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Choosing Safe Cables for Your Boat

If you read last month's column (June 2010 All at Sea: Working in the Engine Room), you know we decided to add an additional 32v battery bank for the house loads on Wired, a vintage Hatteras yacht. This is not a particularly complicated undertaking, but it does require an understanding of basic DC electrical practices and safe work habits, as well as access to good references for guidance on cable sizes, overcurrent protection and the like.

Unfortunately, or perhaps fortunately, depending on your perspective, the time I spent inside the main electrical panel installing the new voltage meters revealed an issue of more immediate concern which would have to be corrected before any new projects would be started.

Like virtually every 30 year-old boat, Wired has had her share of mechanics, electricians and others installing, replacing and otherwise contributing to her systems. Somewhere along the way, perhaps as part of her re-powering, much of her primary cable had been replaced. The original factory wiring is neat and professional, employing high quality marine grade materials.

The "rewiring" is also reasonably neat but unfortunately, instead of employing marine grade materials, someone decided to utilize 600v welding cable. This is common and by now many readers are saying "there is nothing wrong with that." Wrong! This is not an acceptable practice. Don't do it – and if it already has been done, fix it. Now!

If you disagree with me and you're still reading, let me tell you why it's wrong. After all, it says right on it 600 Volts. That's way more than we'll ever ask it to carry. It's big, often bigger than the cables being replaced. If it can handle a welder, what could be the issue? Well, I'll give you two. Two really big ones!

First there's the insulation. The standard welding cable from an auto parts store or electrical supply house has rubber insulation. The use of rubber for insulation is one of the features that make welding cable so flexible and easy to work. Marine battery cable will be insulated with PVC or cross-linked polyethylene. Despite making the cable stiff and heavy, this is an important difference. The reason this matters is that rubber insulation will not survive the constant attack from warm salt water, diesel or gasoline fuel, bilge cleaner, cleaning solvents and heat that are typical in the engine room and bilge of a yacht. Over time the insulation on welding cable will soften and expand. Eventually it will become a gooey, moisture-laden mess allowing water to leak in and current to leak out.

The second reason is equally important. You may already know that marine wire differs from household wire in that the conductors (typically copper) in marine wire are made from multiple strands, whereas household wire is typically a solid conductor. If you really did your homework, you might also know that the difference between marine wire and what you might purchase in an auto parts store is that the copper conductors in "marine cable" are passed through a bath of tin before the insulation covers them up.

This is not done simply to increase the cost. "Tinning" the wire prevents water from wicking up through the capillaries between the strands. This is the second big difference between marine battery cable and welding cable. Welding cable is not tinned and its construction will allow water to wick up inside the insulation where it will corrode the thin copper strands, creating unwanted resistance and weakening the cable.

This truly is a case of "you get what you pay for." Though welding cable can be installed for 25 – 30% of the cost of proper cable, this is not a place where you want to skimp. The results will range from the mere inconvenience of having a sticky mess in your wiring bundle to voltage loss at critical components like your starter – and potentially dangerous or even lethal consequences resulting from compromised insulation or broken conductors.

Replacing six runs averaging twenty feet long with 4/0 including end fittings took about two days and cost around $1,000 in materials: a sound investment in safety, performance and peace of mind.

Peter Patterson is a Canadian Coast Guard certificated Master and an ABYC certified marine technician. He is a former Canadian Yachting Association Instructor/Evaluator and powerboat instructor. Currently he is on trickle charge while he reinvents himself.

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