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post #1 of 18 Old 05-23-2014 Thread Starter
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Voltage drop?

Can somebody explain voltage drop? And how it affects running some new wires in my boat?

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post #2 of 18 Old 05-23-2014
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Re: Voltage drop?

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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Re: Voltage drop?

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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post #4 of 18 Old 05-23-2014
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Re: Voltage drop?

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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Re: Voltage drop?

Further, too small a wire or a corroded connection has it's own extra resistance .the voltage drop (IxR) across this resistance is seen as heat (watts=volts x amps) and dim lights.

Last edited by Capt Len; 05-23-2014 at 11:44 AM.
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post #6 of 18 Old 05-23-2014
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Re: Voltage drop?

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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Last edited by MarkSF; 05-23-2014 at 12:54 PM.
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post #7 of 18 Old 05-23-2014
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Voltage drop?

Quote:
Originally Posted by Don0190 View Post
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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Last edited by Rusty123; 05-23-2014 at 01:37 PM. Reason: spelling
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post #8 of 18 Old 05-23-2014
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Re: Voltage drop?

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.


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Re: Voltage drop?

Quote:
Originally Posted by markofsealife View Post
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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post #10 of 18 Old 05-23-2014
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Re: Voltage drop?

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.

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Last edited by guyfromiowa; 05-23-2014 at 04:32 PM.
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