Electrical theory OT, sort of

[Frank Blackstone]

I have a circuit that is delivering voltage , 90 volts DC, but very low current. Assuming the line size is sufficient, 2/0, and the amperage at the source is 0.15amp what could cause a drop to 0.08amp after going through several connections. Is it the DC volts that travel on the surface of the wire and the amps through the center of the wire? I believe I need to freshen up the connections and protect them from corrosion from the outside elements. I used IDEAL NOALOX anti-oxidant joint compound on the aluminum wire connections several years ago. How long does that protect? Anything better? On a related topic what is good for battery terminal corrosion prevention of copper to lead? I have tried several products but seem to have to clean terminals every 6 months. Thanks, Frank

[Plymouthy Adams]

you can go to the wire gauge chart and get the nominal resistance per size per 1000 foot and do the match..it would however take a good length of wire to drop anything significant..would suspect loose/dirty terminals before the cable..what are you running you have aluminum wiring on?

[Alexander]

what is the load? Amps can only be measured in series circuit.

how are you determining your amperage drop? how are you determining the amperage source? what is the resistance of the load?

the higher the voltage the lower the Resistance. 2/0 wire is more then thick enough to carry the current provided your run is not hundreds of feet.

[grey beard]

Frank,

Sounds to me like you have one or more connections with a voltage drop. Unlike amperage, you can measure for voltage drop with a simple voltmeter and all the components still wired in place. The voltmeter does not have to be connected in series, as does an ammeter. Just run your meter leads from beginning to end of your circuit on either side - pos or neg - and apply current to the circuit. Any voltage drop will be indicated on your meter - sort of like a parallel shunt circuit.

For example, connecting a voltmeter from a vehicle's battery post to the battery terminal - with the two parts still connected - will give you the voltage drop of that connection when you crank the engine with the starter. Pertty amazing what you can find with this sort of test.

This test is so simple to perform that most techs either ignore it or do not know about it, but it is an invaluable diagnositc tool. Remember, every connection is a potential resistance. Connections can also be mechanically tight but electrically loose, in terms of voltage drop. Think about it . . . . ..

Good luck.

[Merle Coggins]

Is it the DC volts that travel on the surface of the wire and the amps through the center of the wire?

Amperage is a measure of current flow. Voltage is a measure of potential difference between the positive and negative terminals.

Think of electricity like water in a garden hose. If the water is turned on but the hose is blocked off at the other end, you will have a certain amount of pressure in the line, but no water flow. In this scenario the faucet is the positive connection and the atmosphere is the negative terminal. So, the potential difference between the pressure in the hose verses the pressure of the atmosphere would relate to your Voltage. But since the hose is not flowing any water, there is no flow rate or Amperage. Relate this to a cable being connected to your battery at one end, but not connected at the other end. You can measure the voltage between the end of the cable and the other terminal to see the potential Voltage, but without it being connected to a circuit there is no current flow, so no Amperage.

Now, let's connect things up to do work. I'll connect the hose to a sprinkler and the battery cable to a lamp. Now you'll have water flow and current flow. Water flow can be measured with a flow meter, which is connected in series to the work load so that the water flows through it. Amperage is measured with an Ammeter in series so that the current flows through it. The water pressure would be checked with a pressure gauge Teed into the line. Voltage is measured with a volt meter Teed into the circuit, (touching the wire or terminal with the probes). With this scenario I can measure the flow of water going through the sprinkler, just as I can measure the current flow through the lamp. And I can connect a pressure gauge to the connection going into the sprinkler and measure the water pressure at that point. Now measure the water pressure after is comes out of the sprinkler and has done it's work (the water on the grass). This is the pressure drop. This is the same in an electrical system. Measure the voltage before and after the lamp and you'll have your voltage drop.

With a single load in the circuit the voltage drop across the load should equal the total potential of the battery. However, if there are multiple loads connected in series the total voltage drop in the complete circuit will equal the battery voltage. In other words, each load with consume some, but not all, of the voltage in the circuit. But the amperage will be constant throughout the entire circuit. The amperage will go up or down depending on the amount of total load (resistance), but it will be the same at any point in the circuit.

That's my electricity 101. It either cleared some things up, or totally confused you.

[Johnny 5]

Aluminum is the worst conductor of all metals. Electricity has to work harder to travel through it than copper so I'd switch to copper connectors for starters. Are you sure the connections are aluminum? They're usually just tinned or plated to prevent corrosion. And yes, DC travels through the conductor, not on the outer surface using the skin effect like AC or microwave. Current flow is positive to negative but electron flow is actually negative to positive. The no-ox stuff you're using is industrial strength. I haven't heard of anything better yet.

[Frank Blackstone]

The connectors are of unknown metal that does not hold a magnet. Maybe copper or brass. The wire is copper braided. the screws holding this connection are stainless steel. The positive side is grounded. The consensus of the gearhead, good ol'boy and electronic geeks seem to be to use P-V silicone sealer on the connections. I cleaned off the grey goop no-nox after several years and it was still goop but no corrosion. Would changing the ground to the negative increase the current? If DC travels through the wire body why do we use braided wire which has more surface area and less body area?

[Don Coatney]

I checked with both Horace Lamb and Oliver Heaviside and what I found was that for DC current it's inside the conductor.

"For DC (Direct Current) in typical conductor material like copper, aluminum, silver, etc., the conductor's charges are flowing distributed uniformly throughout the material."

However for 60Hz AC (alternating current) in typical conductor material, the current is concentrated in the outer few centimeters of the cable. So if your cable diameter is less than one cm or so, then the current flows everywhere inside.

"As the frequency increases, the current tends to get channeled closer to the surface of the conductor. At frequencies well above 60Hz, the current in the center of the conductor can be considered non-existent for all practical purposes. That is why radar units (which operate at frequencies anywhere from 1GHz to 15 GHz or more) usually transmit their eletrical energy through waveguides, which are really hollow tubes, either circular or rectangular. At those frequencies, no current exists anywhere except at the surface, so the hollow tube is effectively just the skin with no center."

This does little to explain why most household AC wire is solid yet industrial AC wire is stranded. Nor does it explain why most automotive DC wire is stranded.

[Frank Blackstone]

Good information, especially about radar, and I'll kkep looking for info about stranded DC. Inquiring minds want to know.