Use of Solder on Mains Voltage Lines Inside a Device

[TheNerd389]

I'm considering soldering some wires that will be operating at mains voltage inside a device. I know that solder is avoided in home wiring, but I'm not familiar with best practices inside devices.

I'm also cautious working with mains voltage lines in general, as I'm aware there are many safety concerns around them that I don't know enough about as a beginner.

So, if I need to solder mains lines inside a device together, specifically in Y and wire-to-wire joints, what should I do to keep things safe? The wires will not be completely secured, and will be moved occasionally by the user.

Is electrical tape and heat shrink sufficient to protect the solder joints? Or do I need to consider other things as well?

[bob91343]

I have no idea why solder might not be okay to use.  I have used it with no problems.  I also have used wire nuts but those need to be the right size and tightened properly.  Unless the crimp type is used.

[Andree Henkel]

You definitely need a kind of "strain relief" for the mains in power cable, there must be not any mechanical stress on the solder joints.

[helius]

Do you know the Western Union splice?
Have you seen what happens to electrical tape after 40 years?
There are some good reasons that crimp connectors, barrier blocks, and wirenuts are used to join wires together, not solder.

[hermitengineer]

So, if I need to solder mains lines inside a device together, specifically in Y and wire-to-wire joints, what should I do to keep things safe? The wires will not be completely secured, and will be moved occasionally by the user.

Use stranded wire.  Solid wire is too brittle if it's going to see movement.  Also, don't let the wire move close to the solder joint.  Just like solid wire, soldered wire is brittle, and even the stranded part near the solder will see stress as the wire is moved around.  Crimped connections have stress trouble near the crimp if the wire bends.  If it's going to move at all, have a wire clamp or something to make sure the joint doesn't see any of that movement or strain.  In fact, any appliance, once ready to close up, will have as much of the wire clamped down rather than flapping in the breeze as they possibly can.

Is electrical tape and heat shrink sufficient to protect the solder joints? Or do I need to consider other things as well?

I hate electrical tape.  It's sticky, it loses adhesion when it heats up, and over time it just becomes a brittle casing.  Heat shrink is better, but most appliances I have opened up used wire nuts or crimped connectors (spade with a locking finger, ring tongue, etc).

[james_s]

No problem at all with soldering mains voltage stuff, look inside any mains powered equipment and you'll find soldered connections. It's not used for household wiring because it would be a time consuming pain vs just using wire nuts or terminal strips.

It's good that you are cautious though, this is something you have to make sure you do right. If you have any doubt in your work, post a picture so people here can evaluate it for safety.

[Ian.M]

Solder fatigues if repeatedly mechanically stressed, and also can melt if a fault causes the wire or terminal to overheat.  Therefore it must *NEVER* be the only thing holding the joint together, (i.e. the joint must be adequately mechanically fastened before soldering), and the wiring must be secured, preferably so that no movement of the wire can reach the joint, but at a minimum so that even if the joint fails, the resulting bare wire end cannot become hazardous.

Although I have solder-spliced a couple of mains leads over the years for personal use, I always took great care to rebuild the mechanical integrity of the cable, with layered insulation bonded in place over a considerable length, and *never* did such work for a client.   IMHO soldered mains connections or splices are unacceptable if user-accessible (without use of tools).

[fordem]

The wires will not be completely secured, and will be moved occasionally by the user.

Once the joint is user accessible, it should be some sort of UL approved plug/socket.

[JustMeHere]

Look for a "strain relief bushing"

 

[Ed.Kloonk]

When using heatshrink over a solder join/splice etc, have your soldering skill high enough that the solder is smooth before you apply the heatshrink. Excess, untidy solder blobs can wear through the heatshrink in situations where there is movement.

If you're apprehensive, consider getting some spare heatshrink, some cable, possibly a flux pen and practice, practice, practice.

Knowing what temp to set the iron is key. You want the solder to flow without burning the insulation, of course.

Avoid using insulation tape on bare mains. I used to repair 240v mains as thick as your finger, upside down in the mud trench after the excavator had dug them up. Solder is fine. I should point out that in the dark ages, the early days of house wiring in Australia, all earth connections in buildings had to be soldered. Later on they relaxed the solder requirement on earth bonding and allowed connectors except in distribution boards. (OZ/NZ only AFAIK)

[Berni]

Solder joints are not designed to handle mechanical stress because they become brittle from repeated bending. So any cables that go outside and can be moved around have to be secured somehow, be it a cable gland, clamp, snaked around a cable holding slot etc...

But there is nothing wrong with using solder for mains. Where solder is generally not used is very high current cables. When currents of >50A are involved it brings with it very thick cables that can get very hot in any spot that does not maintain a good low resistance connection. So those are typically crimped into cable lugs using literally tons of force to cold weld the two together. This makes for a more secure low resistance connection. If solder was used the worse conductivity of it might present extra resistance and if the connection gets hot the solder will melt and let go, leaving the fat high current cable to slip out of the connection and potentially fall onto something else causing a huge short circuit with sparks flying.

[TheNerd389]

Do you know the Western Union splice?
Have you seen what happens to electrical tape after 40 years?
There are some good reasons that crimp connectors, barrier blocks, and wirenuts are used to join wires together, not solder.

I was planning to use the method covered in section 19.4 of this document:
https://nepp.nasa.gov/files/27631/NSTD87394A.pdf

If that's insufficient, I was thinking of doing the one in 19.5.

[TheNerd389]

Solder fatigues if repeatedly mechanically stressed, and also can melt if a fault causes the wire or terminal to overheat.  Therefore it must *NEVER* be the only thing holding the joint together, (i.e. the joint must be adequately mechanically fastened before soldering), and the wiring must be secured, preferably so that no movement of the wire can reach the joint, but at a minimum so that even if the joint fails, the resulting bare wire end cannot become hazardous.

Although I have solder-spliced a couple of mains leads over the years for personal use, I always took great care to rebuild the mechanical integrity of the cable, with layered insulation bonded in place over a considerable length, and *never* did such work for a client.   IMHO soldered mains connections or splices are unacceptable if user-accessible (without use of tools).

The soldered section would be contained inside the device, but a short length of wire will be exposed through the side of the case for connecting to it. Does that make a difference? I don't have a grommet to use to secure it unfortunately.

[TheNerd389]

When using heatshrink over a solder join/splice etc, have your soldering skill high enough that the solder is smooth before you apply the heatshrink. Excess, untidy solder blobs can wear through the heatshrink in situations where there is movement.

If you're apprehensive, consider getting some spare heatshrink, some cable, possibly a flux pen and practice, practice, practice.

Knowing what temp to set the iron is key. You want the solder to flow without burning the insulation, of course.

Avoid using insulation tape on bare mains. I used to repair 240v mains as thick as your finger, upside down in the mud trench after the excavator had dug them up. Solder is fine. I should point out that in the dark ages, the early days of house wiring in Australia, all earth connections in buildings had to be soldered. Later on they relaxed the solder requirement on earth bonding and allowed connectors except in distribution boards. (OZ/NZ only AFAIK)

I'm confident in my soldering skills for this. I've had to solder wires quite a lot over the years, sometimes even cleaning up poor manufacturing for small-batch devices. I just finished soldering the first PCB I've had made, though. (Finally back in school after working as a tech for about 10 years.) Take a look.

I know R11 is crooked. It bothers no one more than me. 

As far as heat shrink, I'd use adhesive lined stuff for added rigidity.

[TheNerd389]

So, if I need to solder mains lines inside a device together, specifically in Y and wire-to-wire joints, what should I do to keep things safe? The wires will not be completely secured, and will be moved occasionally by the user.

Use stranded wire.  Solid wire is too brittle if it's going to see movement.  Also, don't let the wire move close to the solder joint.  Just like solid wire, soldered wire is brittle, and even the stranded part near the solder will see stress as the wire is moved around.  Crimped connections have stress trouble near the crimp if the wire bends.  If it's going to move at all, have a wire clamp or something to make sure the joint doesn't see any of that movement or strain.  In fact, any appliance, once ready to close up, will have as much of the wire clamped down rather than flapping in the breeze as they possibly can.

Is electrical tape and heat shrink sufficient to protect the solder joints? Or do I need to consider other things as well?

I hate electrical tape.  It's sticky, it loses adhesion when it heats up, and over time it just becomes a brittle casing.  Heat shrink is better, but most appliances I have opened up used wire nuts or crimped connectors (spade with a locking finger, ring tongue, etc).

The device in question is a high-efficiency benchtop power supply, if that helps. As far as the type of wire, it's definitely stranded, but I'm not sure the exact spec on it. I'll be modifying a commercial splitter to use in this device.

And yes, I also hate electrical tape. Where possible, heatshrink is my preference.

[TheNerd389]

Solder joints are not designed to handle mechanical stress because they become brittle from repeated bending. So any cables that go outside and can be moved around have to be secured somehow, be it a cable gland, clamp, snaked around a cable holding slot etc...

But there is nothing wrong with using solder for mains. Where solder is generally not used is very high current cables. When currents of >50A are involved it brings with it very thick cables that can get very hot in any spot that does not maintain a good low resistance connection. So those are typically crimped into cable lugs using literally tons of force to cold weld the two together. This makes for a more secure low resistance connection. If solder was used the worse conductivity of it might present extra resistance and if the connection gets hot the solder will melt and let go, leaving the fat high current cable to slip out of the connection and potentially fall onto something else causing a huge short circuit with sparks flying.

Understood. I'll see how well I can tie down the wire between the accessible section and the joint. I don't have a ton of space to work with, though. Current shouldn't be an issue here. The connector I'm using tops out at 2.4 amps, and this project shouldn't be pushing anything over 1 amp. Even that would be limited to rather exceptional circumstances.

[Berni]

Understood. I'll see how well I can tie down the wire between the accessible section and the joint. I don't have a ton of space to work with, though. Current shouldn't be an issue here. The connector I'm using tops out at 2.4 amps, and this project shouldn't be pushing anything over 1 amp. Even that would be limited to rather exceptional circumstances.

A quick easy way to hold down cables in a small area of space is to glue them down.

Two part epoxy can be mixed together and poured where the cable goes trough, forming a big solid blob around it. Being two part makes it evenly harden deep down a thick blob and they will generally stand up to quite a bit of heat before letting go (unlike hot glue). Unless its using some sort of fancy silicone or teflon insulation the epoxy will grip the wire very well even if its just laying straight.

[TheNerd389]

A quick easy way to hold down cables in a small area of space is to glue them down.

Two part epoxy can be mixed together and poured where the cable goes trough, forming a big solid blob around it. Being two part makes it evenly harden deep down a thick blob and they will generally stand up to quite a bit of heat before letting go (unlike hot glue). Unless its using some sort of fancy silicone or teflon insulation the epoxy will grip the wire very well even if its just laying straight.

Funny you mention that. The first wire that came to mind for this if I were starting from scratch was high temp silicone. XD I got some for a project years ago but never got around to using it.

I'm fortunate enough to be able to make design changes to the enclosure. The project is actually for a class, and we're designing the enclosure itself. I can toss in a zip tie point at the appropriate location if I can just find the room to do so. Unfortunately, the entire reason I'm asking about this is because space is incredibly tight in this design.

[TheNerd389]

The wires will not be completely secured, and will be moved occasionally by the user.

Once the joint is user accessible, it should be some sort of UL approved plug/socket.

I'd love to use a proper connector. Unfortunately, that isn't an option for this project. I asked about getting such a connector to work here in my first post to these forums because I couldn't find one. Apparently they just aren't available in the US at all. The only option left is to fashion a harness of sorts using commercial cable assemblies.

[Ian.M]

https://www.eevblog.com/forum/beginners/question-about-the-iec-c7c8-figure-8-connector/ explains a lot about your design constraints.
 
I *really* *don't* like the idea of a trailing daisychain C7 outlet cord, even with a snap-in plastic cord grip to prevent tugging on your internal wiring, let alone without one, as IEC C7 cord end connectors tend to go bad (loose and arcing) with age and use.

What about a panel mount polarised IEC 60320 C8 inlet, with solder lugs, and a panel mount NEMA  1-15R socket for the daisychain outlet?   Then you could make the Y connection at the solder lugs, and use off-the-shelf NEMA 1-15P to IEC C7 cords for the daisychain cable (e.g. https://www.amazon.com/SF-Cable-2-Slot-Polarized-IEC320C7/dp/B01BW0QBY0 ).   I'd put a fuse between inlet and outlet because *idiots*!

[wizard69]

Do you know the Western Union splice?
Have you seen what happens to electrical tape after 40 years?
There are some good reasons that crimp connectors, barrier blocks, and wirenuts are used to join wires together, not solder.

I was planning to use the method covered in section 19.4 of this document:
https://nepp.nasa.gov/files/27631/NSTD87394A.pdf

If that's insufficient, I was thinking of doing the one in 19.5.

That would NOT be acceptable by most electricians, or engineers in charge of mains electrical systems.   I don't even see it as acceptable for low voltage controls.   In a nut shell lead solder should not be used as a mechanical connection solution

Now the question that comes to mind here is why are you designing in soldered main connections in the first place.    It has been a long day for me so I really don't grasp what you are building but I find it odd that you have a need to solder wires like this at all.   Generally the goal is to run wires point to point / device to device and use the termination offered up there.    Sometimes that will be a soldered connection.   If you really need to tie multiple wires together there are plenty of no solder options.   Terminal strips, barrier strips and distribution blocks come to mind.    There are also a variety of crimp and spring loaded connectors that have their uses along with the wire nuts.

As for soldering of connections in residential wiring I'm not sure what the current status is but it can be a real pain to solder stuff in a location with no electrical power.   Given that alternative methods, especially quality and properly installed crimped connections work as well or better than solder there is little point to solder mains wiring.

That is today.    In my house it appears that the electrician that did the work in the early 1950's didn't trust anything.   He literally soldered the wires, wire nutted them, then wrapped in rubber tape followed up with friction tape.  That is a total joy when you want to take a connection apart to extend or modify.

I've also have seen companies go to the extreme when it comes to their grounding network.   In stead of soldering the grounding system was brazed to just about every I-beam post I saw in the building.   I'm not suggesting brazing as a solution for you at all, just that permanent connections are still used in industry.   Personally I wouldn't take such an approach for anything mains related, if I want high reliability it would be either crimped or bolted connections.

[TheNerd389]

https://www.eevblog.com/forum/beginners/question-about-the-iec-c7c8-figure-8-connector/ explains a lot about your design constraints.
 
I *really* *don't* like the idea of a trailing daisychain C7 outlet cord, even with a snap-in plastic cord grip to prevent tugging on your internal wiring, let alone without one, as IEC C7 cord end connectors tend to go bad (loose and arcing) with age and use.

What about a panel mount polarised IEC 60320 C8 inlet, with solder lugs, and a panel mount NEMA  1-15R socket for the daisychain outlet?   Then you could make the Y connection at the solder lugs, and use off-the-shelf NEMA 1-15P to IEC C7 cords for the daisychain cable (e.g. https://www.amazon.com/SF-Cable-2-Slot-Polarized-IEC320C7/dp/B01BW0QBY0 ).   I'd put a fuse between inlet and outlet because *idiots*!

I'd be really worried that the NEMA plug would be used for other purposes. The input current limit to the stack is low by the standards of that connector. The daisy chain connection is only exposed from the top and bottom of the device as well. It will eat into the space available quite a lot to move to a larger plug. It would also risk putting us over budget.

The original idea was to use something like this with only the connectors exposed to the outside: https://www.amazon.com/gp/product/B074DSG7JS/

The strain relief, especially at the Y, makes that particular one nearly impossible to fit in, but the idea is to have a vertical channel running through the middle of the stack that allows users to connect the 70-100mm exposed lengths of the daisy chain together inside the stack. It'd be a pain to set up, but it'd make for a really tidy PSU stack.

What I'm getting is that if I go this route, I should put a substantial amount of effort into physically securing this cable so that it can't move. I should use stranded wire designed for flexibility as much as possible as long as I can come up with a way to secure it "well". I should be very conservative with how I define "well".

I should use a solder joint that can take a substantial amount of punishment, and the tie down points should be able to handle both repetitive and shock loads.

Does anyone know how much force I should design the tie downs to take? Something like the weight of the stack of power supplies times a safety factor of 10? Or the pull force of the connector times a safety factor of 5? I'd appreciate input here.

[tooki]

Seems to me it’d be wiser to either choose a bigger enclosure so you can fit the PSU inside easily; choose a PSU with dimensions that fit better; or keep the PSU external and use a DC input jack.

If you’re going to have an AC pass through, then you either need to fuse it internally, or it needs to handle the full maximum current of an AC circuit, 15A in USA, which a C8 can’t do IIRC. So I’d use an IEC C14 socket (standard PC power inlet). Forget soldering, just invest the $15 in a cheap ratcheting crimper for fully insulated terminals, and use that with connectors with 1/4” (6.3mm) faston spades. You can get stackable ones for paralleling connections.

With that said, I wouldn’t bother with internal daisy chaining. You can easily get C13/C14 Y cables.

[wizard69]

I'd love to use a proper connector. Unfortunately, that isn't an option for this project. I asked about getting such a connector to work here in my first post to these forums because I couldn't find one. Apparently they just aren't available in the US at all. The only option left is to fashion a harness of sorts using commercial cable assemblies.

I'm still trying to figure out what you are doing here.    However you should have no problem getting proper connectors for US standard inlets and outlets in the USA.   This really confuses me as there are multiple manufactures of such hardware Hubble being just one manufacture.  For example Hubble has the 5278 and 5279 inlets and outlets, amongst many other options.   

Even if you go with corded inlets and outlets, all your wiring inside the device needs to be wired UL approved manner with UL approved devices.   At least if you intend to market the device.   Frankly that device should get a UL stamp through their approval process also.

[calzap]

Actually, the US NEC does allow building wire to be soldered as long as the joint is not mechanically or electrically dependent on the solder.  See sections 110.14(B) and 250.8(B).  So, there is little point in doing it.  There is also the risk of an inspector having a “no solder allowed” mindset.  However, welding and brazing are allowed, and are used most commonly on grounding conductors.

Different rules apply inside devices.  There are listed (UL, ETL, CSA, etc.) devices that have solder joints carrying mains voltages or higher.

Mike in California