Date: Tue, 10 Dec 1996 22:49:21 -0800 From: Mike West Subject: Wire Ampacities and Kirchoff Roadkill Electronics or how we come to know Kirchoff. I think this will be a new series on electrical stuff. Ben Franklin, the dicoverer was our kind of guy. You all know the story, he's out flying a damn kite in a lightning storm with the house key up the string. That was to keep the housekeeper locked out, kind of mess with her mind thing. Ben is humming something that faintly resembles "reach up. . . and touch the sky" Zap! goes the lightning, Zing! goes Bens muscles, Whizz! goes the trousers trout, right down his leg. Definitely a man ahead of his time. The housekeeper quit, right then and there. He was the Ambassador to Europe at the time, so he's hangin' with the guys at the old Savoy. Some of the guys were Ohm, Ampere, and Count Etcha Volt. He tells them the story and they all got a laugh. I'm not sure whether Galvani was there or not. So they all went home and played with it a while and wrote "papers" proclaiming their discovery and naming it after themselves. None of them had "real" jobs. Watt was an engineer and eventually had to clean up this mess and equate it to something real, like "horsepower". Well enough frivolous history. These "numbers" below are the ampacity of some of the wire sizes in our "small wonder" machines. In your manuals, under schematics, you will see some small numbers alongside the wires. Those are the metric sizes of the wire used. >From that you can deduce what size fuse goes in your circuit. No, it is not "cool" to have all red ones in there. 0.5mm: (.032") 5 amps 2.0mm: (.063") 15 amps 0.75mm:(.029") 7 amps 2.5mm: (.071") 17 amps 1.0mm: (.045") 9 amps 6.0mm: (.110") 35 amps 1.5mm: (.055") 13 amps 25mm : (.225") 80 amps Now we've done the academic thing we can get back to frivolity. This is a hypothetical situation and never happens to V-dubs, but you might stumble over some guy with a Maseratti and have to play "good samaritan". It's 2 a.m. in the middle of the corn-belt in Iowa, it's dark, it's raining and you're wearing sun glasses. He's daintily poking around under the dash when you arrive and ask the question. His windshield wipers are out. You mosey back to the van and get your trouble light etc. Coffee can full of tools. You poke up in there and do your magic act and come back out with the answer. "Your wiper motor's shot, dude" Can't drive in this rain without one so you offer a ride. Won't leave his car in this terrible place. Offers money . . . . hmmm. Over in the field is a '32 Packard, abandoned, so you wander over for a look. It has a windshield wiper motor, might still work. Back to the Maseratti and look at the physicals, hmmm, it's doable. Still need a resistor for the six volt motor, hmmm. Back over to the Packard where you do a dissection while you think about the other problem. Suddenly a light goes on . . . . and you remember Kirchoff. Kirchoff was a heavy who did some work on series circuits based on Ohms Law. Kirchoffs Laws "The resistance of a series combination is the "sum" of the resistance of the seperate parts". "Current thru a series combination is the same as the current thru each part". "Voltage across a series combination is the "sum" of the voltages across the seperate parts." You chuckle a little and step back and look at the tail-lights. Yes there's still one with a bulb. Looks ok. You hang the wiper motor in with two bolts, fix the mis-aligning and different size shafts with a piece of fuel hose and hose clamps. Now for the master-piece: you go back to the Packard and pull the tail-light socket and bulb and clean it up and wire it in series with the 6 volt motor. The guy is moaning by now but you continue on like a man maddened. Flip the power on and voila, wiper action. the wipers turn while the tail light burns. The guy gets back in his car gingerly, and with dubiety, it's been fixed by a V-dubber after all, smiles wanly, and peels out of there! You laugh hysterically for the next hundred miles. :-) What Kirchoff says back there is if you stick 2 six volt loads in a 12 volt system they work just fine. A tail light bulb will probably carry 4 amps, about the same as a wiper motor so we should be alright there. The second law is the one that gives us the six volts. The third law says the amperage will be what it should be. west, this electricity is fun stuff!