Moderated Discussion Areas
  ContinuousWave: Small Boat Electrical
  Murphy's Law for Electrical Wiring

Post New Topic  Post Reply
search | FAQ | profile | register | author help

Author Topic:   Murphy's Law for Electrical Wiring
jimh posted 08-25-2008 04:48 PM ET (US)   Profile for jimh   Send Email to jimh  
A well-known corollary of Murphy's Law as applied to electrical wiring: Any wire cut to length will be too short.

This weekend I was involved in (yet another) electrical upgrade project on my Boston Whaler boat. I added a marine 12-volt receptacle, which in an automobile is known as a cigar-lighter receptacle. When I was finished with all the mechanical aspects of installing this receptacle--and they're a story in itself--I looked at my existing wire inventory for a suitable length of wire to finish the connection between the receptacle and the power distribution panel. (I am not sure if the small little panel bolted to the bulkhead under my helm console in my 1990 Boston Whaler really qualifies to be given a title like "power distribution panel," but let's run with it.)

Of course, in the hundreds of feet of wire in various gauges, colors, and lengths in my large box of wire, there was nothing suitable. I decided to make an emergency run to WEST Marine to get the proper wire. I was encouraged by my recent finding of a $10-Reward Coupon from WEST Marine; I just uncovered it a few days before in a pile of boat paperwork. So essentially the wire would be free.

On the way to WEST Marine I began to ponder how much wire to buy. The actual distance between the receptacle and the panel is about one foot. With Murphy's Law in mind, I figured I would double that length, and buy two feet of wire. When I got to the store, I noted the price was not as bad as I thought, only $1.64/foot, so I figured I would splurge and get three feet. My reasoning was that any surplus would be useful for something, and I did not want to drive back if I really needed more. I called the sales associate over and asked her to cut three feet of the wire for me.

The sales associate rather casually measured off the wire, using the display rack as a ruler. I questioned her about the length, and she replied that the rack was four-feet wide, so I was getting a little bonus wire.

When I got back to the boat I went about installing the wire. I connected one end to the receptacle, and then, following the cable routing of the existing wiring, I threaded the new cable through several cable-tie points and made it part of the bundle going to the distribution panel.

When the wire dressing was finished, I discovered I had just about the exact length of wire I really needed. In fact, I did not trim even an inch off the new wires. I terminated them with ring connectors and installed the new circuit.

This is a good demonstration of how to plan a wiring job: have about four times as much wire on hand as you think you will need.

Jerry Townsend posted 08-25-2008 05:22 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Well - I don't know Jim - the West Marine "scale" could be way off too. Maybe that is the reason I always buy wire in spools - now, bear in mind, I probably don't have the color selection you have. Hope your cigarette lighter plug project worked out well for you. ------- Jerry/Idaho
jimh posted 08-25-2008 05:41 PM ET (US)     Profile for jimh  Send Email to jimh     
I have spools of 16-AWG marine grade wire, but for this circuit I wanted 12-AWG. And I wanted black and red insulation.

By the way, I used the handy ANCOR wire calculator to select the wire gauge. See the link from

Frank O posted 08-25-2008 06:59 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Funny, I was just down at West Marine on Saturday morning, buying wire to run from my outboard, under the deck and to the console. I noticed they had some spools of 18 gauge/35 feet long, and some of 16 gauge/25 feet long. Given that my boat is 21', I thought either length would be adequate, but went with the 18 gauge as that appeared to be the more appropriate wire diameter. Imagine my surprise when I found I had only about three feet left over after completing the run (which involved some sidways zigzags to get to the under-deck tunnel). So the 4x rule of thumb doesn't sound that crazy ...
swist posted 08-25-2008 07:02 PM ET (US)     Profile for swist  Send Email to swist     
I do low-voltage DC wiring as part of my real job. Our standards require squared-off leads, raceways, and all kinds off other hardware that makes the actual length of wire required significantly more than the point-to-point length, or even the length you might calculate taking the various diversions into account.

After doing this for more years than I can remember, the number of times I have come up short is embarassingly more than it should be. But given the price of copper, you can't simply every time reel off ten times what you think you need.

I still have no idea where all that wire goes...

andygere posted 08-25-2008 07:10 PM ET (US)     Profile for andygere  Send Email to andygere     
I'm glad that I'm not the only one who has experienced Murphy's "magical shrinking wire". I am amazed at how often the extra material I buy is just enough, or even still short of what I actually need. In boat maintenance and repair, leftovers are a good thing. Also, I applaud your requirement for the correct color wire. Even if nobody else ever sees it, it matters.
jimh posted 08-25-2008 08:04 PM ET (US)     Profile for jimh  Send Email to jimh     
There is not much place on a boat for re-fitting with 18-AWG wire. This excerpt is from the American Boat and Yacht Council (ABYC) recommendations on electrical practice:

"Conductors shall be at least 16 AWG. EXCEPTIONS: 1. 18 AWG conductors may be used if included with other conductors in a sheath and do not extend more than 30 inches (760mm) outside the sheath.”

This is quoted at the ANCOR technical web site mentioned and linked above.

seabob4 posted 08-25-2008 09:15 PM ET (US)     Profile for seabob4  Send Email to seabob4     
Frankly, 14GA is the wire I find most useful for most things. I feel 16 is a little light, especially for pumps. Pacer, out of Sarasota has Ancor grade wire at far less than Ancor.

Key to keeping corrosion out of marine wiring? Adhesive lined shrink sleeving.

davej14 posted 08-25-2008 10:18 PM ET (US)     Profile for davej14  Send Email to davej14     
Sometimes I cut it twice and it is still too short.
Frank O posted 08-25-2008 11:02 PM ET (US)     Profile for Frank O  Send Email to Frank O     
There is not much place on a boat for re-fitting with 18-AWG wire. This excerpt is from the American Boat and Yacht Council (ABYC) recommendations on electrical practice:

"Conductors shall be at least 16 AWG. EXCEPTIONS: 1. 18 AWG conductors may be used if included with other conductors in a sheath and do not extend more than 30 inches (760mm) outside the sheath.”

Dang those standards! In my case I was replacing the kill-circuit wire from the engine to the console, as the existing one had a spot where it was grounding, and it was easier just to run an entire new wire. My selection of 18-gauge was based on eyeballing the old wire -- which, of course, was bundled in a sheath.

For the time being, the new wire seems to be functioning fine. I'm thinking of adding some kind of cushioning or strain relief where it turns corners, such as at the exit of the under-deck tunnel in the bilge area. Beyond that, if the engine either stops running -- or won't stop -- I guess I'll know I need to upsize the wire.

contender posted 08-26-2008 02:03 PM ET (US)     Profile for contender  Send Email to contender     
First I agree with seabob, second I ALWAYS purchase something to much, to long. Its not worth it to me to have to go back to the store or get 1/2 done and see I run out, not enough, its just not worth it to me. Measure twice cut once, but you guys should see my store room...good luck
swist posted 08-26-2008 03:44 PM ET (US)     Profile for swist  Send Email to swist     
And a corollary to Murphy's Law for Electrical Wiring:

When you do go into your mountainous pile of scrap wire in the basement, you still won't find the right length, gauge, and color that you need.

Post New Topic  Post Reply
Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.