Posted by Peter Kennedy on 8/4/2018 to Electrical Standards
Production boat builders usually use single conductor wires. These wires are assembled off site into wire harnesses and laid into the boat at a stage of construction where access is most available, usually before the deck is attached. By doing it this way boat builders are doing their best to automate the process. Wire harnesses made up of single conductors are physically more compact, lighter and also less expensive. So it makes total sense for boatbuilders to do it this way.
Compare that with the boat owner doing a rewiring project later in the life of the boat and you will quickly see why single conductor wires don't make as much sense for that application. The wires have to be dragged through holes and conduits, often where they don't easily fit, and in the process they get subjected to a lot of abrasion and potential damage to the insulation. Having an extra jacket is one way to minimize the potential damage to the insulation. So in most cases it makes much more sense to use duplex or triplex wire for rewiring. The only case where single conductor wires usually works out is areas with very confined access like ceiling voids.
There is one other point to consider: The ABYC standards require for AC and DC wires to be bundled separately. The outer sheathing of duplex and triplex wires counts as sheathing, so when using these wires you can run them in the same conduit. If using single conductor wires then AC and DC wires have to be bundled separately.
Is there not a greater risk of a short using duplex cable since positive and negative are side by side for the entire run?
The greatest risk of shorts are when the wires pass through openings or where they chafe against a sharp edge, or where vibration can cause physical damage. The advantage of duplex or triplex wires is that they are double insulated so are a bit more resistant to this kind of wear and tear. There isnt much chafing between the conductors inside the sheath, its more with the outside environment. The ABYC standards are quite particular about routing of wires to try and avoid this kind of damage and of course the reason we have circuit protection is to avoid disaster if it does occur. Overall I would say duplex or triplex wire has a bit more protection.
I have an RV project where I am rewiring everything from scratch. I intend both AC and DC circuits to be in seperate conduits as much as practical, junction box/wire glands to Duplex or Triplex marine wire where conduit is impractical. For my DC circuits, is there an advantage to have Duplex wire running from a common bus to each load? It seems that I would run less wire, and save weight if I ran primary wire (from the breaker) to the place I would transition to Duplex, and have my negative (Duplex) attach to a nearby ground source (Frame). Is this plausible or am I missing something? Not wishing to cut corners, trying to keep an eye on weight. Thoughts?
Cars and trucks are typically wired with the positive wire going to the device and the chassis being the return ground. To make a good connection to the chassis there may be a screw hole to make the connection and that would presumably be made before it is painted.
Its a bit harder to come along afterwards and make a lot of good connections to the chassis, drilling holes and scraping paint comes to a lot of work in a big project and you may find it easier to install duplex wire throughout.