Use the Strain Relief or Weep
It is pretty amazing how often do-it-yourself shore power cords fail. The cord pictured here failed after it was coiled and uncoiled a number of times. In the process, the cord developed a twist which was, of course, transmitted to the connector. The connector cannot rotate once it is plugged in. Since the installer did not properly use the connector's strain relief, the three conductors twisted inside the connector and eventually pulled out of their terminals.
You may also notice that the insulation is melted just behind the business end of the black wire. This says that the connector terminal was not gripping the black conductor very securely. Contact was marginal enough that not all of the strands of the wire were carrying their share of the load. It's like cutting partway through a wire. The resistance at the cut increases because the wire has less cross sectional area.
With light loads there was probably no problem. Switch on the hot water heater, however, and the increased current made the wire hot too. Hot enough to melt the wire insulation a little, not hot enough to overheat the terminal and melt the body of the connector. The exposed strands of wire may be a little long and they were certainly twisted and mashed together. Because they do not have a uniform cross section in this state, it was probably easier to pull them out of the connector terminals.
How to do this right:
[0. Hire an electrician to do the job or follow these instructions very closely.]
1. Strip back the outer jacket of the cable leaving a nice, clean end. That much looks right in the photo. If you nicked the insulation on any of the wires, do it over. Leave the inner conductors about three inches long. Slide the connector backshell up the cable NOW. You will regret it later if you get the whole connector nicely terminated and the backshell is still lying on the ground next to you. Move the backshell far enough up the cable that it will be completely out of the way. Keep it from sliding back to the end of the cable with tape if you have to.
2. Arrange the three conductors so they lie in the same sequence as the colors are at the connector terminals. Sometimes this will just happen to come out right and sometimes you will have to muscle one of the wires through the other two, depending on which end of the cable you happen to be at. The "muscle" part is why you left the wires pretty long. Try to get the wires re-arranged up into the cable jacket because you are going to cut the wires surprisingly short and there will be no room to adjust things afterwards.
3. Open the screws on all three (or four) terminals all the way. They are captive and will not fall out. Strip the insulation off the end of one of the wires and determine just how deep the pockets are in the terminals. It helps to start with maybe 3/4 of an inch of wire exposed and gradually shorten it until the wire bottoms out with the insulation just inside the terminal. Do not twist the wire strands. Cut the wire nice and square at the end. Strip the insulation cleanly without nicking any wire strands. This may take a little practice, but it's necessary. I strip the insulation with a sharp utility knife, taking care to avoid cutting too deep. If you don't quite cut all the way through the insulation all around the wire, but just enough, you should be able to pull off a ring of insulation in one piece, tearing the thin web of insulation next to the wire. To cut all around the wire you twist the wire against the knife and then reposition to get what you couldn't reach the first time. You can also rotate the knife around the wire, but I think that's a little harder to keep the cut at the same place all around the wire. You do not want to end up with a spiral cut. If you do, you may be able to carefully trim it square. There are other methods that work, but I think this one is the cleanest and worth doing even if you have to practice a dozen times to get it right. Did I mention that impatience is why most people can't put on a connector properly?
4. Having determined exactly how long to strip the wires by practicing on something you will not use, figure out how long the three wires will have to be. They must be long enough to bottom out inside the terminals once they are stripped and they must be short enough so the outer jacket of the cable is about a half-inch past the inside of the strain relief. Keep the wires in the right order and cut them all to exactly the same length. They will look pretty stubby and you will wonder how this is going to go together. Fear not, if you have been following instructions. If not, start over. Strip each wire to leave as much exposed conductor as you determined earlier will bottom out in the terminal pocket while leaving maybe 1/8 of an inch of insulation inside the pocket as well. The wires in the photo are perhaps a half-inch too long, which is one of the reasons the connector failed.
5. Take the screws out of the strain relief. You will notice that the arms on the strain relief fold back out of the way. Different brands of connector may have different designs of strain relief. You will just have to figure out an equivalent procedure if that is the case. Carefully slide all three wires into their respective connector pockets at the same time, taking care that all the strands of all the wires go into three respective pockets. Obviously, you will have had to pre-arrange the three wires so they are in the right place because all you can get your hands on is the big cable on one side and the connector body on the other side. Once you get all the wires precisely where you want them, take great care to support everything so they do not slip out again. The more messed up the fine strands become the harder this becomes. Again, DO NOT TWIST THE STRANDS OF THE WIRES. They should all lie parallel to each other exactly as they emerge from the insulation. When you twist them in the vain hope of getting those few unruly strands to stay close to the rest, the wire gets fatter and harder to slide into the terminal pocket. It will also be easier to pull out of the terminal when you are finished.
6. Carefully supporting everything so the conductors remain bottomed out in their terminal pockets, tighten the terminal screws one at a time. You can go around once and make them snug while ensuring all wires are bottomed out. Then go around again and tighten to really grip the wires. A good terminal grip deforms the relatively soft copper conductors slightly. At the very least it flattens out the strand bundle in the jaws of the terminal. There is a limit however. Do not break the screws or use the wrong sized screwdriver and cam out of the screw heads. Screwdrivers very purposefully are not furnished with tee handles to limit how much torque can be applied. OK, now the wires will not fall out of the connector.
7. Assemble the strain relief around the outer jacket of the cable. If you cut the wires too long and not enough outer jacket appears on the inside of the strain relief you might be able to bend the wires a little and get enough strain relief penetration. This is a desperation move and will seriously cut the strength of the connection. Tighten the strain relief screws until the jaws of the strain relief seriously deform the cable jacket. Don't worry about crushing the cable, it is meant to take it. If the jacket goes far enough inside the strain relief the clamps will be biting down on the full, reinforced-with-fillers cable. Do not tighten the screws so much that you break the strain relief, although this is hard to do because most strain relief assemblies bottom out and will not tighten past a certain point. One more thing. Do not even THINK about compensating for a sloppy job to this point and tighten the strain relief on the three wires themselves. They will not be gripped firmly enough and the cable will surely pull out of the connector, as it did in the photo. Worse yet, do not tape the wires and then clamp over the tape. The tape will creep and let the wire slowly slide back as if it were greased.
8. If you have done things correctly so far this is what you have accomplished: Strain on the cable tending to pull it out of the connector will be resisted by the wires securely clamped in the terminals. Metal clamped to metal. That is why you want them all the same length, cut square and fully inserted in their pockets. If one is shorter it takes all the strain and may loosen. The strain is shared by the strain relief, of course. Where the strain relief shines is in resisting twisting motion that wants to rotate the cable with respect to the connector. In the photo you can see that the three wires twisted around each other because the strain relief was not applied properly. Twisting the wires pulls on some strands in each conductor harder than others and may start the process of slipping out. Finally, since the insulation of the three wires ends inside the terminal pockets there is no way anything is going to short out inside the connector. Because you have tightened the screws fully, you have a good connection which will not heat up in use. Connectors for smaller cables rely more on the strain relief to make a good mechanical connection and less on the conductors. When you have AWG 8 or 6 conductors though, they supply much of the strength of the cable in themselves.
9. Make certain that no strands of wire have escaped their contact pockets and are lying along the surface of the connector or are folded back along the insulation. This will probably lead to a short or a shock. If a strand escapes but everything else looks good, cut it off. The rules say you should not cut down stranded wires so they fit into connectors, but I don't think that applies here. The risk of short and then having vaporized metal on the insulator when the wire strand explodes is much worse than a conductor minus a few strands.
10. At last, slide on the backshell and fasten it in place. The only purpose of the backshell on a properly terminated connector is to give you something to grab on to besides the jacket of the cable itself. Maybe to make the connector look nice. Possibly to electrically insulate the connector, but only in the event of failure somewhere else. If you have exposed conductors at the connector body it will provide insulation to the outside of the connector. The backshell should just be a second line of defense, not a cover-up for a sloppy job in the first place.
I'll bet most readers never suspected attaching a shore power cable to a connector involved so much technique. Do it right, it will never fail. At least not at any of the points you worked on. Finally, get the right wire colors in the right connector holes. I have been "bitten" a few times by boats that had the black and white wires reversed in one of the shore power cord connectors. When I see shore power cables with something besides factory installed molded connectors I reach for my power analyzer to ensure the polarity has not been screwed up.
I have seen a number of variations of this problem. In each case one or more of the points I have stressed were violated and the connector either failed or was discovered in the process of failing. In the event you are terminating the mating male connector on the boat, the same considerations apply. If the wire is not long enough to do everything right, it is much better to splice on another couple of feet than to make do and risk having a failure in the future. Yes, I know the backshell/strain relief is hard to get on a bulkhead mounted shore power connector on a boat. If you cannot accomplish it properly, get someone fully qualified to help you. If the owner tells me he's done it, I often open it up to check.
August 17, 2007