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  #41  
Old 02-27-2008
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Quote:
Originally Posted by sailingdog View Post
No, in a house that might be the case... on a a boat, where there are often no switches for a circuit, other than the breaker, the breaker really has to be designed to protect both the wire and the load. That isn't always true, since I do have a few fused switch panels on my boat, to separate out the individual lighting fixtures from a single circuit breaker, but this is not generally the case on a boat.
Yeah.. maybe for lights and pumps but anyone trusting a generically sized (5, 10 15 etc) mechanical breakers for radios, chart plotters, radar, GPS etc is asking for it.. these devices need fuses. I never understood those HUGE breaker panels with an expensive breaker for EVERYTHING.. No shame in using a 15 amp breaker for "electronics"
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Old 02-27-2008
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Quote:
Originally Posted by sailboy21 View Post
Yeah.. maybe for lights and pumps but anyone trusting a generically sized (5, 10 15 etc) mechanical breakers for radios, chart plotters, radar, GPS etc is asking for it.. these devices need fuses. I never understood those HUGE breaker panels with an expensive breaker for EVERYTHING.. No shame in using a 15 amp breaker for "electronics"
I agree - they should be judicially broken up and I actually prefer having a backup to a backup...there is nothing wrong in my book having inline fuses close to the source that further protect.. odds are the main breaker will go - but in the rare case - nice knowing you are providing that additional eh... birth control measure in case the main breaks...
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  #43  
Old 02-27-2008
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Originally Posted by artbyjody View Post
Oversizing wire however actually generates more load resistance to begin with and should not be overly done or practiced as you decrease the overall cpacity of your storage system...which is the making the batteries work harder and deplete faster...
For a 12VDC power system, Jody, that statement is complete hogwash!!

You can oversize the wire to blazes and the only thing it will hurt is your wallet (what, with the price of copper these days) - and increase the size of any potential fault current, meaning a bigger bang if something shorts out. Heck, there's nothing stopping you wiring your entire boat in 1" copper bus-bar should you ever feel so inclined. Will it work better than a correctly-sized cable? No. Will it cost more?. Yes. Is the boat more likely to sink due to excess weight or be raided by copper-recyclers because there's more copper on board?.. Yes. Are my batteries more likely to explode if something goes wrong?... Yes.

.. hence, there's no practical reason to do it - but that doesn't mean it won't work just fine and dandy.

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Originally Posted by artbyjody View Post
CSA= CONDUCTOR CROSS SECTIONAL AREA IN CIRCULAR MILS.
Interesting link. I've been around a while but I've never come across a "circular mil" before. If I didn't know better I'd have thought it might be something you grind flour on!! We'd call it "square millimeters" over here. It's amazing what you learn on Sailnet...
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Last edited by Classic30; 02-27-2008 at 02:47 AM.
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Old 02-27-2008
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Quote:
Originally Posted by Hartley18 View Post
For a 12VDC power system, Jody, that statement is complete hogwash!!

You can oversize the wire to blazes and the only thing it will hurt is your wallet (what, with the price of copper these days) - and increase the size of any potential fault current, meaning a bigger bang if something shorts out. Heck, there's nothing stopping you wiring your entire boat in 1" copper bus-bar should you ever feel so inclined. Will it work better than a correctly-sized cable? No. Will it cost more?. Yes. Is the boat more likely to sink due to excess weight or be raided by copper-recyclers because there's more copper on board?.. Yes. Are my batteries more likely to explode if something goes wrong?... Yes.

.. hence, there's no practical reason to do it - but that doesn't mean it won't work just fine and dandy.



Interesting link. I've been around a while but I've never come across a "circular mil" before. If I didn't know better I'd have thought it might be something you grind flour on!! We'd call it "square millimeters" over here. It's amazing what you learn on Sailnet...
Nice post - like the humor but you are wrong in your basis... "and increase the size of any potential fault current, meaning a bigger bang if something shorts out." you actually want that event to be with the thinnest gauge wire you can - why - the bigger it is the bigger the threat in terms of fire... a smaller diameter wire if it gets overloaded melts , burns etc - has less fuel so to say... minimizing your risk...

Just because it is 12 VDC - trust me - I had a car catch on fire due to the wiring ... proper specs yield proper damage control situations and appropriately more manageable situations...

But sure if you are lax (and eat lucky charms for breakfast) or you perform the wiring in somewhat professional manner odds are that will not happen - but when it comes to recommendations over the internet - or in general - I prefer to err on the side of least is more...
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  #45  
Old 02-27-2008
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Most of the equipment you mention has in-line fuses in their power leads.

Quote:
Originally Posted by sailboy21 View Post
Yeah.. maybe for lights and pumps but anyone trusting a generically sized (5, 10 15 etc) mechanical breakers for radios, chart plotters, radar, GPS etc is asking for it.. these devices need fuses. I never understood those HUGE breaker panels with an expensive breaker for EVERYTHING.. No shame in using a 15 amp breaker for "electronics"
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Old 02-27-2008
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Quote:
Originally Posted by Hartley18 View Post
Interesting link. I've been around a while but I've never come across a "circular mil" before. If I didn't know better I'd have thought it might be something you grind flour on!! We'd call it "square millimeters" over here. It's amazing what you learn on Sailnet...
LOL about the flour grind!!! While all wires have a "circular mil" (unit of area)it is mainly used on the US system for the larger gauge wires. Starting at 250 MCM, it is a direct relation to the circular mil size, which for 250MCM is 250,000 circular mils.
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  #47  
Old 02-27-2008
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Wow, I had no idea what I started. Great thread and I have learned a lot.

The last discussion about cautions on oversizing wire has me scratching my head though. I have reviewed both Nigel Calder's and Don Casey's books and all I find there is to make sure the wire is big enough for the anticipated load. I don't see anything that says to make sure the wire is not too big. In fact I think both say oversizing is good.

For my rewire project the tables say for a 5 amp draw with a 40 foot round trip run 10awg is the wire to use. For my run of lights through the boat on either side this is what I need and will be my longest run. I was going to use this gauge for much of my wiring and then a smaller gauge say 14awg for the running lights which are a long run but will only have a 1 or 2 amp draw.

I don't see anything that says don't use wire that is too big? I know I am probably paying a little more than I need to but felt it easier to just have two spools of wire and pick the one that is larger than needed for each run.
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  #48  
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Maccauley-

You don't want to go too large on the wiring since that costs money and adds a good bit of weight. Sometimes, the fact that you can buy more wire of a given size means you'll save money on that wire, over buying several different gauges of wire in shorter lengths, and the cost difference becomes fairly minimal.
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  #49  
Old 02-27-2008
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Quote:
Originally Posted by artbyjody View Post
Oversizing wire however actually generates more load resistance to begin with and should not be overly done or practiced as you decrease the overall cpacity of your storage system...which is the making the batteries work harder and deplete faster...
Oversizing you have to consider also distance and gauge ensure sure you do not see more than a 3% drop in voltage over the run... (if you can make sense of it - the charts American Wire Gauge table and AWG Electrical Current Load Limits)

A checklist however somewhat related:

Marine Electrical Check List (good site in general)

Not that it is an evil practice -however....really should be avoided...


VD= 2*K*I*L/ CSA

WHERE:

VD= VOLTAGE DROP
I= CURRENT
L= DISTANCE OF THE LOAD FROM THE OUTLET (IN FT)
K= 12 FOR COPPER AND 19 ALUMINUM (FOR MAXIMUM TEMP OF 75 DC)
CSA= CONDUCTOR CROSS SECTIONAL AREA IN CIRCULAR MILS.
Over sizing the wire will DECREASE load resistance as the wiring runs will add less resistance to the circuit as a whole. The equation you posted says exactly that. For a given load, the larger the CSA, the less inherent resistance the wire adds to the circuit, hence the lower the voltage drop.

Oversized wire does not increase the available fault current of a circuit, a protective deveice with a larger rating does. If you size the breaker (or fuse) for the wire, which you should, then the breaker (or fuse) should trip LONG before the wire melts and burns. If it doesn't, there was no point in the protective device in the first place. Minimizing your "risk" by using a small wire that won't burn as long is the definition of false security. How about preventing the wire from burning in the first place? As far as I'm concerened, I don't want any fire on my boat, no matter what size.

As Hartley18 stated, oversizing your wiring, as SD has done (as well as myself) will hurt nothing but your wallet.

Now I certainley agree that if you use grossly oversized wire, and size the breaker or fuse to protect that wire, you have created a situation that will produce more fault current before the protective deveice activates. It does not, however, increase the risk of the wire burning because the wire and breaker are sized to match each other and the breaker will trip long before the wire reaches its burning point.
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I have a Russian motorcycle, and I wish the wires worked evey now and again.

Right now they work every once in a while and blow fuses nearly all the time.

Thicker wire would just tempt me to hang myself with it.
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