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Old 25-03-2008, 03:59   #1
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current share with 2 wires

one of the wiring runs i need sits right on the border of needing the next size up wiring, i realize that doubling the run using the same wire will help the sitiuation but will the 2 wires share the current roughly equally, do i need to take very careful care to amke the runs the same length or will one wire always have less resistance and take the lions share of the current ?
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Old 25-03-2008, 05:40   #2
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If they are of the same gauge, yes. If you are adding a wire in parallel, it is effectively the same as using a thicker wire,
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Old 25-03-2008, 05:46   #3
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If they of different gages then the resistance is not the same and has the potential to be a problem. What is it you are wiring? Things like a windlass have large surges and you don't want to be on the "line". A light bulb just burns a little dimmer. In terms of cost I'm not sure it pays.
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Old 25-03-2008, 06:16   #4
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When resistors (wires are resistors) are connected in parallel, their combined resistance is less than any of the individual resistances. The equation for the combined resistance of two resistors (or wires) R1 and R2:

Combined resistance of two resistors (wires) in parallel:
R = (R1 R2) (R1 + R2)

See the Wire Chart for resistances of different gauges of wire:
Wire Size Charts:
Chart Page 1:
http://www.cruisersforum.com/gallery...=500&userid=79
Notes Page 2:
http://www.cruisersforum.com/gallery...=500&userid=79

And for the published article tutorial, Goto “Ohm’s Law & You”:

http://cruisersforum.com/photopost//...php?photo=1315
http://cruisersforum.com/photopost//...php?photo=1316

or:
http://cruisersforum.com/showthread.php?s=&threadid=372
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Old 25-03-2008, 06:33   #5
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No real problem especially if they are loomed together.
Total resistance is decreased (hence current carrying capacity is increased).

Total resistance of two conductors (or resistors) in parallel is given by: R(total)=(R1xR2)/R1+R2).
Current through conductor is given by Ohms law: Current=Voltage/Resistance.So if they about the same size and length, they will have about the same resistance, therefore they will carry about equal currents i.e. they will equally share the total load.
If one has say ten times the resistance of the other, it will carry about 1/10 of the total current and the lower resistance one will carry 9/10's of the current.
Only real problem is the potential of one of them breaking (becomes open circuit), then the other one will attempt to carry the full load therefore potentially causing a problem

Opps Gord beat me.
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Old 25-03-2008, 07:44   #6
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Given the fact that the two wires need to be the same gauge the biggest issue is the termination of the two. This is the area where you could pick up additional resistance and hence a drop in current carrying capacity. As long as you're very careful to insure that both wires are terminated correctly, no problem.
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Old 25-03-2008, 13:27   #7
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"take the lions share of the current?"
Lions' share, no, probably not, the sharing will be proportional to their sizes (to their resistance actually). Perhaps if you mentioned the specifics, the wire sizes (SAE or AWG gauge? They are not the same.) and lengths, and what the load is?

Fuses (or breakers) are also supposed to be sized to prevent wiring fires from overloads, if you run two wires doubled-up but only run one fuse for the pair--that could be problematic, depending on the exact numbers.
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Old 25-03-2008, 14:19   #8
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... Fuses (or breakers) are also supposed to be sized to prevent wiring fires from overloads, if you run two wires doubled-up but only run one fuse for the pair--that could be problematic, depending on the exact numbers.
I don't understand the concern (above).
For purposes of specifying over-current protection (fuses/breakers), two parallel wires should be considered as one conductor.
I recommend de-rating the total (additive) current-carrying capacity of the pair, by a factor of about 80%.
Notwithstanding, that you can increase a circuit's capacity by paralleling another conductor onto an existing one - I'd recommend installing new, properly sized conductor(s).
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Old 25-03-2008, 14:46   #9
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Let me clarify that. Suppose you take two conductors, each sized for a 15A safe carrying capacity. So you stick a single 30A fuse on that run...or a 20A fuse to be conservative. Now, mice get aboard and chew one of the wires, and the entire load is running through one wire, unknown to you. You've got a 15A-rated wire, and a 20A fuse, and that may start a fire.

Likely? No, maybe not. But haven't we all seen stranger things happen? If there are two wires run together that way, arguably EACH wire should be separately fused, or the common fuse should be rated for less than the capacity of EITHER wire. Which would defeat the purpose of having the extra wire, unless that was there strictly to lower the voltage drop, rather than to carry extra current.

" I'd recommend installing new, properly sized conductor(s)." Yup, me too. Wire is generally cheaper than fire extinguishing.
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Old 25-03-2008, 22:24   #10
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Quote:
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.... Now, mice get aboard and chew one of the wires, and the entire load is running through one wire, unknown to you. You've got a 15A-rated wire, and a 20A fuse, and that may start a fire.

Likely? No, maybe not. ....
" I'd recommend installing new, properly sized conductor(s)." Yup, me too. Wire is generally cheaper than fire extinguishing.
Or the said mouse (or mice) chew through half of the new correctly sized conductor and hey presto, back to square one, potential fire on board.
Moral of the situation - keep mice off ship and keep rats on board (for obvious reasons).

There are very occasionally reasons to split a larger conductor into two smaller conductors in parallel (for instance to pass through an existing connector that doesn't cater for the larger conductor but will handle two smaller conductors or to make the fitting of end terminals easier).
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Old 26-03-2008, 02:59   #11
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Or the said mouse (or mice) chew through half of the new correctly sized conductor and hey presto, back to square one, potential fire on board.
Quote:
Originally Posted by hellosailor View Post
...Suppose you take two conductors, each sized for a 15A safe carrying capacity. So you stick a single 30A fuse on that run...or a 20A fuse to be conservative. Now, mice get aboard and chew one of the wires, and the entire load is running through one wire, unknown to you. You've got a 15A-rated wire, and a 20A fuse, and that may start a fire.
Likely? No, maybe not. But haven't we all seen stranger things happen? If there are two wires run together that way, arguably EACH wire should be separately fused, or the common fuse should be rated for less than the capacity of EITHER wire. Which would defeat the purpose of having the extra wire, unless that was there strictly to lower the voltage drop, rather than to carry extra current.
" I'd recommend installing new, properly sized conductor(s)." Yup, me too. Wire is generally cheaper than fire extinguishing.

There are numerous examples of good practice that will work to keep these scenarios highly unlikely, including maintaining a rodent-free zone around all wiring ...
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Old 26-03-2008, 07:24   #12
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thanks for the answers guys now i have one more question, i have a small portable firdge that suffers voltage drop, i have some excess heavier duty cable can i cut the lead short from the fridge end and plug end and splice my heavier cable into the middle, the fridge end is sealed and i have no chance of getting the cable i have into the plug, are their any concerns with me doing this?
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Old 26-03-2008, 07:39   #13
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Why not take the heavy cable and run a proper run to the fridge outlet. This requires no modification to the fridge and makes all your wiring proper. Splicing a piece in the middle just seems like a problem made worse.

It sounds like you need a 12 volt outlet in a place closer to the appliance. A good quality 12 volt outlet is inexpensive. Wired properly it could serve other purposes too.
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Old 26-03-2008, 07:39   #14
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Not really.
By increasing your installed wire size (inasmuch as you can), over what the OEM used, you can only improve the installation.
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Old 26-03-2008, 13:04   #15
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Northerncat, this is in no way intended to be disrespectful but as your questions indicate that you do not have an electricial background, I have to ask if your wire splicing techniques are "up to speed"? If they are, please disregard this post .

The reason for asking is that a less than high quality splice may introduce unwanted resistance into the fridge circuit that is already suffering from too much resistance (i.e. giving rise to the voltage drop experienced). Especially as I understand you will be adding 4 splices and unless they are very low resistance, they may undo the benefit of the heavier cable.

I am sure there is information galore on wire splicing on CF , however a good solder joint, insulated and mechanically secured is hard to beat unless it is in a high vibration enviroment.

BTW, Cairns is one of my favourite places in Oz, I have been visiting on and off for over 45 years and best couple of hours sailing (ever) was rounding Cape Grafton and sailing into Trinity Inlet in mid 80's.
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