12 volt wiring, polarity, and fuses question.

[Big Dave2]

I have wired up our fish house and have wired all the "hot" wires to 20 amp fuses on a automotive type fuse box. All the ground wires will be hooked together then to the neg. battery pole.

What I am concerned most about is crossed polarity(I.E. neg. to pos) when hooking up the wires to the battery in the dark, or just while not paying attention. On our last house a radio was fried this way.

So my question is this:

Will the fuses on the "hot" wires be enough protection, or should I also put an inline fuse on the ground wire in case of a backwards hook-up?

If the ground wire is inadvertantly connected to the positive battery terminal will this cause damage to the componants or do both have to be hooked up to cause damage?

Are 20 amp fuses sufficient?

Thank you

Dave

[Dave]

20 amp fuses are quite large. Lighting circuits in a house is only 15 amps; and that circuit would run quite a few lights. Also, if you didn't run 12-gauge wire (rated for 20 amps) the wire could melt, if shorted, before a 20 amp fuse would blow.

Check at an electronics shop to see if they have a reverse polarity device (power diode)you can attach to the positive line where appliances would be connected. For DC lights, it wouldn't matter if they crossed.

[bigb56303]

first off 20 amps is way to much especially if your using 12v dc you need to look at what your going to be using and fuse each accordingly, you can get what your curreent draw is by dividing your watts by the voltage your using then fuse it that way, as far as hooking it up correctly to the battery, youll have to be carefull or make it so that you can only hook up one way. fusing the neg is not really a good idea bucause your load with vary depending on what you have on at the time and if your fused to high then you defeat the purpose, good luck if ya have any more ? id be glad to help if i can

[Macgyver55]

Most electronics without internal polarity protection require a VERY small fuse to protect them (3amp max). It would probably be wise to protect them separately.

Depending on your wire size, assuming you are using large enough (at least 16 gauge is required in order to carry a 22 amp load) and how many lights you are using, a 20 amp fuse is not that much out of line. That is the standard size fuse on many vehicle lighting circuits witch generally have a minimum of six bulbs, and usually more on them. Many larger trucks have several more lights than that and often use a 25 amp fuse and 14 gauge wire.

My gauge charts show that 12 gauge wire running 12 volts can carry a maximum of 41 amps. That is a lot! Much more than you would ever need in a fish house.

As far as the polarity issue, I'd suggest putting some type of fool proof connector on the wire ends to prevent accidently crossing them (male/female style) or at the very least marking them very obvously to prevent it. Fuses should always be put on the "load" side of wiring and as close to the power source as possible for maximum protection. Any wiring between the battery and the fuse is unprotected so make sure it is secured and well protected from rubbing and shorting out.

[JohnMickish]

Wait a minute here. You have a chart that states 12 gauge wire can handle 41 amps? Amps are amps and they don't care what the voltage is. If something is rated to carry 20 amps, it's rated the same at 12 volts as it is at 250 volts. Anyways, to split hairs here, it's not the copper wire that is going to fail, it will be the insulation. A high current load (regardless of voltage) creates a lot of heat and that will melt the insulation, causing more problems with exposed copper and when you have more than one circuit with failed insulation you start to find the fun stuff, like when the furnace comes on so does the radio . I hope this cleared things up.

Oh, to answer your question, use smaller fuses, install a battery operated puck light in the area of your battery, mark one wire with white tape and the coreponding teminal area on the battery with white paint. Makes it easier to see. If your still paranoid about screwing it up, purchase a diode pack and if that's all to much $$, spend $10 on a flashlight.

You can also get a battery quick disconnect. Both wires from the batt go into one plug, and the house wires get the coresponding plug. Fool proof.

[Macgyver55]

Here is the chart I referred to. I don't claim to be an electrician but maybe I misinterpreted the chart. Hopefully not, I wouldn't intentionally give out incorrect info. If I did, let me know where I went wrong. Thanks

Load Carrying Capacities

The following chart is a guideline of ampacity or copper wire current carrying capacity following the Handbook of Electronic Tables and Formulas for American Wire Gauge. The rated ampacities are just a rule of thumb. The insulation temperature limit, thickness, thermal conductivity, and air convection and temperature should all be taken into account. The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative. The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle .

[Whoaru99]

If you use a full-wave bridge rectifier between the main power feed wires to the fuse box and the battery, you will not have to worry about how you connect the leads to the battery.

The bridge rectifier will have two leads/terminals designated for connection to AC power; connect these to your battery leads/clips. The bridge rectifier will also have a + and - terminals; connect these to the fish house wiring power feed line.

By nature of the diode arrangement inside the bridge recifier, no matter which way you connect the clips to your battery, the + and - polarity to the wiring will be correct.

The drawback is that you will have about 1.2-1.5 volt drop because of the diodes in the bridge rectifier. Some gear may not operate quite as efficiently with the voltage drop and lights may be a little less bright. Most things will be fine though.

Just be sure to get a big enough bridge rectifer for the load. You can get some fairly high current (30A ?) bridges at Radio Shack.

Kind of makes it foolproof as long as you fuse not too high and connect the bridge rectifier into the circuit correctly.

[sparkyaber]

That chart doesn't look like it came out of the N.E.C. . It seem to me that the chart is for vehicles and does not say any thing about insulation. Unless it is hi temp insulation (310-18, 310-19)........riiiiiiight. Seriously, that chart does look like it is for uninsulated conductors in free air, that is just a guess, but an educated one.

[Macgyver55]

Quote:

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That chart doesn't look like it came out of the N.E.C.

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You are correct Sparkyaber, it didn't. The charts I used were for automotive applications. They are simply a rule of thumb. Since Big Dave said he was wiring 12 volt and using an automotive style fuse box I immediately assumed what is true in automotive circuitry could be applied to his house wiring. I believe it can.

Millions of vehicles on the road routinely use a 20-amp fuse with 12 and even 14 gauge wires on them to protect various circuits without any problem. Many of these circuits are bundled and in often times very hot areas.

He also said that he would be using more than one circuit. I personally don’t think using a twenty-amp fuse for his lighting and other accessories is out of line by any means. We have our entire deer shack wired for 12 volt using 12 gauge wire with a couple 20 amp breakers. We run lights, radio, television, small, pancake fans and phone chargers sometimes all at the same time. We do have a separate 3-amp fuse for the stereo and the TV has one in the cord.

Whatever the case… sorry for dragging out the thread Big Dave, we’ve strayed way off of the original question now.

Hopefully somewhere here you got the answer you were looking for.

[fishermn]

Ok, hopefully this will help a bit.

Regarding the table above: This is a table for rating a single wire in free air (not in a bundle) most likely assuming a higher temperature insulation class that what you are using (I assume you are probably using standard household wiring?)

Regarding your selection of fuses: The 20 amp fuses will protect the wire from being exposed to an extended overload or from a short circuit, but thats about all. As some of the others mentioned, you will need a much smaller fuses (.5 Amp - 3.0 amp) to protect individual equipment (most of these should already have fuse protection...)

Even if you do use smaller fuses this will not gaurantee that you will not damage electronics if you do inadvertantly reverse polarities on the battery connection.

I suggest that you do something simple to avoid the confusion, such as using different style connectors for the positive and negative polarities. As an example, use a good quality clip (like you would find on a battery changer for one terminal) and a spade lug on the other terminal. Take the wing nut off the side of the battery that always gets the clip, and it will make it pretty difficult to mix things up. Or use color coded wires or clips.

[Whoaru99]

Quote:

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If you use a full-wave bridge rectifier between the main power feed wires to the fuse box and the battery, you will not have to worry about how you connect the leads to the battery.

The bridge rectifier will have two leads/terminals designated for connection to AC power; connect these to your battery leads/clips. The bridge rectifier will also have a + and - terminals; connect these to the fish house wiring power feed line.

By nature of the diode arrangement inside the bridge recifier, no matter which way you connect the clips to your battery, the + and - polarity to the wiring will be correct.

The drawback is that you will have about 1.2-1.5 volt drop because of the diodes in the bridge rectifier. Some gear may not operate quite as efficiently with the voltage drop and lights may be a little less bright. Most things will be fine though.

Just be sure to get a big enough bridge rectifer for the load. You can get some fairly high current (30A ?) bridges at Radio Shack.

Kind of makes it foolproof as long as you fuse not too high and connect the bridge rectifier into the circuit correctly.

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