wire termination for screw terminals

[electrolust]

https://reprap.org/wiki/Wire_termination_for_screw_terminals

Is that proper advice?

I'm wiring an LED driver. The terminals in question do have a flat pressure pad, and are tin plated. There is a mains voltage terminal and a control terminal. Both of these are meant to be used with solid wire.

It's clear from inspection that solid wire does not contact the terminal pads very well. Round vs flat, so you get limited contact. There's limited torque you can apply, so you aren't biting into the wire much either -- only enough to prevent pullout. This is a high quality product, so that must be within the design spec.

I'm wondering for the solid wires, should I squash them with pliers to get them flatter, for better contact?

For my stranded connection (to the LEDs), I can see if I just put the wire in, the strands flatten out and I get good contact. But given the way the solid wires connect, I wonder should I tin the stranded, or use ferrules. I would crimp on deutsch size 20 contacts as I have them and have the tooling already. It's equivalent to a small diameter ferrule. But it's clear that in the flat terminal, I would get better contact area with just the bare wire.

3.5A over 18AWG.

As I will be installing "many" of these in a difficult to access area, I want to ensure a stable connection.

[electrolust]

Maybe it matters, this isn't literally a screw with a flat washer-like pad on it. it's a PCB mounted terminal strip. I suppose the mechanical aspect is the same either way.

[themadhippy]

If it will fit fold the wire back on itself so its double the size in the terminal

[Gregg]

From my personal experience:
For stranded wire a crimped ring or fork terminal gives the best possible connection; uninsulated crimps generally are easier to get a good crimp connection and don’t take up as much physical room. They are also much easier to stack more than one under a terminal screw.
If you insist on winding the stranded wire around a screw terminal, strip the end long but don’t remove the stripped part of the insulation; twist the strands tighter together and the stub of insulation will keep them together and provide a better end to help keep the strands under the screw head; after tightening, give it the tug test, wiggle it around and re-tighten; you’d be surprised at how much the copper strands compress together after manipulating them.
For solid wire: copper work hardens and flattening it seems like a good idea, but pliers seldom are smooth or parallel meaning you might not get as good of a connection as if you let the screw flatten the wire.  Lightly crimping and soldering crimp connectors with solid wire might be a good solution if planning on stacking several under a screw. 
On some old installations I have seen the stranded wire looped around a brass grommet and crimped to make a rudimentary ring terminal.

[calzap]

I second Gregg's suggestion to use crimped ring or fork terminals on the wires.  Be sure to use a crimper that applies a set force (i.e. squeeze until it releases type).  Don't use the squeeze-until-it-feels-right, cheaper type of crimper.

Mike in California

[Vovk_Z]

https://reprap.org/wiki/Wire_termination_for_screw_terminals
Is that proper advice?

Yes, in general.

I'm wondering for the solid wires, should I squash them with pliers to get them flatter, for better contact?

No.

[Vovk_Z]

I wonder should I tin the stranded, or use ferrules.

To tin the stranded wire is a little outdated way (it can be ok only if you have not another way). There is now ferrules or forks (etc.) present so use them. It is enough to crimp them with a good crimping tool (there is no need to solder or crimp-and-solder).

[Ian.M]

*NEVER* tin stranded wire for direct connection to a screw terminal (of any sort).  Solder cold-flows under the typical clamping pressure of a screw terminal and the connection *will* loosen and become unreliable.  The only possible exception to this is tinning the very tip of the wire, to aid keeping all the strands together, as long as the screw clamp bears on the bare copper strands behind the tinned tip and doesn't bear on the tinned part.

[poorchava]

The advice from that link is generally correct. One this I don't entirely agree with is stranding wires before inserting them into a crimp terminal. Especially when using the 'folding wings' type of crimp.

[Vovk_Z]

*NEVER* tin stranded wire for direct connection to a screw terminal (of any sort).  Solder cold-flows under the typical clamping pressure of a screw terminal and the connection *will* loosen and become unreliable. 

Solder itself cold-flows of cause, but solder with copper wire cold-flows about an order less, so with a minimal amount of solder it may work quite good very long (with up to several amperes current).
I can agree that *NEVER* screw untinned stranded wires (without tin/solder), because stranded copper wires "cold-flow" and loosen very much.

[MarkF]

Crimp terminals are recommended.

[tooki]

*NEVER* tin stranded wire for direct connection to a screw terminal (of any sort).  Solder cold-flows under the typical clamping pressure of a screw terminal and the connection *will* loosen and become unreliable. 

Solder itself cold-flows of cause, but solder with copper wire cold-flows about an order less, so with a minimal amount of solder it may work quite good very long (with up to several amperes current).
I can agree that *NEVER* screw untinned stranded wires (without tin/solder), because stranded copper wires "cold-flow" and loosen very much.

It’s more likely vibration causing stranded wire to loosen.

Either way, tinning stranded wire is a big no-no, since it’ll loosen even without vibration.

The correct answer is “ferrules”.

[Vovk_Z]

It’s more likely vibration causing stranded wire to loosen.

- No, stranded wire loosen without any vibration. I've seen it many times.

[calzap]

Building wiring in conduit in the U.S. usually involves stranded wire.  Termination at bus bars and breakers usually means inserting the wire in a metal surround and tightening a set screw.  I've noticed that in a few months/years, the set screw needs additional tightening.  Not sure if this is "flow" or the strands being pushed into a more geometrically compact arrangement or the screw backing out a bit or a combo.

When wiring a receptacle or switch that only has screw terminals, most electricians don't use a crimp-on terminal.  The usual trick is to cut the insulation about 35 mm from the end, and slide the insulation so as to create about a 15 mm gap.  This keeps the strands from splaying.  Then wrap the exposed section around the screw shaft, tighten the screw and snip the excess wire.

Often though some or even most of the strands will be squeezed out from under the screw as it's tightened.  Sometimes there are only a few strands are making contact.   Then a conscientious electrician will redo it ... not all of them are conscientious ... whatever the boss and inspector allow is good enough.  And eventually, the screw will need retightening no matter how good the initial installation.  I always use a crimp-on terminal to connect stranded wire to a screw terminal.  It's just as quick, maybe quicker, and more secure.

Mike in California

[wizard69]

The correct answer is depends. 

There are a number or different terminals Strips  designs   Which is the best solution depends upon those designs.   

Setscrew type terminal strips and stranded wire are best assembled with wire ferrules. 

Clamp type blocks need ring or fork terminal for the termination of stranded wire and specifically not wire ferrules.   In case you are wondering clamp type screws have (usually) a captured square washer under the screw head.  These types of terminals work best when the load on each side of the screw is even.    Thus I prefer ring or fork terminals but they may not be required.  A variation of these terminals are designed to terminate bare wire copper or stranded and can be seen in home wiring devices like sockets and light switches are designed for bare copper wire. 

Another type of terminal block just has a pan head screw.  These absolutely need a ring terminal or fork terminal.  In some cases you can wrap solid copper wire around the screw but this isn’t optimal and you need to do it the right way. 

[duak]

If there's no way to use a crimped termination on a stranded wire that is held down with a simple screw I've found a few things that help:

1.) Leaving a bit of insulation on the end of the stripped section can help for some sizes

2.) if the screw is right handed, ie. the normal one that tightens clockwise, twist the wire strands counter-clockwise before forming it into a hook.

3.) use a washer underneath the head of the screw - the bigger, the better.  I occasionally find products like Solid State Relays that have a square washer with folded wings that help hold the strands in place.  I wish these were available more widely.

[tkamiya]

https://reprap.org/wiki/Wire_termination_for_screw_terminals

The terminals in question do have a flat pressure pad, and are tin plated. There is a mains voltage terminal and a control terminal. Both of these are meant to be used with solid wire.

It's clear from inspection that solid wire does not contact the terminal pads very well. Round vs flat, so you get limited contact. There's limited torque you can apply, so you aren't biting into the wire much either -- only enough to prevent pullout. This is a high quality product, so that must be within the design spec.

Does the flat pressure pad snugly fit the slot and have a downward bend at all 4 corners?  If this is the case, the terminal may be designed to accept two wires, one on each side.  You'll have to consult with manuals to determine if this is the case.  In those cases, you don't need to make a U hook.  Just straight wire will do.  That little downward bent will help with fraying of stranded wire, too.

As to solid wire not making good connection, it's been this way for many years.  Even 100 amp circuit breaker is made this way.  I think you are over thinking this.  I will not suggest flattening the wire either.  You'll just make a weak point for wire to break.  There are manufacturers of terminals that writes in manual AGAINST using too much torque, as it will squish the wire and possibly damage the terminal. 

As to compression wire end terminals, please note, insulated type is for stranded wires only.  You will not get a good crimp on solid wires.  (even if you use expensive manufacturer supplied tools)   Quality of crimp, if you choose to use them with stranded wires, depends on use of proper tool.  Too much or too little pressure can cause eventual failure.

If I were to do this, I would use stranded wire larger than spec calls for, and crimp an insulated Y terminal with approved tool.  Another way I might do, assuming it is not subject to vibration, is use a solid wire, make an U shape and torque appropriately.  Then bundle the wire for extra measure. 

FYI:  I'm a licensed electrician (but license is from Japan, so it's not valid in USA)

[tooki]

It’s more likely vibration causing stranded wire to loosen.

- No, stranded wire loosen without any vibration. I've seen it many times.

Yes, sorry, I forgot to mention the other thing that causes loosening: thermal cycling.

[Lomax]

I would never put bare stranded wire into a rotating clamping mechanism. And as pointed out by others tinning the wire may lead to problems in the future, especially in a high current/high temperature application. Uninsulated open barrel ring or fork terminals are the gold standard, but I find ferrules also work perfectly well in screw terminals. Considering that the cost & time for adding them is insignificant I can't see why one wouldn't.

[wizard69]

It’s more likely vibration causing stranded wire to loosen.

- No, stranded wire loosen without any vibration. I've seen it many times.

This very true and frankly I suspect this is why some are saying tinned wires are subject to loosening over time.  The reality is if you work in a plant with a decent Electrical / Electronic PM program, you will be running around once a year torquing screws down until the stabilize.    Vibration can of course be an issue but working in a plant where there is basically no machine vibration I can safely say that vibration is only part of the problem.   A a result I've seen various issues and can safely say what is right depends upon what is being used at the time.

For example set screw type connections work really well with wire that has had a ferrule put on it.   This especially if you have to terminate a very small wire into a larger terminal.   Ferrule allow the wire to act like it is solid.   

On the flip side ferrules really seem to make things worse on barrier type terminal strips.   That is strips that have wire clamping washers under the clamp screw.   Here it is better to have ring or fork terminals.   It seems like these types of terminals work much better when the load is balanced under the screw.   They also work with bare stranded wire fairly well as long as the diameter of the wire is not excessive.   If you try to clamp large diameter solid, ferruled or even stranded wire, under one side of the screw you will have trouble.  In such a case I've even seen the clamp screw end up bent and unable to secure the wire reliably.   

One other thing tinned wire is not wire with globes of solder on it.   Tinned wire is effectively plated with solder.   I've never seen tinned wire causing a problem by itself.   It however doesn't really offer any advantages either.   If you put globes of solder on the wire then you will certainly have issues.   In any event any place where tinning might offer a benefit, wire ferrules are easier and provide additional advantages.

[Lomax]

On the flip side ferrules really seem to make things worse on barrier type terminal strips. That is strips that have wire clamping washers under the clamp screw.

The black rigid barrier/terminal blocks often have square washers, typically with the corners bent down. These handle bare stranded wire reasonably well, and ferruled wire perfectly. Round washers can be problematic for both, depending on the design, and are better served with ring or fork terminals. The type pictured below destroys stranded wire though, and should really only be used with solid core or ferrules. Or perhaps not at all.

[electrolust]

that's the type on this particular board. i ended up going with ferrules, they seem to be working great but of course in the span of just weeks so would bare wire.