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Old 22-11-2007, 00:32   #1
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wire chart ?

I am sure it is just me but in the study hall wire size chart, why is there a difference between the mm 2 and the cm 2 ? for example 16 guage is bracketed as 1.31 mm 2 but in the next column it has 2,336 cm 2 . What does this corelate too ? I have rolls of wire here at work that are of a standard automotive size for light runs and it has .75 mm writen on the label. What does it all mean...........
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Old 22-11-2007, 02:08   #2
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Cooper
cm 2 stands for circular mils. an area represented by a wire 1 thousanth of an inch in Diameter. This usually throws people who have not had training in the electrical fields. Here is a link to a good explanation.Circular Mils Explained Circular Mils Explained - Resources - Blue Sea Systems
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Old 22-11-2007, 04:38   #3
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Your .75 mm wire has a cross-sectional area of 0.75 square mm (0.0011625 sq. inches, and a diameter of about 0.99 mm ∅ (0.039 inches ∅).

This VERY SMALL wire has a maximum current rating of about 6.0 Amps for chassis wiring, and shouldn’t be used in any application aboard a boat.
Such small wires have little mechanical strength, and will introduce unacceptable voltage drops in even very short runs.

I don’t recommend wire sizes of less 1.0 mm
(#18 AWG)
for ANY USE aboard a boat.
And you’ll note that the smallest wire size my chart lists is #16 AWG (1.31 mm), which is my preferred minimum size.
The nearest appropriate standard metric size would be 1.5 mm.
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Old 22-11-2007, 10:37   #4
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FYI, the USCG minimum wire size for Subchapter-T inspected vessels is 14 AWG. This is for the reason that Gordon mentions above about strength. I once had to replace all my instrument gauge wire with 14 AWG wire, much of which carried less than an amp of current....what a pain that was.
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Old 22-11-2007, 17:25   #5
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Glad you all brought up the subject. I've heard that one can use automotive wire and even lamp cord or extension cord wire for wiring aboard. I've heard this from folks who were trained electricians. Is there a real advantage to using marine tinned wire?
JohnL
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Old 22-11-2007, 19:59   #6
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Originally Posted by SkiprJohn View Post
Glad you all brought up the subject. I've heard that one can use automotive wire and even lamp cord or extension cord wire for wiring aboard. I've heard this from folks who were trained electricians. Is there a real advantage to using marine tinned wire?
JohnL
Ordinary automotive or household wire will corrode. The copper gets covered in black gunge which doesn't conduct electricity, and makes soldering nearly impossible, so connecting the ends of these wires to anything becomes difficult to do and the connections become very unreliable. Eventually the copper just turns to green dust.

The houshold or auto stuff might last a few years, but someone down the line will have to replace it all one day.
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Old 22-11-2007, 22:54   #7
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I bought a boat long ago where the owner had used auto wiring. Some of the wires were swollen, under the insulation, so bad they were twice the size of the original. They were cracked and covered in green powder (copper oxide).

The worse thing is you can cut them back and back and back. They'll be black the full length of the wire.
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Old 22-11-2007, 23:19   #8
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I saw a boat once where the original owner had used Romex and wire nuts in a few places.
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Old 22-11-2007, 23:34   #9
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Romex
What's Romex??
You can use plain copper. Not recommended when tinned is easily available. But it's not easily available where I am. So if you do use plain copper, you need to ensure your terminations are well sealed and protected. I either crimp or solder dependign on the type of terminal used. I then place a piece of heatshrink over the terminal and place a small dab of hotmelt glue on the crimp/solder connection. Then I heatshrink down the shrink and it squeezes onto the glue and seals the joint. If that is to much bother, then tinned wire is a must.
When I make a termination, not only is the terminal/cable protected, but I also fit the terminations into a small plastic sealable box. For insance, all the wires from the bilge pump, float switch and high water alarm which equals 6 connections are all housed in a small plastic box screwed to the bulkhead and it has a water tight lid screwed onto it.
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Old 23-11-2007, 01:02   #10
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Romex is regular house wiring used in the US, and other places too I'd imagine. It has two or three insulated SOLID copper wires, and one uninsulated copper wire all inside an outer layer of insulation. It gets run throughout a house through holes drilled in the studs and stapled in place - no conduit. Comes in 12 and 14Ga, not sure what that is in metric, but is rated for 20 and 15 amps respectively.
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Old 23-11-2007, 01:56   #11
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Arrrr, OK, now I understand.
Way back years ago, it used to be used here in NZ and was called "twin & earth". Today's more modern version is called TPS. It has all three insulated conductors, one being the earth and an outside jacket, making it double insulated. All 230V wiring must use this stuff by law, even on boats. I am not aware of any tinned version for marine use here. I certainly would not use it for 12V applications and would cringe at the idea.
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Old 23-11-2007, 11:12   #12
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Thanks all for the good information. In the past without this forum I've followed the lead of folks who really gave me some bad advice. It has cost me a lot in time and cash. Its good to know that I need to get some marine wire.
Kind Regards,
JohnL
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Old 23-11-2007, 11:30   #13
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The point may be getting missed about why "Romex" type solid core wire is such a bad idea. Boats move and get wet...houses generally don't. Stranded wire that is crimped at the terminal is much more secure, and waterproof if done correctly than solid core wire which is terminated with wire nuts or twisted 180 degrees and screwed down. I'm sure the vast majority of people in here know this. I just didn't want to see anyone go down to Home Depot to save money on their boat wiring.

Also, Ancor sells tinned crimp terminals with the heatshrink custom fit to the terminal. All you need is a heat gun and something that is waterproof such as Tefgell over the bare metal to displace the oxygen which completely stops the oxidation. This may sound a little obsessive but I like to dip the bare wire into the Tefgell before crimping down....there is no way water is going to get in there. There is still plenty of metal to metal contact with a good crimp.

ADHESIVE LINED HEAT SHRINK RING TERMINALS
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Old 23-11-2007, 17:29   #14
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Thanks all.....If you dont ask the questions.....

cheers
Martin
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Old 24-11-2007, 21:38   #15
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I personally think the need for tinned DC wiring is over rated. Its absolute need should be driven more by location, how dry the boat is and budget rather than being the unbreakable rule many proclaim on forums.

Inside a dry boat, as many modern boats are, it is not necessary at all in my opinion and for wiring inside main/distribution panels should not be required in any boat (because if wet/damp in there one has a bigger problem than whether the wiring is tinned or not). Notably most equipment one buys, including things such as float switches and bilge pumps do not come with tinned cable. Tinned is obviously preferable for runs inside the mast and outside where it is likely to be exposed to water but an expensive waste of money for large interior cabling such as from batteries and alternators.

One commonly sees the claim that coaxial cable for radios should be tinned. In my opinion that is not needed at all, the reason being that if the shield is wet enough to need the tinning to prevent corrosion then the cable is already lossy for RF and should be replaced - perhaps any corrosion from lack of tinning is a valuable indicator of such wetness. The tinning serves no purpose other than to instill a false sense of confidence.

What has not been stated in this thread is that all runs of DC cabling should be sheathed ie the two conductors insulated and that all contained in another insulating and protective sheath (along similar lines to the TPS AC cable that Alan mentioned). For safety reasons this is far more important than tinning but the requirement is rarely mentioned. It is not possible though to get sheathed large cables such as used for charging and main conductors from batteries and probably not desirable for free airflow and the good concept of giving some seperation to the +ve and -ve cables in order to reduce the possibility of arc accidents in the future due to insulation fretting.

In our own boat which was specified by me all DC cabling is, however, tinned (its cost and availability was not an issue) except for that in the DC panel and large x-section area cables such as for the high output alternator, the batteries, invertor, windlass, etc. All AC cabling is untinned. After 11 years there is no discoloration on any of it, but the boat is a dry one. For the high quality commercial builds for others I have managed I have expected similar from the builder.

I trust that has been useful to some.
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