDs. I used to keep more but it takes a lot of space and something I can rotate out.


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## Jlarson (Jun 28, 2009)

We use 79 colors on builds, field wiring pulls are a diffrent story, any color on a spool goes then lol


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## SteveBayshore (Apr 7, 2013)

When we build cabinets we always have scraps of 16 gauge tray cables with 7, 12, 19, 30, or 37 conductors, all colored. Our scraps are 100' long minus. If more than 36 conductors are needed, we'll color code separate bundles. We used to stock colored spools with tracers from Manhattan Cable but that became too expensive 15 or so years ago.


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## MoscaFibra (Apr 15, 2021)

just the cowboy said:


> No they were wrap arounds and they fell off.


I remember using the self laminating labels the first time, and loving them. At some point, they started to suck and now I hate them.


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## just the cowboy (Sep 4, 2013)

mburtis said:


> I've gone back and forth on wire colors. Highly detailed color codes may make sense for oems. From an inventory and simplicity point it seems a more basic approach with good labeling would be just as good, ie black for ungrounded conductors, white for grounded conductors, green for ground, maybe blue for ungrounded 24 v.
> 
> Safety wise it's an interesting topic. If it was truly universal than complex coloring may improve safety, such as if you see yellow you know it's foreign power, blue is low voltage,etc. However nothing really mandates any of this so there's nothing keeping a guy from running 120v in blue. Which could cause an issue if somebody just started messing in panel without looking at it close enough and just assumed blue was low voltage. If you open a panel and it's just black, white, and green you would have to take the extra 5 min to study the wiring. Good labeling would be essential which rarely happens.


If you follow the NFPA79 color code and do matching terminal stripes it works great. YOU are the owner of YOUR system. Make it the right standards and hold others to it. It will save time troubleshooting and be safer.


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## gpop (May 14, 2018)

If you want to make life a little easier stick a copy of the print to the inside of the door as well as a drawing of the relay bases being used including the pin/screw numbers. 
Honestly nothing worse then having to climb in the panel to see the nearly invisible numbers on a relay block then working out what wires go where especially on timer relays. 

Years ago we use to carry a little book someone before us made (small ring binder). It had the relay wiring diagram on the left page and the base on the right page with a idiots guide.


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## just the cowboy (Sep 4, 2013)

gpop said:


> If you want to make life a little easier stick a copy of the print to the inside of the door as well as a drawing of the relay bases being used including the pin/screw numbers.
> Honestly nothing worse then having to climb in the panel to see the nearly invisible numbers on a relay block then working out what wires go where especially on timer relays.
> 
> Years ago we use to carry a little book someone before us made (small ring binder). It had the relay wiring diagram on the left page and the base on the right page with a idiots guide.


I can’t stand when you order relays and bases from the same company and pin and base numbers don’t match.


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## mburtis (Sep 1, 2018)

just the cowboy said:


> I can’t stand when you order relays and bases from the same company and pin and base numbers don’t match.


That would be incredibly annoying. I can never see the numbers on the bases with the relay in anyway. So then you pull the relay out and discover what it does...


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## mburtis (Sep 1, 2018)

When they upgraded the plant plcs a couple years ago I got to talking to the senior engineer that helped build the plant back in the 90s. We got talking relays and he was talking about how back in the day they used to have these huge books from the relay manufactures giving all sorts of data on contact ratings and other design values dependent on mounting orientation and all sorts of other details. My old school nerd brain wants to find one now.


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