Shore power feed to manual transfer switch - crisis avoided

[richard5933]

Just a brief report on a problem I found last week and what I did to fix it.

Our shore power receptacle and cord were installed using an older style Hubbell connector. Since there is no screw-down locking ring, I've gotten into the habit of checking to make sure it's firmly in place when I walk past it by giving it a little turn to the right. Last week I did that and found it slightly warmer than normal, even though I was only running basic equipment (fan, charger, and some lights - no a/c).

The warmer connector led me to do further inspections. Voltage at the outside of the receptacle was 120vac. Voltage 5 feet away at the manual transfer switch was only 117vac. I'm guessing that the 3 missing volts were to blame for the heat in the connector.

I decided that it was time to bring the coach at least somewhat to the modern world and install a new Marinco 50-amp receptacle. Since Custom Coach had cheated and used the chassis/shell for the ground conductor, I was also going to take the opportunity to upgrade the lead from the receptacle to the transfer switch to 4-conductor. This allowed me to install a proper ground buss bar next to the breaker panel and to properly ground the circuits.

What surprised me was to find that the cable connecting the shore power receptacle to the transfer switch was only 8/3, yet it was being used to carry 50-amps per leg. Really surprising since they used 6-ga conductors between the transfer switch and breaker panel. The insulation on the individual conductors cracked as soon as I bent it to remove the old conductors. Now I suspected that the heat has been a problem in the past and caused the insulation to fail. Luckily I caught this before anything caught fire or caused a short.

New receptacle and 6/4 cable installed.

The whole thing is a good reminder that this type of issue can be hidden anywhere, especially in older coaches and RVs.

[rayin]

Thanks a lot, you just lengthened my "to-do" list when we get home tomorrow. Really, thank you. I check every connection screw for tightness annually, and have replaced the old Marinco shore power receptacle,  but never thought about checking the wiring to the ATS.

[richard5933]

Hope I don't start a controversy with this bit of information, but when I reinstalled things I opted to use crimped ferrules on the ends of the stranded wires before inserting into the screw-down terminals.

Years ago I learned that tinning them was no longer recommended. But, I did not like how the individual strands took a beating when crushed in the terminals. Every time I re-tightened them the strands splayed out further.

Using crimped ferrules is apparently common practice in Europe. I couldn't find anything against the practice, so I got the appropriate kit and went at it. Gotta say, even though it was an extra step, it made for a much neater job in the end.

Time will tell.

 

[kaypsmith]

Neater and longer lasting, I don't always use crimp ferrules but in an overheating or where the wire end gets a lot of pulling situation (in remote controls), they really work great.

[wildebill308]

3 hours ago, richard5933 said:

Years ago I learned that tinning them was no longer recommended.

Just wondering who said that? Before I retired I had to get re certified in solder/soldering. I don't remember the technical term for the classification but it was a aircraft through space craft classification. Nearly everything was tined. 

[manholt]

Bill, when did you retire?

[richard5933]

8 hours ago, WILDEBILL308 said:

Just wondering who said that? Before I retired I had to get re certified in solder/soldering. I don't remember the technical term for the classification but it was a aircraft through space craft classification. Nearly everything was tined. 

I'd have to search again for the sources. I just did a quick search again, and found a few that mention tinning just the ends of the strands to help keep them in shape, but to avoid tinning the part of the stranded wire which will be placed under the screw terminal. Here's one piece about it I found:

The reason for the prohibition is that when you fully tin a multistrand wire fully, the solder wicks between the strands of copper and forms a solid block, part of whose volume is metallic solder. When you clamp the solder and copper bundle you tighten the screw or clamp against the solder block, and in time the solder metal "creeps" under the compressive forces and the join loses tension. The wire can then either pull out or cause a high resistance connection with heating.

There does seem to be an active debate in the electrical world about this, but from what I've seen I've two main points:

For screw terminals, ferrules are the best bet. Second to that is to twist the wires gently and then use the screw terminal. Tinning can lead to a loosened connection over time.

For spring-type clips and terminals, tinning the ends is okay because the spring can adjust if the wire changes shape/size slightly over time.

Connections which may be subject to high heat should not be tinned or soldered, since the high heat will cause the solder to soften and loosen the connection.

[wolfe10]

Probably some confusion here.

For decades, all marine wire is PRE-TINNED.  It is the expensive wire.  The thin tin coating on all strands protects the copper which just loves to turn into COPPER OXIDE (that "green stuff" that doesn't conduct electricity). 

There is a high-end pre-tinned wire: http://www.ancorproducts.com/en/products/wire-and-cable

[richard5933]

1 hour ago, wolfe10 said:

Probably some confusion here.

For decades, all marine wire is PRE-TINNED.  It is the expensive wire.  The thin tin coating on all strands protects the copper which just loves to turn into COPPER OXIDE (that "green stuff" that doesn't conduct electricity). 

There is a high-end pre-tinned wire: http://www.ancorproducts.com/en/products/wire-and-cable

Yeah - marine wire is the 'good stuff' with each strand individually tinned all the way through and through.

What I was referring to in the post above was tinning the ends together before inserting them into the screw terminal, so that when the screw terminal was tightened it had a solid mass to press against instead of a bunch of squirrely loose ends. I was taught to do this by the cigar-smoking ham radio old-timer that showed by brother and me how to build radios when we were kids. I've been doing this ever since without really thinking about if it is still best practice, and now I'm finding that it's not always so. I've actually had to unlearn a lot of what this guy taught us, like to not solder the connection between the grounding wire and ground rod for the ham shack, and to crimp rather than solder battery connector terminals.

Using crimp ferrules on the ends of stranded wire seems to be much more common in the UK and the EU as well as in the marine world than it is in the RV industry. Takes more time and costs more. But, I suspect, in the end provides a more stable and safe connection.

[manholt]

This Post seems to have taken over, for those other posts, that is now by the wayside...like Roland and his dash light problem!  Any way we can reach out and help him?

[wildebill308]

10 hours ago, manholt said:

Bill, when did you retire?

11/01/2012, Ok my certification may have lapsed by now.