wire guage simple question

[patrick_g]

if the wire takes damage like chafing... hopefully the fuse will pop!

soundguy

Please enlighten me (perhaps a daunting task) as to why chafing one wire of a parallel pair would cause a fuse to "pop." Lets say it chafed entirely through such as I cut it in two. All the current (OK, a slight reduction due to increased resistance) is now flowing in the one remaining conductor which is likely now undersized and could dangerously overheat BUT the breaker or fuse would likely not "pop" since its current rating has not been exceeded.

Assume you have a wire capable of safely carrying 10 amps and you parallel it with a second similar wire to handle 20 amps in total (forget about the REAL details of whether the conductors are twisted together, in a conduit or in free air or...) and feed their load via a 20 amp breaker. Lose one of the pair and you are feeding up to 20 amps through a wire we agreed is safe for 10 amps so dangerous overheating is a distinct possibility.

If you assumed that the chafed wire was chafed by contact with a grounded conductor then "popping" a fuse would be a distinct possibility BUT... you didn't say that.

Pat

 

[ModMech]

Ugh, I can't show this well, but I will try.

If you have two circuits that share a common circuit protection device (fuse, breaker, link etc), the protection (or conversely wiring) must be selected so that a short in ANY conducter will cause the protection to become active and stop current flow.

Assume B+ is a battery and "FuseA" is a single fuse rated at say 30A

B+ FuseA(~)______Wire1_________Load"X"____GrndB-
B+ FuseA(~)______Wire2_________Load"X"____GrndB-

If EITER Wire1 or Wire2 becomes shorted to ground, it will fail FuseA, if the circuit is PROPERLY DESIGNED AND PROTECTED. With "home brew" repairs and projects, mine included, this is often not the case.

Again, EVERY conducter attached to a circuit protection device (fuse) must be capeable of carrying the amperage rating of the protection at all times. Or in reverse, the circuit protection on any circuit must be sized so that no wire connected to it will fail (melt) when carrying the full rated amperage of the fuse.

If a wire is "grounded" or "shorted" (to ground), it will try to carry the MAXIMUM current of the entire system. With a 225A batterty, that 16Ga wire will TRY and carry all 225A!!!!! It doesn't matter if there are 10 16Ga wires connected to that 15A fuse, the one that is shorted will carry ALL the load and fail the fuse.

The first part of the question was, I think, what if a B+ wire (wire1) chaffed to another B+ wire (wire 45) on a different circuit and would the resulting overload of the remaining wire (wire2) cause the fuse to fail. All GOOD questions and the answers are...... it depends!

Assuming wire1 is cut, leaving the entire load to wire2, if the circuit is designed correctly it will "pop" it's OWN 10A fuse, but in the example above, wires 1&2 SHARE a common 20A fuse. In that case, you fail the wire to the point of insulation melting and ultimately ground it and fai lthe fuse. LOTS of re-wiring to do!

Assuming wire1 is simply shorted to wire45 and wire45 is a power wire for something else. As long as wire45 and wire1 are similar in size nothing will really happen other than anything powered by one will also turn on the feature of the other. If the fusing is adequate, each wire will power it's load plus that of the other, whenever they are activated. In most cases, power circuits cannot handle much more than their own loads and the additional load will pop the fuse (or CB or link). However, this again assumes proper design where no wire is fused to more than it's rated ampacity, which with modified systems is often not the case.

 

[pat32rf]

(retired service eletrician) In a tractor installation case like this I would use one fuse, knowing that an overload would be caused by a short circuit, in which case even a 40 amp fuse would blow before it melted a #14 wire....

 

[Soundguy]

Please enlighten me (perhaps a daunting task) as to why chafing one wire of a parallel pair would cause a fuse to "pop."Pat

Here you go.. I'll talk slow and use big words.

I was envisioning this scenerio when i posted that comment.

wire pair is going thru a metal panel, like a fire wall, or routed behind a dash cowling, or is simply laying on the top of an engine like I see so many tractor wireing setups.. or it's coming out of the headlamp shell on the side of the hood... no grommets there, just wire insulation on metal.. wire chafes.. circuit now has a 'short' path to chassie ground, fuse pops.

no daunting task.. just lazy electrons...



soundguy

 

[jlrsn]

I'd also solder them vs crimp connectors.
soundguy

This is semi long and for educational purposes if you want to read it.
It has been proven that a soldered wire connection is more prone to breakage due to vibration than a Properly Crimped Solderless Connector. When I state properly crimped I don't mean where a person used side cutting wire cutters (Dikes) or cheap automotive type crimpers to crush (flatten) the solderless connector onto the wire. A properly crimped connection is known as a gas tight crimped connection. When you solder a stranded wire capillary action will cause more than the connection to become a non-flexible solid wire by wicking the solder up the wire. This is why in the aircraft industry this type of connection is not allowed unless specifically called out in the specs. When I give an aircraft an annual inspection as a IA (FAA Inspection Authorization) I won't pass a soldered or improperly crimped connection. Most of the improper crimps found on aircraft are usually done by the aircraft owner, improperly trained aircraft mechanics, or by avionics shops not knowing better. This is why aircraft avionics shops are now required to have at least a Certified Airframe Mechanic (A) on their staff to sign off the work. As a DME (FAA Designated Aircraft Mechanic Examiner) giving the Airframe and Powerplant Practical Exams and issuing the Aircraft Mechanic Certificates (In the U.S. no one has a Mechanic or Pilot's License, only Certificates) I stressed the importance of proper connections. Now the dangers to a tractor tearing up a dirt road at 15mph is not as great as an aircraft flying at Mach 0.95 at 50,000 feet and I routinely find wires just hand twisted around a lug on tractors, cars, etc.
Jim

 

[Soundguy]

This is semi long and for educational purposes if you want to read it.
It has been proven that a soldered wire connection is more prone to breakage due to vibration than a Properly Crimped Solderless Connector.

And a properly soldered connection should be just fine.

'strain relief' is not just something you take an aspirin for...

soundguy

 

[Bedlam]

I agree Solder is fine and has been fine for me for 40 years. Must depend what its used on.Maybe its so cheap people don't like it or maybe because its an "old time fix" so to say..All connection ways have a place.

 

[patrick_g]

I agree Solder is fine and has been fine for me for 40 years. Must depend what its used on.Maybe its so cheap people don't like it or maybe because its an "old time fix" so to say..All connection ways have a place.

Solder is a fine thing in its place and done right. Many more folks do it than do it right. Many solderers have no clue regarding heat induced embrittlement. Ditto for wicking of solder into multi-strand wire for a significant distance from the site where it is applied. A clamp on heat sink immediately behind or on both sides of the area to be soldered can help keep the temp low away from the area to be soldered and reduce the wicking action.

I have some wires to be extended (related to my breaker boxes and a generator installation) and I considered two ways to git 'er done. 1. a soldered joint and 2. crimp-on but splices. The largest wire I have to solder is #4. I have found that #4 wicks heat so quickly that it is hard to get a good solder flow at the intended site and the joint takes so long to do that heat goes up the wire melting the insulation and or wicking solder up inside the wire makeing itr virtually impossible to shape easily, pull it through a conduit elbow or whatever.

If flexibility and vibration are not issues I have been known to use a short piece of copper tubing as a but splice (with or without crimping), applying heat with a torch to the copper tubing and feeding in the flux cored solder at both ends of the tubing. A propane torch or even oxy-acetylene can be used to heat the tube so fast that you can get solder in both ends and get proper flow before the heat or solder flows too far up the wires. The advantages of the copper tube are that it takes the heat well allowing you to use much more intense heat and get the job done without overheating the two wires and the tube protects the small strands from the intense heat of the torch.

I also use short lengths of copper tubing to make ring terminals: insert wire part way into the tube and crimp or hammer it flat. Solder the wire to the tube as above and then drill a hole in the flattened part of the tube that doesn't have wire in it. This is easy to do and makes connections that have lasted me for decades and I have never had a failure. Saves considerable $ too as large ring terminals are expensive and little scraps of copper tubing aren't.

How am I going to splice my #4 and #6 wires? I guarantee it will not be wire nuts.

I have recently purchased a hydraulic crimper with a set of dies to cover 12 ga to double ought. I am not happy with the high cost of store bought butt splices for the larger wires but... I don't have to do all that many.

Pat

 

[jlrsn]

And a properly soldered connection should be just fine.
soundguy

I agree with you Chris when it is not used on aircraft. That's the reason that I said it was for educational purposes. With aircraft I've had vibration so strong that when I was taking a 15' parabolic antenna 500 feet vertically up to a building top mast with a helicopter between the hi-rise buildings of San Francisco that the engine out warning horn started sounding when a stranded wire broke giving me a false alarm and then my #1 radio I was talking on stopped when the radio broke from it's cage and landed on the floor. I personally wouldn't think twice about soldering a connection on my tractor, truck, car, motorcycle, or on a piece of equipment in my machine shop. Then its not that critical.
Jim

 

[Soundguy]

I personally wouldn't think twice about soldering a connection on my tractor, truck, car, motorcycle, or on a piece of equipment in my machine shop. Then its not that critical.
Jim

Since this is a tractor website..... and not an airplane or helicopter website.......


soundguy