Freewill - I don't mean to sound picky. But, a 10 Amp circuit is way overkill for the control power circuit. Since the common relay only pulls about 0.1 amps. Theoretically, a 10 Amp circuit could control 100 relays. I usually recommend a 3 Amp fuse. I think that the "theoritical 30" relays is enough.

When tapping OE systems for control, I always recommend these untrained guys to use a fuse at the tye-in point. Just like tapping the battery. I say this to protect the OE systems. If there was a fault on the "added" system, and since the "added" system would have a much smaller fuse than the OE system, the new fuse would open leaving the OE system in tact and opperational. This is a sort of "Planed Failure" method. And otherwise known as "CYA".

 

Thanks everyone! I'm gonna go out today and get two fuses.

 

Agreed, and good points.

The 10 amp suggestion was about protecting #16 wire. The coil only needs a fraction of that capacity.

Also like the idea of a fuse at the tie-in point.

I had the same thoughts while typing, but kept it to the basics of having fuse sizes and wire sizes that match. Darn posts get too long otherwise

 

Ron, first I want to thank you for keeping me on my toes. I wish that I could be as intellectual as you. And I know that sometimes I'm a little slow. But please bear with me on this point.

Lets say that you have an electric motor whose nameplate indicates a FLA rating of 28 amps. And this motor is connected to a #10 conductor and protected by a 30 amp fuse. Now lets say that this motor is applied a load that cause the amperage rating to exceed the 30 amp fuse by just a few amps. Only the fuse threshold is exceeded. The conductor may be stressed. but the threshold wasnt exceded. So the fuse is blown. After the load is removed from the motor. And the fuse is replaced. The motor is capable of being operated safely again. My point is that a fuse can protect the circuit AND the load. Without the correct size fuse in the above example, both the circuit and the device would have been destroyed.

 

What you are saying is not incorrect. It's perfectly fine to fuse for less power than a circuit can carry, if you want to limit what an appliance can have for current.

The fact is, though, most appliances are internally protected, and get power from a circuit that is capable of delivering much more power than the appliance will consume.
Look at the power outlet on the center stack of your JK. It has a 20 amp fuse, and wire that can safely carry the 20 amps, (which means the wire can carry more than 20 amps).
However, there is absolutely nothing wrong with plugging in an appliance that draws only 1 amp.
If there is a short, or you plug in something that draws more than the wire can handle, the fuse blows before the wire glows.

The same is true of household circuits. You install wire heavy enough to carry the anticipated load, and fuse accordingly, to protect the circuit. It matters not if you plug in an appliance that draws a fraction of the circuit's capacity.

Lots of people install extra circuits in their vehicles. It's important for them to remember the relationship between the wire gauge and the fuse they choose.

 

Ron and JKF - Sometimes I use an "overkill" branch circuit to minimize voltage drop. A 30 amp load might get 8 or 6 cable and 40 amp protection. The long cable run acts like a variable resistor as the load and voltage drop change. The fuse/breaker sees both the load and the cable, and can't sense the load very well.

So the load device has internal protection or gets its own local fuse. The cable delivers higher voltage to the device. Less power is lost along the cable. Life is good.

I use this thinking in my bus where current for big loads travels at 12 volts for 20 - 30 feet. We dry camp for weeks at a time where loosing less power along the branch circuits adds up at the batteries.

Power loss is not the point in this thread of course. I'm just using it as an example of how it can be an advantage to consider the branch circuit and its protection separately from protecting the load device.

 

I understand completely. And this actually helps me to prove my point. That a fuse "IS" capable of protecting both the circuit "AND" the connected device depending on what it is engineered to do. I think that your statement that "fuses only protect the conductor" is a little "Brash". Granted that your not going to protect a single light fixture on a lighting distribution circuit with a single fuse. That fuse will protect the conductor. However, present day HID lighting distribution systems incorporates individual fuses at each fixture. And these fuses are rated to protect the fixture. Also, a single fuse could protect a winch motor. If engineered and installed correctly. A winch motor at full current would eventually open it's fuse before the motor is destroyed. The conductor should be capable of supplying full motor current with minimual damage. However, an electric motor is not capable of maintaining full current for a very long time. So, once again, the fuse is protecting the appliance.

Hey Freewill. I still have a hard time with voltage drop on these smaller voltage systems. Being primarily an AC electrician, the smalles voltage I usually deal with is 24v AC. I understand that bigger is always better when it comes to distribution circuits. But bigger is not always necessary. I know that I am better at practiceing my craft than teaching it. But I always try to help people understand what is necessary for a system to work. I prefer people to have the confidence in knowing that they are using the correct size wire for their instalation. And not just simply use a bigger wire because they are unsure.

 

Boy you said a mouthful with that last sentence. When in doubt, find out!

Most of my experience was industrial. Motors all had a local disconnect with protection. The local protection was sized to the motor with more precision than industry standard fuse sizes could offer; the disconnect was there for service. Its an old habit to think that way even though commercial and residential motors and appliances usually have their own internal protection.

AC allows the use of transformers so its easy to keep the voltage high so that conductors don't need to be all that large and voltage drop is easy to manage. With low-voltage DC we are stuck with using a lot of copper to get much power from A to B. Its the reason vehicles went from 6v to 12v in the fifties, and why big buses run 24 volts. Fortunately Jeeps are small so voltage drop is one less thing to worry about. Wish I could say the same for my bus. I've got some 4/0 runs in there for 100 to 120 amp loads because the voltage drop was 15-20% when conductors were sized for ampacity rather than voltage drop.

I'll say one thing: When you and Ron and I get onto an electrical thread here, the O.P. may get more than they bargained for, but the eventual answer will be dead-on!

 

Hey, if this stuff was easy, we wouldn't get paid so well to do it. And, like you and Ron, I enjoy helping others.

 

I never got TOO much into electrical work, so I've just sitting back, reading and learning from u guys!