Stage Pin, Wiring and a bit of history about them or at least as much history of it's development as I am aware of at this time: There are four main types/periods of stagepin/slip plug design. I will refer to it as old style Union Connector co., but Kliegl, Century, Lighting Equipment Co. And VN also made them amongst other manufacturers in similar or unique styles. Now there is two sizes of stage pin plug used the 20 amp and 60 amp, back when this plug designed there was four sizes of stage pin plug - the 10 and 30 amp types having been discontinued in use. The first stage pin plug was designed before ferrules and crimp on ring terminals, simply put it was a set of pins in plastic or fiber of some sort, that had slight recess pathways molded into them for feeding the wire and cover over it. Nothing like even the newer Union style plug which was a molded plastic/Bakelite material with at least some room for wiring much less terminals. The materials used were not overly strong so most of the body of the plug was bulk thickness of material for strength. Not a lot of space inside the connector for wires, space between conductors or strain relief on the main 12/3 grade S cable fitting type. The strain relief and its hole offered little grip to this type of cable once the cable was warn in and shrunk slightly and as such it was frequent that the jacket would slip out of the strain relief, much less conductor insulations would also frequently be stretched back exposing conductors to each other that with some twisting of the cable that was also easy to do could short the conductors. Conductors could short even if not stretched or the cable twisted, they could short if the jacket were cut back to far and there was moisture added to the too short a space between conductors or if the conductors heated up and flaked off also exposing conductors to a short. Also the pins of this type of connector might have been a little softer because it is more common for the heat expansion slot in the pins to be pushed together or the entire pin itself bent over. A further problem of this time period was there was aluminum conductor instead of copper wire available for use in the theater as a cheaper alternative to copper, and those who use it with a brass pin and screw quickly found galvanetic oxidation problems - dissimilar material problems with touching and passing a current between each other which would oxidize and cause high resistance connections. Aluminum wire since has fallen out of usage thankfully but at the time it was about almost common to find. The outer jackets were similar and you never knew if the cable was copper and rated for full load or aluminum and rated for less of a load, much less what would cause a fire for you under heavy loading. I expect such connectors when new came with a sort of wire cavity washer which allowed the wire to fit and be supported within the cavity and be made into a circle around the screw much similar to Bates connectors coming with ferrules. Such wire washers are not exactly the easiest thing to find on the market and as such wires were at best tinned which would work to some degree in supporting the wires under the pressure of the screw terminal onto it, given some potential problems with cold solders and overheating of the insulation over the wire causing high resistance connections and shorts between conductors. Still it was better than just inserting the bare wire alone into the screw terminal than hoping the screw head would not sever conductors, apply uneven pressure on the bulk of them or allow for a loose terminal once the conductors settled in. Stranded wire has different grades. Building wire while individual strands are a lot larger and fewer than what is on a cable. They can withstand more abrasion and twisting from the screw head, but because the strands are larger, it’s also less flexible. Cable is made with smaller strands so it will move about in general without wearing away at them. It just does not work well without support. This is all given the screws themselves did not come loose be it from strands coming loose making the tension on the screw not sufficient for it to become loose, or just the screw coming loose on its own with or without enough tension. In addition to issues with the wire cavity and how conductors are attached, the plug while well designed enough in becoming a industry standard two pin or three pin, layout, its strain relief cavity was far too close to the attachment point, and only designed to grip a 12/3 grade S cable. Even than it would slip. For this reason, even on the proper wire friction tape was used to bulk up the wire size to make it hold. For smaller gauges of wire, even a 12/2 grade S, there would need to be extra bulking up of the wire thickness to make it hold. Hopefully it was applied correctly or it would slip even still. 12/3 SO is almost 5/8" in dia. Try to insert a 16/3 SJ fixture wire into such a plug and you would have to make up for the difference in a 5/16" cord with something. Could be a jacket on a old cord, could be friction tape, could be almost anything that might or might not work. This plug if done to the old timer standards with trained technique can be safe and rugged to use, but without such experience passed on, can also be very unsafe to attempt to use. An alternative design of the original Union plug was for individual un-fiberglass sleeved asbestos fixture conductors that fed in thru individual holes in the back of the plug. This provided individual pathways for the wires with defined barriers between conductors. The asbestos wire alone was considered to be sufficient alone without any sleeving over it to protect the wires, so it was possible to feed them into the back of the plug from separate holes. Not very workable with modern cable, but it was a good idea except if the wire was braided or banded together, at which point the flexing the strands would have to bend around a sharp corner at the back of the plug that would abrase the wire’s insulation. Other than that, this was a really good plug which was probably the original concept for the single hole version since there is a lot less space in the single hole version. However due to the lack of un-sleeved conductors, such a style is not found anymore. It also has the same problems of strain reliefs not being designed for other than one style of wire - thus needing help to grip the conductors. There was at least a lot more space between terminal and strain relief. A final incarnation of this plug will have been for twofers with individual very thickly insulated wires braided together and exposed. Such things still exist no doubt 20 or 30 years later in some companies a testament to the design and the very good wire used. To wire this plug, you removed the insulation off about 3/4 to 7/8" of wire, than cut 1/8" off the neutral and 1/4" off the ground if there was one. Next right out of the jacket you would fold the hot and neutral at a 90° angle to the jacket, than at 3/8" and ½” bend the conductors back towards the plug’s terminals. This would make a fork shape that would fit within the wire well fairly well with only slight adjustments needed. You would than strip 1/4" to 3/8" of insulation off the wire and form it into a circle for either insertion into the wire washer or tinning. Before the wire was actually inserted into the connector a few layers of friction tape would be placed over the jacket of the plug to make it form up. The new style Union connector that was bakelite a sort of porcelain/plastic type material was a bit more brittle but were more ridged enabling the casting to be less bulk and more room inside the connector for wiring even crimp terminals. Not a lot of more space since the plug was not made longer for more room between strain relief and terminal, it was kept to the same relative size as the original version, but at least the grip on the strain relief was slightly improved and there was room for crimp terminals making for a much safer connector. With some work you could also remove the pin to replace it or in making a grounded plug un-grounded without damaging the plug. Both grounded and ungrounded plugs were available with either age of plug in the end of the old style. The three hole individual conductor Union plug was also carried over into the more modern style and could be found up until asbestos wiring stopped being used and was replaced by cords inside fiberglass sleeves. To wire this plug, as with the above, you wanted to form it into a fork shape, but in this case because you were using crimp terminals, the terminal would do this and your wires were shorter. You would strip no more than 5/8" to ½" of wire, than cut 1/8" off the neutral and1/4" to 5/16" off the ground. You would than fold the neutral and hot over at 90° and strip 3/16" of insulation off the wires. Flag terminals which are ring terminals with the hole in them directly perpendicular to the axis of the wire so they make a sort of flag on the wire along with a stubby ring terminal for the ground were than crimped onto the conductors. You wanted to push the crimp into the insulation by about 1/16" so it bunched up around the crimp and provided some extra insulation for cable stretching/shrinking needs. This also meant that the ground in the center of the wire would about have the insulated terminal touching the outer jacket of the cable if done properly. If you made the wires too long, because the strain relief was so close to the terminals, the jacket would pull out of the strain relief and the conductors would rub up against the sharp edges of the strain relief hole. Unfortunately, the ground crimp might dig into and short out the other conductor’s insulation at this point since it about touched them, but that was a detail still less important than the extra safety improvement this type of plug had over the older style. Once well crimped, the wires would be electrically very safe because they were no longer what was under tension by a screw, nor was heat involved in making them, plus there was a little more room between the stripped portion of the wires. No more forming the stranded wire into circles than attempting to put a wire washer around it, or figuring out how to tin them without destroying the insulation etc. Just a question of getting a proper crimp onto the terminal - especially the flag terminal for finding a crimp tool to do it, or a proper crimp tool for the ground that often would carry all of the strain relief duties for the plug. Wires slipping out of the ground ring, improperly spliced - instead crushed crimps was a common problem. Don’t remember what my teachers used, but it took me years to find a crimp tool that would crimp the terminal properly and not crush or bend the flag part of it. Out of this reasoning, the screwing the crimped terminals to the wall technique and climbing it to test, even soldering the crimp might have sound reasoning behind it’s use in testing. Most people had hammers, and pliers, perhaps a wire stripper, but certainly no Klien #1005 crimp tool with the front cutting tip cut off so it would not damage the flag part of the crimp. Having found my Vatco #1900 tool, I quickly forgot how those that taught me to wire such plugs did it themselves be it proper Stakon tool or smashing it. I knew I needed a well defined tooth at the end of a crimp tool to crimp the terminal, but had no idea of what to use or where to find one. Smashing the crimp terminal while done is not the best option, it will not apply even pressure to all the strands of wire and will often open up the seam in allowing for individual strands to fall out of the crimp and cause a higher resistance. If some strands are not supported but other are, when the wire is pulled one of two things will happen. Either the conductors will resettle allowing more than all the conductors to fall out of the crimp, or those strands with too much pressure on them will break because there is too few of them under high crushing pressure to carry the weight applied in pulling them. Wires falling out of a crimp is nothing new flag terminal or not. I have some professionally built AC Distro racks that use flag terminals with wires falling out of the crimp even with the proper crimp tool used - just without sufficient pressure applied to the crimp. A few hundred such crimps per rack that have to have every wire tugged on now to verify they will not just fall out all by themselfs. While at least you could get the crimp terminals for this plug, they were not cheap or readily available thru a hardware store. Tech departments would perhaps buy a hundred at a time, but no more during the budget year. In other words, in spite of the better way, companies would naturally fall back to tinning the wire and or shoving the wire under the screw heads and hoping the wires did not arc too badly. Given the electrician put any forethought into what they were doing at all as I still constantly see these plugs in service with wires wrapped just around the terminal. When ever you inherit and become responsible for any slip plug, it should be opened up and inspected for proper strain relief and connection. The industry has far too widespread unsafe slip plugs in use in it - new or old of any style. New plugs designs were needed and well known to all as necessary or the industry might just switch to twist lock... as it did about this same time. My experience with the improved stage pin plug was in about the mid eighties though it might have been introduced later. Pinecrest given that’s the actual brand name, I believe was the brand I first saw, than Rosco amongst other companies developed a style of their own. While the plugs were longer, they were more shapely in design so the right angles that snagged on everything were removed from the plug. There was also all kinds of space for proper separation between conductors, conductors that would use the much easier to crimp ring terminal such as on the Union’s ground pin or even the more normal longer ring terminal, and still allow extra room for an improved strain relief. On later forms of this style it was one that could have a strain relief block inserted into it for smaller wires, or even use a Heyco fitting for other sorts of small cords including zip cord. The pins were stronger and much easier to take out of the plug for replacement. Screws were switched from flat head to the much easier to use Phillips screw with even a external lock washer added to prevent the screw from coming loose. Along with time, it also came further stronger and improved materials used for the body and screws in the area of the strain relief that were closer together for more strength and less bending. The front of the wiring cover was also inserted into a self holding recess so it would prevent bending and add strength, if not later a third screw was added to the extended length plug. Vastly improved plug that could even have clear plastic covers applied over the terminals so you could observe what condition they were in. Great improvement to the design, much more expensive unfortunately in the end, but good improved design if the strain relief were a bit less than ideal, and it still took a proper crimp tool to ensure wires would not come loose. Such plugs are still widely available and useful. Union further improved on their old style plug types to follow suite with the same extended length plugs with extra room for wiring and normal ring terminals, however the plug still has right angles to it’s design which snag on stuff when you pull on the cable. To install this cable onto a plug, you will want to strip 1" to 1.1/4" of jacket off the cord and cut 1/4" off the ground if not 1/8" off the neutral and 5/16" off the ground. This when the wire is folded into a fork shape to fit within the channels, will allow the wires to be the proper length without any extra slack in the wire which is hard to align. As with the above flag terminals, strip about 3/16" off the insulation on the conductors, and shove the ring terminals onto the wire so the insulation is pushed back and bunched up around the crimp terminal. This will allow for a little extra insulation to be available should the insulation on the conductor be pulled. Crimp the terminals with a Stakon tool sufficient to hold should it be tested in the above screwing to the wall test, than install it into the terminals. Normal tightness of screw terminals would be 1/4 turn past hand tight meaning 1/4 turn past what is tight without holding the screw driver with a huge amount of force - what is sufficient to hold the screw driver easily, than 1/4 turn more with some force. Advanced Devices/Bates further refined the design and became the industry standard for stage pin plugs. They designed a very good plug that was tapered in shape so it would not snag, had three screws for more support of the cover, and a set of double strain relief blocks that fit into a channel allowing it to fit and adjust to just about any type of wire as long as you were able to read the directions on the package and make some general choices of proper tension off that. It also featured ferrules - tinned copper tubes which did not necessitate a proper crimp tool to install, of which fit into a set screw opening instead of having to wrap around a screw. Much easier for installations in the field with no more than a wire stripper and Phillips screw driver. The plugs were initially cheap to buy and the ferrules for them were cheap enough by the hundred that anyone not lazy could keep them in stock. Ferrules are the imitating factor in this plug, easy to loose, and rarely in your pocket on a ladder, but in the end no different than not having any ring terminals in stock and clamping directly to the wire with it being bad practice. There was even a hole designed into the plug the Rosco type followed so you could insert a tie line thru the plug or bolt groups of plugs together. Much better design, one that a few other companies have copied the design to and “improved” in materials, than moved off shore to make their own very cost effective alternative sold a little more cheap in price yet. The bates plug given a larger size than the old Union with unsupported space in it’s body has been known to develop puncture holes in it, or crack under too much tension on the strain relief. It like all plugs breaks due to the higher hardness plastic necessary for thinner walled plugs, this is the primary thing improved upon in other companies designs for the plug. Often you do get what you pay for in the alternative Bates style plug however between strain reliefs which are not universal parts - thus if you mix brands though the strain relief insert looks the same, a Bates strain relief will not fit inside a Offshore brand plug, and the plastic screws holding it together are slightly smaller in shaft dia. This means the plastic screw holding it together breaks easier on installation and if you use a Bates plastic screw on it, you will not be able to re-install a offshore plastic screw into the plug - it will be stripped out. Overall, most parts such as the ferrule, black oxide plastic screws, and brass screws on the bates plug are readily available thru McMaster Carr or other sources. Should your plastic screw holes strip out, there are other plastic cover holding screws on the market as a direct replacement unlike with the old Union style that you could about get a replacement sheet metal screw for but would about as often crack the body of the plug once installed. There is also a version of the pin available using a 4-pin indent crimp tool to absolutely lock the wire into the terminal instead of having a screw that can come loose. Faster on the labor, cheaper and the assurance it will not come loose for factory install situations, you don’t need a ferrule or set screw anymore. Beyond the ferrule which is easy to install without special tools, the major advantage of this plug over all other stage pin plugs is the strain relief that at least adequately holds all forms of cable. 12/3 SO wire gets no strain relief. 14/3 SO wire gets a single rounded strain relief. 12/3 SJ gets a single flat one, 14/3 SJ and 16/3 SO gets two rounded. 16ga single conductor lamp cord in fiberglass sleeving gets a rounded and flat strain relief insert. And Zip cord or 18ga lamp cord in fiberglass gets two on the flat. People not following directions is the only major problem with this style of strain relief. Things like 12/3 SJ wire put between two rounded strain reliefs will cause the insulation between conductors to become displaced which with flexing can cause a short between conductors. In general the largest problems with the connector strain relief are that the edges are while slightly rounded still sharp enough to cut into rubber, the strain relief being installed either too tight or loose from not following directions, and the low coefficient of friction values on fiberglass sleeving when paired up with plastic allowing at times the fiberglass to slip out of the plug. Displacement and on smaller wires- the flattening out of the wire under the strain relief and the while improved grip of fiberglass sleeving still almost necessitates use of other materials to help it. Should you wish to put the plug on a thermoplastic wire that has less compressive qualities to it, you are better off installing some PVC or Nylon tubing over the cable so it will become a larger wire size and the tubing will compress but still grip the cable better. For say a 18/3 IEC cable such as might be off the back of your computer monitor, it uses a ±1/4" dia. cord. You would want to use a double rounded insert into it, but it still will flatten out the plastic cord more than it will want to be once it gets old or exposed to chemicals in the air such as fog that will break it down. Instead, if you insert a say 1/6" wall tubing matched to the size of the cable over the cord, the tubing will compress as per rubberized wire, yet like a ferrule retain the cord in applying equal pressure to it’s circumference. All thermoplastic wire can benefit by some softer material between it and the plug. That material will also prevent too much flex between the wire and the somewhat sharp edges of the strain relief that can cut into it. Going back to the friction tape, it’s also still of value in keeping that fiberglass sleeving from slipping some 100+ years after the friction tape was invented and scores of years since it was famous for use with Union plugs. Tech people always used to bitch and complain about it’s use, now they bitch about it’s reappearance and often would prefer just replacing the sleeving as it wears out or slips. It on at least the space between cord and fiberglass sleeving will provide when about a double layer of it, enough springiness to mold around the wires and grip the normally fiberglass wire to fiberglass sleeving to plastic junction that has been known to slide. 2" Of the friction tape with both ends under the strain relief will allow sufficient gripping power of the round and flat strain relief inserts to grip the wire without adverse damage to it, and prevent it from slipping out. This as opposed to a double wrapped cable tie inside the connector that could grip and prevent the fiberglass from slipping out but is more a question of getting the cable tie tight enough. If that fiberglass slips all the strain relief duties fall upon the set screw tension on the ferrule wrapped conductors - primarily the neutral. A 16 ga wire is not strong enough not to break under tension. You need an adequate strain relief. Friction tape above when followed thru with a double layer plus some of it above the fiberglass sleeving will allow for a very tight grip on the wires when used with a double rounded strain relief insert that will hold really well and also like the PVC tubing, prevent the wire from bending too close to the connector which is a common reason not only the fiberglass sleeving wears out and cuts, but the inner conductors also break and wear. Exposed friction tape however is a magnet for dirt and goeyness when fresh however. It will hold the cable well but has to be balanced with the above cleanness of the plug. To install the wires onto this plug, you don’t just strip the jacket and insert the wires with ferrules onto the set screws. There are issues such as that bolting thru hole between ground and hot that get in the way, plus the actual distance any one wire has to travel to get into position is different on a flat connector than it would be from a equally distant Edison plug. If you make the neutral and especially hot wires stretch to their terminals without adjusting for length, you are asking for something to come loose if not short out. Strip 1.1/4" of wire off the jacket. Cut the Neutral for 1/4" and the ground for ½" shorter. To strip the wire, use a quality pair of diagonal side cutting pliers in pinching the cable sufficient to grab it and pull or rock it back some off the conductors, than cut it. Grab another part of the insulation, pull it back and cut it. Continue on with that two more times, than bend the wire so the jacket breaks apart with stretching. Look at the wire you are bending to ensure you did not nick the insulation over conductors. If the jacket does not pull apart freely, while the jacket is bent, press down lightly with the dikes and it should cut without cutting into the insulation below. Pull the insulation free after most of it is cut. Next form the conductors into a fork with the hot being slightly wider apart from the ground than the neutral. This has the advantage of pre-forming the wires so they align with the holes they will go into and pre-arranging the wires conductors so the ones on the outside of the bend are shorter than those on the short side of the bend. Do this and no wires will be pulled out of the ferrule. Don’t pre-bend your wire and once it is bent, it will have conductors inside the ferrule of different lengths which will not be compressed upon evenly. Strip 1/4" off the insulation and insert a ferrule onto the wire. Grab the flair out of the ferrule with the pliers or wire gripping front of the stripper at the flair out, and push the ferrule down on the wire by 1/16" so conductors are pushed out the front of it and the insulation bunches up around the ferrule. On fixture cord, strip 5/16" and don’t bother pushing down on it. In either case, with the tip of the stripper at the flair, compress or sort of pre-crimp the ferrule to the wire. This will hold the wire to the ferrule but not obstruct it from fitting into the terminal. Wire will be sticking out the top of the ferrule, cut it off so there is no strands sticking out which will prevent the wire from sitting well inside the terminal hole. It is the extra wire after the bend to fork shape that if within the terminal will prevent sufficient contact. Once this is done it’s easy to insert wire with ferrule as one into all terminals of the plug and tighten them down with the 1/4 turn tension over hand tight. The ferrules provided with stage pin plugs are designed for 12 ga wire and this can become a problem. It’s as if you were sticking the 12ga wire into the opening of the pin without a ferrule. 16ga wire without the proper size of ferrule will not receive the same pressure on all conductors as above with cold solder and unsoldered wires into screw terminals on other types of plug. Only now the problem of doing so is worse than the head of a screw tightening onto a wire, we are talking about the tip of the screw cutting into conductors. At least only those conductors directly under the screw. Those without direct pressure of the screw get no tension and provide a high resistance connection. Inserting wire into a too large ferrule is not a cure to this either because the wire has nothing wrapping it’s circumference in applying equal pressure around all strands much less the 12ga ferrule is just going to flatten out and since it’s not full of wire, the screw will most likely just cut right thru it and any strands of wire under it. It’s possible to cut your wires instead of attaching them to the set screw. Any wire when doubled up will about equal a wire gauge three sizes larger in size or three sizes small in number than the original gauge. Take the above 16ga wire, when you strip double it’s length, than fold it up back upon itself, it now equals a 13ga wire which with the fold is plenty close to the 12ga of the wire ferrule. Tipple fold a 18ga wire into a 12ga ferrule and it’s the same principal. This works well for the above two sizes of wire in making them occupy the space of the Bates style ferrule. It does not work so well for 14ga wire unless you again double the length, but in the last 1/4" of stripped wire, re-strip the wire down to a 16 or 18 ga. than fold it back. Remember while current travels down the circumference of the wire not it’s inside higher resistance, it also flows down the path of least resistance. In other words, while broken or stripping the outer conductors off a wire might on other things cause extra resistance, in this case since its just used as filler for the ferrule, the current will not receive extra heat by doing this. Instead the current will flow into the properly stripped section of the wire first without worrying about the cut into conductors. Fold those remaining strands over and use them for filler on the ferrule. There is also a problem with tinned wire such as you will find on fixture wire. It’s the same relative size as cable if at times less strands for thicker more heat resistant sizes, but in general heat wire is tinned copper coated and it is not as flexible much less able to bend as easily. This wire will not bend back on itself without stripping it down to a smaller size sufficient to fit within the ferrule. Even than it’s difficult to fit within a ferrule. Fixture wire is very difficult to make work with a 12ga ferrule. Most just ignore there is a problem, yet every time they open up a plug used for a fixture and find a ferrule and strands of wire within it stuck within the stage pin plugs terminal, they must say to themselves there is something wrong here. Every time they see a melted down plug or one with loose terminals, they assume someone did not tighten down the screws enough. It’s possible, but given such tension is standard practice unless using power tools without verifying the proper tension, it is more likely the wire just settled or broke within the far tool large ferrule and the set screw was now not providing sufficient tension on the wire. That when coupled with cables that had badly applied strain reliefs allowed the wires to slip out of the terminal and arc at worst, or just create a high heat resistance condition. Happens all the time with 60amp stage pin plugs on 5Kw lighting equipment. Yes copper expands and contracts, but if there were sufficient retention around the strands of wire in the first place, they would not resettle and cause a loose connection for higher resistance. It’s bad to put too much tension on a terminal as it also causes resistance, but by far easier to end up with the wire once warm expanding and moving itself so as to accommodate for the larger size. Than next time the plug is used, with it’s wires no longer tight because they have contracted, they are a high resistance connection. There is two really good cures for the smaller wires in stage pin plug problem. First is to use crimp on plugs given you have a four pin indent tool and don’t mind throwing out the plug if the wire needs to be removed. Or you can use the proper size of ferrule in the terminal, than since it’s too small for the 10ga wire hole, on the bates plug, inserting the 16ga ferrule into the 12ga ferrule so both the wire is encased within the proper ferrule and the outer ferrule is sufficient in size to clamp down under the screw terminal - at this point even with a ferrule of double wall thickness so given the smaller size of the individual strands, the conductors are no longer cut by the twisting screw pressure on them. To that extent, insulated end ferrules with a PVC sleeve that fits over the insulation on the wire are cheap and have the benefit of allowing no exposure of bare wire outside of the plug should the insulation shrink back some. Simple process requiring an extra part kept in stock and an extra step in installation of the plug, but it’s benefits are simple should you wish a really good connection. By the way don’t be touching the ferrule more than necessary, much less especially the stripped wire much. The oils from your fingers like with the effects of them on halogen lamps will create resistance if not corrode the wire. Just a slight detail that will require deoxidant on connections to currents normally higher than 20 amps, but still important as a practice. Finally, a note on using plugs for twofers. Lots of companies just shove double the size of wire into a plug’s terminals and call it a twofer. Remember that this is theater and we are held to a higher sense of doing what is right than what is in your house. The NEC specifically bans the use of plugs/connectors as interconnection devices. This means a plug cannot nor will ever be made to make such a thing easier to do much less should ever be used. Think about this as the plug twofer is constructed. You certainly cannot fit two pieces of 12/3 even SJ cable into the strain releif without well overstressing it. You can about do so with two cords of MTW wire inside fiberglass, but even than it’s really clamping down unsafely onto the wires. The plug is rated for 20 amps. Any cables used under that 12/3 will not be sufficient for a perspective load given they don’t go to a fixture. The manufacturers could design a double wide strain relief, but than they would be responsible for doing something against the NEC that states something like any cord mounted splices need to be within vulcanized or other approved cord mounted cable mountings. Doubt the plug could than get a UL listing in not being code compliant. I have seen one bates plug with the strain relief ground out for a larger size, thought it was interesting and well done, but than realized that once you modify a connector it’s your complete liability over the plug. Should it cause a fire which is directly responsible from the plug, you not the manufacturer are now liable for it. Done all the time in the field. Someone will cut up one of my cables and shove both cords into a plug. Sometimes they will strip extra jacket away from the cable than tape up the exposed conductors so they can fit the cover on. Other times they will no doubt Vise grip the cover into place and absolutely smash the wires dangerously together. Some will even add a cable tie to the cords coming out of the strain relief so they when bent apart don’t at least instantly get cut jackets from the plug. Almost nobody but the most skilled of tech person is able to get the wires into the pin hole, even without crimped and folded individual ferrules used. After all, nobody has a handy ferrule crimp tool with them which being square once crimped would not work well, much less they don’t have access to double wire ferrules which are designed to house double wires within one insulated ferrule. This will fit within the opening and won’t have the problems of stray strands needing to be cut or falling outside the opening. In other words, it might seem simple to make a twofer out of a plug, but avoid doing so if at all possible, it’s going to be a high resistance connection and one the plug not only is not designed for but one that is against your local codes. No more real notes other than if assembling more than one plug at a time or working with a buddy, it’s by far faster and safer to do it in an assembly line. Cut all cords of insulation, arrange the wire next step, than strip and install the ferrules. While you do this, your buddy preps the plugs. When he or she is finished, he starts putting the wires into the terminals and screwing them down. You take the plugs than and install the covers after verifying each wire went to the proper terminal. Even than always test your work after assembled. But if you break your work up into simple one to three part steps, over a number of devices, it’s very easy to do the same thing every time without any mistakes you forgot to either do or look at.