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Old 05-12-2015, 08:56 AM
RUSSELL RUSSELL is offline
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Electrical Connectors, Wire Length, and Voltage Drop

Something has me a little puzzled and I thought I throw this out there for general thought.

Neil Dutton posted a link which described how the electrical system associated with an engine works; the link was actually automotive but I cannot see how the principles are any different for us. At any rate, the point was that the battery discharges while starting and supplies the current to the starter motor and all other devices until the engine is running and the alternator is rotating at a sufficient RPM. At this point the battery is actually a load and all current utilized everywhere within the system is supplied by the alternator (again, when running at sufficient RPM - changes back at some low RPM).

If I understand this correctly, because electrons break free from the outer valences of their atomic orbits and move at the speed of light, every point of a wire is the same for our purposes (i.e., moving fifteen feet along a wire at speed of light is equivalently being in the same place for our purposes). Thus, it seems to me that if we measure the voltage to ground at any point in the system then the voltage should be whatever the alternator is putting out (again, only concerned with the running RPM situation).

However, we know this is not the case. Again, if I understand it correctly, this is because there are no "perfect" conductors - very material has some resistance. Some materials have very low resistance (conductors); some materials have very high resistance (insulators). I assume we use copper wire because it has low resistance, bends easily, is plentiful and easy to produce, and is resistant to corrosion. My understanding is that marine wire is "tinned" to improve the corrosion resistance.

So wire has some resistance, and so force (voltage) is required for current to flow. Since energy can neither be created nor destroyed, this force is transformed to heat. We know that energy has been transformed because there is a "voltage drop" - if we measure the voltage at one of the wire versus the other end, they are not the same. If we use Google we can find charts and formulas based on Ohm's law and the properties of the materials that tell us how much voltage drop there will be and how much heat will be generated for a particular wire gauge at a particular length.

Finally, my question: what about connectors? No connection can be perfect and must add resistance. Do similar charts and formulas exist for each added connection?

In planning my re-wiring, I used the wire chart for a 3% voltage drop to determine wire gauge and then moved down one AWG (i.e., used the next larger diameter size wire). So, I think I am fine.

I think I have more connections and more wire than is optimal - but, I find masses of wire confusing. I think I am increasing the resistance of the system for the sake of being able to understand where things are going and what they are doing. I think this is negligible, but then it occurred to me that I did not account for the connections.
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Old 05-12-2015, 09:24 AM
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Quote:
Finally, my question: what about connectors? No connection can be perfect and must add resistance. Do similar charts and formulas exist for each added connection?
Not really....

http://www.dmctools.com/Products/crimping_facts.html

"A proper crimp will have resistance equal to or less than the conductor."

'Proper' being the operative term. A bad crimp may have much more resistance. Infra red imaging is used commercially to look for bad terminations in electrical boxes - the higher resistance causes heating (and potentially fire).

The magic being you can ignore the crimp for voltage drop calculations, as long as you are using a good crimping tool, and perform some pull tests on your finished work. The pull test is the practicable way to prove your crimper is set properly.
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Old 05-12-2015, 09:48 AM
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Talking

Neil has posted on numerous occasions that he prefers to avoid connections where practical, and he cited the problems with Catalina "plug ins". I think using good terminal blocks and posts is not so much of a problem. I would only add that in building your electrical "tree" be wary of "cobble-ons" at the end of a circuit; that is where voltage can be lost dramatically. Where voltage stability is important run lines direct from a main buss which is being fed directly by the alternator.
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Old 05-12-2015, 09:51 AM
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Russel,
Jeff has it right - ignore the connectors for sizing the system.
The fly in the ointment is later in time when the system starts to act up. Then the whole thing becomes suspect, with the prime suspects being those close to the bilge, exposed to salt air, or subject to vibration. Voltage drop across a connector or over the length of a cable will identify the part of the system which is not acting 'as new'.
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Old 05-12-2015, 10:13 AM
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A little follow on.... You obviously want to do this properly - that's great, and you're asking the right questions.

Don't scrimp on the lugs or wire. UL 1426 listed marine wire is tinned for corrosion resistance. Marine lugs are close-ended on the ring side and have adhesive heat shrink so air or water won't get in. Both are considerably more expensive than household stuff. It hurt$ when you need to remake a connection. But they are worth it, IMHO.

PS. Simplify. I went down this rabbit hole and had fun, but every time I touch the boat I try to simplify. The opportunity is in the design phase.

“Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.”
― Antoine de Saint-Exupéry, Airman's Odyssey

YMMV.
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Last edited by BunnyPlanet169; 05-12-2015 at 10:21 AM.
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Old 05-12-2015, 11:04 AM
RUSSELL RUSSELL is offline
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OK - did not know that about connections.

"The proper tool for the job" - every time I try to do something I learn this again! Guess I better get at those 2 AWG connectors again!

Yes, I am using tinned marine wire, tinned ring shank connectors with heat shrink, and as with the wire all bus bars and terminal posts are larger gauge than required.

The Catalina 30 in the vintage I have (and Neil has) has an automotive trailer plug connector that would lay under the liner in a place you could not get to and absorb moisture. Mine broke in my hands when I pulled it out. OEM not so good on that one!
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Old 05-12-2015, 01:45 PM
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Maine Cruising forum has some great "how-to" articles on his site - this one should be required reading for all boat owners!



http://www.pbase.com/mainecruising/wire_termination
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