I would like to get some opinions on this. I am doing maintenance/upgrades on my boat, some of which is electrical. I have been trying to study up on all the marine standards in order to avoid mistakes. I have found tons of rules and standards for safety, and most of the new ones for me concern the need to prevent any kind of spark that would ignite gas fumes.But with all the strict safety precautions I was completly suprised when I found out the specs for wire gauge and current capacity.I found a site listing ABYC standards ( http://www.pkys.com/Reference.htm ) which tells me that I can run 20 amps through an 18ga. wire. This was very suprising in that electrical codes (for dry land) limit you to 7.5 amps to prevent overheating! Even worse is the voltage drop you get if you actually try to run 20 amps through 18ga.He comes the math. I looked up the standard resistance for 18ga copper wire and got 6.6 ohms / 1000 feet ( http://www.bnoack.com/index.html?htt...esistance.html ), or .0066 ohms/ foot.Suppose I have something that draws 20 amps that has 10 feet of wire from the fuse block, and it is 15 feet of wire from the fuse block to the battery. (Not hard on a 20 foot boat).Total of 25 feet of wire, but you have to consider the path from the battery to the device and back, for a total of 50 feet of wire. Total resistance is .33 ohms (doesn't sound like much yet, does it?)Voltage drop = resistance x current.Voltage drop = .3 x 20 ampsVoltage drop = 6.6 voltsVoltage to device = 12.6 volts from battery  6.6 volt dropVoltage to device = 6 volts Wattage dissapated in wire = 6.6 volts x 20 ampsWattage dissapated in wire = 132 watts.Don't know about you, but trying to run a 12 volt device with only 6 volts and dissapating 132 watts in a wire is a BIG NONO in my book.And remember, this assumes all you connections and fuses are perfect!!!Please, I would like some comments. (But I will be running heavier cable)
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Wiring, wire size, and current capacity
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Re: Wiring, wire size, and current capacity
Jack,I really had a hard time believing the chart on the link you gave, so I went in search of another chart. That appears to be an accurate chart according to several websites I looked at. Personally, I would never try to run 20 amps through an 18 AWG wire either.I sell electrical goodies (AC type stuff) to contractors, and 12 AWG is the norm for 20 amps at 120V.I rewired my old boat also, and as I have just learned, I WAY overengineered the job. Looks I went way overkill on the wire guage. Oh well, it's not like the wire is really expensive, and I would certainly rather be safe than sorry!Here is one of the most complete pages I found. web pageBarefoot waterskiing...we never said it was easy...we just said it was cool!

Re: Wiring, wire size, and current capacity
Originally posted by Jack L:Voltage drop = .3 x 20 amps
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Re: Wiring, wire size, and current capacity
Jack  your reading the standard in a vaccum. Consider this:ABYC E9.15.8 Conductors used for panelboard or switchboard main feeders, bilge blowers, electronic equipment, navigation lights, and other circuits where voltage drop must be kept to a minimum, shall be sized for a voltage drop not to exceed 3 percent. Conductors used for lighting, other than navigation lights, and other circuits where voltage drop is not critical, shall be sized for a voltage drop not to exceed 10 percent.So you could never be allowed to install 18 guage wire in a circuit where you had a voltage drop of 50%.On the same page as you posted, click on 'Excerpts from Standards'.
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Re: Wiring, wire size, and current capacity
not doubtin' that you read what you read.. but it's not right an is wrong..big diff from ac to dc an what is lackin' from your 'scientific' calculations is voltage.. the more volts ,,the smaller the wire.. the less volts ,, the bigger the wire.. 12 volts is about as 'less' as it gets..ever think about becomin' an electrical engineer..
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Re: Wiring, wire size, and current capacity
Just to reiterate Rabbit's point:Technical Data  American Boat & Yacht Council Standards for Boats Conductors shall be at least 16 AWG (except 18 AWG may be used as internal wiring in panelboards). Conductors shall have a minimum rating of 600 volts (E8.14.1.1). All conductors and flexible cords shall meet the requirements of the applicable standards of Underwriters Laboratories, Inc. (E8.14.1.6). Flexible cords shall have a minimum of 300 volts (E8.14.1.2).
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Re: Wiring, wire size, and current capacity
Here is a good general discussion of boat wiring by Don Casey: http://www.boatus.com/boattech/casey/05.htm
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Re: Wiring, wire size, and current capacity
Got a good thread going here  thanks for the replies.sangerwaker One thing to note is that with 120v stuff there is not only a maximum current for a wire size but they want a maximum voltage drop of 2% (if I remember correctly). If the wire is long enough you have to up the gauge. Trying to run gear at an undervoltage is one of the fastest ways to need replacements. It is really painful to see a large electric motor try to start when someone failed to take motor starting currents into account 18 rabbit  You read it wrong. I never used percentage. the .3 (typo  should have been .33) is the resistance in ohms of the wire. If you put 20 amps across .33 ohms you get 6.6 volts of voltage drop across the wire.Also, the voltage drop problem is much bigger with 12 volt systems than with 120 volt since you are concened with the percentage drop, and 3% of 12 volt is much smaller than 3% of 120 volt.I do not believe that AC vs. DC is a difference here. I think what you mean is 120v systems vs 12v systems. (For those of you who are sticklers for accuracy there is a difference due to what is called the skin effect but it does not become significant until the frequency becomes 1MHz or more).Paul Moir  Great point. My concen is that while you and I know to consider voltage drop, how many other people do. I doubt very many have even heard of it.It may have just been a stupid oversight on my part, but voltage drop limitations do not seem to be very evident when I look at the web sites. You had to click on "Excerpts from Standards". From what I can see this "excerpt" is a limiting factor more often in wire gauge than total amperage.Would love to see it more out front.crab bait  We think of the more volts the smaller the wire because to deliver the same power if you double the voltage you half the current. At home you use 240volt for air conditioners, ovens, etc. to keep the wire gauge reasonable.But that is not what is happening here. We started with a fixed current (20 amps in our example) and calculate the wire needed due to that (and percentage of voltage drop).BTW  if you think 12 is as less as you get you should see a 300 amp 5 volt system. And I have worked on designs of systems from microvolts (receivers) to 20 kilovolt, from frequencies from DC to 40GHz.Thanks everyone so far, hope we get more interest  I think this is a topic that may boat owners don't fully understand.
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Re: Wiring, wire size, and current capacity
It may have just been a stupid oversight on my part, but voltage drop limitations do not seem to be very evident when I look at the web sites.The values shown on the ampacity table are the maximum safe amperages which the conductor can carry on a continuous basis. They do not apply to intermittent starting loads such as motor start currents.
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Re: Wiring, wire size, and current capacity
Originally posted by Jack L: I would like to get some opinions on this. I am doing maintenance/upgrades on my boat, some of which is electrical. I have been trying to study up on all the marine standards in order to avoid mistakes. I have found tons of rules and standards for safety, and most of the new ones for me concern the need to prevent any kind of spark that would ignite gas fumes.But with all the strict safety precautions I was completly suprised when I found out the specs for wire gauge and current capacity.I found a site listing ABYC standards ( http://www.pkys.com/Reference.htm ) which tells me that I can run 20 amps through an 18ga. wire. This was very suprising in that electrical codes (for dry land) limit you to 7.5 amps to prevent overheating! Even worse is the voltage drop you get if you actually try to run 20 amps through 18ga.He comes the math. I looked up the standard resistance for 18ga copper wire and got 6.6 ohms / 1000 feet...Suppose I have something that draws 20 amps that has 10 feet of wire from the fuse block, and it is 15 feet of wire from the fuse block to the battery. (Not hard on a 20 foot boat).Total of 25 feet of wire, but you have to consider the path from the battery to the device and back, for a total of 50 feet of wire. Total resistance is .33 ohms (doesn't sound like much yet, does it?)Voltage drop = resistance x current.Voltage drop = .3 x 20 ampsVoltage drop = 6.6 voltsVoltage to device = 12.6 volts from battery  6.6 volt dropVoltage to device = 6 volts Wattage dissapated in wire = 6.6 volts x 20 ampsWattage dissapated in wire = 132 watts.
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Re: Wiring, wire size, and current capacity
Ron7000,We have an interesting discussion going here. I suspect we only have the real diehards left after all this math.There is an error in your math. When you totaled up the voltage drop you forgot Rw3. This will give you another 2.22v drop, making it not 10.1 v to the device, but 7.88v.You are also using 13.5v as the source voltage, taking charge voltage into account. I used the battery voltage of 12.6 plain. If you had started with 12.6 the voltage would be closer to 6.98v by your calculations. We are still differennt by about 1 volt, but we do agree that that is no way to power equipment.Another factor is heat  the wires are heating up. I think you showed about 1.3w/ft of wire using the gauge mentioned. Maybe not too much for a single wire.Now consider that you have a wire out and back, typically bundled as a pair. That makes 2.6w/ft.To that add the fact that wires are typically bundled together. Even more heat.Another problem is that your calculations assume that the device is a fixed resistance. That is not the case a lot of times. Light bulbs have a lower resistance when cold then when hot. Electronic devices have voltage regulators that work to maintain constant voltage, and they do this by drawing more current at lower voltages to keep the power constant. This makes the problem even worse. When all is said and done, I think we agree that there are probably a lot of boats out there with wiring that owners have put in that can cause real problems.
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Re: Wiring, wire size, and current capacity
sorry, there were errors in my post. I went back and editted in bold. Biggest error was I didn't add up Resistance Total right, should be 1.03 ohms not 0.964 ohms. That in turn threw everything off.I think you got confused with my 13.5 being volts. It was amps. That value is now 12.621 when using the correct resistance total of 1.03 ohms.
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Re: Wiring, wire size, and current capacity
Originally posted by Jack L:We have an interesting discussion going here. I suspect we only have the real diehards left after all this math. ya think? I used the battery voltage of 12.6 plain. If you had started with 12.6 the voltage would be closer to 6.98v by your calculations. I don't think so. I'd have to do the math. What caught my eye and made be deicide to post was your calculations showed about 50% drop in voltage to the device. It's less than 25% actually. The killer is all the resistance in the circuit, but there is one good thing about it will explain below.We are still differennt by about 1 volt, but we do agree that that is no way to power equipment.CorrectAnother factor is heat  the wires are heating up. I think you showed about 1.3w/ft of wire using the gauge mentioned. Maybe not too much for a single wire.Now consider that you have a wire out and back, typically bundled as a pair. That makes 2.6w/ft.To that add the fact that wires are typically bundled together. Even more heat.Another problem is that your calculations assume that the device is a fixed resistance. That is not the case a lot of times. Light bulbs have a lower resistance when cold then when hot. Electronic devices have voltage regulators that work to maintain constant voltage, and they do this by drawing more current at lower voltages to keep the power constant. This makes the problem even worse. When all is said and done, I think we agree that there are probably a lot of boats out there with wiring that owners have put in that can cause real problems.
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Re: Wiring, wire size, and current capacity
The temperature rating for even the cheapest modern insulation is pretty high. Without looking a a piece of Ancor wire I suspect marine wire is even higher. Yes you should worry about (somewhat) undersize or overlength wires heating because that will increase their resistance even more and you may reach a point of unacceptable voltage drop, but I think you will find the main cause of meltdowns and fires is unfused shorts. Proper fusing of lowvoltage wiring, boats and elsewhere, is a more recent concept than for high voltage. In my field, we use a lot of 12VDC wiring and the National Electrical Code has only relatively recently (last 10 years) talked much about low voltage wiring. A lot of old boats are underfused  either by design, or by incompetent amateur work.
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