Fuse off of Battery

[Steve B]

So I've read and found what I think I'm looking for but want to ask to make sure. If I am running a hot lead off of the battery I should use 10gage wire and make sure there is a 30amp fuse? Is my thinking correct?

[Thane]

A 30 amp circuit breaker is typically used for the tow vehicle to trailer connection. A circuit breaker will automatically reset as opposed to blowing a fuse which would require a little maintenance to correct.

I am adding a 12V exterior plug on my trailer and am using what you describe, a 10 gauge wire and in-line fuse. I will use a 15 amp fuse for increased safety. I don't plan on running heavy duty items, just an electric cooler and other cigarette lighter compatible items. I will encase the 12V plug in a PVC pipe contraption to make it water tight when not in use.

[Steve B]

Oh yeah, I forgot, do I run the lead off the (+) or (-) side of the battery or doesn't it matter?

[Steve L.]

Run the lead from the positive to the circuit breaker and then on to the equipment. Otherwise, the items remain "hot" at their terminals like to a wrench or screwdriver that accidentally touches a new ground.

[Steve L.]

As I read this again this morning, I may have been a little too brief to be clear in my first response.

Electricity basically flows from pos to neg. A circuit breaker or fuse protects everything downstream of itself. If the fuse blows on the way back to the battery because it's in the negative line right at the battery, all the wires prior to the fuse are still live and could be shorted to some alternative ground. There are all sort of things you could short to while trying to fix what is wrong.

Keep in mind that the circuit breaker in the positive side protects only things on its loop or circuit. It should be in the only line that goes to the positive terminal of the battery. If you have more than one wire to the positive terminal, then the circuits on the other lines aren't protected.

My trailer came with a thermally reset breaker. It will trip in an overload (due to heat) and then when (and if) it cools off, it resets. This is handy because it's in an awkward spot to get to. But I have to keep in mind that it will reset and that the circuit can become "live" again suddenly. These are inexpensive, come in various amp ratings and are usually readily available in auto parts stores.

[Byron Kinnaman]

As I read this again this morning, I may have been a little too brief to be clear in my first response.

Electricity basically flows from pos to neg.

This theory was disproved many many years ago. Current flow is from Negative to positive. (Ben got it wrong)

A circuit breaker or fuse protects everything downstream of itself. If the fuse blows on the way back to the battery because it's in the negative line right at the battery, all the wires prior to the fuse are still live and could be shorted to some alternative ground. There are all sort of things you could short to while trying to fix what is wrong.

The main reason you want to fuse the positive side is... the negative side is usually connected to large pieces of metal, like the frame and appliance cases. This is convention that is used because automobiles standardized on a "negative" frame system, that is the negative side of the 12 Volt DC is connected to the vehicle frame. Notice I didn't use the term "ground". The term "ground" is misunderstood more than any other term I know of.

[Steve L.]

I'm comfortable with my response as written. I think Mr. Kinnaman is confusing electron flow with the definition of current flow.

http://www.mste.uiuc.edu/murphy/Hole...tricFluid.html
http://cnx.org/content/m0011/latest/
http://iss.cet.edu/electricity/pages/b14.xml
etc., ad nauseam

From the physics I can find, the drift velocity of electrons in a 12ga wire carrying 10 amperes of current is something on the order of 31 inches/hour. I believe the actual difference in the defined current flow and actual electron movement is something akin to a distinction without a difference.

But if you feel this distinction is important and adds to the discussion, more power to you (pun intended).

[Byron Kinnaman]

I'm comfortable with my response as written. I think Mr. Kinnaman is confusing electron flow with the definition of current flow.

http://www.mste.uiuc.edu/murphy/Hole...tricFluid.html
http://cnx.org/content/m0011/latest/
http://iss.cet.edu/electricity/pages/b14.xml
etc., ad nauseam

From the physics I can find, the drift velocity of electrons in a 12ga wire carrying 10 amperes of current is something on the order of 31 inches/hour. I believe the actual difference in the defined current flow and actual electron movement is something akin to a distinction without a difference.

But if you feel this distinction is important and adds to the discussion, more power to you (pun intended).

OK, What's current????

[Pete Dumbleton]

Just to add some theoretical/practical confusion to the mix, I have several friends in the boat maintenance business and they tell me that European boats (30-50 feet that they work on) have all the fuses and circuit breakers on the negative side of the battery system...

[Byron Kinnaman]

Just to add some theoretical/practical confusion to the mix, I have several friends in the boat maintenance business and they tell me that European boats (30-50 feet that they work on) have all the fuses and circuit breakers on the negative side of the battery system...

It really doesn't matter which side is fused when it's just wires. The same current, same number of amps, has to flow in both the negative side and positive side through any powered item. Where it does matter is when one side is connected to a lot of metal, such as your automobile. The convention today, negative is connected to the chasis of the car. That hasn't always been the case. That convention is carried through to trailers and other things.

Most of the confusion is caused by the fact that Ben Franklin got it wrong and the term "ground". Early telegraph used the earth (ground) as the return path, because Ben had it wrong the negative potential was the ground potential. Confusion and misunderstanding has continued to this day. I see it almost every time somebody starts talking about electricity on any forum or mail list.