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bell ringer
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Basically:

there is resistance in the wires, the more current that flows though the wire the more resistance there is, this results in lower voltage at the end
 

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Think of it like a garden hose. Long hose, small diameter, less water comes out the end. Same length, larger diameter, more water comes out.

WADR, this is basic electrical 101. You might want to buy a book. Charlie Wing;s is a very good one.

Blue Sea Systems - Innovative electrical systems - Built to last has some good technical literature on their website.
 

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All above correct. Start with 12 V battery wire out, device, wire back. The total voltage drop around the loop is 12 V. Smaller wires more voltage drop, less voltage across the device.
 

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Don Casey's Complete Sailboat Maintenance Manual goes into great detail on voltage drops, calculating the correct size of wire, and so on.

There's a simple calculator here :

Voltage Drop Calculator Genuinedealz.com

But basically for running wires in your boat, the following rules apply

16 gauge is the thinnest allowable on a boat
The breaker should be sized to protect the wire, ie trip below the wire's rated current
Rated currents vary with where the wire is (see the table at the above link, wire is derated in engine compartments)
The wire should be sized to give less than the allowable voltage drop, based on the current drawn by what you are running off of it
You can also place fuse holders near to the device (Blue Sea Systems make some nice ones)

ABYC says that voltage drop for critical devices (VHF, GPS, etc) should not exceed 3%. For others, 10%.

Fuse holders protect the device, breakers protect the wire

For wire runs I prefer boat cable, which is 2 core with an extra protective sheath. Red and yellow are the approved colours now for DC + and -.

I usually keep a good length of 2 core, 14 guage, red and yellow, boat cable on the boat, which suffices for most jobs.

So let's say we have a VHF radio that can draw 6A. Using the calculator above, and entering 14 gauge wire, a 10ft wire run, 12V DC, we get a drop of 2.59% which is just acceptable. For a 20ft wire run we have to use 10 gauge wire to stay below 3%.

14 guage wire is rated for 35A outside the engine space, so a typical 20A breaker will protect it fine.
 

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Basically:

there is resistance in the wires, the more current that flows though the wire the more resistance there is, this results in lower voltage at the end
Slight correction: resistance of wiring is constant (at a given temperature). Though it is correct that a higher current through a given resistance yields higher voltage drop (in accordance with Ohm's Law: V = I x R).

In a 12 volt circuit, the total voltage drop (by definition) is 12 volts. That voltage drop is split amongst every component through which current flows: wire, connectors and loads, in proportion to each component's resistance relative to the total circuit resistance. Ideally, the entire 12 volts should be available to the load. In order to approach this ideal, the voltage drop across non-load components (wires and connectors) should be as low as possible, which means that wiring should be adequately sized and connectors should be well designed and installed.

In general, larger wiring is preferable, but there is a point of diminishing returns, which is where the thumb rules and wire size tables are valuable.
 

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I thank the members of the Sheerly Academic Answer Brigade. ;)

I shall now make a reply on behalf of the non-academics.... :)

By the time a wire has done a long lap around the boat from the battery bank it will have dropped a fair bit of voltage, say HALF a VOLT. This is nothing to worry about.

However, if at the end of that long wire run is a difficult bit of equipment, for example, a fridge or freezer, you may be on the cusp of getting a problem.

So if you are running new wires and its going to be a long run: Battery Bank to Switchboard, to other side of boat then look carefully at the wires already installed and try to buy a thicker one.

If you go stupidly thick they will not fit through the existing conduit and will be very expensive.

If your new wiring is for a shorter run, or for a small drawing unit just buy the same size wire as you already have.


Mark
 

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i thank the members of the sheerly academic answer brigade. ;)

i shall now make a reply on behalf of the non-academics.... :)

by the time a wire has done a long lap around the boat from the battery bank it will have dropped a fair bit of voltage, say half a volt. This is nothing to worry about.

However, if at the end of that long wire run is a difficult bit of equipment, for example, a fridge or freezer, you may be on the cusp of getting a problem.

So if you are running new wires and its going to be a long run: Battery bank to switchboard, to other side of boat then look carefully at the wires already installed and try to buy a thicker one.

If you go stupidly thick they will not fit through the existing conduit and will be very expensive.

If your new wiring is for a shorter run, or for a small drawing unit just buy the same size wire as you already have.

Mark
saab?
 

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You can measure voltage drop instead of calculating or WAGing it. Use a digital voltmeter. Connect one lead to one end of the wire that is supplying the current. Attach the other lead to the other end of the same wire. With the circuit energized the voltmeter will show the actual amount of difference between the voltage at one end of the wire and the other end of the same wire. That voltage reading is the voltage drop. Ideally the voltage difference would be zero, but only if the amount of current could travel through that particular wire and encounter a very small amount of resistance. If the wire is open, that is to say, not connected or broken, the voltage drop will be the same as the source voltage. Of course, that can be explained by the fact that the open circuit has very high resistance and no current flows through it.
Summary: Zero voltage drop is ideal. Small amount of voltage drop is usually acceptable. High voltage drop indicates high resistance. Also, the sum of all of the voltage drops in a series circuit must equal the source voltage.

Ready? GO!
 

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It makes a whole lot of difference when you measure the voltage drop under load. Not just the drop in the bare unloaded wires, but the actual drop when the device at the far end is pulling power. Drop under load will usually be much higher than drop on bare wires. Neither one is good.
 

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Iowa-

It makes a whole lot of difference when you measure the voltage drop under load. Not just the drop in the bare unloaded wires, but the actual drop when the device at the far end is pulling power. Drop under load will usually be much higher than drop on bare wires. Neither one is good.
I'm pretty sure he was describing measurement under load. In fact, his methodology doesn't really serve any purpose if the circuit is not under load, since there would be no current flow.
 

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Rusty, even with no load but the multimeter, that's still "a" load, and current will indeed still flow. If it didn't, you couldn't measure anything. The problem is that modern electronic multimeters are very high impedance, very low load, which is a mixed blessing.

Put on one on the leads from a solar panel 1/2 hour before sunset and you may measure 12 volts. Now hook up a light bulb...and watch the voltage drop to 7 volts.
 

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Rusty, even with no load but the multimeter, that's still "a" load, and current will indeed still flow. If it didn't, you couldn't measure anything. The problem is that modern electronic multimeters are very high impedance, very low load, which is a mixed blessing.

Put on one on the leads from a solar panel 1/2 hour before sunset and you may measure 12 volts. Now hook up a light bulb...and watch the voltage drop to 7 volts.
You are right - my statement that no current would flow in a no load condition is a bit of a simplification, since the meter draws a minuscule amount. However, my point was merely that Iowa's useful suggestion to measure the voltage drop across a component (like a wire) would only be meaningful in a loaded condition (as he mentions) for precisely the reason you state in your solar panel example.
 

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It doesn't have to be very complicated. This online calculator AWG by wire length/amps calculator
does it for you - just fill in the blanks.

Wire ampacity - the current it can handle - is here Allowable Amperage in Conductors - Wire Sizing Chart - Blue Sea Systems
Marine rated tinned wire is 105 degrees C rated.

While ABYC allows 16 awg I use nothing smaller than 14 awg with the possible exception of led lights. 14 awg is cionsidered a 15 amp circuit and for a higher load than that the wire should be larger.

Fuses or breakers protect the wire and should be close to the source. Starting at the batteries you fuse for their wire size. When you go to a smaller wire you need a smaller fuse as the larger fuse will not protect the smaller gauge wire downstream. Basic principle is the fuse has to blow before the wire catches fire.

Some items - notably electronics - have inline fuses in their pigtails. These are sized to protect the equipment which they sometimes do and sometimes don't.

I agree Charlie Wing's book is the best for an amateur and is up to date.

Boatowner's Illustrated Electrical Handbook: Charlie Wing: 9780071446440: Amazon.com: [email protected]@[email protected]@http://ecx.images-amazon.com/images/I/[email protected]@[email protected]@51ic0iynkYL

While you can measure voltage drop in an existing wire it is of no use when deciding what size wire to use for a new circuit.
 

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BOOKS

Every so often folks ask: "What book should I buy to learn electrical stuff about my boat?"

The appropriate answer is: "Depends on how you like to learn."

Go to a chandlery and read a few in person, and start by buying one that you think suits your "level" and is "readable" to you."

A good starting list is provided by West Marine in the their online Advisors (and usually in their catalogs, one of which should be in your house and the other on your boat).

Recommended Books on Marine Electrical Systems | West Marine
 

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Discussion Starter · #17 ·
Thanks, all! I've got the Casey book and the Adlard Cole book, just been a while since I've dug them out. Seems like every book I get on a subject there's 5 more recommendations for others, lol.
 

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Thanks, all! I've got the Casey book and the Adlard Cole book, just been a while since I've dug them out. Seems like every book I get on a subject there's 5 more recommendations for others, lol.
Doesn't matter how many you have if you don't read any of them...:confused:
 
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